Gaius R. Shaver

Plant community responses to experimental warming across the tundra biome

Recent observations of changes in some tundra ecosystems appear to be responses to a warming climate. Several experimental studies have shown that tundra plants and ecosystems can respond strongly to environmental change, including warming; however, most studies were limited to a single location and were of short duration and based on a variety of experimental designs. In addition, comparisons among studies are difficult because a variety of techniques have been used to achieve experimental warming and different measurements have been used to assess responses. We used metaanalysis on plant community measurements from standardized warming experiments at 11 locations across the tundra biome involved in the International Tundra Experiment. The passive warming treatment increased plant-level air temperature by 1-3°C, which is in the range of predicted and observed warming for tundra regions. Responses were rapid and detected in whole plant communities after only two growing seasons. Overall, warming increased height and cover of deciduous shrubs and graminoids, decreased cover of mosses and lichens, and decreased species diversity and evenness. These results predict that warming will cause a decline in biodiversity across a wide variety of tundra, at least in the short term. They also provide rigorous experimental evidence that recently observed increases in shrub cover in many tundra regions are in response to climate warming. These changes have important implications for processes and interactions within tundra ecosystems and between tundra and the atmosphere.

Changes in the Carbon Content of Terrestrial Biota and Soils between 1860 and 1980: A Net Release of CO"2 to the Atmosphere

Changes in land use over the past two centuries have caused a significant release of CO2 to the atmosphere from the terrestrial biota and soils. An analysis of this release is based on amounts of organic carbon within an ecosystem following changes such as harvest of forests; it is also based on rates of changes, such as conversion of forest to agriculture, deduced from agricultural and forestry statistics. A model is used to calculate the net amount of carbon stored or released each year by the biota and soils of 69 regional ecosystems. Some of the changes, such as afforestation, the growth of harvested forests, and buildup of soil organic matter, result in a storage of carbon; others, such as harvest of forests and increase in pasture and agricultural areas, result in a loss of carbon to the atmosphere. According to this analysis, there has been a net release of carbon from terrestrial ecosystems worldwide since at least 1860. Until 1960, the annual release was greater than release of carbon from fossil fuels. The total net release of carbon from terrestrial ecosystems since 1860 is estimated to have been 180 x 1015 g (a range of estimates is 135-228 x 1015 g). The estimated net release of carbon in 1980 was 1.8-4.7 x 1015 g; for the 22 yr since 1958 the release of C was 38-76 x 1015 g. The ranges reflect the differences among various estimates of forest biomass, soil carbon, and agricultural clear- ing. Improvements in the data on the clearing of tropical forests alone would reduce the range of estimates for 1980 by almost 60%. Estimates of the other major terms in the global carbon budget, the atmospheric increase in C02, the fossil fuel release of C02, and the oceanic uptake of C02, are all subject to uncertainties. The combined errors in these estimates are large enough that the global carbon budget appears balanced if the low estimate for the biotic release of carbon given above is used (1.8 x 1015 g released in 1980) with the higher estimates of oceanic uptake. If higher estimates for biotic release are used, then the carbon budget does not balance, and the estimates of oceanic uptake or of other factors require revision.

Ecosystem carbon storage in arctic tundra reduced by long-term nutrient fertilization

Global warming is predicted to be most pronounced at high latitudes, and observational evidence over the past 25 years suggests that this warming is already under way. One-third of the global soil carbon pool is stored in northern latitudes, so there is considerable interest in understanding how the carbon balance of northern ecosystems will respond to climate warming. Observations of controls over plant productivity in tundra and boreal ecosystems have been used to build a conceptual model of response to warming, where warmer soils and increased decomposition of plant litter increase nutrient availability, which, in turn, stimulates plant production and increases ecosystem carbon storage. Here we present the results of a long-term fertilization experiment in Alaskan tundra, in which increased nutrient availability caused a net ecosystem loss of almost 2,000 grams of carbon per square meter over 20 years. We found that annual aboveground plant production doubled during the experiment. Losses of carbon and nitrogen from deep soil layers, however, were substantial and more than offset the increased carbon and nitrogen storage in plant biomass and litter. Our study suggests that projected release of soil nutrients associated with high-latitude warming may further amplify carbon release from soils, causing a net loss of ecosystem carbon and a positive feedback to climate warming.

