ANALYTICAL MODEL OF STEMWOOD GROWTH IN RELATION TO NITROGEN SUPPLY

We derived a simplified version of a previously published process-base d model of forest productivity and used it to gain information about t he dependence of stemwood growth on nitrogen supply. The simplificatio ns we made led to the following general expression for stemwood carbon (c(w)) as a function of stand age (t), which shows explicitly the mai n factors involved: [GRAPHICS] where eta(w) is the fraction of total c arbon production (G) allocated to stemwood, G is the equilibrium valu e of G at canopy closure, lambda describes the rate at which G approac hes G, and mu(w) is the combined specific rate of stemwood maintenanc e respiration and senescence. According to this equation, which descri bes a sigmoidal growth curve, c(w) is zero initially and asymptoticall y approaches eta(w)G/mu(w) with the rate of approach dependent on lam bda and mu(w). We used this result to derive corresponding expressions for the maximum mean annual stemwood volume increment (Y) and optimal rotation length (T). By calculating the quantities G and lambda (whi ch characterize the variation of carbon production with stand age) as functions of the supply rate of plant-available nitrogen (U-o), we est imated the responses of Y and T to changes in U-o. For a plausible set of parameter values, as U-o increased from 50 to 150 kg N ha(-1) year (-1), Y increased approximately linearly from 8 to 25 m(3) ha(-1) year (-1) (mainly as a result of increasing G), whereas T decreased from 2 1 to 18 years (due to increasing lambda). The sensitivity of Y and T t o other model parameters was also investigated. The analytical model p rovides a useful basis for examining the effects of changes in climate and nutrient supply on sustainable forest productivity, and may also help in interpreting the behavior of more complex process-based models of forest growth.