Global Warming and Terrestrial Ecosystems: A Conceptual Framework for Analysis

Emissions of greenhouse gases are expected to 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 CO2 on plants and ecosystems (Koch and Mooney 1996, Schulze et al. 1999). There are important differences, however, between increases in atmospheric CO2 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 experi mental results and predicting effects of warming on ecosystems. Projections of global warming from General Circulation Models (GCMs) are now familiar to both scientists and nonscientists. Knowing, however, that the mean global temperature will increase by 1.0–3.5 °C tells us little about how temperatures will change in a particular location or how the ecosystems in that location will respond. For instance, temperature increases are likely to be greater at higher latitudes (Houghton et al. 1995, 1996), where initial conditions are below 0 °C most of the year and growing season temperatures are only a few degrees higher. Even a small increase in average temperature at higher latitudes could increase both the length of the unfrozen period and degree-day accumulations by large percentages (Billings 1987). The implications of a given increase in temperature will also depend on the initial temperature at a specific location because the rate of many biological processes in relation to temperature typically peaks at some intermediate temperature. An increase from an initially low temperature may cause an increase in photosynthesis, for example, while an increase from an initially high temperature might cause photosynthesis to decrease (Larcher 1995). Finally, even as the average temperature increases, some parts of the globe are actually expected to experience cooling (Houghton et al. 1995, 1996). The complexities of these patterns of change in temperature contrast with the increase in atmospheric CO2, for which there is no large temporal or spatial variation either in current levels or in the rate or pattern of increase. The direct effects of warming on ecosystems will also be more complex than the direct effects of increased CO2 because temperature impacts virtually all chemical and biological processes, whereas the direct influence of CO2 is almost entirely limited to leaves (photosynthesis, stomatal aperture, and perhaps respiration; Koch and Mooney 1996). For both warming and CO2, however, the greatest obstacles to understanding lie in the web of indirect effects resulting from interactions among processes affected directly by environmental change. These interactions lead to feedbacks that are sometimes positive and sometimes negative, so the responses to temperature change or CO2 can be expected to vary among ecosystems in both magnitude and direction depending on the properties of the dominant species, interactions among species, and the initial physical and chemical environment. The effects of a temperature increase on the carbon budget of an ecosystem provides an example of this complexity (Figure 1). Net ecosystem production (NEP), defined as the overall carbon balance of an ecosystem over some time period (Woodwell and Whittaker 1968, Mooney et al. 1999), has two major components, net primary production (NPP) and heterotrophic respiration (Rh). NPP, the principal input of carbon to the ecosystem, is the net result of CO2 fixation by photosynthesis and CO2 loss by plant respiration. The product of NPP is new organic matter, which accumulates first in plants as living biomass and is eventually transferred to soils as litter and to animals and decomposer organisms as food. Rh represents the loss of carbon from the ecosystem by respiration of animals and decomposers; the products of Rh include CO2 and other inorganic carbon products (e.g., CH4). Both NPP and Rh are affected directly by temperature change. Both are usually increased by warming, although Rh often increases more rapidly in the short term (Woodwell 1995). In addition, warming can affect NPP and Rh indirectly by altering the ecosystem's moisture regime, nitrogen availability, length of its growing season, or species composition (Figure 1). Warming-driven changes in moisture, nitrogen, or species composition may also have intermediate effects on other ecosystem processes or states (e.g litter quality and quantity, which affect both Rh and nitrogen mineralization), leading to multistep indirect effects including losses of carbon through fire or leaching and changes in the balance of NPP and Rh (Melillo et al. 1990, Chapin et al. 1997). For many ecosystems, the indirect effects of a temperature increase on carbon balance are likely to be more important than the direct effects. Nutrient-limited tundra and northern forest ecosystems, for example, are much more responsive in the short term (1–10 years) to changes in nutrient availability, which is likely to increase with soil warming, than to the direct effect of increased temperature (Clark and Rosswall 1981, Chapin 1983, Jonasson and Shaver 1999). In many dry ecosystems, increased evaporative water loss at higher temperatures, resulting in dryer soils, may strongly limit soil and plant processes so that potential temperature-driven increases in process rates are not achieved (Mooney et al. 1999, Saleska et al. 1999). Finally, changes in temperature may alter species' competitive interactions and the activity of herbivores and pathogens (Mooney et al. 1991, Smith et al. 1997, deValpine and Harte In press), leading to changes in light, water, and nutrient limitations with complex long-term effects on carbon turnover, NPP, and Rh (Herbert et al. 1999). Warming will affect essentially all ecosystem processes and organic matter pools but at different rates. Because the processes and pools are linked to each other through biogeochemical cycles, the magnitude and even the direction of the net change in the ecosystem may vary over time. For example, NPP is affected by processes operating over a wide range of time scales, from the very short-term responses of leaf-level photosynthesis to the long-term changes in storage and turnover of soil nitrogen stocks (Figure 2). Although in most ecosystems the immediate effect of warming may be to increase NPP through increased photosynthesis, in the longer-term the uptake and accumulation