Individualistic Growth Response of Tundra Plant Species to Environmental Manipulations in the Field

In undisturbed arctic tussock and wet meadow tundras we increased air temperature with a plastic greenhouse, increased nutrient availability by NPK fertilization, and decreased light intensity with shade cloth to determine the factors limiting growth of tundra plants. After 2 yr of these manipulations we measured growth of each major vascular species and one moss species. Each species showed a different pattern of growth response to alteration of light, air temperature, and nutrient regimes, indicating that no single factor limits growth of all species in these communities. Growth of canopy species (Betula nana, Ledum palustre, Carex bigelowii, and Eriophorum vaginatum) was reduced by experimental shading more than was growth of understory species (e.g., Vaccinium vitisidaea and Rubus chamaemorus). Species typical of nutrient—rich sites (Betula nana, Rubus chamaemorus, and Polygonum bistorta) generally responded more to nutrient addition than did species typical of nutrient—poor sites (e.g., Empetrum nigrum), although there were species characteristic of fertile sites (Salix pulchra) and infertile sites (Ledum palustre) which did not show this pattern of nutrient response. Species that grow in warm hollows between tussocks showed less growth in response to increased air temperature than did canopy species. We suggest that lack of a single common factor limiting growth of all species in tussock and wet meadow tundras implies that (1) each species is individualistically distributed, as described by the continuum model of community organization, (2) as a result of competition and/or distinct evolutionary histories, the growth of each species is limited by a different combination of environmental factors, and (3) production by individual species varies greatly from year to year, but production by the whole vegetation is more stable, because years that are favorable for growth of some species cause a compensatory decrease in growth of other species.

Resource-based niches provide a basis for plant species diversity and dominance in arctic tundra

Ecologists have long been intrigued by the ways co-occurring species divide limiting resources. Such resource partitioning, or niche differentiation, may promote species diversity by reducing competition. Although resource partitioning is an important determinant of species diversity and composition in animal communities, its importance in structuring plant communities has been difficult to resolve. This is due mainly to difficulties in studying how plants compete for belowground resources. Here we provide evidence from a 15N-tracer field experiment showing that plant species in a nitrogen-limited, arctic tundra community were differentiated in timing, depth and chemical form of nitrogen uptake, and that species dominance was strongly correlated with uptake of the most available soil nitrogen forms. That is, the most productive species used the most abundant nitrogen forms, and less productive species used less abundant forms. To our knowledge, this is the first documentation that the composition of a plant community is related to partitioning of differentially available forms of a single limiting resource.

EFFECTS OF TEMPERATURE AND SUBSTRATE QUALITY ON ELEMENT MINERALIZATION IN SIX ARCTIC SOILS

We compared the effects of temperature on rates of microbial respiration, N mineralization, nitrification, and P mineralization in soils from six arctic ecosystems located along a toposequence on Alaskas North Slope. Soils from these ecosystems were incubated aerobically in the laboratory for 13 wk and at temperatures representative of field values during a typical growing season. Rates of C and N mineralization were insen- sitive to temperature between 30 and 90C but increased by factors of 2 or more between 90 and 15?. For both C and N, differences in mineralization rates among soils were greater than differences due to incubation temperature within single soils. This suggests that the quality of soil organic matter varies widely among these ecosystems and is more important than soil temperature differences in controlling rates of these processes in the field. Nitri- fication occurred in all soils, even at 30, but there were large differences among soils in nitrification potentials. Overall differences in P mineralization between soils were small. Rates of P mineralization, however, decreased with increasing temperature in soils from some sites and increased with temperature in others. Carbon respired during the 1 3-wk incubations ranged between 1.5 and 8% of total soil organic C across soil types and incubation temperatures. In contrast to the relatively high C mineralization rates in these soils, net N and P mineralization rates were very low and were likely due to high microbial demands for these nutrients. High microbial demand for mineral nutrients can severely limit plant N and P availability in arctic soils.

