Ecosystem Theory Required to Identify Future Forest Responses to Changing CO₂ and Climate

Summary

The current anthropogenic disturbances in global environmental systems require immediate study and understanding. Yet, piecemeal and applied approaches to solving these interrelated problems hold little promise because the underlying processes of environmental change and ecological response are not understood. Development of an appropriate and valid body of theory is desperately needed. The nature of certain ecosystem theory requirements is revealed by model experiments on responses by forest ecosystems to future climate and CO₂ effects. Increasing concentrations of atmospheric CO₂ potentially could generate multiple and even opposing effects on forests. Greenhouse experiments have shown that enhanced CO₂ positively affects woody seedling growth, and that these effects may also occur in saplings and mature trees under elevated CO₂ concentrations. Yet, today’s close geographic correspondence between certain climate variables and forest distributions suggests that climate changes resulting from future CO₂ increases could destroy many currently existing forests. The potential response of forests to these conflicting forces was examined using a computer model of tree growth and forest stand development. The model is composed of abstractions from current ecological theory and uses knowledge of forest dynamics to simulate simultaneous changes in CO₂ and climate, and the known responses of forest stands to these variables. Model runs of several hundred years were constrained by a climate and CO₂ scenario which changed annually, as suggested by current energy use projections. Simulation results suggest that initial forest response to both climate and CO₂ changes may be minor because of tree longevity. In the long term, the positive effects on forests, simulated as CO₂ fertilization, were overwhelmed by the negatve effects, simulated as drastic changes in the climatic status quo. Yet, the value of the simulations is severely diminished by weaknesses in the theoretical underpinnings of the models. Particularly (but not exclusively) constraining is the absence of a cohesive, tested theory to predict (l) forest ecosystem behavior under chronically changing environmental regulation, and (2) tree species behavior along boundaries of geographic ranges.