Terrestrial Ecosystems in a Changing World

In situ studies of communities suggest that the interactions between biodiversity, atmospheric CO₂ concentration, and C cycling are very complex. Rising CO₂ will probably act on plant diversity through a number of very indirect pathways; e.g., by altering the relative availability of resources such as water and nutrients thereby altering competitive interactions among plants. Second, changes in plant community structure due to rising CO₂ concentrations, particularly in highly dynamic systems, may be as or more important in determining biomass responses than the direct effect of elevated CO₂. Finally, species loss resulting from global change might reduce productivity as well as responsiveness to elevated CO₂, although the response will largely depend on species identity.

Over all, simple scenarios invoking a clear and simple biospheric loop between atmospheric CO₂ and terrestrial biomass due to CO₂ fertilization need to be abandoned and replaced by an understanding of a complex system involving both biotic and abiotic feed-back loops. Our ability to either manage or predict the relationship between diversity and ecosystem function is still limited.

As we show above, a failure to incorporate migration limitations into models of vegetation response to climate change greatly compromises their predictive capability, and the uncertainty due to migration is therefore substantial. Species range shifts have been a ubiquitous response by plant species during Pleistocene climate change, and early signs of this response are evident in modern assemblages. Recent work has increased our understanding of the dispersal limitations to migration rate, but there has been far less focus on the issues which govern population establishment and growth rate, especially at the edge of species’ ranges.

Finally, promising approaches are being developed that address the issue of how human transformation of landscapes will modify migration rates, and that combine mechanistic migration models with spatially explicit models of species geographic ranges at spatial scales relevant to simulating plant propagule dispersal and demographic behavior. These approaches will provide useful insights into biodiversity change under climate and landuse change scenarios. However, the potential increase in spatial resolution of DGVM simulations, and their increasing capacity to simulate more finely defined plant functional types, will allow them to provide an independent alternative assessment of the role of migration in determining the future structure and function of the ecosystems of the Earth.

DGVMs exploit the power of modern computers and computational methods to yield a predictive description of land ecosystem processes that takes account of knowledge previously developed through long histories of separate disciplinary approaches to the study of the biosphere. The degree of interaction between the different scientific approaches still falls far short of optimal; thus, DGVM developers have a responsibility to be aware of progress in several disciplines in order to ensure that their models remain state-of-the-art. We have presented a series of case studies of the evaluation of DGVMs that demonstrate the predictive capability that current models have achieved. Nevertheless, there are plenty of unresolved issues — differences among models that are not well understood, important processes that are omitted or treated simplistically by some or all models, and sets of observations that are not satisfactorily reproduced by current models. More comprehensive “benchmarking” of DGVMs against multiple data sets is required and would be most effectively carried out through an international consortium, so as to avoid duplicating the large amount of work involved in selecting and processing data sets and model experiments. We have also presented a series of case studies that illustrate the power of DGVMs, even with their known limitations, in explaining a remarkable variety of Earth System phenomena and in addressing contemporary issues related to climate and land-use change. These case studies encourage us to believe that the continued development of DGVMs is a worthwhile enterprise. Finally, new directions in Earth System Science point to a range of aspects in which DGVMs could be improved so as to take account of recently acquired knowledge, such as experimental work on whole-ecosystem responses to environmental modification and new understanding of the functional basis of plant traits; complemented by an effort to represent semi-natural and agricultural ecosystems and the impacts of different management practices on these ecosystems; and extended to include processes such as trace-gas emissions, which are important in order to understand the functional role of the terrestrial biosphere in the Earth System. Together, these potential developments add up to an ambitious research program, requiring the economies of scale that only an international collaborative effort can provide.

