Under the frame of the International Geosphere-Biosphere Programme (IGBP) the “Core Projects” Global Change and Terrestrial Ecosystems (GCTE) Focus 2, the Biospheric Aspects of the Hydrological Cycle (BAHC) International Project Office in collaboration with the IGBP International Global Atmospheric Chemistry Projects (IGAC), Biomass Burning Experiment (BIBEX), held a workshop to develop a strategy to integrate ecosystem disturbance into dynamic global vegetation models. The workshop main thrust was on fire as disturbance, but included discussion of other disturbances, such as insect and disease outbreak, land use, and extreme weather events. The workshop, hosted by the Potsdam Institute of Climate Impact Research in Potsdam Germany was held 22-24 June 1998.
Disturbance plays a major role in shaping and maintaining many of the Earth’s terrestrial ecosystems. In fact, many ecosystems depend on fire for their very existence. As example, the prairie’s of North America would be wooded Savannah if it were not for grazing and fire. Global Change is expected to result in changed distribution of current ecosystems, changed composition of those ecosystems, and in creation of new ecosystems. The International Geosphere Biosphere Programme, through the Core Projects Biospheric Aspects of the Hydrological Cycle, International Global Atmospheric Chemistry, and Global Change and Terrestrial Ecosystems recognized that disturbance needed to be included in the modeling efforts of each project. Three main themes were recognize: impact of disturbance on carbon pools, vegetation change, and feedbacks to the atmosphere. This strategy was based on the fact that biomass burning influences atmospheric chemistry, that feedbacks of energy, water and trace gases to the atmosphere are influenced by vegetation, and that changes in the composition of ecosystems have direct impact on the carbon pool, on biodiversity, and health and productivity of the land. Disturbance includes fire, insect, disease, drought and flooding, land conversion, land use, air pollution, and introduction of exotic species. While it will be necessary to ultimately include all disturbances, the Potsdam workshop limited itself to fire. This strategy is based on the fact that there are no process driven models for all disturbances, and that fire has a number of reliable models with which to begin the process of introducing disturbance into dynamic global vegetation models. While this workshop limited itself to fire, a great deal of consideration was given to the fact that the model shell must be able to include other disturbances in the future. As a result, the strategy was to focus on a hazard function which would lead to effects of disturbance. The hazard function is basically a probability statement of risk of effects. This approach is equally valid for all forms of disturbance.
Workshop Recommendations and Challenges
The inclusion of disturbance models within DGVMs creates a number of unique challenges for model development, calibration, and verification. These challenges include:
Optimum model formulation for disturbances cannot be currently specified. Therefore, alternative model approaches must be systematically implemented, tested and compared. Criteria for comparison of disturbance modules should be based on the adequacy of their representation of the disturbance regime and subsequent effects of disturbances on vegetation.
Model comparison is dependent upon the adequacy of data describing vegetation structure, land-use, and reconstruction of historical climate and fire history. Some of these data may never be available at a global scale. Therefore, construction of such data for different regions (e.g., boreal, tropical, savanna, etc.) should be developed as case studies for model comparison.
Plant functional types (PFTs) used in DGVMs are not yet specifically designed to account for responses to disturbance. Detailed consideration must be given to the possible need to expand the definition of PFTs to include disturbance effects.
Inclusion of fine-grained details of vegetation response to disturbances within coarse-grained DGVMs is difficult from both a practical and theoretical standpoint. Theory suggests that predictions across temporal and spatial scales is possible for single attributes (i.e., either mean, variance, or extreme disturbance events), but prediction in shifts of disturbance regimes are difficult to characterize by simple models alone.
A general disturbance framework for inclusions within DGVMs should account for multiple disturbance agents. The present challenge is to consider both fire and insect disturbance(and the interaction between fire and insect effects)within different vegetation types.
Because fire and insect effects are contagion processes, the simulation of coarse-grained dynamics of disturbance effects may benefit from fine-grained descriptions of the spatial heterogeneity of vegetation and land-use.
The spatial scales associated with both data and models for development and testing of prediction of global change are arbitrary. Systematic investigation of aggregation errors and resulting prediction bias associated with inconsistencies in scales between models and data is a needed to insure the adequacy of current descriptions and future predictions.
Plant ecophysiological responses to multiple disturbances are poorly understood, making the interaction between fire, insects and plant physiology difficult to simulate. Response functions that describe changes in DGVM ecophysiological parameters as a consequence of disturbance are needed to accurately simulate vegetation dynamics.
Identification of disturbances which may act at global scales to affect patterns of growth and productivity of vegetation remains a significant challenge for DGVM models.
Specific research tasks need to be designed to address the above issues in order to insure that model projections adequately and reliably reflect changes in vegetation dynamics as a consequence of global climate change, land-use change and disturbance.
A detailed paper resulting from the workshop is entitled “Strategy for a fire module in global dynamic vegetation models” and will be published in one of the forthcoming issues of the International Journal of Wildland Fire.
Michael A.Fosberg BAHC Core Project Office Potsdam Institute for Climate Impact Research (PIK) Telegrafenberg P.O.Box 60 12 03 D – 14412 Potsdam Germany