Evaluating progress toward closed forest models based on fluxes of carbon, water and nutrients.

Closed system models are defined as mathematical models of systems having specified boundaries within which all flows into and out of the system are accounted for. Closure is obtained experimentally when we can measure all the flows and do not depend on residuals. The meeting on which this volume is based discussed a range of models and approaches to modeling, and the possibility of achieving closure. There was general agreement that we can develop closed system models of the water balance, carbon cycle and nutrient fluxes at the stand level. Confidence in our ability to account for all the flows is greatest for water, decreasing progressively for carbon and nutrients. The priority areas for research on the carbon balance are belowground processes, foliage dynamics and respiration. The problems requiring particular attention in relation to the water balance are the measurement of interception losses, lateral flow in the soil and evaporation from snow. Areas warranting particular research attention in relation to nutrient fluxes through forest stands are the rates, and the controls on rates, of nutrient uptake by trees, and rates of mineralization with emphasis on the importance of microbial processes at the ecosystem level. Most models are written for uniform conditions. Forests are not uniform so the problem of heterogeneity, and how to deal with it in models, requires considerable attention, as does the question of how to scale up, to deal with large areas. There are a great many forest models of all types and the continual development of new ones may not be an effective use of research resources. There is a need for some assessment of the range of models currently existing, or under development, and for moves toward a directed strategy of model structure and development.