Biodiversity, Distributions and Adaptations of Arctic Species in the Context of Environmental Change

Royal Swedish Academy of Sciences

Effects on the Function of Arctic Ecosystems in the Short- and Long-Term PerspectivesAuthor(s): Terry V. Callaghan, Lars Olof Björn, Yuri Chernov, Terry Chapin, Torben R.Christensen, Brian Huntley, Rolf A. Ims, Margareta Johansson, Dyanna Jolly, Sven Jonasson,Nadya Matveyeva, Nicolai Panikov, Walter Oechel, Gus ShaverSource: Ambio, Vol. 33, No. 7, Climate Change and UV-B Impacts on Arctic Tundra and PolarDesert Ecosystems (Nov., 2004), pp. 448-458Published by: Springer on behalf of Royal Swedish Academy of SciencesStable URL: http://www.jstor.org/stable/4315526 .Accessed: 11/10/2011 20:18

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Climate Change and UV-B Impacts on Arctic Tundra and Polar Desert Ecosystems

Effects on the Function of Arctic Ecosystems in the Short- and Long-term Perspectives Terry V. Callaghan, Lars Olof Bjorn, Yuri Chernov, Terry Chapin, Torben R. Christensen, Brian Huntley, Rolf A. Ims, Margareta Johansson, Dyanna Jolly, Sven Jonasson, Nadya Matveyeva, Nicolai Panikov, Walter Oechel and Gus Shaver

Historically, the function of Arctic ecosystems in terms of cycles of nutrients and carbon has led to low levels of primary production and exchanges of energy, water and greenhouse gases have led to low local and regional cooling. Sequestration of carbon from atmospheric CO2, in extensive, cold organic soils and the high albedo from low, snow-covered vegetation have had impacts on regional climate. However, many aspects of the functioning of Arc- tic ecosystems are sensitive to changes in climate and its impacts on biodiversity. The current Arctic climate results in slow rates of organic matter decomposition. Arctic ecosystems therefore tend to accumulate organic matter and elements despite low inputs. As a result, soil-available elements like nitrogen and phosphorus are key limitations to increases in carbon fixation and further biomass and organic matter accumulation. Climate warming is expected to increase carbon and element turnover, particularly in soils, which may lead to initial losses of elements but eventual, slow recovery. Individual species and species diversity have clear impacts on element inputs and retention in Arctic ecosystems. Effects of increased CO and UV-B on whole ecosystems, on the other hand, are iikely to be

small although effects on plant tissue chemisty, decomposition and nitrogen fixation may become important in the long-term. Cycling of carbon in trace gas form is mainly as co2 and CH4. Most carbon loss is in the form of CO2, produced by both plants and soil biota. Carbon emissions as methane from wet and moist tundra ecosystems are about 5% of emissions as CO2 and are responsive to warming in the absence of any other changes. Winter processes and vegetation type also affect CH4 emissions as well as exchanges of energy between biosphere and atmosphere. Arctic ecosystems exhibit the largest seasonal changes in energy exchange of any terrestrial ecosystem because of the large changes in albedo from late winter, when snow reflects most incoming radiation, to summer when the ecosystem absorbs most incoming radiation. Vegetation profoundly influences the water and energy exchange of Arctic ecosystems. Albedo during the period of snow cover declines from tundra to forest tundra to deciduous forest to evergreen forest. Shrubs and trees increase snow depth which in turn increases winter soil temperatures. Fu- ture changes in vegetation driven by climate change are therefore, very likely to profoundly alter regional climate.

INTRODUCTION

The impacts of changes in climate and UV-B radiation on species (1) and on the numerous and complex interactions among species (2) determines the spatial structure of vegetation, such as height of the canopy, trophic interactions and community composition in terms of biodiversity. Spatial structure interacts strongly with the functioning of ecosystems. In the Arctic, ecosystem function represented by cycles of nutrients and carbon and exchanges of energy, water and greenhouse gases, has pro- vided a range of feedbacks to regional climate. Sequestration of carbon from atmospheric CO2 in extensive, cold organic soils and the high albedo from low, snow-covered vegetation in winter have historically led to low primary productivity and local and regional cooling. On the other hand, the extensive wetlands of the Arctic emit significant quantities of the radiatively strong greenhouse gas, methane. The processes that control these feedbacks, such as photosynthesis, decomposition, and vegetation canopy characteristics, are sensitive to changes in climate, and to some extent, changes in UV-B radiation. They also depend on biodiversity at the plant functional level which is itself responsive to climate changes (3). Consequently, changes in climate and UV-B radiation are expected to impact the functioning of Arctic ecosystems with potential implications for primary productivity and local and regional climate.

In this paper, we assess the likely impacts of changes in climate and UV-B radiation on the functioning Arctic ecosystems.

We define ecosystem function in terms of: - carbon and nutrient cycling including dissolved organic car-

bon (DOC) export; - soil processes; - controls on trace gas exchange processes; - primary and secondary productivity; - water and energy balance.

We focus on processes at the plot (single m2) scales: impacts of changes in climate and UV-B on processes at the landscape and regional scales are assessed by Callaghan et al. (4, 5). The paper is part of an holistic approach within the Arctic Climate Impacts Assessment (ACIA) (6) to assess impacts of climate change on Arctic terrestrial ecosystems (7).

