- Site Description
- Description of Research
The vegetation is a mosaic of types ranging from evergreen heaths on drier sites, to wet sedge tundra in flat lowlands, to riparian deciduous shrubs. The most abundant vegetation type is cotton grass-tussock tundra. Despite the dry climate, the permafrost prevents deep water drainage, cold temperatures lower the evapotranspiration, and the soils are usually moist to soggy. Vegetation composition on any site depends upon time since glaciation, soil type, exposure, drainage and topography (Walker et al. 1989).
There are four distinct terrestrial ecosystem types that differ greatly in both vegetation growth form and soil type: tussock tundra, heath, wet sedge tundra and deciduous shrub stands. The vegetation types are typical of major river valleys of the foothills region of the North Slope of Alaska, in particular along the Sagavanirktok, Kuparuk, and Atigun Rivers. The "shrub sites" are strongly dominated by deciduous shrubs, mainly willows (Salix spp.) and birch, "heath sites" by evergreen shrubs, and "wet sedge sites" by rhizomatous graminoids and "tussock tundra" by graminoids, deciduous shrubs, and evergreen shrubs in roughly equal abundance.
Tussock tundra is the most common vegetation type of the northern foothills of the Brooks Range (Bliss 1956, Wein and Bliss 1973, 1974, Batzli 1980, Miller 1982, Shaver and Chapin 1986). Tussock tundra occurs on gently rolling topography with silty to gravely soils, both glaciated and unglaciated. The soils are always moist and are unevenly covered with an organic mat 0-30 cm thick, underlain by a silty mineral soil. The maximum depth of thaw in the tussock tundra is also 30-50 cm, so in many locations the soils do not thaw to the mineral layer. The microtopography is dominated by tussocks of Eriophorum vaginatum, which may be 10-30 cm tall. Our site at Toolik Lake has been described previously (Chapin and Shaver 1985b, Shaver and Chapin 1986).
Heath vegetation is on well-drained, deeply thawed (>2 m depth) mineral soil with a thin but variable (0-10 cm) organic layer. The heath vegetation is found on exposed, well-drained ridges throughout the North Slope (Britton 1966, Walker et al. 1982). The topography is flat, but the soil was rocky, with a very thin (<1 cm) organic mat at the surface. The height of the vegetation was <10 cm, usually <5 cm.
Wet sedge tundra (Brown et al. 1980) covers the flat areas with impeded drainage. Wet sedge tundra has been intensively studied where it forms a broad expanse on the arctic coastal plain (1978, Brown et al. 1980); it is characterized by minimal topographic relief, high soil water content (often with standing surface water through much of the summer), a peaty organic mat 10-50 cm thick over a silty mineral soil, shallow depth of soil thaw (25-30 cm), and low plant stature (usually <20-30 cm). Soil frost phenomena such as ice-wedge polygons are common. The peaty soils of the wet sedge tundra have the highest water content and often lie beneath 1-5 cm of standing water during early and late summer.
Deciduous shrub stands are near the river and other well-drained sites on deeply thawed (>1 m) alluvial soils. These stands are similar to the tall (1-4 m) stands of willows and other deciduous shrubs that commonly occur on gravely river bars and well-drained floodplains throughout the North Slope (Bliss 1956, Bliss and Cantlon 1957, Robus 1981, Walker et al. 1982).
All ecosystems, except perhaps the riverside willow, are underlain by continuous permafrost. Soils are Inceptisols except in the wet sedge tundra and the riverside willow ecosystems where soil orders are Histisol and Entisol, respectively.
Description of Research
There are three major components to the terrestrial research of the Arctic LTER, including: (1)Observations and Monitoring, (2) Long-term, Whole-Ecosystem Experiments, and(3) Process, Species, and Community-level Studies. These components are linked through additional, separately funded (4) Synthesis and Scaling efforts that aim to apply our results to long-term, large-area predictions of stability and change in the arctic landscape. Finally, within the last 5 years we have begun an active program of (5) Cross-Site Comparisons in which we seek to test the generality of our results by direct comparisons with data from other sites, both in the Arctic and at lower latitudes.
