Focus 1: Ecosystem Physiology and Global Change

[Leader: James Ehleringer]
[Officer: Diane Pataki]

Introduction

The primary aim of Focus 1 is to understand and model the effect of global change on primary ecosystem processes such as the exchange of carbon, water and trace gases with the atmosphere, element cycling and storage, and biomass accumulation or loss. In the long term, linkages need to be made between the above processes and community dynamics.

A central thesis of Focus 1 is that the ways in which ecosystems function - their physiology - will be strongly affected by the combined and interactive suite of atmospheric and climatic changes, such as elevated CO₂, temperature, N deposition, and changes in precipitation. The analysis will take into account land-use practices that change species composition and land cover.

In order to address these goals, Focus 1 is organized into four activities (Figure 2). The first emphasizes the direct effects of elevated CO₂ concentration and temperature on plant physiology and ecosystem function; interactions with other global change drivers (e.g., N deposition) are also considered The second activity examines the response of biogeochemical cycles of carbon, nitrogen, and other elements to global change, and how nutrient limitation may constrain ecosystem responses to atmospheric and climatic changes. The third activity studies changes in water and energy fluxes of terrestrial ecosystems, and the controls of such processes. The fourth activity integrates the first three through model sensitivity analyses and intermodel comparisons using experimental data sets as they become available; a global synthesis of whole ecosystem pools and fluxes will be produced through both data syntheses and modeling exercises.

Figure 2

Activity 1.1: Ecosystem Response to Elevated CO₂ and Temperature
[Leader: Christian Körner]

There is a critical need for information concerning ecosystem-level responses to increasing levels of atmospheric CO₂ and air temperature, and their interactions with dynamic resources, especially nitrogen and water. A necessary part of this information includes the complex suite of feedbacks that structure and regulate ecosystems, including changes in nutrient cycling, water availability and relations with other ecosystem components (e.g., herbivory and decomposition).

Objectives

Task 1.1.1: Elevated CO₂ Impacts on Ecosystem Function, including Interactions with Temperature and Nutrient Changes
[Leader: Richard Norby]

The main focus will be placed on the effects of elevated CO₂ on ecosystem function, and whenever possible, on the net effect of elevated CO₂ along with other drivers of climatic and atmospheric change, such as temperature, N deposition, precipitation, and ozone.

Implementation

Whole-ecosystem manipulative experiments are encouraged to be initiated that will simultaneously vary CO₂ concentration and other critical controllers of ecosystem processes, such as nutrients and water. The primary goal of these ecosystem experiments is to identify and quantify the mechanisms underlying the ecosystem responses. The experiments of Activity 1 will integrate techniques for elevating CO₂ concentration, such as controlled environment chambers, field open-top chambers, FACE (Free-Air CO₂ Enrichment technology), and natural CO₂ springs. However, use of FACE technology and the naturally occurring vegetation around CO₂ springs are particularly encouraged for new projects.

Optimally, all ecosystem experiments will be of sufficient duration and spatial scale to detect responses developing over the life cycle of dominant species (e.g., competitive relationships), changes in strongly buffered ecosystem components (e.g., soil carbon and nutrient pools), feedbacks between changes in plant properties (e.g., tissue chemistry, architecture, and phenology), and other ecosystem functions such as decomposition and nutrient cycling, water and energy balance, and herbivory. CO₂ treatments will employ current ambient CO₂ concentration and at least one elevated treatment (550 ppm or 700 ppm); non-linear responses should be quantified whenever feasible.

A major effort should be directed at setting up CO₂ enrichment experiments in biomes that are critical for understanding the global carbon cycle, but which have not been sufficiently studied (or have not been studied at all) with regard to potential responses to increasing atmospheric CO₂. These include such major ecosystems as boreal forest, savanna and both humid and dry tropical forests. Strong links with Focus 4 will be developed to study the effects of elevated CO₂ on biodiversity and its effects on ecosystem functioning.

