Plant species diversity, ecosystem function, and pasture management—A perspective

Plant species diversity, ecosystem function, and pasture Management—A perspectivePlant species diversity, ecosystem function, and pasture management—A perspective
M. A. Sanderson1, S. C. Goslee1, K. J. Soder1, R. H. Skinner1, B. F. Tracy2, and A. Deak3
1USDA-ARS Pasture Systems and Watershed Management Research Unit, Building 3702 Curtin Road, University Park, PA 16802, USA (e-mail: [email protected]); 2Crop Sciences Department,
University of Illinois, Urbana, IL, USA; and 3Crop and Soil Science Department, The Pennsylvania State University, University Park, PA, USA. Received 30 June 2006, accepted 28 February 2007.
Sanderson, M. A., Goslee, S. C., Soder, K. J., Skinner, R. H., Tracy, B. F. and Deak, A. 2007. Plant species diversity, ecosystem function, and pasture management—A perspective. Can. J. Plant Sci. 87: 479–487. Grassland farmers face many challenges in pasture management including improving sustainability, reducing inputs of fertilizers and pesticides, and protecting soil resources. In this paper we provide our perspective on managing plant diversity within and among pastures as one tool to aid producers in meeting these challenges. Pasture ecosystems can be highly diverse, with a complex array of organisms contributing to ecosystem functioning. Within the broad range of plant and animal biodiversity in pastures, plant species diversity may be the most amenable to manipulation or management. Reported benefits of plant diversity in grasslands include: increased forage production, greater ecosystem stability in response to disturbance, and reduced invasion by exotic species such as weeds. Some view diversity as a sort of insurance policy where different species contribute in their own time or can take the place of species that fail from stress or mismanagement. Using mixtures of several forages in pastures, in some instances, can improve forage yield and reduce weed invasions. Pasture management for increased plant species diversity, however, is not simply mixing and planting as many forage species as possible. The kinds and amounts of different forage species along with their arrangement within and among pastures at the farm scale are critical features that must be considered. Tools must be developed to determine the appropriate species mixtures for varying soils, landscapes, climate and purposes to fulfill multiple functions for producers.
Key words: Grazing ecosystem; forages; diversity; ecosystem function; ecosystem services
Sanderson, M. A., Goslee, S. C., Soder, K. J., Skinner, R. H., Tracy, B. F. et Deak, A. 2007. Diversité des espèces végétales, fonction de l’écosystème et gestion des pâturages – un point de vue. Can. J. Plant Sci. 87: 479–487. Pour les agriculteurs des prairies, la gestion des pâturages pose maintes difficultés parmi lesquelles comment rendre ces derniers plus durables, comment réduire l’usage des engrais et des pesticides, et comment préserver les ressources du sol. Dans cet article, les auteurs donnent leur point de vue sur la gestion de la diversité des espèces végétales dans le pâturage et entre les pâturages en tant qu’outil susceptible d’aider les producteurs à surmonter ces difficultés. L’écosystème des pâturages varie considérablement, un ensemble complexe d’organismes concourant à son fonctionnement. Parmi la grande variété de plantes et d’animaux qui peuplent un pâturage, c’est la diversité des espèces végétales qu’on pourrait sans doute le mieux modifier et gérer. Parmi les avantages de la diversité végétale dans les prairies, mentionnons une plus grande production fourragère, une meilleure stabilité de l’écosystème advenant une per- turbation et une résistance accrue à l’envahissement par des espèces exotiques telles les adventices. Certains voient dans la diver- sité une police d’assurance, les différentes espèces contribuant à l’ensemble le moment venu ou se substituant aux espèces qui cèdent au stress ou aux erreurs de gestion. Dans certains cas, un mélange d’espèces fourragères dans le pâturage accroîtra le ren- dement et freinera l’arrivée des mauvaises herbes. La gestion du pâturage en vue d’une plus grande diversité végétale ne se résume toutefois pas à mélanger et à semer autant d’espèces fourragères que possible. La nature et le nombre d’espèces fourragères ainsi que leur disposition dans le pâturage et d’un pâturage à l’autre à l’échelle de l’exploitation sont des aspects essentiels dont il faut tenir compte. On doit élaborer des outils pour déterminer le mélange adéquat d’espèces en fonction du sol, du relief, du climat et de l’intention recherchée si l’on veut satisfaire aux multiples besoins des agriculteurs.