The design of our terrestrial research incorporates a combination of comparisons among sites that differ in their physical setting (e.g., topography and geologic history) and biota (plant functional types) with long-term manipulations of climate and nutrient inputs in each of these different sites (Table 1). Since 1988 we have maintained a series of observations and experiments in which contrasting tundras, dominated by different mixes of plant functional types, are subjected to identical manipulations of nutrient inputs, air temperature, and shading (light reduction). Comparisons among treatments within each tundra type have taught us a great deal about the roles and interactions of climate and biogeochemistry (e.g., Chapin et al. 1995, Shaver et al. 1998, Gough et al. submitted). Additional comparisons of the responses of contrasting plant species and functional types to a common suite of manipulations in different sites have taught us how differences in species function affect overall ecosystem characteristics (e.g.,Bret-Harte et al. 2001, submitted, Shaver et al. in press, Gough et al. submitted.)
A simple, general conceptual model of climate/carbon/nutrient interactions in arctic terrestrial ecosystems (Shaver et al. 1992; Fig. 1) helps us to integrate our work on contrasting tundra types and plant functional types. This model also serves as a conceptual starting point for much of our simulation modeling (e.g., McKane et al. 1997a, 1997b, Rastetter et al. 1997a, Herbert et al. 1999) and for our cross-site comparisons and syntheses (e.g., Shaver and Jonasson 1999, Shaver et al. 2000, Jonasson et al. 2000, 2001). Our work is also linked conceptually to the lakes, streams, and land/water components of the Arctic LTER, viewing terrestrial ecosystems as sources and controls on water and element inputs to aquatic systems (e.g., Kling et al. 1991, 1992, 1995).
Terrestrial ecosystem research at Toolik Lake began in 1976, and by the time the LTER project was funded in 1987 an extensive background of data on tundra biogeochemistry and ecophysiology was already in place (Shaver 1996a, 1996b). Our descriptive data base grows yearly as a result of annual, intensive harvests of one or more tundra types in the Toolik Lake region. These harvests allow development of detailed descriptions of species composition, plant biomass allocation, primary production, and biomass turnover in a wide range of tundras dominated by different plant forms and varying at least 10-fold in productivity. Over the past 20 years, we have gradually described C, N, and P budgets and standing stocks for all the major vegetation and soils types in the study area (e.g., Shaver and Chapin 1991, Giblin et al. 1991, Chapin et al. 1995, Shaver et al. 1998, Gough et al. 2000, submitted). These data sets are all stored in the Arctic LTER data base and are widely used both in our own modeling and synthesis efforts and in cross-site comparisons (described below).
Repeated intensive harvests over the past 20 years have also allowed us to define the "normal" range of annual variation in aboveground production and biomass in moist tussock tundra, the dominant ecosystem type at our study site, and to determine whether the vegetation is changing as a result of the general warming that has occurred over this period. For example, total aboveground biomass in 2000 was the highest we have ever measured and it also included a higher proportion of the deciduous shrub, Betula nana than in any previous harvest (Fig. 2). Both of these changes are expected to occur as a result of climatic warming (e.g., Chapin et al. 1995). However, for both total biomass and Betula biomass the most similar year to 2000 was 1982, the earliest year of our record, leading us to the conclusion that if the vegetation is changing in response to climate the magnitude of the change is small relative to typical year-to-year variation.
Table 1. Experimental Designs for Terrestrial Research of the Arctic LTER Project.
|Location||Year Started||Ecosystem Type||Treatments||Major Harvests, sampling|
|Toolik Lake||1981||Moist Tussock||N+P Fert||1982, 1983, 1984, 1989, 1995, 2000|
|N, P, N+P,
Greenhouse, Shade, Greenhouse+N+P,
|Tussock: 1995, 1996, 2002
Wet: 1992, 1993, 1994, 2001
Heath: 1992, 1993, 1996
|Herbivore Exclosure||Annual monitoring of flowering only|
|1992||Moist Tussock||15N Addition||1992, 1993, 1998|
|Tussock: 1999, 2000 (CT only), 2001 (CT only), 2003|
|N, P, N+P,
|Tussock: 1999, 2000, 2001|
|1997||Moist Tussock||Species removal,
|2003 or 2004|
|Sag River Topo-sequence||1984||Moist Tussock
|N, P, N+P,
Labile C (starch, sawdust),
Yearly monitoring of key biological variables in relation to weather allows further insights into long- and short-term climatic controls. For example, year-to-year variation in flowering of the arctic cotton grass, Eriophorum vaginatum, is a spectacular event on the North Slope of Alaska, but early work showed that this variation is not at all related to the current year's weather (Shaver et al. 1986). After more than 20 years of monitoring, we are now able to show that it takes two warm years before the year of flowering to increase flowering in the year of observation (Fig. 3), top). Furthermore, we are beginning to see that increased flowering in a given year is also related to increased N mineralization in the previous year, which is also predicted by soil temperatures over two years (Fig. 3), bottom)
In the current funding period (1999-2004) we maintain nine long-term experiments, the earliest of which began in 1981 (Table 1). Because these ecosystems continue to respond to our treatments, we gain new insights about ecosystem regulation with each harvest (e.g., Shaver et al. in press). Our principal efforts, however, have shifted to a new series of long-term experiments, specifically focused on effects of herbivory (biota) and variation in soil acidity (geology/geomorphology). We also have assumed responsibility for maintaining a set of long-term species removal experiments in which vegetation composition is controlled directly.
