© 1996 Heron Publishing—Victoria, Canada
Analytical model of stemwood growth in relation to nitrogen supply
Roderick C. Dewar and Ross E. McMurtrie
School of Biological Science, University of New South Wales, Sydney NSW 2052, Australia / Received March 2, 1995
Summary
We derived a simplified version of a previously published process-based model of forest productivity and used it to gain information
about the dependence of stemwood growth on nitrogen supply. The simplifications we made led to the following general expression
for stemwood carbon (cw) as a function of stand age (t), which shows explicitly the main factors involved: cw(t) = ηwG*/µw(1 – λe–µwt – µwe–λt/λ – µw), where ηw is the fraction of total carbon production (G) allocated to stemwood, G* is the equilibrium value of G at canopy closure, λ describes the rate at which G approaches G*, and µw is the combined specific rate of stemwood maintenance respiration and senescence. According to this equation, which describes
a sigmoidal growth curve, cw is zero initially and asymptotically approaches ηwG*/µw with the rate of approach dependent on λ and µw. We used this result to derive corresponding expressions for the maximum mean annual stem-wood volume increment (Y) and optimal rotation length (T). By calculating the quantities G* and λ (which characterize the variation of carbon production with stand age) as functions of the supply rate of plant-available
nitrogen (Uo), we estimated the responses of Y and T to changes in Uo. For a plausible set of parameter values, as Uo increased from 50 to 150 kg N ha–1 year–1, Y increased approximately linearly from 8 to 25 m3 ha–1 year–1 (mainly as a result of increasing G*), whereas T decreased from 21 to 18 years (due to increasing λ). The sensitivity of Y and T to other model parameters was also investigated.
The analytical model provides 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.
Keywords:
canopy closure, mean annual increment, optimal rotation length, plantation, sustained yield.