BIOGEOCHEMICAL CYCLING: DYNAMICS OF CARBON (C) AND NUTRIENTS

Arctic ecosystems are characterized by low primary productivity, low element inputs, and slow element cycling yet they tend to accumulate organic matter, C, and other elements because decomposition and mineralization processes are even more strongly limited than productivity by the Arctic environment, particularly the cold, wet soil environment (8). Because of this slow decomposition, the total C and element stocks of wet and moist Arctic tundra frequently equal and may exceed the stocks of the

448 ? Royal Swedish Academy of Sciences 2004 Ambio Vol. 33, No. 7, Nov. 2004 http://www.ambio.kva.se

same elements in much more productive systems of temperate and even tropical latitudes (Table 1).

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Low Arctic sites with warmer and dryer soils, and extremely unproductive high Arctic polar deserts and semideserts, have smaller organic matter accumulations (Table 2). Most of the organic matter and element accumulation occurs in soils, while large accumulations of vegetation mass are limited by a lack of tall woody plant forms such as trees, by selection for slow-growing, low, compact plant forms, and by low productivity and low availability of soil-available elements such as N or P. Typically, the majority of the vegetation mass consists of roots and below- ground stems, with aboveground plant mass accounting for less than one third, and sometimes only 5-10%, of the total.

In addition to the large C stocks within the seasonally-thawed, upper active layer of soil (Table 2), an equally large pool of organic C may be held in the upper permafrost, within 1-2 m of the surface (10). While these frozen C stocks are not actively involved in C cycling on a seasonal or yearly basis, in the long- term they do represent an important C sink, and they may be of particular importance if climate changes lead to greater soil thaw or to loss of permafrost (4).

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The largest body of information on organic matter, C, and nutrient budgets of a wide range of Arctic ecosystems comes from the International Biological Program's Tundra Biome Program, which took place during the late 1960s and early 1970s (13). Since then, research on Arctic element cycling has tended to focus on controls over individual biogeochemical processes rather than on compari- sons of overall budgets and element stocks. The recent surge of interest in climate change and feedbacks from the Arctic to the globe has highlighted the relevance and utility of those earlier studies, particularly as currently only a few sites are under study at the whole-system level.

Microbes in Arctic soils contain only one or a few percent of the ecosystem C pool. However, the proportions of ecosystem N and P

are appreciably higher due to high concentration of N and P in the microbial tissue comvared to the concentration in Dlants and soil

organic matter (SOM) (14). The proportions of microbial biomass and nutrient content of the total amount in the soil organic matter are similar to the proportions in non-Arctic ecosystems but, due to the low plant biomass in the Arctic, appreciably higher than the proportion in the vegetation. Data from various Arctic and sub-Arctic sites have shown that microbes commonly contain appreciably less C, slightly less or comparable amounts of N and much higher amounts of P than the entire plant biomass (15-18).

Nutrient mineralization rates are low, however, typically tenfold lower than in the boreal region. The low rate is mainly due to low soil temperatures, and it leads to low supply rate

of nutrients to the plant available pool and nutrient-constrained plant productivity in most Arctic ecosystems (19). The combina- tion of low mineralization rates and high proportion of nutrients in microbes compared to plants leads to possible competition for nutrients between microbes and plants at periods of rapid microbial growth (20). However, microbes are likely also to release a pulse of nutrients during periods of population decline when the cells are lysed and nutrients are leached (8, 21). To predict the microbial effects on the nutrient constrained plant productivity by environmental changes, it is essential therefore not only to understand how the microbial processing rate of the organic matter will change, but also to understand the controls of microbial population sizes and how changes in the populations affect nutrient cycling and interact with plant processes (see also (2)).

Spatial Variability

Although the productivity of the most productive tundra may rival that of highly productive shrub and marsh systems at lower

latitudes, most Arctic systems lie at the low end of the global productivity range. What is striking is the wide range of variation (about three orders of magnitude) of NPP, and standing stocks of organic matter in soils and vegetation within the Arctic (Table 2). In general, productivity and organic matter stocks decrease with temperature and precipitation from South to North, but local variation in productivity in relation to topography is dramatic (often 10-100-fold). Among the most important cor- relates of topographic variation in productivity are the duration and depth of winter snow cover and degree of protection from winter

wind damage, as well as variation in soil moisture, soil thaw, and soil temperature. Local variation in these factors can be nearly as great as that across a wide range of latitudes (8, 22, 23). Lo- cal variation in productivity is also associated with dramatic shifts in the relative abundance of plant functional types including both vascular and nonvascular plants (1-3). Because of this dramatic local variability, primary production and organic matter accumulation are distributed in a mosaic fashion across the Arctic, with a higher frequency of more productive sites (usually wet or moist lowlands) at lower latitudes.