The newly established and developing set of ecosystem experiments employing FACE technology should be encouraged to provide a platform for many investigators from other institutions and disciplines to explore the multiple components of ecosystem responses to elevated CO₂. FACE experiments should employ highly integrative approaches to provide data and make linkages with other Focus 1 activities and tasks, including N and P dynamics, litter chemistry and decomposition, root system structure, activity, and turnover, soil microbiology, soil organic matter, trace gas emissions, and biodiversity issues. Advances in FACE exposure protocol and new scientific opportunities, such as remote sensing and stable isotope analysis, should be shared among FACE researchers through facilitated communication and workshops.

Key topics for experimentation for tasks 1.1.1

The whole ecosystem experiments will be closely linked to the development and operation of dynamic ecosystem models. This linkage will help to formulate hypotheses, guide the interpretation of the results, sharpen the focus on understanding the mechanisms underlying the observed responses, and ensure the broader applicability of the results to other systems (Task 1.4.1).

Proposed Timetable

Task 1.1.2: Increasing Temperature and its Impact on Ecosystem Functioning, especially on Carbon Flux
[Leaders: Lindsey Rustad and Gaius Shaver]

The main focus will be on the impacts of increasing of air and/or soil temperatures on ecosystem functioning, and whenever possible, on the net effect of temperature along with other drivers of climatic and atmospheric change, such as elevated CO₂, N deposition, precipitation, and ozone.

Implementation

As in task 1.1.1 the primary goal is to identify and quantify the mechanisms underlying the ecosystem responses. For this purpose, a wide array of experiments will be proposed using the various techniques available for increasing temperature of air and/or soil, although convective heating is preferred: electric resistance heating, field greenhouses, infrared heating, and temperature-controlled field chambers. Each of these methods has already been shown to be useful in field experiments, but each has different advantages and disadvantages. Natural experiments using thermal gradients, soil/vegetation transplant, and snow removal experiments can also provide important information. The challenge is to design and interpret these experiments in ways that are complementary. Laboratory microcosms may also be used to address specific questions, but emphasis should be placed on whole-ecosystem field experiments. Natural temperature gradients are also important tools.

The analysis of the effects of increased temperature on ecosystems should cover the most important biomes, but a special emphasis needs to be placed on low temperature high-latitude and high elevation ecosystems where the greatest effects of climate warming are to be expected. Existing experiments and networks (such as the ITEX network in the Arctic and the several existing soil heating experiments) must be brought together and protocols developed to maximize complementarity and comparability. A consortium of intensively-studied sites where all major aspects of ecosystem physiology are assayed simultaneously in one place is particularly needed.

Key topics for experimentation for tasks 1.1.2

Proposed Timetable

Activity 1.2: Terrestrial Ecosystem Biogeochemistry
[Leader: David Schimel]

The aim of the biogeochemistry activity is to understand and be able to predict the constraints that the nitrogen and phosphorus cycles place on the terrestrial carbon cycle. This will be achieved by focusing research on the key points of linkage between these three cycles (C, N, P) in order to reveal and quantify the factors controlling the interactions. The key interactions occur at the points where the inorganic nutrients are bound into or released from organic compounds, both during litter decomposition and during carbon and nutrient assimilation. It is recognized that elements other than N and P may also constrain the C cycle, but is assumed that for the major portions of the land surface these two elements have primary importance.

We conjecture, for instance, that (a) tropical forests and woodlands are highly phosphorus-limited and will be incapable of further organic matter accumulation unless greater C availability increases the efficiency of mycorrhizal symbioses, whereas (b) temperate forests are nitrogen limited, and will be able to accumulate more carbon than at present as a result of industrial nitrogen deposition (50% of the N deposition in Europe is anthropogenic). The significance of this activity is that the capacity of the terrestrial biosphere to absorb CO₂ from the atmosphere is limited by the availability of the other elements needed in the construction of living issues. Further, the stability (turnover time) of belowground pools of non-living organic matter, and thus the capacity of ecosystems to store or release CO₂, is influenced by nutrient supply. One of the several impacts of global land use change is a perturbation of the nutrient cycles through soil loss, fertilization, industrial deposition, and tillage.

