The harvested side of edges: Effect of retained forests on the re-establishment of biodiversity in adjacent harvested areas

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Review

The harvested side of edges: Effect of retained forests on the re-establishment of biodiversity in adjacent harvested areas

Susan C. Baker a,⇑, Thomas A. Spies b, Timothy J. Wardlaw c, Jayne Balmer d, Jerry F. Franklin e,Gregory J. Jordan a

a University of Tasmania, School of Plant Science, Private Bag 55, Hobart, Tasmania 7001, Australia b US Department of Agriculture Forest Service, PNW Research Station, 3200 Jefferson Way, Corvalis, OR 97331, USA c Forestry Tasmania, Division of Research and Development, GPO Box 207, Hobart, Tasmania 7001, Australia d University of Tasmania, School of Geography and Environmental Studies, Private Bag 78, Hobart, Tasmania 7001, Australia e School of Environmental and Forest Science, College of the Environment, University of Washington, Seattle, WA 98195, USA

a r t i c l e i n f o

Article history:Received 14 January 2013 Received in revised form 18 March 2013 Accepted 19 March 2013

Keywords:Forest influenceEdge effects Variable retention ClearcuttingDispersalRe-colonisationNatural disturbance

a b s t r a c t

Most silvicultural methods have been developed with the principal aim of ensuring adequate regenera- tion of commercial tree species after harvesting. Much less effort has been directed towards developing methods that benefit the re-establishment of all forest biodiversity. The concept of ‘forest influence’relates the probability of species re-establishment to the distance from mature forest. This idea is central to contemporary retention forestry practices as well as connectivity theory in natural landscape manage- ment. Some species from all major forest biodiversity groups respond to forest influence following har- vesting, however, the temporal and spatial scales of forest influence are mostly poorly known. This paper reviews global knowledge of mechanisms and scales at which forest influence operates, and shows that these are highly variable. Important general factors and mechanisms that underlie the ability of organ- isms to re-establish include qualities of retained elements, dispersal capacity, suitability of habitat con- ditions, and interspe cific interactions, all of which may vary with distance from intact mature forest.Forest influence may enable species to persist in harvested areas through buffering of microclimate,and/or assist re-colo nisation via proximity to source populations or essential habitat elements. Although foresters have often applied a ‘‘rule of thumb’’ that the extent of forest influence is within one tree height of mature forest, existing scientific literature provides little evidence of a universa l relationship between canopy height of retained forest and re-e stablishment success. One-tree-heig ht-from-retention guide- lines can help plan harvest layouts, but only as long as plans allow for variation in re-establishment suc- cess among species and groups. The evidence from this review is that variability in harvest layout s will positively benefit biodiversity conservation in managed forest landscapes.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 3. Retention silviculture and forest influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

3.1. Overview of retention silviculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 3.2. Silvicultural management of forest influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4. General factors and mechanisms leading to forest influence on biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 4.1. Qualities of retained elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 4.2. Dispersal limitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

0378-1127/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.foreco.2013.03.024

Abbreviations: VR, variable retention; ECM, ectomycorrhizal fungi.⇑ Corresponding author. Tel.: +61 3 6235 8306; fax: +61 3 6226 2698.

E-mail addresses: [email protected] (S.C. Baker), [email protected](T.A. Spies), [email protected] (T.J. Wardlaw), [email protected](J. Balmer), [email protected] (J.F. Franklin), [email protected] (G.J.Jordan).

Forest Ecology and Management 302 (2013) 107–121

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4.3. Microclimatic gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1114.4. Distance to critical habitat elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 4.5. Interspecific interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 4.6. Temporal factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

5. Impacts on different groups of organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 5.1. Vascular plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 5.2. Bryophytes and lichens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 5.3. Vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

5.3.1. Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 5.3.2. Birds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 5.3.3. Amphibians and reptiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

5.4. Invertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 5.5. Fungi and microbes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

6. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6.1. Mechanisms and scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6.2. Comparison with edge effects into unlogged forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 6.3. Harvesting patterns and forest influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

7. Knowledge gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 7.1. Recommendations for sampling designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 7.2. Recommendations for future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

7.2.1. Mechanisms of forest influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 7.2.2. Spatial and tempor al scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 7.2.3. Harvest design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

1. Introductio n

The direct and indirect effects of nearby mature forest on dis- turbed areas are known as ‘forest influence’ (Keenan and Kimmins ,1993). These edge effects are diverse and complex, but some important aspects relate to dispersal of individuals and seed, aswell as microclimat ic gradients such as shading. Proximit y toundisturbed mature forest may assist forest organisms to re-estab- lish in areas disturbed by timber harvesting or natural disturbance.The magnitude and distance of forest influence varies between species and environmental characterist ics, such as slope, aspect,latitude and microclimat e (Keenan and Kimmins , 1993 ). Forest influence may be positive or negative, depending on the factor orspecies involved (Bradshaw , 1992 ).

In forests managed for wood production, foresters have tradi- tionally develope d harvesting techniques designed to ensure prompt and adequate regeneration of commercial tree species.However, the degree to which harvesting practices enable other organisms to be sustained in the post-harvest forest is much less well understood . Understandi ng the mechanisms, spatial scales and ecological impacts of forest influence on biodiversity (the topic of this review) can assist forest ecologists and managers in design- ing ecologically sustainable forest harvesting practices, predicting recovery from natural disturbance s, and mitigating the adverse ef- fects of forest fragmentation .

Historically, forest conservation efforts have focussed more onreserving areas of unharvested forest, and much less on the re- establishment of biodiversity in harvested areas. However, it isbecoming increasingl y clear that harvested areas are important for maintenanc e of regional biodiversity, and that characterist ics of the surroundi ng landscape affect the population sustainabili tyof many species (Lindenmayer and Franklin, 2002 ). Adequate con- servation in managed forests requires both reservation and appro- priate managemen t practices on those areas available for harvesting (Lindenm ayer and Franklin, 2002 ).

Forest influence is also relevant to the management of forests subject to natural disturbance . In temperate and boreal forests,large-scale natural disturbances like wildfire, insect outbreaks

and windthrow kill stands of mature trees, although enough usu- ally survive to enable re-establish ment of a new tree cohort and as- sist re-establi shment by other biodiversity (Turner et al., 1998;Lindenm ayer and Franklin, 2002; Dale et al., 2005 ).

Both timber harvesting and natural disturbance can result in lo- cal elimination of some species that are adapted to conditions inmature forest (Lindenmaye r and Franklin, 2002; Whelan et al.,2002; Baker, 2006 ). Re-colonisation of the regenerating forest,and the subsequent establishment of viable populations by these species, depends on dispersal either from nearby intact mature for- est or from surviving individuals within the harvested area. The effectiven ess of this dispersal depends on species’ life history traits and the availability of suitable habitat. Dispersal limitation can constrain colonisation by species from all taxonomic groups (Bull-ock et al., 2002 ). Indirect effects of proximity to forest edges on bio- diversity also affect habitat suitability. For example, shading by the nearby mature forest may allow some species to persist and/or re- establish in otherwis e hostile harvested areas by providing sites that are buffered from extreme environm ental conditions.