The spatial distribution of productivity and organic matter in the Arctic is broadly predictable in relation to temperature,

Ambio Vol. 33, No. 7, Nov. 2004 C Royal Swedish Academy of Sciences 2004 449 http://www.ambio.kva.se

soil moisture and other soil factors such as pH, topography and snow cover (24-27). The proximate controls on C cycling in these ecosystems, however, are much more closely tied to the inputs and turnover of other elements, especially N and P (28). Because N and P inputs (by deposition, fixation, or weathering) are low, even where rapid photosynthesis is possible C cannot be stored in organic matter any faster than the rate of N or P accumulation. Thus, for example, in the Canadian high Arctic where a large portion of the surface is bare-ground, C fixation and accumulation is closely tied to low, wet areas where anaerobic soil conditions favor N fixation (29). In other Arctic systems, such as Alaskan wet and moist tundras, the total amounts of N and P in soil organic matter may be very high but, due to slow rates of decomposition, the availability of these elements to plants is low, thus leading to low productivity despite high element and soil organic matter stocks (21).

Temporal Variability

Year-to-year variation in biogeochemical cycles has received little attention in the Arctic, although a few multiyear records of ecosystem C exchange, N deposition, and C and N losses at the watershed or catchment level do exist (30-33). Clearly, the trend over the Holocene, at least, has been one of overall accumulation of elements in organic matter since the loss of the glacial ice cover, but the variation in rates of accumulation (or loss) at the scale of years to decades is particularly poorly understood.

The net C balance of Arctic ecosystems, as in any terrestrial ecosystem, may be positive or negative depending on the time scale over which it is measured and the environmental conditions during the measurement period. This dynamic balance is called Net Ecosystem Production (NEP) and is defined as the difference between two large, opposing fluxes, Gross Ecosys- tem Production (GEP, or gross ecosystem photosynthesis) and Ecosystem Respiration (RE), both measured in units of C mass or moles per unit area and time. Ecosystem Respiration (RE) has two major components, RA (autotrophic or plant respiration) and RH (heterotrophic or animal plus microbial respiration). Each of these components of NEP has different relationships to current temperature, moisture, and light conditions. Because they are measured at the whole-system level, all three components are also a function of the current functional mass or surface area of organisms as well as the organisms' current nutritional status. Thus, for example, even though NEP must be positive over the long-term for the large C accumulations in tundra ecosystems to occur, on a daily or seasonal basis NEP swings from strongly negative (at "night", even under the midnight sun, or in winter) to strongly positive (at midday in midsummer). These daily and seasonal fluctuations have been measured at an increasing num- ber of Arctic sites in recent years (30, 34-37).

NEP may also vary sharply among years, and may be either positive or negative on an annual basis. Recent work in Alaska (30) indicates that although C was accumulating in wet and moist tundras in the 1 960s and 1 970s, (i.e. "negative" C balance) during much of the 1980s and 1990s there was a net loss of C from these ecosystems in both winter and summer (i.e. "positive" C balance). In the late 1 990s, the summer C balances turned again so that the ecosystems were net sinks, but it is not yet clear whether the C balances for the full year have returned to net C accumulation (Fig. 1; see also Callaghan et al (al ( t also is not clear whether the shifts in NEP that have occurred over the past 40 years, from C sink to C source and are related in any direct way to weather, because the entire period has been one of general warming in northern Alaska (4). Modeling studies (e.g. 38-40) suggest that in the short term (within one or a few years) the response of RE (both RA and RH) to temperature is more rapid than the response of GEP, leading to a short-term loss of C (negative NEP) with warming. In the long-term, however, the interaction between temperature and

soil nutrient availability might increase GEP sufficiently to cause an eventual return to net C accumulation (positive NEP). There is also evidence from manipulation experiments (41, 42) and latitudinal gradients (43) that air warming can result in soil cooling after long periods. The mechanism is that air warming leads to increased leaf area indices that intercept a greater proportion of incoming radiation before it reaches the soil surface, thereby leading to soil cooling.

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Figure 1. Long-term trends in summer net CO flux, temperature, and precipitation for Alaskan coastal we4 sedge tundra (44, 45).

Species and functional type composition of the vegetation are keys to long-term change in productivity, because of differences in nutrient use and allocation, canopy structure, phenology, and relative growth rates among plants (46, 47). Large differences exist, for example, in the rate at which tundra plants can respond to changes in weather and climate, due to differences in allocation to stems versus leaves or to secondary chemistry versus new growth (48), in the ability to add new meristems (49), and in the constraints on the amount of growth that can be achieved by a single meristem within a single year (i.e. determinate versus in- determinate growth). Species and functional types also differ in their phenology of growth and thus in their ability to take advan- tage of a change in the timing and duration of the growing season. For example, moss-dominated ecosystems in Iceland have limited ability to respond to climate change without a complete change to a vascular plant-dominated community (50) whereas shrubs and small trees already present in sheltered, moist de- pressions on the North Slope of Alaska seem to be already ex- panding their distribution (51).

The chemical composition of primary productivity (leaves versus wood, secondary chemistry, species composition) is important as a long-term feedback on productivity and its re- sponsiveness to climate change. It is also important in terms of both animal community composition and secondary (herbivore) productivity. C:N ratios, lignin and protein content, and tannin, resin, and phenolic content are all important in determining for- age quality (2) and the susceptibility of plant lifter to decomposi-

450 C Royal Swedish Academy of Sciences 2004 Ambio Vol. 33, No. 7, Nov. 2004 http://www.ambio.kva.se

tion, and thus the remineralization of essential limiting nutrients like N and P (52).

Despite the critical importance of NPP together with NEP and considerable research on these parameters, additional field measurement and focused process studies are needed to resolve is- sues relating to the different methodologies used for measuring NPP and NEP (53, 54). Also, results from different methodologies need to be reconciled (55).