Task 1.2.1. Nutrient Constraints on Organic Matter Accumulation in Terrestrial Ecosystems
[Leaders: Peter Högberg and David Schimel]

Objectives

Implementation

The approach is to build on two existing experimental platforms: (1) ecosystem-level elevated CO₂ experiments (GCTE-Elevated CO₂ Consortium) and (2) the IGBP megatransects. In addition, nutrient addition experiments in natural ecosystems will be identified, resampled and analyzed. This will include as many existing long-term trials as possible, but probably also requires the initiation of N x P x micronutrient factorial experiments in the moist tropics (Miombo, LBA and South-East Asia transects). A global literature review of fertilization trials will be also made.

Task 1.2.2. Global Patterns of Litter Chemistry and Decomposition
[Leader: Cheryl Palm]

The GCTE approach to modeling terrestrial ecosystems is strongly based on the concept of plant functional types (PFTs). One of the key considerations in allocating PFTs should be tissue chemistry, including nutrient content and the content of organic compounds with different degrees of resistance to decomposition. The principal way in which terrestrial carbon cycle models are constrained by nutrients is to ensure that the C:N and C:P ratios in plant tissues remain within the observed bounds. Furthermore, the rate of decomposition of plant tissues is directly and indirectly controlled by their tissue nutrient contents.

Dead plant material is a significant component of the terrestrial carbon pool, with a turnover rate in the order of years. A portion of the carbon in litter becomes incorporated in the soil organic matter, which is the largest terrestrial pool and the one with the slowest turnover. Several models exist which describe the decomposition of plant tissues as a function of climate and tissue chemistry. These models were generally developed for a small range of plant tissues, gathered from a particular region. Globally robust models need to be developed and tested.

Objectives

Implementation

A global database will be built for litter chemistry representative of all terrestrial biomes; the database will be used to define functional types and understand the controlling factors of litter chemistry. This database will also include litter and decomposition data from manipulative experiments dealing with elevated CO₂, nutrients fertilization, temperature, etc. The task then will proceed by holding model intercomparisons using shared data sets and by conducting a global litter exchange experiment.

The IGBP Transects will play an important role in the implementation of Activity 1.2. The IGBP transects are a system of large-scale networks of research sites spanning the major gradients of anticipated global change. At present twelve are proposed, of which the majority are either in existence or in advanced stages of planning. They fall into four groups with three 'continental replicates' in each group: the high-latitude transects covering the temperature gradient between the tundra and boreal forests; the semi-arid tropical transects spanning the precipitation gradient between the deserts and the tropical forests; the transects of land-use intensity in the humid tropics; and the temperate region land use and moisture transects.

The transect concept was developed within Activity 1.2 and formed the basis of its previous implementation plan. The concept has now been accepted by both IGAC and BAHC as a suitable vehicle for monitoring and experiments within their areas of interest as well, and has therefore grown beyond the scope of GCTE Activity 1.2 alone. Activity 1.2 will continue to use the transects as a major platform for its tasks; for instance, as a way to ensure global coverage of litter tissue chemistry and N and P limitation.

Proposed Timetable

Activity 1.3: Effect of Changes in Vegetation on Carbon, Water, and Energy Fluxes
[Leader: James Ehleringer]

This activity aims to improve our understanding of the controls on carbon, water, and energy fluxes between the terrestrial biosphere and the atmosphere under global change. Emphasis will be placed on ecological processes that link organism-level processes to whole system function.