Global Warming and Terrestrial Ecosystems: A Conceptual Framework for Analysis

raise global mean temperature over the next century by 1.0–3.5 °C (Houghton et al. 1995, 1996). Ecologists from around the world have begun experiments to investigate the effects of global warming on terrestrial ecosystems, the aspect of global climate change that attracts the most public attention (Woodwell and McKenzie 1995, Walker and Steffen 1999). The effort to understand response to warming builds on a history of investigations of the effects of elevated CO 2 on plants and ecosystems (Koch and Mooney 1996, Schulze et al. 1999). There are important differences, however, between increases in atmospheric CO 2 and temperature change, both in the temporal and spatial patterns of change and in how they affect ecosystems. The scientists involved in temperature change research have had to face new technical and conceptual challenges in designing and interpreting their experiments (Schulze et al. 1999). In this paper we describe these challenges and present a conceptual framework for interpreting experimental results and predicting effects of warming on ecosystems.

Production: Biomass Relationships and Element Cycling in Contrasting Arctic Vegetation Types

Primary production, plant biomass, plant element content, and various mea- sures of turnover and element cycling were compared among four contrasting tundra vegetation types near Toolik Lake, Alaska. The study sites were selected to represent extreme examples of the wide variation in plant growth form composition that is typical of northern ecosystems. The aim of the research was to determine whether vegetation types that differ in their dominant plant growth form also differ in their production: biomass relationships and overall patterns of element use. The four sites included tussock tundra, a deciduous shrub-dominated riparian tundra, an evergreen heath tundra, and wet sedge tundra. Biomass and element content (N, P, K, Ca, and Mg) were determined for both vascular and nonvascular plants, and production estimates were obtained for vascular plants. Production and biomass of most tissues were determined by quadrat harvest methods, with additional, separate determinations of stem secondary growth and belowground rhizome growth as components of net primary pro- duction (NPP). Production, biomass, and element content of roots were not determined. Vascular plant biomass (excluding roots) varied by nearly 9 x among sites, from 217 to 1877 g/m2. At least 50% of the vascular biomass at all four sites was belowground stems, but the relative allocation to leaves vs. aboveground stems varied greatly. When mosses and lichens were included, total biomass varied by only 5 x among sites, and lichens were in fact the most abundant plant form at the evergreen heath site. The element content in vascular biomass of these communities varied by 8-21 x, depending upon the element; including mosses and lichens, element content varied by 6-12 x. Primary productivity of vascular plants also varied sharply among sites, from 32 to 305 g m-2 yr-I (not including root production). Leaves were the largest single component of NPP at all sites, but the relative importance of above- and belowground stem growth varied considerably. The element requirements of vascular NPP varied by 9-17 x, de- pending upon the element. Despite these order-of-magnitude differences among sites in biomass, production, and element requirements, and the dramatic variation in allocation patterns, there was re- markably little difference among sites in the overall production: biomass relationships and element turnover. These overall similarities were due to the dominant effect of stems and rhizomes in the whole-plant and whole-vegetation biomass and element budgets. The wide range of leaf turnover rates and leaf production efficiencies was compensated at the whole- plant level by stem processes, especially element storage. Thus, we conclude that plant growth form composition is not a good indicator of whole-vegetation biomass turnover rates and patterns of element use, although growth form composition is related to total production and biomass.

Global Change and the Carbon Balance of Arctic EcosystemsCarbon/nutrient interactions should act as major constraints on changes in global terrestrial carbon cycling

n the cold, arctic climate, global warming due to greenhouse gas accumulation in the atmosphere might be expected to increase both primary production and heterotrophic (mainly soil) respiration. Which of these processes will increase more or more rapidly? The answer to this question is critical in understanding the effects of warming on the net carbon balance of arctic ecosystems and of the earth itself. If primary production increases faster than heterotrophic respiration, carbon will be removed from the atmosphere and will accumulate on land. If the reverse happens, carbon will be lost to the atmosphere. Eventually, a new equilibrium may be reached, but this equilibration could take decades or even centuries. Meanwhile, dramatic losses or gains of carbon may occur over the entire arctic region (5.7 x 106 km2; Oechel 1989), with potentially important feedbacks on the global atmospheric concentration of

Arctic ecosystems in a changing climate: An ecophysiological perspective

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