N and P budgets were developed for several Arctic sites during the IBP studies 30 years ago (56), but complete documenta- tion of inputs and outputs of any element other than C has not been attempted since then for any Arctic site. Part of the prob- lem is that individual N and P inputs and outputs in Arctic ecosystems, such as N fixation, N deposition, denitrification, rock weathering, or losses in streamflow, are even smaller than the amounts annually recycled by mineralization of organic matter (Fig. 2; 57, 28, 58), except perhaps in the high Arctic (29). Thus, very long-term records are needed to evaluate the significance of interannual variation in N and P budgets, while most studies of the component processes last only 1-3 years.

I IN -- -

(MW - OsSdflutIsm

Figure 2. Nitrogen budget for wet sedge tundra at Barrow Alaska (28; adapted from Chapin et al. (56)). Numbers in boxes are N stocks in g m-2; numbers in parentheses are N fluxes in g m-2 yr-.

Inputs/Outputs, Primary Production and NEP

The dominant C input to Arctic ecosystems is by photosynthesis of vascular and nonvascular plants which in total sums to Gross Ecosystem Production, GEP. The relative (apparent) importance of various controls on primary production differs depending on whether one looks at the leaf level, the canopy level or the whole vegetation, and at daily, seasonal, or decadal time scales (59). Carbon inputs at the leaf level are clearly limited in the short term by generally low irradiance and consequent low temperatures during usually short, and late, growing seasons (Fig. 1 in ref. 1), despite a wide range of specific photosynthesis-related adaptations to the Arctic environment (3). Photosynthesis of Arctic plants is also often sensitive to changes in CO2 (in the short term), moisture conditions, and snow (UV effects are vari- able and comparatively small). Although Arctic plants in general are well-adapted to the Arctic climate, there still is considerable variation in the responses of photosynthesis to microclimate among plant functional types. In the longer term and at the level of whole vegetation canopies, however, C inputs are limited by generally low canopy leaf areas, leaf phenology/duration, and light interception (59). Canopy leaf area is low because low soil nutrient availability, particularly N, limits the ability of the vegetation to develop a large, photosynthetically efficient leaf area (60), and it also limits the ability of the vegetation to use newly- fixed C in new growth, because growth requires adequate sup- plies of multiple elements in addition to C (8, 28). It is also low

because of the low stature of the vegetation, which prevents development of a multilayered canopy. Other environmental factors such as wind and soil disturbance also limit C gain. Storage of photosynthate and nutrients acquired in previous years plays a key role in determining a current year's productivity (61).

Carbon outputs from Arctic ecosystems occur via a wider ar- ray of processes and are regulated very differently from C inputs (see below). The dominant form of C loss is as CO2, produced by both plants and soil biota. Autotrophic or plant respiration (RA) typically accounts for about half of GEP on an annual basis (53, 59) but follows a very different seasonal and daily pattern. Het- erotrophic respiration (RH), mostly by soil organisms, accounts for most of the other half although in the long-term the sum of these two must be slightly less than GEP if C is to accumulate in soil organic matter. RH includes both CO2 and CH4, the latter produced anaerobically in wet soils (see below). Much of the CH4 produced in Arctic soils is oxidized to CO2 before it reaches the atmosphere; net CH4 emission thus is normally only a fraction of CO2 emission in Arctic soils (less than 5%), but methane is a much more powerful greenhouse gas than CO2. Other aspects of carbon balance are important yet difficult to quantify. Examples

are plant root respiration, the sloughing of dead material from roots, root exudation, and the growth and respiration of microorganisms intimately associated with plant roots.

Most of the respiratory CO2 and CH4 losses from Arctic systems move directly to the atmosphere. A significant fraction of these gases, however, travels in dissolved forms in soil water, eventually moving into streams and lakes where they are re-

leased to the atmosphere (62, 63). Additionally, soil and surface waters contain significant amounts of dissolved organic forms of C, much of which is eventually consumed by aquatic microbes, producing more CO2 (Chapter 8 in ref. 6). Together, these losses to aquatic systems may add up to a significant component of the net C balance of Arctic systems. Synoptic, simultaneous analysis of aquatic C losses at the same time, place, and scale as direct atmospheric exchanges has not been completed, but estimates of aquatic C losses suggest that these may equal as much as 20- 30% of GEP.

Wintertime CO2 losses are a second major gap in our knowl- edge of C losses from Arctic ecosystems. Although wintertime CO2 losses have long been recognized (64), more recent research indicates that these losses are not only larger than was previously thought, but also that they may be the product of significant respiratory activity during the wintertime (3, 54, 65-67) in which recently fixed carbon is respired (68).