Although studies of edge effects on biodivers ity in managed for- ests are common, these studies have almost always been focused on gradients from the harvest unit into the unlogged forest and ig- nored the forest influence gradients into harvested areas (Harperet al., 2005 ). This orientation is related to the interest in impacts of logging on the conservati on of biological diversity in adjacent areas. While there is a large literature on recruitment of seedlings into gaps (including micrometeo rological gradients), the focus ofgap studies is usually from a silvicultural rather than biodiversity perspecti ve (Gray and Spies, 1996; Coates, 2002 ). However, con- temporary forest managemen t is based on the premise that it isimportant to integrate wood production with conservati on objec- tives within the harvest site. Under these new approaches, ecolog- ical research and practical application have usually focused onmaintain ing refugia of mature-forest species and structures within harvested sites without explicit consideration of the role this retention plays in re-establish ment of biodiversity in harvested areas (Baker, 2011 ). Nevertheless, ‘rules of thumb’ are sometimes applied to facilitate forest influence in retention forestry systems

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(Mitchell and Beese, 2002; Baker and Read, 2011 ). Such efforts are constrained by inadequate understand ing of the ecological processes.

Our synthesis of forest influence builds on Bradshaw’s (1992)re-view of forest influence which focused on silvicultural aspects such as regeneration establishment, stand developmen t, felling damage and burning along with aesthetics. Bradshaw (1992) provided little discussion of how forest influence can impact biodiversity, except where animals impacted on production outcomes (e.g. seedling browsers). The impact of forest influence on biodiversity is there- fore the focus of our review. The aim of this paper is to review the current literature to assess the different components that make up forest influence, their relationship to biologica l diversity and their role in conserving biological diversity in forests managed for wood production. An understand ing of how and why forest influence affects re-establi shment by biodiversity can assist ecolo- gists and managers develop practices that encourage re-establi sh- ment of mature-forest species in disturbed areas. We summarise the limited information on the spatial extent of forest influenceand describe common factors and mechanism s underlyin g forest influence, and their relevance to different elements of biodiversity.Until there is better empirical data on the spatial and temporal scales of forest influence, general knowledge of factors and mech- anisms that underlie biodiversity responses should assist manag- ers to predict the efficacy of different silvicultural options for biodiversity conservati on.

2. Methods

We have reviewed a wide range of ecological information onthe processes and scales of forest influence on major groups of for- est biodiversity, primarily focusing on research that directly relates to forest harvesting. The paper first outlines retention forestry methods and their value for increasing forest influence in har- vested areas. We then review and synthesise what is known about the underlying factors and mechanis ms of forest influence, and their effects on particular biodiversity groups. Finally, we highlight key knowledge gaps that require further research effort.

We focus on harvestin g of native forests in temperate and bor- eal regions where one managemen t aim is to conserve biodivers ity at the landscape scale, regardless of whether sites are regenerated naturally or artificially, such as by planting or seed sowing. The lit- erature was largely derived from studies of retention forestry along with a variety of silvicultural systems that would traditionally have been classified as even-aged, including clearcutting, gap cutting,seed tree, and shelterwood. However , our review is relevant more

broadly, including to reduced-im pact logging in the tropics (e.g.Ruslandi et al., 2012 ), since, as Bradshaw (1992) points out, distinc- tions between even-age d versus uneven-aged managemen t are artificial, especially when one takes a global overview.

We aimed to achieve a broad review of the topic, and can only consider each part briefly. Only the most relevant papers were in- cluded, omitting much detail. The papers were selected on the ba- sis of personal knowledge and database searches. Search terms included, but were not limited to: variable retention, green-tree retention, forestry, clearcutting, harvesting, dispersal, re-colonis a-tion, re-establishme nt, biodiversity. Search terms identifying the individua l biodivers ity groups were also used. We also approached regional experts in retention forestry to obtain additional references .

3. Retention silviculture and forest influence

The proportion of harvested area influenced by the edge is animportant characterist ic of any silvicultu ral system (Bradshaw,1992), although historically the focus was usually on maximising tree establishment, survival and growth. However , retention for- estry approaches also have biodivers ity conservation as a key objective, and thus maintaining forest influence over a large pro- portion of the harvested unit is often a primary consideration. Toillustrate the concept, Fig. 1 contrasts the hypothetical zones of for- est influence in a clearcut and a retention forestry site.

3.1. Overview of retention silviculture

Retention forestry was developed in western North America based on insights into species recovery following the volcanic eruption of Mt St. Helens in 1980. Refuges enabled some organisms to survive within the blast area and distance to these source pop- ulations influenced the pattern and rate of re-establishme nt (Daleet al., 2005 ). Retention silvicultural systems are flexible approaches to forest harvesting, in which small patches of forest (with aggre- gated or group retention) and/or scattered individual structura lelements such as trees, snags and logs (with dispersed retention)are retained, for the long-term, across a harvested area (Franklinet al., 1997; Lindenmaye r et al., 2012 ). Many silvicultural prescrip- tions actually retain both forest patches and individua l structures (mixed retention). Diverse forms of retention silviculture, com- monly named variable retention (VR) and green tree retention,are now used to balance economic, social and conservation objec- tives in many parts of the world (Gustafsson et al., 2012 ). Although Franklin et al. (1997) did not use the term, the underlying

Coupe boundary

Protected

Area with influence

Aggregates

Fig. 1. The concept of forest influence illustrated with schematics of a harvested coupe situated next to a strip of forest protected within a reserve. The site was harvested with either the aggregated retention form of variable retention (VR) (left) or traditional clearcutting (right). The variable retention site contains unharvested aggregates which are designated for long-term retention. These, and the adjoining strip of reserved forest, are expected to provide buffered microclimatic conditions and re-colonisation sources for seeds, spores and animals from old forest to more recently harvested areas. According to some definitions of VR, more than 50% of a site should be within one-tree- height of long-term retention (putative zone of forest influence). Under this definition, areas surrounding the site that are not designated for long term-retention, are not considered in forest-influence calculations. The example shows that VR sites with uncut aggregate have greater areas within one-tree-height of retained forest – retaining 23%of the area in aggregates results in 51% putative forest influence, compared to only 6% in the clearcut site.

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principles of ‘forest influence’ (sensu Keenan and Kimmins, 1993;Beese et al., 2003 ) are implicit in their primary objectives for reten- tion forestry. These principles are lifeboatin g of biota, structural enrichment of post-harvest stands, and enhanced connectiv ity.Retention (‘‘lifeboating’’) of species and structures (e.g. logs, snags,cavity-bearing trees) from the pre-harves t stand may allow some mature forest species to persist through the early stages of stand developmen t (Beese and Bryant, 1999; Baker et al., 2009; Stephens et al., 2012 ), and then provide a source for re-establi shment ofspecies into the harvested area (Hill and Read, 1984; Luoma et al., 2006 ). Structural retention moderate s habitat conditions,making the harvested matrix a less hostile environm ent for some species, e.g. through shading of harvested areas by retained trees (Heithecker and Halpern, 2006, 2007 ). Reductio n in the distance between unlogged habitats or important structural features isbeneficial, both for species moving among areas of suitable mature forest, and for species re-establi shment into harvested areas (Chan-McLeod and Moy, 2007; Söderström, 2009 ). Thus, regener- ated forest stands enriched with retained biological and structural features may enable re-establishme nt of some species earlier inthe rotation (Fisher and Bradbury, 2006 ). However, the extent towhich it is possible for a retention area to help will depend onthe current successional stage and pathway for the particular pre-harvest ecosystem, which will be impacted by previous natural disturbance and harvest history. Restoration of mature forest con- ditions may be required in some second or third growth forests (Bauhus et al., 2009 ).