Task 1.3.1: Root Distributions and Carbon and Water Fluxes
[Leader: Robert Jackson]

Roots play a central role in nutrient and water dynamics, both in providing a fundamental mechanism allowing for temporal separation of resource capture and loss activities at the ecosystem level and in structuring the competitive interactions among ecosystem components. The purpose of this task is to understand the importance of root distributions, and below-ground processes in general, on ecosystem carbon and water fluxes. An equally important goal is improving the representation of below-ground phenomena in global models. Both of these goals will be examined in the context of such drivers as elevated CO₂, nutrient deposition, and land-use change (particularly changes in plant functional types). This task consists of three components: literature syntheses, field projects, and modeling integration.

The goals of this task are also important to the Biosphere Aspects of the Hydrological Cycle (BAHC) project. Several joint GCTE-BAHC projects and meetings are already planned. In addition, a joint project with GCTE and DIS (Data Information Systems) is proposed to generate global rooting maps. Incorporating rooting depth into a General Circulation Model (GCM) will also be done jointly with Activity 1.4.

Objectives

Implementation

Literature syntheses will be an important component of early implementation. These syntheses will include the analysis of root and mycorrhizal attributes globally and their relationship to environmental variables (e.g., precipitation, soil texture). The data will be made available to scientists through an internet database allowing the continuous gathering and dissemination of information (X-roots project). These data will be used to improve the representation of below-ground phenomena in ecosystem and global models, and for understanding the controls of belowground NPP. Field campaigns will also be initiated along IGBP transects to examine the role of altered root distributions on carbon and water fluxes.

Proposed Timetable

Task 1.3.2: FLUXNET - An Integrated Long-term Carbon and energy Flux Network of Terrestrial Ecosystems (Joint task with BAHC)
[Leader: Riccardo Valentini]

The study of CO₂ and H₂O flux interactions between terrestrial vegetation and the atmosphere has been made more routine by recent developments in eddy covariance techniques (long-term measurements, footprint analysis, software and hardware standardization, etc.). Historically this technique has been used primarily for intensive, short-term land surface experiments. However, new opportunities are emerging for extension to larger spatial and longer temporal scales, since it is now practical to make continuous CO₂ and water flux measurements on a seasonal basis with hourly resolution. A network of European forests is currently in operation (EUROFLUX, MEDEFLU) and similar efforts exist in North America (AMERIFLUX), Australia (OZFLUX,), EUROASIA NET, Japan, and in the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). The FLUXNET initiative offers a valuable framework for new research on ecosystem responses to global change and the effects of land use and/or cover change on biosphere-atmosphere interactions.

Task 1.3.2 will focus on two areas within the FLUXNET framework. First is the development of a functional classification of terrestrial vegetation in terms of carbon, water and energy fluxes with the atmosphere. There is an emerging body of evidence which suggests that we can consider terrestrial ecosystems as functional units with respect to mass and energy exchange with the atmosphere. Properties emerging at canopy level, including energy partitioning, maximum canopy assimilation, and light-use efficiency, are overall ecosystem properties influenced by vegetation type, by efficiency of biogeochemical cycling, and by climate. By taking advantage of the existing FLUXNET framework, it may be possible to scale emerging properties across ecosystems and thus classify vegetation into functional units. In conjunction with Activity 1.4 and Focus 2, an overall goal in this activity is to use a classification scheme approach to evaluate global-level effects of changes in vegetation distribution on biosphere-atmosphere interactions.

The second area of focus is the effect of annual and interannual climate variability on biological/ecological processes that affect biosphere-atmosphere exchange. Many of the processes driving water and carbon fluxes at ecosystem level - phenology, leaf area and biomass production - are strongly dependent on seasonal changes in climate. Furthermore, episodic events (e.g., temperature extremes, ENSO, drought, fire) are important driving variables influencing ecosystem interactions with the atmosphere. Questions concerning direct biosphere responses to interannual climate variability are of particular relevance. For example, how do terrestrial carbon fluxes differ in ENSO (El Niño - Southern Oscillation) and La Niña years? How does interannual variability affect leaf development and ecosystem carbon uptake and source/sink distribution?