There is considerable biological potential to reduce greenhouse gas emissions from agricultural soils but many factors prevent the full biological potential being realized. When considering greenhouse gas mitigation, it is important to consider all of the greenhouse gases together as a management practice suitable for reducing one gas may increase emissions of another. Successful greenhouse gas mitigation options for agricultural soils will likely be those that provide other economic and environmental benefits and win-win strategies should be targeted. In the long term, soil-based greenhouse gas mitigation options (including carbon sequestration) can play only a small role in reducing the gap between projected emissions and the reduction in emissions necessary to achieve atmospheric CO₂ stabilization. Nevertheless, since it is critical to reduce greenhouse gas emissions over the next 20–30 years to achieve CO₂ stabilization within a century, and since there is no single solution, soil-based greenhouse gas mitigation options should form part of a broad portfolio of measures aimed at reducing greenhouse gas emissions.

Our limited knowledge on the dynamics of managed and unmanaged ecosystems has curtailed our ability to predict the effects of global change on the ecosystem goods and services that societies rely upon for their wealth and development. Global change terrestrial transects have proved to be an important and useful scientific approach to study the spatial and temporal dynamics of multiple drivers and complex responses. Although a number of studies on global change and terrestrial ecosystems have been done during the recent decade, much remains to be learned of the interactive effects of multiple drivers and their spatial and temporal dynamics which this chapter has presented. In addition, it is recognized that the study of global change not only needs fully effective cooperation of scientists in China, but also needs connection with relevant studies world-wide.

In order to improve integrative global change studies in China and develop the capability to predict the responses of terrestrial ecosystems to global change in China, the following research fields will require further development over the next decade: (1) mechanisms driving responses of terrestrial ecosystems to global change; (2) shifts and adaptation of vegetation and ecosystems to global change; (3) index system of plant functional types of China combined with remote sensing techniques; (4) soil responses to global change; (5) parameterization and scaling techniques in models to study ecosystem dynamics from landscape to regional scales; (6) development of coupled models of the carbon balance, and vegetation dynamics, and atmospheric circulation and climate in order to simulate responses and feedbacks of terrestrial ecosystems to atmosphere and climate changes; and (7) establishment of an information system of terrestrial ecosystems including continuous observations, databases, ecosystem models, and expert systems to observe and predicts changes in terrestrial ecosystems, and to inform policy makers.

A GCTE core project “Global change impacts on terrestrial ecosystems in Monsoon Asia” (TEMA) has been carried out from 1995 to 2003. This chapter overviews the TEMA-employed unique approach of integrating across different scales, i.e., from a plant leaf to watershed budgets, targeting on the eastern Asian region. We particularly focused on the linkage between physiological processes of foliage canopy and landscape-scale processes of plant demography and plant community dynamics, where individual plant processes were integrated from physiology, and we projected the change in geographic pattern from individual plant processes. We evaluated the watershed unit where freshwater chemistry provides a signature of biogeochemical characteristics of terrestrial ecosystems. Stream chemistry controls the trophic condition of lake ecosystems, which can contribute to global change particularly through methane emission. Integration at the scale of watersheds will contribute, within the scope of the new GLP in relation to LOICZ, to the validation of the impact of environmental change on human society, and the impact of human activities on watershed-scale environments.

The Global Land Project (GLP) represents the joint, land-based research agenda of two major global change science programmes: (i) the International Geosphere-Biosphere Programme (IGBP), which originally focused mainly on biophysical processes in the Earth System through its Global Change and Terrestrial Ecosystems (GCTE) core project, and (ii) the International Human Dimensions Programme through its core project on Land-Use and Land-Cover Change (LUCC). The focus of the new project includes people, biota, and other natural resources (air, water, and soil). The strategy presented here critically emphasizes changes in the coupled human and environmental system, which is an extension of the ecosystem concept to explicitly include human actions and decision-making. The research planning builds upon the extensive heritage of global change research including the research discussed in the other chapters in this volume. The Global Land Project is designed to promote greater integration of social and biophysical sciences to meet the current challenges to coping and adapting to global change impacts the world is facing today and the near future. The sustainability of the coupled human-environment system and of ecosystem services is highly vulnerable to global change impacts as we move toward Earth System dynamics not yet faced by our societies.