Each summer we complete a major harvest of one or more of our experiments (Table 1). The general aim is to harvest each experiment at least once every 3-6 years, but the schedule is kept flexible to allow coordination of our harvests with collaborating, independently-funded projects. These independent projects also use our experiments for more intensive, focused research at the process, species, or community level. By coordinating our ecosystem-level production, biomass, and nutrient budget harvests with detailed process research on the same plots we try to maximize the potential for integration of data collected at different scales.
Herbivore exclosures: The central hypothesis of this experiment is that herbivores play an important role in controlling tundra plant species composition, but are less important as direct controls on productivity or nutrient turnover at 1-10 year scales. We also hypothesize that herbivores induce changes in species composition more rapidly under fertilization, but that fertilized plots withor without herbivores will converge to similar species composition after about 10 years. In July 1996 we set up herbivore exclosures in acidic tussock and dry heath tundras. These exclosures were established within the existing LTER experimental areas, set up in 1987, on plots that had been randomly located as part of the original design but left unused for future treatments (n=4 in tussock tundra and n=3 in dry heath). There were two nested levels of exclosure (microtine exclosures nested within caribou exclosures), combined factorially with NP fertilizer addition. We harvested these plots in 1999 and currently plan to repeat the harvest in 2003. Results of our first harvest, in 1999 (Fig. 4)), indicate that after three years of treatment there is greater productivity in the unfertilized plots where herbivores were excluded, and that most of the increase in productivity is by graminoids, the favored food of the local microtine rodents. This is a more rapid change than we initially predicted. Later harvests (the next is scheduled for 2003) will determine whether this trend continues.
Non-acidic versus acidic tundra: The broad aim of these experiments is to determine whether sites differing in pH but with similar production, biomass, and functional type composition to the acidic sites we studied previously, will differ in response to the same set of manipulations. A particular focus is on the importance of initial species composition to the overall ecosystem response, because many of the species most responsive to our manipulations of acidic tundra do not occur in non-acidic tundra (Gough et al. 2000).In July 1997, we established new experiments in non-acidic tussock tundra on the northwest shore of Toolik Lake. The new experiments are identical in design to the existing experiments in acidic tussock tundra and other sites that have been maintained since the start of the LTER project (Chapin et al. 1995, Shaver et al. 1998; Table 1). Three experimental blocks were set out at each non-acidic site. In both sites, factorial NxP fertilizer treatments were applied, using the same fertilizers and rates of application as in the existing sites. In the non-acidic tussock site, we also set up a factorial greenhouse x N+P fertilizer experiment identical to earlier experiments in other sites. A major harvest of the non-acidic tussock tundra experiment was completed in the summer of 2000. This harvest occurred in the third year of treatment and is thus directly comparable with our 1999 harvest of the exclosure experiment (Fig. 2)) and with our 1983 harvest of our original greenhouse/shade/fertilizer experiment (Chapin et al. 1995).
Response to Liming/Acidification:This experiment focuses on links between soil acidity and ecosystem characteristics such as species diversity, productivity, decomposition, and N and Pcycling. In 1998 we set up a new experiment in both acidic and nonacidic tussock tundra in which we are attempting to manipulate soil acidity directly by adding lime and elemental sulfur. We have not yet harvested the experiment but we continue to monitor it for obvious changes and view it as an important opportunity for future process-oriented research. Any harvest will be coordinated with a harvest of another long-term liming experiment at the Sagavanirktok River toposequence, begun in 1985 (Table 1).