3.2. Silvicultural management of forest influence

The concept of forest influence is explicitly factored into rules for designing VR site layouts in Tasmania and coastal British Columbia. In these regions, a VR site is one in which the majority (>50%) of the cutblock (British Columbia) or harvested area (Tas-mania) is within one canopy tree-height of long-term retention,with the implication that this is the extent of forest influence(Fig. 1; Mitchell and Beese, 2002; Baker and Read, 2011 ). Under this definition, only long-term retention counts as providing for- est influence. This recognises that re-establish ment may be de- layed until conditions become suitable in the harvested area,and areas available for harvesting will not provide this ongoing service. Of course, in reality, adjacent areas may in fact contrib- ute to forest influence to a certain extent. This definition arose in part as a method to distingui sh VR from clearcutti ng, based on Keenan and Kimmons’ (1993) definition of a clearcut as hav- ing approximately the area of a circle of four tree heights indiameter. However, this definition of forest influence is not widely applied outside of British Columbia and Tasmania,although in some regions there are guidelines for maximum dis- tances between retention trees or aggregat es (Baker, 2011 ).Encouraging foresters to explicitly manage for forest influenceis hindered by the generally poor understand ing of biodiversity responses.

Although mature forest influence is often considered to dimin- ish significantly at distances greater than one tree length from the edge (Mitchell and Beese, 2002 ), the actual temporal and spatial scales of forest influence are still poorly known, although some studies indicate that canopy height is not directly scaled to dis- tance of forest influence. For example, studies of a variety of eco- system variables at the Sicamous Creek Trial in mainland British Columbia (Huggard and Vyse, 2002 ) recorded very narrow zones of forest influence approximat ely 3–6 years after harvesting. Those authors concluded that using a ‘one or two tree heights from edge’rule for forest influence is not appropriate for most biodivers ity inthat ecosystem, and advocated reducing the distance between re- tained patches.

4. General factors and mechani sms leading to forest influenceon biodiversity

Proximity to source populations can be especiall y important for re-colonis ation by species that have been locally eliminated byharvestin g or other disturbance . However, the biotic and abiotic environm ents also affect the probabili ty of re-establi shing in dis- turbed areas. Many environmental factors vary systematically with distance from edge and contribute to forest influence (see Sec- tion 4.3). There are also many factors contributing to forest influ-ence through the process of re-colonisation. Fig. 2 illustrate ssome of the factors and processes involved with forest influence.

Forest influence will only be significant for a proportion of spe- cies. Some species will survive on the disturbed areas in the firstplace, e.g. survival of some vascular plant and bryophyte species via a soil-stored diaspore bank or vegetative regeneration (Halpernet al., 2005; Stark et al., 2006; Caners et al., 2009 ) or of inverte- brates inhabitin g legacy coarse woody debris (Hjältén et al.,2010). For other species, long-distance dispersal abilities remove the necessity for a local propagule source, provided habitat condi- tions are suitable for re-colonisati on. The relative importance ofthese processes varies among forest types and successiona l stages,for example, plants from stable habitats generally have seeds with lower persisten ce in the soil (Thompson et al., 1998 ).

Major factors likely to influence re-establishme nt success in- clude the functiona lity of retained elements as source populations (see Section 4.1.) and traits of organisms including dispersal ability (see Section 4.2), habitat specificity (Halpern et al., 2012 ), tolerance to altered microclimatic condition s (Martinez Pastur et al., 2013 ),relative density (Solarik et al., 2010 ) and reproductive output and success, competition and survival (Siipilehto, 2006 ). Time since disturbance will also be an important factor affecting the scale and degree of re-establishme nt (e.g. Tabor et al., 2007 ). All of these factors may interact, complicating prediction of individual species responses . It is also possible that processes and time scales for for- est influence may differ relative to the underlyin g growth rates ofdifferent forest types and the quality of retained forest elements .The following sections describe some of the main mechanisms that underlie biodiversity responses to forest influence. More details for different biodiversity groups are given in Section 5.

4.1. Qualities of retained elements

If retained elements are to facilitate colonisation of harvest sites, it follows that they must have influencing characterist ics.The seral stage of the forest will affect the species composition and reproducti ve maturity of the species within it. Thus, retained second growth may, at least until it ages, be a poorer population source for re-establi shment than current oldgrowth. To operate as population sources, retained elements must be sufficiently large to sustain mature forest species for enough time to enable re-col- onisation to take place (e.g. see Section 6.3). Depending on the spe- cies concerned, size requiremen ts may relate to aggregate size tominimise edge and area effects (Halpern et al., 2012 ), windthrow (Steventon, 2011 ), and regeneration burn impacts (Scott et al.,2012). Likewise, patch size will determine habitat suitabilit y inrelation to the home ranges of some animals (e.g. Stephens et al.,in press ). The size and age of individua l habitat elements may also be important; e.g. older, larger trees may provide more seed or cav- ities for vertebrates (Palik and Pregitzer, 1994; Koch et al., 2008 )and large logs may contain appropriate rotten wood types for cer- tain saproxylic invertebrates and fungi (e.g. Yee et al., 2006 ). The total amounts of mature forest retained in the landscape may also affect the capacity of retained patches to sustain viable source pop- ulations, and thereby affect forest influence. Wardlaw et al. (2012)

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found that mature forest sustained viable source populations ofbirds and vascular plants in landscapes that varied widely in the amounts of mature forest retained. However, populations of some species of flighted beetles in retained mature forest decline in land- scapes with little remaining mature forest.

4.2. Dispersal limitation

Logging removes some species from harvested areas; either di- rectly, through tree harvesting, or indirectly, if species either die ordisperse elsewhere in response to unsuitable habitat condition s.Re-colonisati on of harvested areas by such species therefore de- pends on dispersal from source populations outside the logged area. Hence, the extent of forest influence is likely to be related to dispersal distances for species with relatively poor dispersal capacities. This will relate to the dispersal modes employed by dif- ferent kinds of organism. Most dispersal of plants and fungi is via specialised life history stages (seed, spores or vegetative propa- gules). Re-colonisation is a function of the distance these propa- gules travel from the parent (Matlack, 1994a; Tabor et al., 2007 )and the time to reproductive maturity (Matlack, 1994a ). A high proportion of insects have a flying adult life-histo ry stage to enable dispersal, although many other animals lack life history phases specialised for dispersal, and instead depend on adult or juveniles flying or moving across the ground. Dispersal strategies vary greatly within taxonomic groups, and many species may be dis- persed by multiple mechanism s (e.g. Higgins et al., 2003 ). Even for groups of species with generally similar dispersal mechanis ms,there is much variation in relative dispersal ability, e.g. differing

aerodyna mic propertie s of seeds of wind dispersed plants (Mul-ler-Landau, 2010 ).

4.3. Microclimat ic gradients

Microclimatic conditions in disturbed areas can be strongly re- lated to edge proximity, often resulting in better habitat for ma- ture-fore st species near the disturbance edge (e.g. Dynesius et al.,2008). In particular, areas near edges may be shadier, moister, less windy, with lower vapour pressure deficit, lower air and soil tem- perature s, less subject to temperature extremes, and experience delayed snow melt (e.g. Davies-Colley et al., 2000; Huggard and Vyse, 2002; Redding et al., 2003; Huggard et al., 2005; Heithecker and Halpern, 2007 ). Although the extent of microclimat ic buffering differed among variables and also among studies, most gradients inthese studies appeared to be strongest within approximately 10–20 m of edges. However, as shown for soil temperat ure and mois- ture (Redding et al., 2003 ), gradients within the edge transition zone are likely to be minor in comparison to the strong contrast be- tween unlogged and cleared conditions, especially in the early years after disturbance . Gradients in microclimat e can affect pro- cesses such as evapotransp iration, moisture availability to plants,nitrogen mineralisation and CO2 efflux, and therefore ecosystem function (Redding et al., 2003, 2004 ).

Heithecker and Halpern (2007) found steeper gradients intransmitted light along transects from aggregates into harvested areas at a site with taller trees, indicating a relationship with can- opy height, although the 20–30 m zone of reduced solar radiation was less than the height of dominant/co- dominant trees, and was

Fig. 2. A conceptual diagram illustrating forest influence.