Inputs and outputs of N have been relatively little-studied in the Arctic, largely because early work suggested they were small relative to standing N stocks and internal recycling, and thus were less important, at least on a short-term basis (28). In the long-term, however (e.g. on a scale of several decades or more), understanding of N inputs and outputs is essential to understanding how the total pool sizes of N change over time. Changes in standing stocks of N are closely tied to the accumulation or loss

Ambio Vol. 33, No. 7, Nov. 2004 c Royal Swedish Academy of Sciences 2004 451 http://www.ambio.kva.se

of organic matter and C in the Arctic (29). N enters Arctic ecosystems by atmospheric deposition and

by microbially-mediated N fixation (Fig. 2). N deposition rates are low in the Arctic relative to other parts of the world, mostly because the atmosphere is cold enough that it cannot hold the high concentrations of N species such as nitrate (NO3) that fall on lower latitudes. Thus N deposition can account for only about 5% or less of the annual plant N uptake requirement in Alas- kan wet sedge tundra (56), although this might increase with increased industrial activity at lower latitudes. In regions such as northern Scandinavia that are subject to N deposition from lower-latitude anthropogenic sources, however, N deposition may be greater than 0.1 g m-2 yr', which if continued for many years is sufficient to affect plant growth and productivity (69). N fixation rates are usually assumed to be of similar magnitude, although the only relatively recent studies (70, 71) indicate that, at least in the high Arctic, N fixation might account for more than 10% of plant requirements with the remaining 90% supplied by recycling from the soils.

N losses are also poorly-known. There have been no recent, published studies of denitrification in the Arctic; although anaerobic soils might be expected to have high potential for denitrification, the generally low rates of nitrate production in tundra soils suggest that this is also a small component of the annual N budget. Possible spring losses of N in the form of N20 have been suggested (72) but not yet verified in the Arctic. N losses in streams have been monitored at several locations, and are of roughly the same magnitude as N deposition (31).

Responses to Climate Change

Responses of element cycles in Arctic ecosystems to climate change factors have been studied in multiyear manipulation experiments in several contrasting ecosystem types (73, 74; Fig. 3). These experiments include manipulations of air temperature, CO2. light, water (both excess and deficit), nutrients and UV-B radiation. One common observation from these experiments is that although short-term responses to single factors like CO2 or warming are measurable and often significant, these responses are often not sustained due to other limitations. A general conclusion is that nutrient limitation dominates the multiyear responses and is linked to changes in other factors (e.g. temperature and water) through their indirect effects on nutrient mineralization and availability to plants. In wet systems, watertable depth and soil drainage are critical variables limiting nutrient turnover in the soil; increases in C turnover in these systems are not linked

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to increases in C accumulation because increased C accumulation requires increased N and/or P supply (30, 42, 75). A large pool of nutrients exists in organic matter, and may drive large changes in organic matter stocks if the nutrients can be miner- alized and not leached from the system. Similarly, short-term increases in photosynthesis and growth in response to high CO2 are often not sustained due to nutrient limitation (1, 76-78).

The results of several manipulation experiments indicate that nutrient mineralization stimulated by increased soil temperatures will probably not be sustained in the long-term. Warming of soil causes an immediate increase in soil respiration in laboratory studies, but few Arctic field studies have shown increased mineralization rates in response to air warming (see 79) and longer- term field studies show an acclimation to increased temperature (78, 80). Some studies have shown soil cooling in response to air warming in experiments (41) and along latitudinal gradients (43). Air warming stimulates leaf-area development (81) and greater LAI would be expected to intercept thermal radiation before it reaches the soil, leading to soil cooling. In addition, organic matter in lower soil profiles is less responsive to temperature increases than surface layers (68, 82), again suggesting that any temperature-induced mineralization is likely to be tran- sitory.

Overall, however, multiyear experiments suggest that the most responsive Arctic ecosystems to climate change will probably be those in which the environmental change is linked to a large change in nutrient inputs or soil nutrient turnover, and/or large changes in leaching or erosional losses of soil-available nutrients (Fig. 2). Effects of UV on overall organic matter cycling are generally unknown, but not unimportant. Recent work by Ni- emi et al. (83) showed that UV-B decreased methane emissions from a peat-land in northern Finland while three studies show an effect on UV-B on Sphagnum growth (84-86) with potential implications for carbon sequestration. Long-term responses of biogeochemical cycling to increased CO2 and UV-B are small in magnitude but are likely to lead to longer-term changes in biogeochemical cycling and ecosystem structure (1, 2). However, most of our understanding of UV responses is based on species- and tissue-level research.

Biodiversity and Species Effects on Biogeochemistry

Does the species or growth form composition of the vegetation have any impact on biogeochemistry of Arctic ecosystems, or is biogeochemistry largely regulated by climate and resource availability irrespective of species composition? Although only partial answers to this question are currently available, there are at least five main mechanisms by which species composition are likely to have important consequences for biogeochemistry. These are: i) Species composition will probably affect the rate of change

in ecosystems in response to environmental change, through differences in species potential growth, reproduction, and dispersal rates (e.g. 49).

ii) Species are likely to affect nutrient availability and C cycling through differences in the turnover of elements in their living tissues and in the decomposability of their dead parts (87, 88).

iii) Species will probably affect element accumulations in living plants through differences in their biomass allocation patterns and in the element concentrations and element ratios in their biomass (88-90).

iv) Species are likely to differ in their effects on snow accumulation and snowmelt, surface energy balance, and soil temperature regimes, with important feedbacks on element cycles (91, 92).

v) Physiological mechanisms, for example species of wetlands that act as conduits for methane transport from soil to air (Fig. 4; 83, 93, 94).


All five of these species effects have been documented in Arctic systems, although it is often uncertain how to scale-up from small experimental communities to larger units of the landscape.