Species Removal Experiment: The aim of this experiment is to isolate the effects of individual plant species or functional types on ecosystem productivity, biomass, and element cycling by selective removal of target plants. The experiment was established in 1997 by our colleagues M.S. Bret-Harte and F.S. Chapin, III, and gradually we have assumed responsibility for its maintenance because it complements nicely our manipulations of light, temperature, and nutrient resources. Because species removal treatments are crossed with a NP fertilizer addition that is identical to our other experiments, this experiment also provides us with the opportunity to isolate fertilizer effects from species effects on biogeochemistry. A major harvest of this experiment is planned for 2003 or 2004, depending on when additional funding is obtained for detailed process studies on these plots. Thus far, annual monitoring has shown that removal of individual species often has consequences for other species, indicating that the tussock tundra community is tightly integrated through a wide range of direct competitive interactions (e.g., competition for soil N) and species effects on microenvironment (shading, soil temperature). The response to fertilizer in these plots is similar that we observed on our long-term fertilizer experiment (only about 50 m away downhill), but in these plots we also have a significant grass component, which is much more rapidly responsive.
Ongoing Experiments: Repeated observation and sampling of long-running experiments allows us to determine whether the initial magnitude and trajectory of ecosystem response to environmental change is sustained over the long term, and to examine interactions among slow- and fast-responding components of overall ecosystem change. Repeated observation also allows us to interpret long-term change in response to our manipulations in the context of long-term "normal" or "background" variation in ecosystem states and processes. For example, in the summer of 2000 we completed our fifth harvest of a 20-year old fertilizer experiment (the 6th harvest of control plots for this experiment). After 20 years it was clear that fertilized plots were distinctly different from control plots, not only in terms of their total production and biomass, but that they also differed functionally in terms of the relationships between productivity, leaf mass, and leaf area (Fig. 5)). Remarkably, leaf mass in the fertilized plots was significantly lower than in the controls in both 1995 and 2000, while productivity in the fertilized plots was more than double that of controls. How is this possible? The main reason for this result is that the fertilized plots are not strongly dominated by dwarf birch, Betula nana, which has much thinner leaves (i.e., a much higher specific leaf area). Because birch has thin leaves, it can produce more than twice the leaf area of controls with a lower total leaf mass. This leads also to very different overall pattern of canopy and aboveground N allocation, with a much more efficient photosynthetic return per unit canopy N (Shaver et al. in press). It also allows much greater proportional allocation to woody stem growth, greatly increasing both the height of the canopy and the total aboveground biomass (Bret-Harte et al. 2001, submitted). This interpretation of the causes and implications of a change in plant functional type composition is radically different from earlier hypotheses (e.g., Chapin and Shaver 1985) and would not have been possible without long-term experimental evidence.
Several independently-funded projects coordinate their data gathering with LTER-supported research, usually by additional sampling of LTER experiments. The LTER experiments are designed to accommodate this additional sampling, mainly by setting up and manipulating much larger plots than would otherwise be needed, and by incorporating extra untreated plots for additional sampling or future treatments. Typically, the LTER-supported harvests provide the ecosystem-level budgets that provide context for evaluation of more focused process- and species-level data collection. The LTER funds are also used to support small, focused pilot studies that are intended to expand the overall scope of research and to fill gaps in the research program. Current efforts include the following:
International Tundra Experiment (ITEX): This project, supported by NSF-OPP, began in 1995 and will continue at least through 2004. By sampling LTER experimental plots, the aim is to understand and isolate the role of species in controlling ecosystem function and response to disturbance. An initial hypothesis was that RESOURCES (element stocks and cycling rates) control long-term NPP and organic matter accumulation; but species composition is not important to average NPP and turnover along large gradients of resource availability. However, SPECIES do determine the RATE of response to changes in resources, especially the initial response. Examples of this work are Donie Bret-Harte's work on shrub growth (Bret-Harte et al. 2001, in review) and Laura Gough's work on diversity/productivity relationships (Gough et al. 2000).
Soil-plant interactions: An ongoing theme of our research has been the constraints on C cycling imposed by C/N interactions, and variation in those constraints with topography (geology/geomorphology), species composition (biota) and nutrient inputs (fluxes). Current foci are: (1) C respiration/N mineralization interactions, and (2) below-ground C inputs, as root production and root exudation (NSF OPP 96 15563). In collaboration with other NSF-DEB and NSF-OPP projects, harvests of our plots provide biomass and production measures that complement process-level measurements of soil respiration, N mineralization, and the distribution and movement of stable isotopic tracers (13C and 15N) through plants and soils (Giblin et al. 1991, Nadelhoffer et al. 1991, Nadelhoffer et al. 1995, Shaver et al. 1998, Johnson et al. 1996 and submitted). During the summer of 2001, we will begin a major study of the movement of C from plants into the soil, using a 14C labeling approach, in control and fertilized plots of wet sedge tundra. In 2002, we will continue the work in moist acidic tussock tundra. This work is under the direction of Dr. Knute Nadelhoffer, with George Kling and Loretta Johnson.