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narrower still for effects on temperature. In some cases the scale ofshading effects also varies significantly with the orientation ofedges to solar radiation (Huggard et al., 2005; Heithecker and Halpern, 2007; Prevost and Raymond, 2012 ), which can result indifferential biodiversity responses and tree re-establish ment onshaded and exposed edges (e.g. LePage et al., 2000; Bolibok and Szeligowski, 2011 ). The slope of the site can also modify the shad- ing effect of border trees on the radiation regimes (Prevost and Raymond, 2012 ). Differences in microclimat e between logged and unlogged forest are likely to dissipate rapidly as the regenera- tion ages, in particular when the canopy closes.

Sensitivity of biodiversity to edge proximity may vary among sites along temperature , moisture or productivity gradients, for example edge proximity may be less important on cooler, moister sites for species that respond strongly to microclimat ic conditions (Chan-McLeod and Moy, 2007; Rosenvald and Lõhmus, 2008 ).

4.4. Distance to critical habitat elements

Proximity to specialised habitats will also determine the capac- ity of some species to occupy certain sites. For example, many birds and mammals depend on trees with nest cavities (e.g. Gibbons and Lindenmaye r, 1997; Cooke and Hannon, 2011 ), and invertebrate species with saproxylic larval phases require snags or logs (Hjälténet al., 2010; Grove and Forster, 2011 ). Similarly , the presence ofcertain ectomycorr hizal fungi in harvested areas is likely to be di- rectly associated with the roots of retention trees, thus limiting the distance into harvested areas where they might be able to inocu- late seedlings (Luoma et al., 2006 ). The unlogged edges will also provide inputs of certain habitat elements and nutrients into har- vested areas, for example coarse woody debris, bark and leaf litter.

4.5. Interspecific interaction s

Species occurrences may be positively or negatively correlated with the occurrence of other species, so that forest influence onone species can create forest influence on another. This may beespecially relevant to species with strong interactions, e.g. mycor- rhizal fungal association with tree roots (see Section 5.5), specialist herbivores, or plants and fungi relying on other species for dis- persal. In some cases the forest influence effect could have negative consequences , e.g. reduced growth of regeneration or increasedrisk of browsing, predation, pests and disease (Bauhus et al., 2009 ).Competition within and between species for resources may also lead to changes in habitat occupancy in relation to distance from edge (e.g. Dzwonko, 1993; Coates, 2000; McCoy et al., 2004 ). Pre- dation risk may be higher further from the cover of mature forest,e.g. for some herbivorous mammals (Kremsater and Bunnell, 1999;While and McArthur, 2005 ). Patterning of herbivores can then lead to indirect effects on vegetation communities, through, for exam- ple, greater browsing of seedlings of preferred food plants nearer to edges.

4.6. Temporal factors

The temporal component of forest influence is poorly under- stood. It nevertheless seems reasonable to say that time since dis- turbance will be an important factor in re-establi shment ofharvested areas, particularly for dispersal-limited species and spe- cies requiring habitat conditions (e.g. microclimat e) that develop over a delayed period. However, the likely rates of progression for forest organisms that benefit from forest influence are largely unknown. For species with limited mobility, spread into harvested areas may be gradual or intermittent, and in some cases this may further depend on time to reproductive maturity (Matlack,1994a; Brunet et al., 2000; Tabor et al., 2007 ). Species succession

could additionally cause changes to inter-specie s interactions such as competition or predation.

Unlike the edge influences of logged areas on adjacent forest,which tend to diminish with time since edge creation (Matlack,1993; Harper and Macdonald, 2002 ), the forest influence distance into harvested areas may increase with time, as species progres- sively disperse and establish further into harvested areas (Brunetet al., 2000 ). This rate of progression may depend on species’ dis- persal abilities, and in some cases, e.g. plants, to their age of repro- ductive maturity. At a certain point in time, initially limited species may become fully established in harvested areas such that forest influence gradients are no longer apparent. The successiona l stage of the adjacent retained forest will also determine the species that are available to re-colonis e nearby harvested areas.

5. Impacts on different groups of organisms

The scales and mechanisms of forest influence are highly vari- able both within and between different biodiversity groups. The amount of previous research effort also varies considerably be- tween and within these groups.

5.1. Vascular plants

A number of studies have shown significant relationship s be- tween vascular plant recruitment and distance from edge (e.g.Matlack, 1994b; Asselin et al., 2001; Huggard and Vyse, 2002; Ta- bor et al., 2007 ). Seed dispersal ability, and factors determining establishm ent (gradients of microclimate, competition and herbiv- ory pressure ) may all lead to forest influence on plant species compositi on.

Plant dispersal patterns, and therefore dispersal limitation, vary depending on plant functional traits such as dispersal method (Dzwonko, 1993; Matlack, 1994a; Clark et al., 1998 ). Matlack(1994a) suggested a generalised ranking of migration rates of for- est herbs and shrubs depending on dispersal mode:ingested > adhesive �wind P ants P none. Seed dispersal bywind of several tree species showed an exponential decline with distance from source, and was related to tree height, with deposi- tion at a distance equivalent to five forest heights of around 3%(Greene and Johnson, 1996 ) to 14% (Tabor et al., 2007 ) of edge depositio n. Proximit y to edge may be more important for wind- dispersed species with relatively large seeds, since these tend tobe less well dispersed (Greene and Johnson, 1993 ). For animal-d is- persed species, flying fauna and large herbivores like deer are likely to disperse seeds over longer distances than small ground-dwe lling mammals (Matlack, 1994a; White et al., 2004 ), while ant dispersal may occur over relatively short distances (Dzwonko, 1993; Mat- lack, 1994a ). Proximit y to perching sites in mature forest or dis- persed retention trees will also affect animal dispersal of plant seeds.

There is clear evidence that distance-rel ated changes in physical or biotic environment contribute to forest influence for plants. Insome cases (e.g. LePage et al., 2000 ) these gradients can vary be- tween exposed and shaded edge aspects. Light intensity was re- lated to edge responses of certain plant species (Dzwonko, 1993 )and moisture stress is a factor for Korean red pine establishment (Lee et al., 2004 ). Shade-toler ant plants, such as many late-seral species, may be more common nearer to edges than further into re- cently harvested sites (Battaglia et al., 2004; Huggard et al., 2005 ).Vegetatio n gradients in relation to distance from an old field bor- der were found to co-vary with gradients in canopy cover, soil moisture, soil reaction and soil nitrogen (Brunet et al., 2000 ).LePage et al. (2000) attributed harsh microclimatic conditions to

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explain reduced tree seedling dispersion into clearcuts relative togroup cuts, since substrate availability was similar.

There are a number of examples illustrating how inter-specificinteractions can lead to gradients in plant occupancy with distance from edge. Reduced below-ground competition and reduced shad- ing can result in increased growth rates further from edges,although this will vary depending on species’ shade-toleranc e(Dzwonko, 1993; Coates, 2000; York et al., 2003; McCoy et al.,2004). Predation on seeds or seedlings can affect survival along edge gradients (e.g. Holl and Lulow, 1997; Lee et al., 2004 ). For example, numbers of the Australian marsupia l herbivore, the pade- melon, in recently established plantations decreased with distance from unlogged forest, resulting in increased survival of eucalypt seedlings further from the edge (Bulinski and McArthur, 2000;While and McArthur, 2005 ).

