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Vegetation plays a part in controlling climate and in turn responds to climatic change. Therefore, projections of future climates must include the responses of vegetation, both in terms of function and distribution. Unfortunately, assessments of future climatic impacts on vegetation are still considered with equilibrium vegetation models models which exclude processes involved in vegetation dynamics, such as succession and disturbance. The commonly held view that future spatialchanges in temperature will outstrip the potential for species to migrate has no theoretical underpinning, particularly as the important processes of disturbance and tolerance of climatic change are not included. In order to assess the appropriate rates of vegetation change it is necessary to implement dynamic vegetation models. Such models are now being completed but provide their own problems associated withthe inclusion of poorly understood processes and with methods of testing. [ABSTRACT FROM AUTHOR]

Dynamic global vegetation - biogeochemistry models are required to predict the likely responses of the terrestrial biosphere to anticipated future global environmental change and for improved representationof an active vegetation surface within general circulation models ofthe Earth's global climate system. Testing the predictions of such models is essential to their development prior to use in a predictive capacity. The climate change experiment (CLIMEX) has exposed an entire catchment of boreal vegetation to elevated CO2 (560 ppmv) and temperature + 3 deg.C in summer, + 5 deg.C in winter) for the past three years and has a considerable archive of pre-and post-treatment measurements of both CO2 and water vapour fluxes of the vegetation, catchment runoff and soil nutrient status. These data have been used to test the predictions of the University of Sheffield dynamic global vegetation model (SDGVM) for the same site using historical records of climate as input. Comparisons of observations and predictions at the scale of individual leaves and whole ecosystems are generally favourable, increasing our confidence in the application ofthe model to forecasting the responses of the terrestrial biosphere to various global change scenarios. The SDGVM has been used to predict the future responses of the ecosystem at the site into the year 2003 AD. The results indicate rather small changes in leaf area index and catchment runoff but quite large increases in net primary productivity. The model predictions are now open to testing further as the CO2 and temperature treatments continue in the CLIMEX greenhouse. [ABSTRACT FROM AUTHOR]