Objectives

The long-term objective of this task is to contribute ecological expertise to the further development of FLUXNET and to the interpretation of eddy covariance flux measurements. Specific objectives include:

Implementation

One of the first jobs of this task will be to collect standardized baseline ecological data for each of the FLUXNET sites. This database should include (i) site and regional data characterizing the site in terms of its representativeness of the larger region and the globe, and (ii) ecological data important for interpreting the flux measurements. Such information should include the land-use history of the site, the vegetation composition and successional state (digitized and stored in a spatially explicit form), the natural disturbance regime, and soil structure and chemistry.

The work to develop a functional classification of vegetation (not plants) in terms of water and carbon exchange can build on two existing efforts. First, most groups involved in FLUXNET or in modeling biosphere-atmosphere exchange processes already use land-surface classification schemes, some of which are linked to remotely sensed data. Second, GCTE's Focus 2 has carried out a project on plant functional types, which has developed classification schemes based on plant structure, photosynthetic pathway, and response to environmental factors. Collaboration between these two communities should lead to progress in developing a standardized functional classification of vegetation in terms of water and carbon fluxes.

To initiate research on the influence of climate variability on ecological controls of biosphere-atmosphere exchange, the Task Force will need to establish measurement protocols to ensure that information relevant for vegetation response to seasonal and interannual variations in climate is gathered at the FLUXNET sites. An analysis of that information will yield a first estimate of those ecosystems especially sensitive to climate variability and of the possible effects of ecosystem responses on biosphere-atmosphere exchange processes.

GCTE can also contribute to FLUXNET by (i) providing advice on the selection of new sites, especially those located on the IGBP Terrestrial Transects, (ii) reviewing protocols for ecological studies in the operational networks; (iii) interpreting the flux measurements in terms of ecological controls and assisting with the extrapolation of flux measurements to larger spatial and time scales.

Proposed Timetable

Task 1.3.3: Stable Isotope Integration of CO₂ and H₂O Fluxes
[Leader: James Ehleringer]

Ecologically-relevant information is contained within the isotopic composition of carbon dioxide and water vapor as they flux between the biosphere and the planetary boundary layer, eventually mixing with the remainder of the atmosphere. There is a need to better understand source-sink relationships between ecosystems and the atmosphere on a global basis, especially along gradients of water availability and/or land-use change. The stable isotope composition of both carbon dioxide and water contain integrated information relevant to scaling ecosystem physiology to the landscape and regional levels, especially when combined with ecosystem-level flux measurements. It is the gas exchange between the terrestrial and oceanic surfaces with the atmosphere that contributes to both the seasonal and annual variations in atmospheric carbon dioxide concentration and isotopic composition. Isotopic analyses may prove pivotal in deciphering the influence of annual and interannual climate variability on biological/ecological processes that affect biosphere-atmosphere exchange. Many of the processes driving water and carbon fluxes at ecosystem level - phenology, leaf area and biomass production - are strongly dependent on seasonal changes in climate. Furthermore, episodic events (e.g., temperature extremes, ENSO, drought, fire) are important driving variables influencing ecosystem interactions with the atmosphere. Questions concerning direct biospheric responses to interannual climate variability are of particular relevance. For example, how do terrestrial carbon fluxes differ in ENSO (El Niño - Southern Oscillation) and La Niña years in regions influenced by this climatology? How does interannual variability affect leaf development and ecosystem carbon uptake and source/sink distribution? For example, how do terrestrial carbon fluxes differ? Attention should be focused on incorporating and integrating these isotopic signals into global-change research programs in order to better quantify the dynamics and sensitivity to change of different ecosystems, as well as to partition potential differential responses of above-ground and below-ground components.

Objectives

Implementation

This task can be implemented by providing a focus which brings together interests at the ecophysiological, ecosystem flux, atmospheric, and global modeling levels. At present, there is not a close coordination or integration of isotopic and flux measurement research into a unified program. We envision the incorporation of stable isotope analyses of carbon dioxide and water into developing eddy-covariance FLUXNET network (AMERIFLUX, EUROFLUX, OZFLUX/KIWIFLUX, LBA, ASIAFLUX, etc.) as well as into efforts along the IGBP megatransects. Simultaneously, workshop opportunities will be developed to bring together ecosystem through global modeling interests to ensure that future research efforts will be able to provide useful data for assessing the global carbon cycle.

