Thinking about efficiency of resource use in forests

The growth of forests can be described as a function of the supply of resources, the proportion of resources captured by trees, and the efficiency with which trees use resources to fix carbon dioxide. This function can be modified to explain wood production by subtracting the allocation of biomass to other tissues and to respiration. At the scale of leaves and seconds, rates of net photosynthesis typically show declining marginal gains with increasing rates of light absorption and transpiration. However, these trends may not represent those that occur at the scale of forests and years, owing to more complete biomass accounting (including costs of synthesis and maintenance of tissues), interactions among resources, and adaptation of biomass partitioning to optimize resource capture and use. Patterns in the growth of forests, across environmental gradients or silvicultural treatments, demonstrate that the efficiency of resource use at the scale of forests and years can increase with increasing rates of resource use. Case studies from Eucalyptus plantations indicate that more productive sites tend to have higher efficiency of resource use than less productive sites, and silvicultural treatments may increase both resource supplies and efficiency of resource use. The questions raised here apply to all forests, but the level of confidence in our general conclusions remains limited by the small number of studies available with complete estimates of rates of resource use and production.