5.2. Bryophytes and lichens

Some studies have demonstrat ed relationship s between bryo- phytes and lichens and distance into harvested areas (e.g. Peckand McCune, 1997; Dettki et al., 2000; Baker, 2010 ) while others have not (e.g. Hylander, 2009; Rudolphi and Gustafsson, 2011 ).For both these groups, long-distance dispersal up to the scale ofmany kilometr es (e.g. Lattman et al., 2009 ) is thought to occur pri- marily via spores, whereas asexual propagules or fragments con- tribute mostly to local dispersal (e.g. Coxson and Stevenson,2007; Lobel and Rydin, 2009 ) and in a few cases to long-distance dispersal (Pohjamo et al., 2006 ). Peck and McCune (1997) notedthat species producing large fragments are likely to disperse short- er distances than species producing smaller fragments.

The shading received by harvested areas from retained trees may be particularly important for some bryophytes and lichens be- cause of their sensitivity to microclimatic conditions, in particular moisture (e.g. Dovciak et al., 2006; Dynesius et al., 2008; Rudolphi and Gustafsson, 2011 ). Lõhmus et al. (2006) found that desicca- tion-resista nt lichens were more robust to harvesting impacts than bryophytes. Lõhmus and Lõhmus (2010) found that microclimat e-caused declines in epiphytic bryophyte populations stabilised be- tween 2–3 years and 5–6 yearspost-harv est, while lichen numbers increased during the same period.

Many bryophyte and lichen species are most commonly found on specific substrates such as bark, dead trees, tree fern trunks and logs (Roberts et al., 2005; Turner and Pharo, 2005; Rudolphi and Gustafsson, 2011 ), which in some cases might be more com- mon as a result of inputs from nearby retained forest.

5.3. Vertebrates

Proximit y to mature forest impacts some vertebrate species inharvested areas (e.g. Steventon et al., 1998; Huggard and Vyse,2002). Unlike plants, terrestria l vertebrates lack life history phases specialised for dispersal, and instead depend on adult or juveniles flying or moving across the ground. Most terrestria l vertebrate spe- cies are relatively well-dispersed , so that forest influence on these organisms may be more related to microclimat e, distance to criti- cal habitat features, food distribution or interspecific interactions ,especially predator avoidance.

5.3.1. Mammals The preferenc e of pademelons for edges discussed in Section 5.1

applies more widely among macro-herb ivores. Thus, deer and elk browse more heavily near forest edges, e.g. within approximat ely 200 m from edges (Marcot and Meretsky, 1983 ) and use the for- ested side for cover, which is thought to relate to predation risk and forage abundan ce (Kremsater and Bunnell, 1999 ). Linear ele- ments such as the edges between logged and unlogged forest can

create foraging habitat for bats, with elevated activity near these edges (Grindal and Brigham, 1999; Law and Law, 2011; Webala et al., 2011 ). Proximity of nearby forest may also impact species crossing between areas of mature forest (Marcot and Meretsky,1983; Selonen and Hanski, 2003 ). Thus, retained trees in clearcuts allowed movement of flying squirrels, while open gaps > 100 m ex- ceed their gliding ability (Selonen and Hanski, 2003 ). Proximit y tohollow-b earing trees is also expected to be important for many Australia n marsupials (Gibbons and Lindenm ayer, 1997 ). In some cases the benefits of structural retention may be delayed. For example, hollow-bearing trees retained within harvested sites were rarely used for dens or nests by brush-ta iled possums 8–10 years following harvest, but were used at 17 years following harvest (Cawthen and Munks, 2011 ), presumably increasing the likelihoo d that the possums would forage in the surroundi ng har- vested area.

5.3.2. Birds Forest influence effects have occasionally been documented in

studies of birds, with species either preferring (Steventon et al.,1998) or avoiding (Schlossberg and King, 2008 ) harvested forest near edges. In some cases, edge preference by birds may relate toelevated abundance of invertebrate prey (Helle and Muona,1985). Interspecific interactions may also lead to preferences for condition s further from intact mature forest. Predation or nest-par- asitism may be increased closer to bird perch sites in mature trees (Kremsate r and Bunnell, 1999 ). Certain birds have been found tonest preferent ially on retention trees in logged sites rather than in unlogged forest, possibly due to lower risks of nest predation (Rosenval d and Lõhmus, 2008 ).

Edge responses can also relate to habitat condition s such as the amount of herbaceo us growth (Preston, 2006 ), the distance that birds need to fly to key habitat structure s like nesting or perching sites in mature trees (Schieck and Song, 2006; Rosenvald and Loh- mus, 2007; Atwell et al., 2008 ) and the amount of suitable habitat in the wider landscape (Betts et al., 2006; Wardlaw et al., 2012 ).For example, Betts et al. (2006) suggested aural cues may beimportant attractors for conspeci fics. This may result in aggrega- tions of higher bird density in landscapes with more retained ma- ture forest. Preston (2006) considered that group retention advantag ed forest birds by providing greater connectiv ity, with forest patches in greater proximity facilitating movement around the landscape. For example, a study of gap-crossin g decisions offorest birds across cleared agricultural areas found that birds rarely ventured >25 m from forest edges, and would instead take longer routes under forest cover (Bélisle and Desrochers, 2002 ).

5.3.3. Amphibia ns and reptiles Amphibian abundan ces have been found to decline with dis-

tance into clearcuts (deMaynadie r and Hunter, 1999 ). This is likely to relate to the narrow habitat tolerances of these organisms,which often prefer shaded, moist conditions (deMaynad ier and Hunter, 1998; Maguire et al., 2004; Chan-McLeod and Moy,2007). Specific habitat elements, e.g. coarse woody debris, plant cover or leaf litter condition s, can also be important (deMaynadierand Hunter, 1999; Maguire et al., 2004 ). In some cases, these hab- itats vary in abundance with distance from edge, e.g. as a conse- quence of retention trees falling into harvested areas. Many pond-breedi ng amphibians inhabit upland terrestrial sites in the non-breed ing season. Forest microclimat e can be particularly important for juvenile dispersal from aquatic habitats, especiall ysince amphibians generally have limited mobility compared toother vertebrates (deMaynadie r and Hunter, 1999; Patrick et al.,2006; Chan-Mc Leod and Moy, 2007 ). The distance between aggre- gates was shown to affect movement patterns of dispersing red- legged frogs in British Columbia; frogs had to be within 5–20 m

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of aggregates for increased probability of movement in that direc- tion (Chan-McLeod and Moy, 2007 ).

Less is known about the responses of reptiles to silvicultural practices than amphibians, and no studies have directly assessed gradients of edge response in harvested areas. Based on studies of logging impacts, it appears likely that reptiles may respond toedge gradients in relation to microclimate such as the amount ofsunlight reaching the forest floor (Goldingay et al., 1996; Lima et al., 2001; Alexander et al., 2002 ). This may be important for rep- tiles, especially in cool climates where basking is used to increase body temperatures (Alexander et al., 2002 ). Inputs of habitat ele- ments from edges, including leaf litter habitat and logs as basking platforms, could also lead to forest influence on reptiles (Brownand Nelson, 1993; Alexander et al., 2002; Todd and Andrews,2008).

5.4. Invertebrates

Forest influence has been documented in a number of inverte- brate orders: beetles, spiders, gastropods and both ant and non-ant hymenop terans. Overall, research has been strongly biased towards the ground habitat, which is readily sampled bypitfall traps, and towards spiders and especially beetles (but see Helle and Muona, 1985; Siira-Pietikäinen and Haimi, 2009 ). Forest influence responses may be related to dispersal ability (Jonsellet al., 1999; Koivula, 2002; Jonsson and Nordland er, 2006 ), micro- climatic gradients and canopy openness (Koivula, 2002; Lemieux and Lindgren, 2004 ), or availability of food or specialised habitats (Nordlander et al., 2003; Jacobs et al., 2007; Hyvärinen et al.,2009; Siira-Pietikäinen and Haimi, 2009 ). Serving as source habi- tats, edges can contribute to re-colonisation by ground beetles after disturbance (Molnar et al., 2001; Koivula, 2002 ). Different invertebrate orders and functional groups vary in their response to harvesting practices and forest influence (Helle and Muona,1985; Siira-Pietikäinen et al., 2003; Jacobs et al., 2007; Hyvärinenet al., 2009 ), and specialist species and those of higher trophic lev- els may be especiall y sensitive (Jonsson and Nordland er, 2006 ).