Mots clés: Écosystème des pâturages, fourrages, diversité, fonction de l’écosystème, services d’écosystème
Multifunctionality in grassland agriculture recognizes bene- fits beyond food and fiber production including such non- market benefits as land conservation and maintenance of landscape structure (Boody et al. 2005; Lemaire et al. 2005). A multifunctionality focus is evident in some USA farm programs. For example, the US Department of Agriculture (USDA) National Organic Standards emphasize pasture uti- lization not only for feed production but also for animal well being and product quality (USDA 2006). The USDA-
Natural Resources Conservation Service (NRCS) Conser- vation Security Program rewards farmers for managing for multiple functions in addition to production, such as soil conservation, water quality protection, and carbon seques- tration (USDA-NRCS 2006).
The principal traditional functions of pastures have been to capture solar energy in the form of plant biomass to feed ruminants, and the efficient cycling of nutrients and water. Producers will need to consider managing for additional ecosystem functions such as enhancement of carbon seques- tration, mitigation of greenhouse gas emissions, and to cap- italize on new opportunities to diversify the forage-livestock system to achieve these outcomes in the future.
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Presented at the 2006 annual conference of the Canadian Society of Agronomy and the Canadian Society for Horticultural Science.
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Managing forage species diversity to take advantage of variability in land and climate is an ecological approach to enhancing the multifunctionality, productivity, and sustain- ability of pastures (Hervieu 2002; Sanderson et al. 2004; Hector and Loreau 2005; Isselstein 2005). Forage and grass- land systems must be productive, energy efficient, low cost, and resistant to stresses such as defoliation, drought, or weed invasion. They must also be in a form and structure acceptable to the grazing animal and of adequate nutritive value to meet animal production goals. Fulfilling these func- tions probably will require more than growing many forage species together. More likely it will entail a multi-scale approach with different forages and combinations of forage species distributed across a farm according to site suitabili- ty and goals of the producer. However, producers need to know more about managing forage species diversity. For example: Can producers maintain diverse swards under cur- rent management? How do producers chose and combine the appropriate species and decide where best to place them in the farm landscape? Does increasing sward diversity involve a tradeoff in productivity versus ecosystem function?
The concept of using multi-species swards is not new as evidenced by Darwin’s mention of growing several species of grasses to produce more herbage than growing one species alone (Darwin 1872). Practical farming texts in the late 1800s adopted Darwin’s insights and recommended the use of multi-species swards (Flint 1888). More than a cen- tury later, new insights on the importance of plant species diversity in natural grasslands have renewed interest in the use of greater plant species diversity in forage and grazing lands (Tilman et al. 1999; Minns et al. 2001).
In this paper, we discuss our perspective on managing multiple forage species in pastures as a means of meeting multiple functional goals. Others have reviewed the ecolog- ical literature related to plant species diversity in natural and managed grasslands (Sanderson et al. 2004; Hector and Loreau 2005; Isselstein 2005). We begin with an overview of the plant species diversity of temperate-region pastures. Next, we discuss a series of studies that examined how botanical complexity of pastures influenced herbage and animal production and other ecosystem characteristics such as stability and resistance to invasion. Finally, we conclude with an approach to incorporating plant species diversity in pastures based on principles developed from the science of ecosystem restoration and landscape ecology.
PLANT SPECIES DIVERSITY IN PASTURES Pasturelands may appear homogeneous in plant species but closer examination of pastures at different scales reveals a wide variety in composition. For example, in the Northeast USA, plant species richness in pastures varies across plant community, pasture, farm, and regional scales (Table 1; Tracy and Sanderson 2000a, b; Goslee and Sanderson 2005), with the number of species increasing as the area sampled increases. About one-half of the plant species iden- tified in northeastern USA pastures were native. The aver- age number of plant species in pastures was 32 per 1000 m2
with a range of 9 to 73 species per 1000 m2. Most pastures,
however, were dominated by a handful of cool-season grass- es and legumes (Fig. 1; Goslee unpublished data).
Surveys of cool- and warm-season pastures and hay meadows in the tallgrass prairie region of Kansas demon- strated differences in total and native species richness among sites (Jog et al. 2006). Cool-season pastures and hay meadows had an average of 21 native species and 32 total species per site. Warm-season pastures had an average rich- ness of 55 native species and 73 total species per site. Warm-season hay meadows had an average richness of 70 native species and 85 total species per site.