Soil biota and community structure: A new community-oriented initiative for 1999-2004 is to determine effects of our long-term experiments on soil invertebrate and microbial communities. For the past three summers Dr. John Moore and students have come to Toolik Lake to help develop techniques for sampling and describing invertebrate communities across the full suite of sites and experimental manipulations. In 2001, we will extend these descriptive studies to include bacteria and fungi, with NSF Postdoctoral Fellow Laura Broughton. This work is also being used by Moore and collaborators as part of their multibiome comparison of soil trophic structure and diversity.
The LTER project and its data management/data archive capabilities are used to facilitate synthesis of terrestrial research in several ways. One way is by direct integration of diverse data sets, all collected from the same sites and experiments. We have used this approach successfully in the past to construct and compare C, N, and P budgets for vegetation and whole ecosystems (e.g., Shaver and Chapin 1991, Shaver et al. 1998). A second approach is to use simulation models, both as a means of gaining insight into process interactions and their implications for overall ecosystem function, and as a means of developing long-term and large-area predictions of change in the arctic landscape. We have used the MBL-GEM model, for example, to evaluate the hypotheses in our general conceptual model of C-N interactions (Fig. 1)) and their relative importance in regulating the changes we see in our experimental plots (McKane et al. 1997a). These insights have then been used to develop longer-term predictions of changes in tundra C stocks and C-N interactions in response to climate change (McKane et al. 1997b, Rastetter et al. 1997, Hobbie et al. 1998; Fig. 6)). Several models have used the LTER data base in the development of large-area predictions of arctic C cycling at various scales ranging from the Kuparuk River drainage to the entire terrestrial arctic (e.g., Hobbie et al. 1998, Williams et al. 2000, Clein et al. 2000, McGuire et al. 2000). Finally, our data on plant functional types have been used to test theory of species effects on element cycling, using the MEL model developed by Ed Rastetter (Rastetter and Shaver 1994, Herbert et al. 1999, submitted).
Within the Arctic, the ITEX project involves comparative analysis of plant growth in response to experimental warming at over 25 sites throughout the Arctic. A recent meta-analysis including our data from Toolik Lake (Arft et al. 1999) showed that the responses to warming by individual species at Toolik Lake are similar to those at other arctic sites; this is a key step towards understanding the limits to extrapolation of site-specific results to the entire Arctic. Current ITEX research is focused on analysis of multi-year changes in community composition in response to experimental manipulations at ITEX sites, including Toolik Lake (M. Walker in preparation).
More recently, we have begun a detailed analysis and comparison of results of greenhouse/shade/fertilizer experiments similar to ours at Abisko, Sweden, and on Svalbard. Qualitative comparisons indicate that C-N interactions operate in similar ways at Toolik and Abisko (Shaver and Jonasson 1999, Jonasson et al. 2001). We have completed two detailed comparisons of Toolik and Abisko, one focused on plant secondary chemistry (Graglia et al. in press) and one on soil organic matter, microbial biomass, and soil N turnover (Schmidt et al. in press). With recent funding from the NSF Cross-site competition, we plan a series of reciprocal site visits and modeling analyses to further extend these comparisons; this will start with a visit to Toolik Lake by European ecologists in August 2001, followed by a second workshop at Abisko, Sweden, in 2002. Working with our Scandinavian and British colleagues, we also plan to develop a new model of climate-nutrient interactions and especially the role of temperature in regulating the response of arctic ecosystems to global warming.
Finally, we collaborate with several global networks and organizations to place our research in the context of global patterns of regulation of ecosystem processes and structure. Within the US LTER network, we were active participants in the recent synthesis of productivity-diversity relationships at LTER sites (Waide et al. 1999, Gross et al. 2000, Gough et al. 2000b) and also contributed data to the synthesis of temporal dynamics of primary production at LTER sites (Knapp and Smith 2001). We are active participants in the IGBP-GCTE Network of Ecosystem Warming Studies (NEWS), and helped organize a recent synthesis of ecosystem warming responses (Canadell et al. 2000, Shaver et al. 2000, Rustad et al. 2001,; Fig. 7). These activities will continue with annual workshops over the next five years.