Proposed Timetable

Activity 1.4: Integrating Activities
[Leader: Louis Pitelka]

Global change will lead to the simultaneous alteration of a number of environmental variables. The suite of studies undertaken in Focus 1 - the whole-ecosystem CO₂-enrichment studies, elevated temperature studies, the biogeochemical studies on the IGBP Transects and elsewhere, and the water and energy flux studies - are designed to provide insights into how global change will affect key ecosystem processes involved in carbon balance, nutrient dynamics and hydrologic cycling. The final requirement in Focus 1 is to bring together the results and understanding from many studies to enable us to predict the net effects of global change on ecosystem function, especially the carbon balance of terrestrial ecosystems. The goal of Activity 4 is to ensure that results from other Focus 1 activities are effectively synthesized and incorporated into patch, regional, and global models. The objective of Task 1.4.1 is to promote linkages between Focus 1 researchers and developers of ecosystem models, while the objective of Task 1.4.2 is to stimulate data analysis and interpretation activities in order to take full advantage of Focus 1 experimental results.

Task 1.4.1: Integrating Ecosystem Physiology into Regional and Global Ecosystem Models
[Leader: Chris Field]

The results of field research carried out in the first three Activities of this Focus will be used to improve and test ecosystem-scale models of carbon, nutrient and water cycles that are useful for global change research. Such models will also be valuable in guiding the design and execution of the experiments themselves. Special emphasis will be placed on the integration of our improved understanding of ecosystem physiology into regional and global models of ecosystem functioning.

In addition, this integrating Task will provide the direct link from Focus 1 to the other three Foci. Specifically, it will provide much of the experimental and process model foundation for the incorporation of improved physiology in both patch-scale ecosystem dynamics models and Dynamic Global Vegetation Models (DGVMs) in Focus 2. It will link to Focus 3 by providing a direct comparison with the ecosystem-type process models to be developed for agricultural and managed forest systems. Finally, it will provide the patch-scale process models needed to underpin the Focus 4 work on the relationship between ecological complexity and ecosystem functioning, and the global change impacts on that relationship.

Objectives

The long-term objective of this task is to improve integrated carbon, nutrient and water models at the patch to global scales to predict how global change will affect the physiology of terrestrial ecosystems in the decades to century time-frame. Specific objectives include:

Implementation

The CMEAL (CO₂ Models/Experiments Activity for Improved Links) project, already part of the GCTE Core Research Program, is a good initial base on which to build the research effort of this Task. CMEAL aims to incorporate the latest findings of elevated CO₂ research into ecosystem models. As the original CMEAL project comes to a close several follow-on activities are planned. First, a workshop will bring together the CMEAL team with those involved in warming experiments in order to explore applying a similar approach to better linking warming experiments with models. Second, in cooperation with Focus 2, a workshop will be held to bring together forest gap modelers with experimentalists and ecosystem modelers to discuss representation of physiology in gap models. This workshop will include CMEAL participants, and a major objective is the development of improved gap models that are sufficiently rigorous in the treatment of tree physiology to be appropriate for addressing climate change issues.

As experimental work in other areas of Focus 1 begins to generate new results, these will be incorporated into ecosystem models, as appropriate, by workshops and follow-up collaborative activities involving both experimentalists and modelers. This is an ongoing effort of the Vegetation/Ecosystem Modeling and Analysis Project, and there will be ongoing links with this Focus 2 project. The FLUXNET (e.g., Ameriflux) initiative offers another good opportunity for links between modelers and experimentalists, especially in the use of detailed carbon, water and energy flux measurements from around the world as a validation tool. The modeling outputs may also give useful guidance for the further development of the observational work on FLUXNET. Task 1.4.1 plans to utilize FLUXNET data for model validation will be coordinated with the related efforts of Activity 1.3 and BAHC to maximize the ecological value of these data.