In most studies showing forest influence in invertebrates , spe- cies affiliated with uncut forest became less common with distance into cut areas (Tim Work, personal communicati on; Helle and Muona, 1985; Pearsall, 2003; Klimaszews ki et al., 2008 ). However ,in one study (Pearce et al., 2005 ) species that were more common near edges were usually associated with open rather than mature forest habitats, suggesting that in this case the edge response re- lated more to differences in habitat conditions than re-colonisati onfrom the nearby mature forest.

5.5. Fungi and microbes

There is a growing body of knowledge of forestry impacts onectomycorr hizal (ECM) fungi and some symbiotic nitrogen fixingbacteria, but little research into free-living fungi, viruses, bacteria,protozoans and blue-green algae. The soil microbial communi ty issensitive to changes to microclimate such as temperature and soil moisture availabili ty (Marshall, 2000; Grayston and Rennenb erg,

2006), and could therefore be expected to respond to microclimat icedge gradients. Since many soil organisms are sensitive to temper- ature and moisture extremes, retention forestry practices that re- sult in microclimat ic buffering are expected to benefit these groups. For example, Grayston and Rennenb erg (2006) found that thinning had different effects on microbial biomass, activity and population structure depending on site aspect, and attributed these differenc es to variation in soil moisture availability. Some groups have poorer dispersal and colonisat ion abilities, including nitrogen fixing species and plant pathogen species with partial depende nce on suitable hosts (Bissett et al., 2010 ), and thus might also be sensitive to distance from edge for dispersal. Redding et al.(2004) found soil nitrate content and net nitrification (but not ammonification) increased markedly beyond 2–6 m into harvested forest from clearcut edges, while Hope et al. (2003) did not docu- ment an edge effect on litter decompositi on. These results suggest forest influence distances may be small for microbial activity.

Forest influence from edges or individual trees has been rela- tively well quantified for ECM fungi. Because of their symbiotic relationshi p with tree roots, close proximity of host seedlings tothe roots of retained trees appears to be the main mechanism underlyin g relationship s with distance into harvested areas (e.g.Cline et al., 2005; Luoma et al., 2006 ). Narrow zones of forest influ-ence of generally less than 10 m have been documented in several studies of mycorrhi zal root colonisation and/or diversity (Parsonset al., 1994; Durall et al., 1999; Hagerman et al., 1999a,b; Outer- bridge and Trofymow , 2004, 2009; Luoma et al., 2006; Jones et al., 2008 ). Although a small sub-set of species have effective long-dist ance colonisation via spores, local dispersal, either byspores or mycelial spread among host roots, is more common (Peayet al., 2011 ). Outerbridge and Trofymow (2004) found that old- growth trees provided greater inoculum potential than retained second growth trees. In a related study, the same authors found that increased levels of green tree retention increased overall ECM fungi richness and root colonisation over longer distances (Outerbrid ge and Trofymow, 2009 ).

6. Synthesi s

6.1. Mechanisms and scales

Although several mechanis ms underlie forest influence, it isimportant to re-emphas ise that many individual species will not be subject to these factors because they are well adapted to re- establishi ng in harvested areas. However, such species are rarely of conservation interest because they tend to be favoured byanthropoge nic processes . Several important processes and mecha- nisms relate to the biodiversity responses to retained forest influ-ence. These vary from species to species, but based on the foregoing literature review, some general hypotheses can be posed about which mechanisms are particular ly important for various taxonomi c groups (Table 1). Although dispersal limitation may be the primary limiting factor for some types of biodiversity (suchas some plants and flightless invertebr ates), for others,microclimat ic gradients (e.g. for lichens, bryophytes, reptiles and

Table 1Hypothesised most important mechanism s relat ing to re-establishment of harvested areas in relation to distance from edge for major groups of forest biodiversity.

Limiting factor Taxonomic group

Vascular plants Bryophytes and lichens Mammals Birds Amphibians and reptiles Invertebrates ECM fungi

Dispersal � � � �Microclimate � � �Habitat elements � � � � � � �Interspecific interactions � � � �

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amphibians), the proximity to, and amount of, key habitat ele- ments (e.g. for saproxylic invertebr ates), or interactions with other species (e.g. for mycorrhizal fungi) may be of similar or greater importance . Habitat elements appear to be important for at least some species from all the taxonomi c groups we assessed , confirm-ing the value of structural retention as a valuable forest manage- ment tool to encourage re-establi shment of biodiversity.

Estimate s of the distance that forest influence extends are sum- marised in Fig. 3. Because of differences in design, sampling effort and analysis approach, these measures cannot be directly con- trasted (Harper and Macdonald, 2011 ), but do serve to provide apicture of the relative distances for the various biodiversity groups.The documented scales vary from less than 10 m to approximat ely 200 m, and there was variability within, as well as between, major biodiversity groups (Fig. 3). More mobile biodivers ity groups (e.g.mammals and birds) show generally greater forest influence dis- tances than others, such as ECM fungi, for which distance to roots of residual trees appears to be a plausible mechanis m. Except for vertebrates, estimates of forest influence did not exceed 100 m inextent, although this is possibly biased by the scales (spatial and temporal) of assessme nt.

Most studies were carried out during the first few years after harvesting, with the notable exception of the retrospective study of bryophytes by Baker (2010) in a 48 year-old clearcut. The large size of this clearcut also enabled sampling much further from the edge (to approximat ely 500 m) than the majority of studies, making detection of long edge gradients possible if present. However, the size of cutting units usually prohibits assessment over such long distances. It is therefore important to keep in mind that estimates of depth of forest influence will be within the context of the maxi- mum distance from edge sampled, and may be limited by the size ofthe cut area which may prevent detection of longer gradients.

Research is required to determine how forest influencechanges with time. However , we hypothesise that the strength and distance of forest influence will increase over time, at least initially, for species where the primary mechanism for influenceis dispersal limitatio n (Matlack, 1994b ). By contrast, for species for which microclimate limits establishment, forest influenceeffects may be most pronounced in the short-term , but at some stage the entire regenerati ng forest will reach suitable microclimat ic conditions for sensitive species to establish. For example, we would expect that for species limited by microcli- mate, gradients in forest influence will diminish after canopy closure.

We observed in our review that canopy height of edge vegeta- tion was rarely reported, precluding analysis of the relationship be- tween this measure and forest influence distance. Height of canopy trees along with crown size and width may play a role in determin- ing the extent of the zone of forest influence for some factors;either directly, e.g. through dispersal of tree seed, or indirectly through microclimatic amelioration. For example, it is possible that canopy height sets an approximat e upper limit for forest influenceon vascular plants, for which seed dispersal is likely to be a primary limitatio n.