Terrestrial research of the Arctic LTER project contributes to overall project goals through a combination of long-term observations, long term experiments, and focused research on key processes, species, and communities. Together, these activities have produced an increasingly integrated and continuously growing data base that is used to explain and to predict the relative sensitivity and responses of contrasting arctic tundra ecosystems to climate change and other disturbances. The LTER data base is used in a wide range of modeling activities, at multiple spatial and temporal scales, and also in multisite comparative research both within and outside the Arctic.
Within the current funding period our broad priorities include the development of a new overall conceptual model of carbon and nutrient cycling in tundra ecosystems that considers more explicitly the importance and variability of N inputs and outputs in tundra ecosystems including the major forms of organic and inorganic N. Opening up the N cycle (and its links to the water cycle) will also strengthen our links to the land-water and aquatic components of the Arctic LTER program. Within the terrestrial systems, we need to strengthen and expand our research on the role of herbivores and soil invertebrate communities in biogeochemical cycles. All of this work is designed eventually to lead to better predictions of change in arctic ecosystems based on fundamental knowledge of controls on ecosystem structure and function as exerted by physical setting and geology, climate, biota, and biogeochemical fluxes.
Fig. 7. Results of a metaanalysis comparing responses to experimental warming at a wide range of forest, grassland, and arctic sites. Data from the Toolik Lake tussock (TLKTUS), wet sedge (TLKSED), and dry heath (TOOLIKDH) sites are included, as are several Abisko sites (codes beginning with "AB"). With the exception of aboveground plant productivity, which showed a greater positive response to warming in colder, drier ecosystems, the magnitude of the response of these variables to experimental warming was not generally significantly related to the geographic, climatic, or environmental variables evaluated in this analysis.
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Shaver, G.R., and F.S. Chapin, III. 1995. Long-term responses to factorial NPK fertilizer treatment by Alaskan wet and moist tundra sedge species. Ecography 18: 259-275.
Shaver, G.R., and S. Jonasson. 1999. Response of arctic ecosystems to climate change: Results of long-term field experiments in Sweden and Alaska. Polar Biology 18:245-252.
Shaver, G. R., N. Fetcher and F. S. Chapin, III. 1986. Growth and flowering in Eriophorum vaginatum: Annual and latitudinal variation. Ecology 67:1524-1525.
Shaver, G. R., K. J. Nadelhoffer and A. E. Giblin. 1991. Biogeochemical diversity and element transport in a heterogeneous landscape, the North Slope of Alaska. pp. 105-126 In: M. G. Turner and R. H. Gardner (eds.), Quantitative Methods in Landscape Ecology. Springer-Verlag, New York.
Shaver, G. R., W. D. Billings, F. S. Chapin, III, A. E. Giblin, K. J. Nadelhoffer, W. C. Oechel and E. B. Rastetter. 1992. Global change and the carbon balance of arctic ecosystems. BioScience 42:433-441.
Shaver, G.R., J.A. Laundre, A.E. Giblin, and K.J. Nadelhoffer. 1996. Changes in vegetation biomass, primary production, and species composition along a riverside toposequence in arctic Alaska. Arctic and Alpine Research 28:363-379.
Shaver, G.R., L.C. Johnson, D.H. Cades, G. Murray, J.A. Laundre, E.B. Rastetter, K.J. Nadelhoffer, and A.E. Giblin. 1998. Biomass accumulation and CO2 flux in three Alaskan wet sedge tundras: Responses to nutrients, temperature, and light. Ecological Monographs 68:75-99.
Shaver, G.R., J. Canadell, F.S. Chapin, III, J. Gurevitch, J. Harte, G. Henry, P. Ineson, S. Jonasson, J. Melillo, L. Pitelka, and L. Rustad. 2000. Global Warming and Terrestrial Ecosystems: A Conceptual Framework for Analysis. BioScience 50: 871-882.
Waide, R.B., M.R. Willig, G. Mittelbach, C. Steiner, L. Gough, S.I. Dodson, G.P. Juday, and R. Parmenter. 1999. The relationship between primary productivity and species richness. Annual Review of Ecology and Systematics 30:257-300.
Williams, M., E.B. Rastetter, D.N. Fernandez, M.L. Goulden, and G.R. Shaver. 1997. Predicting gross primary productivity in terrestrial ecosystems. Ecological Applications 7:882-894.