Plant species richness in traditionally managed temperate grasslands in the European Alps averaged 74 species per 1000 m2, whereas more intensively managed grasslands contained 54 species per 1000 m2 (Spiegelberger et al. 2006). The trend of intensified management (higher fertil- ization rates, more frequent cutting and grazing, greater stocking rates) in Europe since the 1960s has reduced species richness and resulted in a relatively uniform plant community (Peeters 2004).
Pastures can be very diverse ecosystems, but many com- ponents of this biodiversity cannot be easily managed or directly manipulated for production (e.g., herbage, meat, milk, fiber) purposes. Management practices can influence insect and soil animal abundance and diversity but it is dif- ficult to affect these communities directly or intentionally (Bardgett and Cook 1998; Kruess and Tsharntke 2002). Plant species diversity may be the component of biodiversi- ty most easily managed in pastures. Species can be added or removed by typical farm management practices and the result can be readily observed and monitored. The plant community is of particular interest because of its central role as primary producer in the pasture ecosystem.
The question remains whether increasing the botanical complexity of mixed swards would be beneficial to produc- ers in terms of herbage or animal production, stability or uniformity of production, among other ecosystem functions of importance in pasture. These issues are discussed in the next sections.
MIXED SWARDS AND HERBAGE PRODUCTIVITY There are several reports on the herbage production of for- age mixtures in small plots and pastures. In a clipped small- plot study, there was no herbage production benefit from planting forage mixtures of four or more species (Tracy and Sanderson 2004b). In a grazed small-plot study, 13 forage mixtures (combinations of two, three, six, and nine species) were compared under management-intensive grazing. The six- and nine-species mixtures yielded more forage than the two- or three-species mixtures; however, the main reason for greater yields was the inclusion of red clover (Trifolium pratense L.; Deak et al. 2004). When mixtures were com- pared with grass and legume pure stands under rotational stocking all mixtures provided higher DM yields compared with pure grass stands (fertilized with 168 or 252 kg N ha–1). A multilocation (28 sites) European study of forage mix- tures and monocultures in clipped plots revealed that mix- tures often yielded more than the highest yielding monoculture (Kirwan et al. 2007).
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Ecological research suggests that natural grasslands of greater species richness are more resistant to stresses, such as drought, then less diverse plant communities (Tilman and Downing 1994). In a small-plot experiment where rainout shelters were used to control moisture availability, a five- species mixture containing chicory (Cichorium intybus L.), orchardgrass (Dactylis glomerata L.), Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) had 89% greater yield under drought, 61% greater yield with normal mois- ture, and 43% greater yield in excessively wet treatments compared with a white clover-Kentucky bluegrass mixture (Skinner et al. 2004). Increased yield was primarily due to the robust growth of chicory, which dominated the mixture. In addition, white clover growing in the five-species mixture had improved leaf water relations and greater relative growth rates than white clover growing in the two-species mixture.
Two pasture-scale studies demonstrated greater herbage production for complex mixed swards compared with a sim- ple grass-legume mixture (Sanderson et al. 2005; Skinner et al. 2006). In a 4-yr on-farm study, an 11-species mixture of several grasses, legumes, and chicory yielded more herbage than did an orchardgrass-white clover sward (Fig. 2; Skinner et al. 2006). In addition, the 11-species mixture had greater root biomass and a greater proportion of the root system located in deeper soil layers than the simple grass-legume mixture. Mid-day photosynthetic rates during the summer
were also greatest in the 11-species mixture. In another grazing trial, 1-ha pastures of six and nine-species mixtures of grasses, legumes, and chicory yielded more herbage than an orchardgrass-white clover mixture in a dry year but not in a wet year (Sanderson et al. 2005). In both studies, the yield benefit resulted mainly from including highly produc- tive, drought-tolerant species. An economic analysis showed that using complex forage mixtures could be more profitable than using a grass monoculture (Sanderson et al. 2006a). A major disadvantage reported in the small-plot and pasture-scale studies was that nearly one-half of the planted species in the complex swards did not persist beyond 3 or 4 yr, indicating that species presence was not very stable in these mixtures.
MIXED SWARDS AND GRAZING ANIMAL PRODUCTIVITY
Grazing animals greatly influence plant species composition and diversity in grazing lands; however, few studies have examined how botanical complexity of swards affects ani- mal performance (Soder et al. 2006). The few published studies on grazing forage mixtures have dealt mostly with mixtures of one grass and one legume, with contradictory results in both dairy cattle (Wedin et al. 1965; Harris et al. 1997; Phillips and James 1998; Rutter et al. 2004) and sheep del Pozo et al. 1997; Wright et al. 2001).