Based on this review, we suggest some hypotheses aboutcondi- tions under which forest influence may be maximised, in the hope that further research will test these ideas, along with the relation- ship between forest influence and height and structure of retained forest:

� forest influence should be of greater relative importance for late-succ essional species, particularly those species with poorer dispersal abilities and requirements for shaded microclimat iccondition s, than for early-successiona l species;

1 10 100 1000

VASCULAR PLANTS Balsam fir regeneration, Asselin et al. 2001

Cool temperate rainforest trees, Tabor et al. 2007Vascular plants, B. Beese pers. comm. 2012

Hemlock seedlings (S facing), LePage et al. 2000Hemlock seedlings (N facing), LePage et al. 2000

Spruce seedlings, LePage et al. 2000Douglas-fir height, York et al. 2003

Giant sequoia height, York et al. 2003Ponderosa pine height, York et al. 2003

CRYPTOGAMS Lichen litterfall, Peck & McCune 1997

Moss cover & richness, B. Beese pers.comm. 2012Bryophytes, ground & log substrates, Baker 2010

VERTEBRATES Deer and elk, Marcot & Metetsky 1987

Syberian flying squirrel, Selonen & Hanski 2003Avifauna, Wardell-Johnson pers. comm. in Bradshaw 1992

Red-legged frogs, Chan-McLeod & Moy 2007Amphibians, deMaynadier and Hunter 1998

INVERTEBRATES Forest interior spiders, Larrivée et al. 2008

Ground-active invertebrates, T. Work, pers. comm. 2010Carabid beetles, Pearsall 2003

Pine weevil feeding, Norlander et al. 2003ECM FUNGI

ECM fungi from aggregates, Jones et al. 2008ECM fungi at aggregates, Outerbridge & Trofymow 2004

ECM fungi from dispersed trees, Luoma et al. 2006ECM fungi from cutblock edges, Durall et al. 1999

ECM fungi from gap edges, Parsons et al. 1994

Approximate distance into harvested area (m)

Taxonomic group

Fig. 3. Estimates of the extent of forest influence into harvested areas for different types of biodiversity. Distance into harvested area is displayed on a log scale, and the midpoint is displayed when a range was given. Differences in study designs, time since harvest, silvicultural systems and gap sizes and approaches to analysis and interpretation mean that these comparisons are illustrative only. (See above-mentioned references for further information .)

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� seral-stage, species composition and condition of the retained mature forest elements are likely to affect their functiona lity as a source population, with intact late-seral areas providing forest influence gradients for a higher proportion of species;� forest influence should be of greater relative importance for

habitat specialists than for generalists ;� mature forest influence should be of greater depth and magni-

tude where local site conditions in the harvest area favour re- establishment from mature forest, such as shaded aspects and positions down slope and downwind of mature edges.

6.2. Comparison with edge effects into unlogged forest

The dominant processes and mechanism s underlyin g forest influence are related to, but different from, those that underlie edge effects into unharvested forest (see Harper et al., 2005 ). Edge effects in unlogged forest relate to removal of the buffering influ-ences of intact vegetation, whereas forest influence on harvested areas relates to additional influence from the retained forest. Thus,while edge effects in unlogged forest often result in increased evapotransp iration, decomposition and growth, and reduced can- opy cover and tree density near edges (Harper et al., 2005 ), forest influence may result in the opposite effects on the same variables for harvested areas near edges. Other factors, such as composi- tional diversity, amount of coarse woody debris, and rates of dis- persal or nutrient cycling, are also likely to show contrasting gradients either side of edges. These distinctions could then result in differences in the relative distances, magnitudes and time-spans of edge responses by biodiversity either side of boundari es, and could affect different suites of species. Nevertheless, certain attri- butes of forest may render them particularly susceptibl e to both edge effects into unlogged forest, and forest influence on the har- vested side. In particular , the magnitude and distance of edge ef- fects are considered to be emphasis ed in forest systems in which the mature forests naturally experience rare disturbance, contain few pioneer species and have much taller and more closed cano- pies than the logged forests (Harper et al., 2005 ). We hypothesise that the same is true of forest influence. Research studies that as- sess gradients both sides of edges would be particular ly useful for making direct comparisons of scales and mechanism s of edge responses either side of boundaries.

6.3. Harvesting patterns and forest influence

Forest influence will impact biodiversity responses to a certain extent under all silvicultural systems (Bradshaw , 1992 ), but is par- ticularly relevant to the objectives of retention forestry ap- proaches . The choice of aggregat ed versus dispersed retention pattern, and distances between retained elements may have strong implication s for re-establi shment success. In particular , since aggregat es maintain a more intact assemblage of the species pres- ent within sites, they might be expected to better facilitate re-col- onisation compared to dispersed retention which typically only retains trees and snags. However, for species where microclimat icameliorati on is more important, the greater degree of shading from dispersed retention may be beneficial. Retained tree densities im- pact the degree of microclimat ic forest influence with dispersed retention. For example, a study of dispersal of canopy lichens found a positive relationship between cyanolichen litter biomass and the number of retained trees (Peck and McCune, 1997 ). Since trees are usually not retained within harvested areas of aggregated reten- tion sites, establishment of some bryophytes and lichens occupy- ing habitats developing on mature trees will be delayed until trees reach sufficient age to provide that habitat, which may not occur within the time-period of a silvicultu ral rotation. For species that prefer these substrates, dispersed retention may facilitate establishm ent in harvested areas (Dovciak et al., 2006; Lõhmuset al., 2006; Caners et al., 2010 ). Dispersed retention is also ex- pected to be beneficial for mycorrhizae associate d with retained trees.

Because of their isolation and small size, dispersed trees and snags and small aggregates are particularly susceptible to edge and area effects (e.g. Aubry et al., 2009; Baker et al., 2009; Lefort and Grove, 2009 ). These legacies are also more susceptible towindthro w (Scott and Mitchell, 2005; Jonsson et al., 2007 ) and regenerati on burn impacts (Scott et al., 2012 ). These factors may thus compromise their ability to retain species, and in turn limit their function as a source of re-introduction.

For a given retention level with aggregated retention, there will inevitabl y be a trade-off between the potential loss of habitat qual- ity if using more small or narrow aggregat es versus the decrease dratio of influence to aggregate area resulting from using fewer lar- ger, less edge-effected, aggregates (Fig. 4). Simulation modelling byChan-Mc Leod and Moy (2007) shows that doubling the retention

Fig. 4. A variable retention site with 23% retention in aggregates illustrates the impact of aggregate size on the amount of putative forest influence. Sites with smaller aggregates have higher levels of calculated forest influence (i.e. areas within one-tree-height of long-term retention). However, smaller aggregates are more edge-affected and susceptible to windthrow and regeneration burn impact. This may impact their functionality for both lifeboating and for providing actual forest influence on species and ecological processes.

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level or halving patch size for the same retention level (i.e. having more but smaller aggregat es) had similar effects on decreasing the inter-patch distances (i.e. increasing forest influence). By contrast,patch shape (circular compare d to rectangular) had relatively little effect, apart from slightly greater inter-patch distances with circu- lar aggregat es. However, this modelling did not consider the im- pact of patch size on functionality of retention.

7. Knowledge gaps

As much as anything, our review highlights what is not known about biodiversity responses to forest influence, and although wespeculate about likely responses, a number of questions remain unanswered.

Research is needed to better understa nd scales and the mecha- nisms underlying the forest influence response and test the hypotheses posed in Section 6. Although more work is required for all taxonomic groups, some groups are particularly poorly stud- ied, e.g. soil microbial community (other than ECM fungi), reptiles,and most invertebrate orders. Sampling a number of biodiversity groups along transects within the same silvicultural experiment,and applying the same analysis approach across taxa, would enable direct comparison of scales of influence. The designs of most re- search experiments do not allow us to easily tease out these under- lying mechanisms, and, as a result, ascribing cause is at best based on correlation with habitat conditions. Although measuring rela- tionships with habitat variables is useful for understanding biodi- versity responses, this approach could be complemented with manipulative research experiments specifically designed to allow determination of the relative importance of different mechanism s.