Finally, another follow-on to CMEAL will be an ongoing effort to fill gaps in our ability to understand and quantify the carbon balance of large regions. A major objective will be to develop improved models that incorporate processes that are important for carbon balance but are not adequately represented in current models. These include land use, disturbance, succession, and physiological saturation. Another objective will be to evaluate and constrain modeled estimates with data from observation. This effort will follow the model of CMEAL by involving a number of workshops (bringing together modelers with experts on particular processes) combined with ongoing work by a post doc.

The next phase of Focus 1 will place more emphasis on the interactions between global change drivers, including elevated CO₂ and temperature, as well as disturbance and land use. The role of this Task is to ensure that this improved understanding is incorporated into ecosystem models at all scales.

Task 1.4.2: Synthesis and Analysis of Experimental Data
[Leader: Louis Pitelka]

Results from Focus 1 experiments and related research done to date are mixed and often inconsistent. This means that it is difficult to develop generalities understandable to decision makers or quantitative relationships that can be incorporated into models. It is likely that some of these inconsistencies could be resolved, or at least better understood, through a more thorough and coordinated analysis of results across all appropriate experiments. Three specific examples of situations that could benefit from more coordinated analyses or meta-analyses include the vast and expanding mass of data on effects of elevated CO₂ on plants, the less extensive but very important data on carbon and nitrogen budget within experimental units, and the problem of time scale-dependent responses of experimental systems.

Objectives

The long-term objective of this task is to improve understanding of ecosystem physiology under global change by analysis, interpretation and integration of experimental results. Specific objectives include:

Implementation

There now exists an enormous number of data sets on the responses of plants and ecosystems to elevated CO₂, yet relatively few definitive and quantitative conclusions can be derived due to the variation in results. New techniques for the meta-analysis of data on effects of elevated CO₂ have been developed and preliminary analyses show that this approach holds promise for identifying common patterns. The next phase will be to accumulate additional data sets, especially from the most recent experiments, and to apply the techniques of meta-analysis to them. In addition to identifying common patterns, this analysis will help to identify those aspects of response that need further investigation.

Much of the controversy over the interpretation and scaling up of results from elevated CO₂ experiments and from observations of CO₂ fluxes results from the different time scales of key vegetation processes and of the experiments themselves. For example, some experiments are stopped before the early positive response phase of the system has been completed, and others involve ecological systems in early successional rather than mature stages. Dissimilar or contradictory results from different experiments could be, at least partially, the consequence of examining responses over very different time scales relative to the development of individual plants and plant canopies. One problem is that the models currently used to simulate responses of terrestrial ecosystems to global change can not easily be applied to evaluate experimental results or predicts outcomes of experiments. This is often because of the short time frame and various artifical aspects of many experiments. One objective for this Task will be to explore ways to modify models to make them more useful for evaluating experiments without compromising their applicability to more natural ecosystems. As part of this Task workshops or related activities will be organized to evaluate elevated CO₂ experiments in terms of the length of the experiment or observation compared to key plant and vegetation time scales, such as the life history and demography of the species, and the successional stage of the vegetation stand.

Until recently there was insufficient effort devoted to measuring entire carbon or nitrogen budgets within experimental units. There now is a much greater emphasis on this, but there are still problems in measuring all pools and fluxes, and in interpreting those measurements that are made. There are two areas in which analysis of C and N budgets are likely to lead to rapid advancement of understanding. The first is the work of the elevated CO₂ consortium, in which recent ecosystem-level experiments have identified belowground carbon pools as the likely fate of the additional carbon assimilated. Confirming this local missing sink directly by measurement is very difficult, so a careful analysis of the C and N budgets of these experimental units may yield further insights into the fate of the additional carbon the controls on the system response to elevated CO₂.

Proposed Timetable

[Focus 2] [Focus 3] [Focus 4]