Comparing light interception with stand basal area for predicting tree growth

Empirical and process-based tree growth models have been developed concurre ntly; however, their growth predictions have rarely been compared directly. A major difference between the model types is the explicit quantification of foliage biomass as a key variable in process-based models. The aim of th is work was to test if this difference has a significant impact on model be havior, especially when simulating silvicultural practices such as intensiv e thinning. A method was developed to evaluate leaf area and light interception of the mean tree of an even-aged stand from yield table data for Norway spruce (Pi cea abies (L.) Karst.) in the northern French Alps. Two scenarios were anal yzed: (1) a closed stand where leaf area was limited by a maximum leaf area index-represented by young, dense stands, and (2) an open stand where leaf area was limited by the height of the crown base-represented by old, spars e stands. Light interception was calculated based on interpolation between a closed stand (Beer-Lambert law) and an isolated tree (light interception proportional to leaf area). This approach was then used to build a growth model in which competition wa s described by the ratio of light intercepted by a mean tree of the stand t o light intercepted by an isolated tree of the same size. This process-base d model was compared with a simpler empirical model in which competition wa s described by stand basal area. Both models fit well to yield table diamet er increment data, the simpler model being slightly better. Simulation of l ong-term growth, interspersed with thinning, revealed differences between t he models. The empirical model was sensitive to thinning and simulated a di scontinuous growth pattern, whereas the model based on light calculation sh owed a smoother growth response to thinning. Simulations of heavy thinning in a dense stand highlighted these differences. The empirical model simulat ed heavy thinning in a dense stand unrealistically: after thinning, trees i mmediately grew as fast as trees of similar diameter in an unthinned stand at the same density. In contrast, leaf area played a regulatory role in the model based on light interception: trees with short crowns, as a result of a previous period of growth at high density, benefited little from an incr ease in light following thinning. It is concluded that models based on phys iological or ecological processes have qualitative behaviors different from those of classical empirical models. This is especially important when mod els are used to make extrapolations far from reference data, for example, t o forecast the long-term effect of a new silvicultural strategy.