Species richness or diversity itself is likely to also affect biogeochemistry of Arctic ecosystems, although the magnitude of the effect is hard to judge. There is a weak positive correlation between productivity and vascular species richness in Arctic vegetation, but, like most vegetation, richness declines when productivity is increased artificially by fertilizer addition or other disturbance (95). Recent evidence suggests that Arctic plants obtain their N from diverse sources in the soil (96-98) and that the relative abundance of different species reflects different abilities to acquire the different forms of N (99). These latter studies suggest that diversity will probably increase productivity in Arctic vegetation by increasing total uptake of different forms of a strongly limiting element, N. Partial support for this conclusion comes from experiments involving removal of individual species from Arctic vegetation, in which the remaining species failed to increase in abundance (100-103).

Vascular plants affect directly the substrate availability for methanogens and have the capa- bility to transport gases between the anaerobic parts of soils and the atmosphere (83, 94, 104, 105). Different vascular plant species have different effects, however, and the vascular plant species composition in wet tundra ecosystems may be a key determinant for the scale of CH4 emissions (Fig. 4; 94, 106). Changes in species composition per se caused by climate warming-and increased UV-B radiation-are likely to cause a

change in CH4 emissions adding to the direct effect of a changing soil climate (83).

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Figure 4. Seasonal mean methane emission from a high Arctic fen in NE Greenland (Zackenberg) plotted against leaf biomass of Eriophorum scheuchzeri, Dupontia psi- losantha, Carex subspathacea and total leaf biomass of the three species. The re- gression lines represent linear fits. The figure shows that the minor constituents of the total vascular plant biomass (Carex and Eriophorum) seem to be "driving" net methane emissions from the site suggesting that shifts in vascular plant species composition alone could lead to significant effects on trace gas exchange (94).

Role of Disturbance

Disturbances are expected to increase with climatic warming, mainly through thermokarst (4) and possibly also through increased fire in some northern ecosystems, and insect pest out- breaks in sub-Arctic forests (2). In general, physical disturbance to Arctic ecosystems results in greater soil warming and perma-

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Ambio Vol. 33, No. 7, Nov. 2004 ? Royal Swedish Academy of Sciences 2004 453 http://www.ambio.kva.se

frost thawing, which tend to increase soil organic matter and nutrient turnover. Typically, productivity of the vegetation increases dramatically although soil respiration also increases. It is not yet clear whether the increased plant growth is sufficient to compensate for losses of soil organic matter. In the long-term, however, Arctic landscapes should gain OM in both soils and vegetation on disturbed sites. The timing and trajectory of these changes are key unknowns for future research.

SOIL PROCESSES AND CONTROLS OVER TRACE GAS EXCHANGES

Diversity of Trace Gases

During the last decade, trace gases, their production, emission, and consumption have attracted considerable attention from the scientific community. The reason is that

i) most of these gases belong to the category of "radiatively active", i.e. they affect heat balance and induce the "greenhouse" effect responsible for climate instability and warming;

ii) the atmospheric concentration (mixing ratios) of these gases underwent remarkably fast changes after the industrial revo- lution (e.g. atmospheric methane has been growing with an annual rate of 0.8-1%, which can have a significant impact on the biosphere even beyond the greenhouse effect);

iii) most trace gases are intermediate or end products/substrates of key biogeochemical processes, and this is why monitor- ing these gas species can be used for early detection of any anomaly in ecosystem functioning.

Table 3 lists the major trace gases and their potential impacts on ecosystems. Not all of the listed gases are of primary importance for Arctic terrestrial ecosystems. In the present review, our attention will be restricted to CO2, (see above) CH4 and N20.

Soil and Ecosystem Processes Responsible for Gas Emissions

Trace gas exchange with the atmosphere occurs through a set of coupled soil/ecosystem processes, including i) production of substrate(s) for processing by trace gas-pro-

ducing organisms; ii) conversion of substrate to respective gaseous species in par-

allel with gas consumption (Table 4); iii) mass transfer of produced gas to the free atmosphere, which

includes three main mechanisms: molecular diffusion, vascular gas transfer (i.e. through plant "conduits"), and ebullition (i.e. bubble formation).

Tal pe=-2OUSSSwOr~u c*a* RLCtvor80W4

Fo~K ~~ttnH~pe+.an Fo~Atfm 3Dtacn

. -

Substrates are formed through one of three processes: i) decomposition (hydrolytic breakdown of plant litter, oxidation, fermentation); ii) nitrogen mineralization, and iii) photosynthesis + photorespiration. However, the starting point for almost all substrates is the primary production of organic matter through plant photosynthesis or (occasionally) bacterial chemosynthe- sis. There are two main flows of C-substrates from plants: i) via plant litter formation with lignocellulose as a main resis- tant component; and, ii) continuous supply of readily available C monomers (root and foliage exudation). The chain of events leading to the formation of immediate precursors of trace gases can be long and intricate (117). It is worthwhile to note that the most successful simulation models of trace-gas emissions include vegetation or primary productivity modules.