of nitrogen in plant biomass (particularly in forests) might reduce the possibility for further increases in NPP by reducing nitrogen availability. Eventually, changes in species composition or litter quality might lead to further increases or decreases in NPP through changes in plant biomass (and nutrient) turnover or changes in litter decomposition and nitrogen mineralization. In some ecosystems (e.g., boreal forests), the increasing fuel load associated with biomass increase may increase the likelihood of fire and therefore the proportion of stands with low carbon storage on the landscape. Thus, the principal long-term mechanisms of regulation of NPP and overall carbon balance with respect to temperature may be very different from the short-term mechanisms (McKane et al. 1995, Braswell et al. 1997, Rastetter et al. 1997). The time scales of response of individual processes should also vary among ecosystems, adding further complexity to long-term prediction of changes caused by warming (Figure 2). For example, in ecosystems where the dominant species in the vegetation are long-lived trees, changes in species composition might have relatively minor effects on NPP in the first few decades of warming, but changes in biomass allocation within the existing dominants might have major effects. Principal controls over nitrogen availability in such forests might lie in controls over turnover of existing large soil nitrogen pools. On the other hand, the changes in species composition in an annual grassland with a relatively small soil organic matter pool could be more important to NPP, and changes in soil nitrogen turnover less important, within the first decades. Ecosystem warming experiments, if continued over several years, can capture much but not all of this changing sequence of responses to temperature change (Figure 2). Changes in basic metabolism (such as photosynthetic acclimation to temperature) may take place over less than 1 year and will be missed by annual sampling schedules, but these short-term responses may have less net impact on long-term change in ecosystem properties than the warming-induced rearrangement of ecosystem carbon and nutrient stocks and species composition, which takes place on a time scale of 1–100 years. Some changes, especially those occurring over very long time scales (e.g., soil profile development and organic matter accumulation) or large spatial scales (changes in fire regime or long-distance movements of large herbivores or timberlines) will not be picked up by experiments and must be investigated in other ways. Despite the complexities of ecosystem responses to warming, an important goal of ecosystem science should be to predict which kinds of ecosystems are more or less responsive to warming, and to identify the characteristics of ecosystems that cause them to be more or less responsive. Our present ability to do so is limited, but evidence indicates that ecosystem responses to warming are strongly affected by initial conditions, including: Stocks and initial turnover rates of labile carbon in the soil Stocks and initial turnover rates of labile nitrogen in the soil Relative size of the carbon pools of plants and soil Dominant form of nitrogen in the soil (e.g., organic nitrogen, water and regime composition and turnover rates of plant and litter of and turnover rates of dominant plant species The effects of these initial conditions on the response of ecosystems to warming can be by changes in NPP, or Rh over time in ecosystems that have has to as by the and in in forest a long-term soil warming in the soil to soil temperatures at a of by °C (Figure et et al. 1995). this the important initial conditions small labile soil carbon pool vegetation carbon pool large soil nitrogen pool nitrogen sequence These initial conditions how the ecosystem to soil warming over such that a pattern of changes in carbon storage results (Figure et al. in the labile carbon pool is rapidly by leading to a short period of net carbon In nitrogen is transferred from the which has a low to the which has a high of nitrogen results in net carbon storage at the ecosystem although there is a loss of soil carbon et al. 1991, 1997). there is no loss of nitrogen from the ecosystem because the has an the had a or a nitrogen could be from the by leaching or and so reduce the nitrogen to carbon Eventually, the labile and nitrogen pools in the soil decrease to the where very little nitrogen can be from the soil to the this the ecosystem may to carbon overall because of increased losses from plants and soils (Figure may be if nitrogen or are or if plant biomass is to such as a or and is nitrogen (Figure In a tundra ecosystem at in northern responses to warming small to increase temperature the growing season by °C (Figure The soil organic matter pool of this ecosystem is and even more labile carbon and nitrogen than the soils of many northern forests et al. and nitrogen in tundra is strongly limited by low soil temperatures and high soil moisture et al. Thus, if soil organic matter turnover is increased to warming, there is a high potential for of nitrogen from soils low to vegetation high resulting in net carbon storage with little or no net change in ecosystem nitrogen stocks et al. however, is strongly with soil moisture, with a relatively small temperature response if the soil is et al. results et al. 1995, Shaver et al. and (McKane et al. Rastetter et al. a response of to warming in this tundra ecosystem that is to that at as long as soil moisture not change (Figure results have also in the long-term and by and (e.g., and 1997, 1999, et al. 1997). The initial response to warming is a large increase in leading to a decrease in for the first soil however, is by increased nitrogen leading to increased plant nitrogen uptake and increased NPP increases in NPP are limited by other (e.g., NPP to increased litter which eventually to higher and to years. Changes in in response to warming of tundra may be very different if there is an between warming and soil moisture (Billings et al. and 1999). Warming with soil moisture will cause a long-term loss of both carbon and nitrogen because of large increases in Rh with losses of nitrogen by from the especially in where increases such nitrogen losses may include large of organic