7.1. Recommendatio ns for sampling designs

Different studies inevitably use different methodologi cal ap- proaches to address specific study questions, and forest influencestudies can locate plots randomly or along transects, either uni- formly or with concentrated sampling effort nearer the edge. Care- ful thought to the sampling design is merited in every case, since adesign that was perfect for one study, may not be ideal for a similar research project. The advice on sampling effort provided by Harperand Macdonald (2011) is likely to be equally relevant to studies offorest influence. However, future meta-analysis and/or synthesis ofthe magnitud e and scale of forest influence would benefit from reporting the following:

� the height of the forest either side of the boundary , the time since harvest, and the approximate expected age of canopy clo- sure, as well as an estimate of the successiona l stage of the mature forest;� how estimate s of the extent of forest influence were derived,

the error associated with these estimates and the magnitude of changes. If practical, consider using the existing randomiza- tion test approaches for estimating the extent of forest influence– see Harper and Macdonald (2011) for an approach for single species data, and Millar et al. (2005) for NCAP analysis of com- munity data; and � the proportio n of species that appear to be sensitive to forest

influence, and reflect on whether they share any common attri- butes that contrast with species that are not sensitive to edge proximity.

7.2. Recommendatio ns for future research

Considerabl e knowledge gaps about forest influence remain,especially related to mechanisms, spatial and temporal scales,and harvest design. We briefly characterise those gaps below:

7.2.1. Mechanisms of forest influenceA better understand ing of the relative importance of factors and

mechanis ms that underlie forest influence can assist managers predict responses for new situations. For example, it is unclear ifthe disturbance regime of the forest type affects species responses to forest influence. In landscapes with less frequent disturbance s,the species pool contains a larger portion of slowly dispersing spe- cies or species with preference for closed canopy habitats. On the other hand, forest influence may not be that important in distur- bance-pr one landscapes where most species are well adapted tore-colonis ation of disturbed areas and presence of undisturbed for- ests is not a strong control on post-distur bance colonisat ion and growth.

Differences in life history characteri stics may also explain vari- ation in species responses to forest influence. For example, wewould hypothesise that forest influence would have more of an ef- fect on late-successi onal species needing to re-establish in har- vested areas compared to early-seral species that might bedisadvantag ed by shaded conditions near edge. Combinati ons ofspecies life history attributes may also render some more sensitive to forest influence. For example, species that are dispersal-limited and sensitive to high solar radiation or temperature extremes could be more sensitive to forest influence than those that do not share this combinati on of characteristics.

Micro-habitat needs of species may also explain differences inresponses of species to forest influence. Species that require coarse woody debris or particular types of litter may only be present inharvest units near forests where those habitat elements are pres- ent. It may be possible to test for the effects of those elements using manipulative experiments that either maintain existing mi- cro-habit ats in logging units or add them to the units where they have been eliminated by the disturbance .

7.2.2. Spatial and temporal scales As with many ecological processes, forest influence effects are

likely to change with spatial and temporal scale, and research onthis topic as well as long-term studies have been lacking (Rosen-vald and Lõhmus, 2008; Bauhus et al., 2009 ). The effects of harvest unit size and size of remnant patches on forest influence have not been studied. We also lack knowledge of how contrast between the structure of the disturbed patch and the adjacent forest patch af- fect forest influence. The temporal aspects of forest influence are particular ly important since the recovery time of microclimat eand biota within disturbances will control the rate and pattern ofharvest activities in landscapes where maintaining biodiversity isan objective. The successional dynamics of post-disturban ce com- munities may be controlled by the sequence of species invasions ofthe disturbed patches. In other words, the first forest species to col- onise the sites may control the rate of succession and the compo- sition of subsequent colonisers (Read and Hill, 1988 ). It is quite likely that some species will recover rapidly across the harvest unit while other may move quite slowly and perhaps never recover be- fore the next harvest event. It will be important to know the rela- tive rates among species, to identify the species with the most vulnerabl e populations to the cumulative effects of harvesting across landscapes over time. Some studies suggest that some ofthese slow species may be lichens and bryophytes (e.g. Kantvilasand Jarman, 2006 ).

7.2.3. Harvest design Many harvest unit design questions require additional research.

One important one is the use of rules-of-thum b such as the ‘one- tree-heig ht from long-term retention’ rule that is sometimes used to characterise edge effects and design managemen t systems. The validity of this rule will vary and managemen t designs may need totake into account other rules that fit better for particular taxa or

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processes. The relative merits of retaining aggregates compared todispersed trees in units are not fully understood and are likely tobe dependent on the details of the aggregates and the patterns ofthe individual trees. Some simple modellin g of the rate and pattern of spread of organisms with different life history characterist ics under different harvest designs could provide some insights toguide managers. Such research could also identify thresholds for retention densities and aggregate sizes that optimise the trade- off between lifeboating and influence functionality . Another area of research interest would be comparing forest influence from har- vest residuals such as aggregates to natural fire residuals. There issome research indicating these residuals have different character- istics and habitat value (Gandhi et al., 2004; Dragotescu and Knee- shaw, 2012 ), so it is reasonable to expect that they might not becomparable in how they provide forest influence.

8. Conclusion s

Forest influence is an important emerging field of applied forest ecology. We argue that managing for forest influence is as impor- tant to conservation outcomes of forest management as is mini- mising edge effects into unlogged forest. The limited amount ofresearch into forest influence has shown that proximity to retained forest does impact a proportion of forest biodiversity. Other than for vertebrates, in most cases influence was estimated to extend <100 m, but in some cases much shorter still, e.g. generally <10 m for ECM fungi. Therefore harvested areas of retention for- estry sites designed with increased forest influence as a manage- ment objective should develop biological communi ties richer inmature-fore st affiliated species sooner than clearcuts . The distance and underlying mechanism s of forest influence vary broadly among taxonomic groups, as well as between individual species within taxonomic groups, and even potentially among individuals within species. Further research is needed to better understand these variables so that forest managers have better information to help with site planning. However, since different species may be favoured at different distances from edge, and no single size of cut area will be ideal for all types of biodiversity (Bradshaw,1992), variation in harvest layouts and gap dimensions is recom- mended. The rule of thumb that forest influence extends one tree height into harvested areas is an easily applied management tool that ensures variable retention cutblocks have greater forest influ-ence than clearcuts. However, it should not be assumed that one tree height is directly scaled to actual depth of forest influencefor biodiversity.

The paucity of research on edge gradients into harvested areas to-date, combined with long time-frames of ecological responses to forest harvesting, mean that forest managers are by necessity having to impleme nt harvesting practices in the absence of de- tailed knowledge of likely biodivers ity outcomes. Priorities for fu- ture research include more studies of the spatial and temporal scales and identifying factors that underlie biodiversity responses.The vast majority of studies on forest influence and retention for- estry more broadly (Rosenval d and Lõhmus, 2008 ) are short-term,thus long-term outcome s are poorly understood . Ongoing monitor- ing of silvicultural trials is therefore of great importance.

Acknowledgeme nts

We wish to thank numerous forest ecologist s from around the world for email discussions and suggestio ns for relevant literature.In particular Bill Beese played a critical role in bringing our atten- tion to the ecological importance of forest influence, and how itcould be factored into harvest planning. We also acknowledge staff at Forestry Tasmania involved with policy development to

encourag e forest influence at harvest. Journal referees including Karen Harper provided detailed and thoughtful comments which considerabl y improved the manuscr ipt. Funding for this project was provided by ARC linkage grant LP100100050 with support from Forestry Tasmania and the Forests and Forest Industries Council. A World Forest Institute fellowship supported SCB in Port- land during part of the project. Robyn Scott produced Figs. 1 and 4.

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