Trace Gas Transport

There are three main transport mechanisms: i) molecular diffusion; ii) vascular transport of gas through plant roots; and iii) ebullition. Vascular transport can be described as a diffusion process through plant root aero-parenchyma, which is a continuous network of gas-filled channels. Vascular transport is two or three orders of magnitude more rapid than diffusion in water. Ebullition is probably the most difficult process to simulate and describe mathematically due to its stochastic nature. In northern soils, ebullition and vascular transport were shown to be the major transport mechanisms accounting for up to 98% of the total emissions (118).

Environmental Controls on CH4 Fluxes

Methane is produced from anaerobic decomposition of organic material in waterlogged, anaerobic, parts of the soil. Wet and moist tundra environments are known to be significant contribu- tors to atmospheric CH4 (119, 120). Methane is formed through the microbial process of methanogenesis which is controlled by a range of factors most notably temperature, the persistence of anaerobic conditions, gas transport by vascular plants as well as supply of labile organic substrates (105, 106, 121, 122). Figure 5 shows the variety of controls on CH4 formation rates at different spatial and temporal scales. Methane is, however, not only being produced but also consumed in the aerobic parts of the soil. This takes place through the microbial process of methanotrophy, which can even take place in dry soils with the bacteria living off the atmospheric concentration of CH4 (72, 117, 122, 123). Methanotrophy is responsible for the oxidation of an ap- proximated 50% of the CH4 produced at depth in the soil and as such is as important a process for net CH4 emissions as the

process of methanogenesis is in itself. The anaerobic process of methanogenesis is much more responsive to temperature than CH4 uptake, so soil warming in the absence of any other changes will accel- erate emission (which is the difference between production and consumption), in spite of the simultaneous stimulation of the two opposing processes.

Apart from temperature, water re- gime, and plant cover, methanogenic bacteria are strongly affected by biological interactions within the soil community. Competition with acetogenic, and sulfidogenic bacteria for H2 (the outcome of this competition depends on the affin- ity to H112, the temperature, and density of the various populations) determines the pattern of gas formation not only quanti- tatively, but also in qualitative terms. For

454 c Royal Swedish Academy of Sciences 2004 Ambio Vol. 33, No. 7, Nov. 2004 http://www.ambio.kva.se

A- I

Figure 5. Controls on methanogenesis (redrawn from Walker et al. (1 24)).

example, ecosystems can be a source of CR4 (if methanogenic bacteria prevail), or H2S and other sulfides (under the domina- tion of sulfate-reducing bacteria), or acetic acid (if a large population of acetogens is present).

Early empirical models of northern wetland/tundra CR4 exchanges suggested sensitivity to climate change (125, 126). A simple mechanistic model of tundra methane emissions including the combined effects of temperature, moisture and active layer depth also suggested significant changes in CH4 emissions as a result of climate change (127). More complex wetland methane emission models suggest that winter processes have a strong influence on net annual emissions of CR4 (128). Variations in CH4 emissions at the large regional-global scale are driven largely by temperature (1 26, 129) with importaint modulating effects of the vascular plant species composition superimposed (1 30, 106). An initial warming is, hence, expected to lead to increased CR4 emissions, the scale of which though depends on associated changes in soil moisture conditions and the secondary effects of changes in vegetation composition (4).

Controls on N20 Fluxes

The simulation of 20 emission requires consideration of the combined N and C cycles, because the substrates of denitrification include electron acceptors (nitrate) as well as oxidizable C substrates. The ecosystem-atmospheric fluxes of N20 are associated with fundamental transformations of nitrogen in the soils, namely the processes of nitrification and denitrification. Very few field studies of 20 fluxes are available from the Arctic (72) but very small releases of Ne20 are expected from Arctic soils due to their general nutrient limitation. One issue of potential great importance in the Arctic though is early spring fluxes of n20. Denitrification has been found to take place even below the freezing point (i31, 132). During freezing and thawing, carbon is liberated and this may increase denitrification activity in the soil (133). During the spring thaw of the soil, significant parts of the annual emission of N20 can take place (134). The early spring fluxes may also explain the significant potential N2 production measured in fertilized plots on a sub-Arctic heath (72) on soils that showed no emissions from control plots. Hence, as with Co4nthe winter and "shoulder" season processes are generally very important but at the same time the least well understood.

Actmicnecosystem exii thces leargseth seasonalthage in enitifc-

eusrgyecanges ofanheretra ecosystem-topei bluecase of thae large-

icoinge raitfidmnatrsomation, to summere wh n the eoytmasorbs,

moist in omthing gnradi uren iiation. Aot9% Ofnte isenerg aborbed-

during summer133isDtransfeheredrto theatmospthere woithth resifcat

transferred to the soil in summer and released to the atmosphere in winter (135). Consequently, Arctic ecosystems have a strong warming effect on the atmosphere during the snow-free season.

Vegetation profoundly influences the water and energy exchange of Arctic ecosystems. In general, ecosystems with high soil moisture have greater evapotranspiration than dry ecosystems, as in any climatic zone. Arctic ecosystems differ from those at lower latitudes, however, in that there is no consistent relationship between CO2 flux and water vapor flux, because vascular plants account for most CO2 flux, whereas mosses account for most water vapor flux (136). This contrasts with other major biomes on Earth, where these two fluxes are strongly correlated (137, 138).