nitrogen, the product of In this increases in nitrogen uptake and NPP in are to for nitrogen and carbon losses in leaching and so that even the ecosystem eventually to an where NPP Rh over there is a net loss of carbon to the (Figure In an grassland at in the a of to the impact of global warming on ecosystem carbon and soil water In an et al. in in warming et al. Both in several different including a and a of the to those for the tundra warming with soil moisture (Figure but the net carbon losses even at an increase in plant biomass the conditions, but major in the carbon of the in years) the of the carbon in this particular The increases in which the of organic matter and of plant but these increases in carbon fixation to the carbon through increased These increases in decomposition caused by greater soil biological activity in by increases and changes in of the et al. These large losses in soil carbon stocks the soils are expected to at a carbon to the pattern in and by the soils at the of the In a ecosystem in Harte and have to both soil and vegetation (Figure changes in NPP and et al. 1995, Saleska et a 1999). In contrast to the a principal the changes in is the effect of the vegetation on species rates and composition and 1995, deValpine and Harte In loss of soil carbon in the of the soils of Saleska and of initial loss of soil organic matter not by a warming-induced increase in heterotrophic respiration because the positive effect of a temperature increase on Rh by the effect of soil this initial loss of soil organic matter by the effect of warming on the rate of litter input to the In warming a from more to less litter by is more than litter by the which has a higher Thus, the loss of soil organic matter is likely to be a with the litter input from an increase in soil organic matter over the long and in an overall pattern of change in to that in responses in the warming to changes in soil organic matter, and vegetation species composition a Harte and have that the of and can a short and long-term components, of how climate change soil organic The research how the impacts of warming on carbon can be by the indirect effect of a warming-induced in species composition on the and quality of litter input to the In ecosystems where the of the soil is warming about by increasing plant may cause increases in In tundra and ecosystems at warming with small caused such an increase in plant (Figure in by increases in plant nitrogen increased almost within et al. 1999). The response of biomass to warming at to that in the tundra at although the response at greater and NPP increased changes in species the response the in initial in carbon stocks not because the not The term than years) response is expected to and an at a net carbon accumulation rate to the has by increases in biomass in response to warming not be a process over not in the ecosystems where the plants to the limit of their For instance, an increase in biomass to that at place over several in a high however, an and growing season, much of the increase in and The loss in biomass because in the and the plants the in et al. a effect of are likely to be of in to the of such carbon pools in the living plant biomass may between of accumulation and at net while soil organic matter is up the of plant warming experiments have a of including and and 1). Some experiments have only of the have the ecosystem for only of each or In most the of warming is by and (e.g., availability of of an and the initial of the the expected changes, but all have results with have begun to differences, and patterns of response among the ecosystems Although there is to designing experiments that a and of expected change, also is that different warming different of the overall warming research should include a of in ecosystem research should also include to individual ecosystems or and are to the for a global understanding of ecosystem responses to climate change et al. 1999). The in different and to the effects of warming on plant and and 1997, et al. 1999). of Ecosystem Warming is as of the and of the warming experiments to however, have place in and ecosystems, which are and and where NPP is strongly Although the processes that the warming responses of these ecosystems are important they may not in other ecosystem In and dry including or ecosystems, and in ecosystems, indirect warming effects through changes in and soil will a major also might the direct effects of warming on NPP to be relatively in these because temperatures are for NPP might even decrease to in the ecosystems or experiments and are to the range of ecosystems including of vegetation with plant (e.g., annual and are also for the of from different of temperature effects on ecosystems. among different have also important et al. 1997). Some relatively new are to among ecosystems and to predict their responses to temperature change within a (e.g., Rastetter et al. 1991, et al. 1997). can also be to for and patterns in For example, we might to the average effects of warming on NPP a of and the to which those effects in at different latitudes or with different moisture Ecologists have begun to to from many to these kinds of and 1995, and 1999), including plant responses to warming in et al. 1999). Finally, there is a to results over time scales and over and An to do so is to with of ecosystem properties and processes environmental such as the et al. 1999). The are to longer-term to responses of vegetation and soils to environmental conditions, whereas warming experiments responses over the to from experiments and may be in relatively changes in the experiments from For example, global of primary production and litter major to have that both are by and Whittaker Although short-term responses of NPP and Rh to warming may not have the with as do ecosystems, the might be a of expected change in the and should be with of and processes that have as of ecosystem change. The of global with and other an to and these ecosystem long-term research and in many relatively and or even less not be very although quality may be (e.g., et al. 1995). to and for might be but the resulting be responses of species, and biogeochemical processes to climate change could be decades than they be Changes in temperature are now occurring with other of global