The decline of forest productivity as stands age: a model-based method for analysing causes for the decline

For closed canopy forests, both above-ground net primary productivity (ANPP) and wood yield decline as stands age. However the physiological mechanisms responsible for the decline are not well understood. Understanding of the causes of the decline and incorporation of aging mechanisms into models of forest production are essential both for sound forest management and for reliable prediction of changes in terrestrial carbon storage under altered climates. To investigate causes for declining net primary productivity (NPP) an ecosystem model G'DAY was modified to include aging mechanisms associated with three main current hypotheses for NPP decline. These hypotheses are: (1) sapwood maintenance respiration increases as stands age, reducing the availability of carbon to support growth; (2) stomatal conductance and hence photosynthetic rates decline; and (3) soil nitrogen availability declines due to nitrogen (N) accumulation in woody litter. A model-based method was developed for determining the relative importance of three mechanisms for NPP decline in forest stands. The method involves a decomposition of simulated model output into three components, each related to one aging mechanism. The method is illustrated by parameterizing G'DAY for young (40 year-old) and mature (245 year-old) stands of Pinus contorta in Colorado USA. Results from this method of analysis indicate that: 1. The G'DAY model without aging mechanisms cannot reproduce the observed changes in NPP as stands age. When the above three aging mechanisms are switched off, G'DAY shows only transient changes in NPP lasting less than 20 years. When the aging mechanisms are incorporated in G'DAY, the model simulates declining NPP on a scale similar to that observed in the field. 2. The gradual decline in NPP following canopy closure is sensitive to assumptions about aging mechanisms and is particularly sensitive to assumptions about soil N availability and declining photosynthetic rates. We identified key areas of model uncertainty requiring further experimental clarification. Here we highlight two inadequately understood processes: soil N immobilization associated with woody litter accumulation and changes in carbon allocation as stands develop. (C) 2000 Elsevier Science B.V. All rights reserved.