Within tundra, vegetation strongly influences winter energy budget through its effects on snow depth and density. Shrubs increase the snow depth by reducing the velocity of blowing snow and reducing sublimation rates; models suggest that in northern Alaska this shrub-induced reduction in sublimation can increase ecosystem-scale winter snow accumulation by 20% (91). Shrubs also cause snow to accumulate within shrub patches and to be depleted from shrub-free zones, increasing the spatial heteroge- neity of snow depth. Snow within shrub canopies is deeper and less dense, which reduces heat transfer through the snow-pack and increases winter soil temperatures by 2?C relative to adja- cent shrub-free tundra. Warmer soil temperatures beneath shrubs may increase winter decomposition and enhance nutrient availability, forming a positive feedback that promotes shrub growth (91).

Midsummer vegetation feedbacks to regional climate are de- termined largely by midsummer patterns of water and energy exchange (139). Midsummer albedo is greatest in sedge communities, whose standing dead leaves reflect much of the incoming radiation (139, 140). Evergreen forests and forest tundra, in con- trast, have a particularly low albedo because of the dark absorp- tive nature of evergreen leaves and the effectiveness of complex forest canopies in capturing light (135, 141). Deciduous-dominated tundra and forest tundra canopies are intermediate in albedo and therefore in the quantity of energy which they absorb and transfer to the atmosphere (141, 142). A larger proportion of the energy transfer to the atmosphere occurs as sensible heat flux in forests, forest tundra, and shrub tundra than in wet tundra (135, 139, 140, 143, 144).

All Arctic ecosystems exhibit greater ground heat flux during summer (5-15% of net radiation) than do temperate ecosystems (generally close to zero), due to the strong thermal gradient between the ground surface and permafrost and the long hours of solar radiation (139). Ground heat fluxes are reduced in tundra ecosystems with a large leaf area, which shades the ground surface (140), or where the ground cover is highly insulative, as with Sphagnum mosses (145). Grazing and other processes causing surface disturbance increase ground heat flux and thaw depth (24). Future changes in vegetation driven by climate change are very likely to profoundly alter regional climate.

CONCLUSIONS

Many aspects of the functioning of Arctic ecosystems represented by cycles of nutrients and carbon, and exchanges of energy, water and greenhouse gases, are sensitive to changes in climate and biodiversity. Historically, sequestration of carbon from atmospheric C2 in extensive, cold organic soils has led to low primary productivity and the high albedo from low, snow-covered vegetation in winter has led to local and regional cooling. This paper has focussed on climate and UV-B impacts on ecosystem functioning at the plot (single in2) scales. At these scales, it is clear that climate-driven changes in ecosystem function, whether mediated through physiological responses of existing


species or changes in biodiversity, have the potential to feedback to the climate system. However, it is the upscaling of these processes to the landscape and regional scales that will quantify the expected feedbacks from terrestrial ecosystems to the climate system (4, 5). At these larger sales, shifts in major vegetation zones responding to climate change become important drivers of ecosystem function and feedbacks.

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146. Acknowledgements. We thank Cambridge University Press for permission to repro- duce this paper. TVC and MJ gratefully acknowledge the grant from the Swedish En- vironmental Protection Agency that allowed them to participate in ACIA. We thank the participants, reviewers and particularly the leaders of the ACIA process for their various contributions to this study.

Terry V Callaghan Abisko Scientific Research Station Abisko SE 981-07 Sweden terry. [email protected]

Lars Olof Bjotrn Department of Cell and Organism Biology Lund University, Solvegatan 35 SE-22362, Lund Sweden lars_olof bjorn@cob. lu.se

Yuri Chernov A.N. Severtsov Institute of Evolutionary Morphology andAnimal Ecology Russian Academy of Sciences Staromonetny per. 29 Moscow 109017 Russia Isdc@orc. ru

Terry Chapin Institute ofArctic Biology University ofAlaska Fairbanks, AK 99775 USA terry. chapin@uaf edu

Torben Christensen Department of Physical Geography and Ecosystem Analysis GeoBiosphere Science Centre Lund University Sweden torben. christensen@nateko. lu.se

Brian Huntley School of Biological and Biomedical Sciences University of Durham UK [email protected]

Rolf A. Ims Institute of Biology University of Tromso N-9037 Tromso, Norway r. a. ims@bio. uio. no

Margareta Johansson Abisko Scientific Research Station Abisko, SE 981-07, Sweden scantran@ans. kiruna.se

Dyanna Jolly Riedlinger Centre for Maori and Indigenous Planning and Development PO. Box 84, Lincoln University Canterbury New Zealand dyjolly@pop. ihug. co. nz

Sven Jonasson Physiological Ecology Group Botanical Institute, University of Copenhagen Oester Farimagsgade 2D DK-1353 Copenhagen K, Denmark [email protected]

Nadya Matveyeva Komarov Botanical Institute Russian Academy of Sciences Popova Str. 2 St. Petersburg 197376 Russia [email protected]

Nicolai Panikov Stevens Technical University Castle Point on Hudson Hoboken, NJ 07030, USA npanikov@stevens-tech. edu

Walter C. Oechel Professor of Biology and Director Global Change Research Group San Diego State University San Diego, CA 92182 [email protected]

Gus Shaver The Ecosystems Center Marine Biological Laboratory Woods Hole, AM 02543 USA [email protected]


Biodiversity, Distributions and Adaptations of Arctic Species in the Context of Environmental Change

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