change. will not be to understand and predict ecosystem responses to temperature change the interactions with the other of global environmental change (Koch and Mooney 1996, et al. 1997, et al. 1997, and Mooney 1999). For example, increases in atmospheric CO2 may plant and ecosystem water both of which may in responses to of the growing season may increase with warming, or may decrease as increases, altering NPP, and other of the globe are increased levels of nitrogen and the of the nitrogen of an ecosystem is an important in responses to other environmental et al. Changes in and regime may also influence responses to For instance, over very large may temperature or with climate change changes in fire could be caused by and affect ecosystem responses to The of species will also be affected by both warming and change. Some interactions between warming and other global changes are more than For example, of or pathogens that the dominant plants and increase fire lead to different results than the changes in response to warming must some will not be to all important of global change will be to include more than or two in to temperature (e.g., CO2 or nitrogen as (Schulze et al. 1999). through the of of experiments, and should be to a understanding of the most important interactions et al. Although much current global change research on feedbacks from terrestrial ecosystems to climate through greenhouse important feedbacks also through changes in vegetation and water in the as for and with the and changes in and in response to warming will change the and et al. of the most important and effects of changes in the is its the of that is through increased will increase net by the The increased net must be through or a of major or warming of the and the increased decreases soil moisture, and species changes result in stomatal of water the result will be increased to the et al. the of such a change the temperature in the reducing The increased soil temperature and from this vegetation change as as the to be for the climate change. further loss of soil moisture in dry will decrease with more as understanding of these of feedbacks on climate change those operating through changes in and water is less than that of feedbacks through greenhouse and should be a high for Because temperature has a direct effect on virtually all ecosystem processes, responses of terrestrial ecosystems to global warming will be even more complex and to predict than responses to increased atmospheric CO2 a conceptual framework for and prediction of responses to warming is to this For the principal of this framework are as a of the major of complexity of the ecosystem warming response and as a of the major on long-term on relatively short-term is to further understanding of how initial biogeochemical and species characteristics might be to long-term of to climate change. a framework as a to new also of the temporal sequence of ecosystem responses to temperature change because is that long-term responses to warming and increased CO2 may in both magnitude and direction from initial experiments are a for this a of experiments in ecosystem is including both and more are the long-term and the development of research that will spatial and of on paper to the and of and to the of the of Warming The with the of the for and a by the of the and the by by through a to and by and temporal temperature changes will not be with most warming by but These changes will over a range of temperature and annual temperature that are greater than the expected average temperature change. and indirect Because temperature virtually all ecosystem processes, indirect temperature effects resulting from interactions among processes are likely to be very important, especially in the long time of ecosystem processes have different time for response to temperature. Thus, overall ecosystem responses to change are not of long-term responses because they only of the response a of initial The temperature change to different ecosystems will different responses depending on initial on the temperature response and on initial biogeochemical conditions and composition sequence of The dominant controls over ecosystem response to temperature will change over as different processes change at different rates. changing sequence will not be the in all ecosystems. direction as as magnitude of losses or of carbon by the ecosystem may be over time as indirect effects and changing of the ecosystem changes can affect Changes in species composition and will affect ecosystem primary carbon and nutrient through changes in parts and turnover and vegetation is from initial include initial temperature regime, initial carbon and nutrient pool and turnover and initial of and indirect effects of temperature on net primary production heterotrophic respiration and net ecosystem production (NEP), the major of carbon between terrestrial ecosystems and the (Woodwell and Whittaker NPP carbon from the whereas Rh carbon to the is the of these two and represents the net of carbon between an ecosystem and the the of the ecosystem, is positive NPP is than Rh and is NPP is than In this of cause and effect by which temperature NPP and Rh but are not to the magnitude or of an is to that temperature NPP and Rh directly and whereas changes in are the net result of the effects on NPP and soil organic scales of response to temperature change by ecosystem processes and of the processes or in the affect net ecosystem production either directly or For they are soils, and The is to how different processes and to temperature change at different the overall ecosystem response result of the individual responses and their may be very different in the long-term the other processes and could be to this respiration; soil organic Changes in net ecosystem production the net of carbon between an ecosystem and the over time in response to There is a net carbon uptake by the ecosystem is is negative, there is a net carbon The different in response (and long-term to warming by different ecosystems, as in the Warming of forest In this are the soil and by the on the The in tundra increased of and vegetation The in 1999, of Shaver in place and at the Jonasson of for warming of terrestrial ecosystems

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