Canadian on-farm research in the 1940s demonstrated greater milk production (4300 kg ha–1) on complex pasture
Table 1. Biodiversity of pastures and conservation grasslands in several states of the northeastern USA
Number of pastures Cumulative plant Item or sites sampled species identified Average per site Range Source
Herbaceous plants 144 310 32 9–73 Goslee and Sanderson (2005) Herbaceous plants 34 280 34 12–60 Adler et al. (2004) Soil seedbank 36 54 10 4–18 Tracy and Sanderson (2000a)
Frequency
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Narrowleaf plantainCurly dock
Fig. 1. Relationship between frequency and abundance of individual plant species in pastures of the northeastern USA.
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swards [alfalfa (Medicago sativa L.), white clover, orchard- grass, smooth bromegrass (Bromus inermis Leyss.), timothy (Phleum pratense L.), meadow fescue (Festuca pratensis L.), Kentucky bluegrass, and Canada bluegrass (Poa com- pressa L.)] than on a dual-purpose hay/pasture mixture of timothy, alfalfa, and red clover (3000 kg ha–1; Clark and Poincelot 1996). Grazing research with lactating dairy cows in the mid 1960s, on the other hand, indicated that there was no benefit in milk production to planting a complex mixture of grasses and legumes for grazing (Wedin et al. 1965).
Cows that grazed a mixed sward of white clover and perennial ryegrass had higher (22.1 kg cow–1 d–1) milk pro- duction than cows that grazed a ryegrass monoculture (18.9 kg cow–1 d–1; Phillips and James 1998). However, when offered a choice of the perennial ryegrass monoculture and the mixed sward of white clover and perennial ryegrass, cows failed to completely select a diet that supported the highest milk production (20 kg cow–1 d–1).
Milk production and ingestive grazing behavior (grazing time, biting rate, and grazing jaw movements) were similar for high-producing dairy cows grazing swards of varying species richness, from a simple orchardgrass-white clover mixture to a complex sward containing nine species (grass- es, legumes, and chicory; Table 2; Soder et al. 2006, 2007). Differences observed in forage yield, botanical composition, and structure of the swards (Sanderson et al. 2005, 2006b) were expected to produce greater variation in grazing behavior (Baumont 2005); however, the high rate of herbage intake by these lactating animals may have reduced their selectivity (Rutter et al. 1997).
A summary of 24 experiments in the USA indicated that beef steer gain from grass-legume pastures averaged 0.14 kg d–1 more than from grass + N pastures (Burns and Standaert 1985). Two recent studies have addressed beef production from mixtures of several grasses and legumes. A mixture of 14 native grass, legume, and forb species produced margin- ally better beef cattle gains compared with a mixture of six native grasses and one clover in Saskatchewan (Iwaasa and Schellenberg 2005). In the Midwestern USA, performance of beef calves grazing swards planted with three to eight for- age species was similar among mixtures, but animal gains varied among the 3 yr of the study (Tracy and Faulkner 2006). Stocking rate and overall forage yield best explained variation in cattle performance in that study.
The few studies conducted on the performance of animals grazing mixed swards of several grasses, legumes, and forbs have been done with all forage species growing together. Spatially separate combinations of monocultures grazed in particular sequences and at specific times matched to the grazing animal’s nutritional needs may be a more targeted way to use forage species diversity in grazing systems (Rutter 2006). Further research is needed to gain a greater understanding of how characteristics of a wider range of individual plant species in mixed swards affects grazing behavior of animals, and how the resultant grazing behavior affects sward production and diversity.
ECOSYSTEM STABILITY Plant biodiversity theory predicts that increased diversity contributes to the stability of ecosystems (Tilman 2001). Evidence is lacking, however, for a clear-cut effect of plant species diversity on the stability of temperate pastures because there are very few published studies. In a small-plot study, planting mixtures of up to 15 species of legumes, forbs, and grasses did not improve forage yield or yield sta- bility (Tracy and Sanderson 2004a). Most of the mixtures decreased in species number during the 3-yr study and became dominated by perennial grasses. Plant species rich- ness and evenness were weakly associated with the stability of sheep production (coefficient of variation in annual car- rying capacity) in New Zealand pastures (Scott 2001). Stability of forage and animal production on temperate graz- ing lands in southern Australia was not related to plant species richness (Kemp et al. 2003).
INVASION RESISTANCE Although weedy unsown species are frequently present in pastures, many of these species are readily grazed by live-
Hay
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Root biomass to a 60-cm depth Avg of hay and grazing treatments
Number…