A SIZE-DISTRIBUTION-BASED MODEL OF FOREST DYNAMICS ALONG A LATITUDINAL ENVIRONMENTAL GRADIENT

A geographically extended model of the dynamics of tree size structure of forests is proposed to simulate the change of forest zonation alon g latitude in response to global environmental change. To predict the response of forests to global change, it is necessary to construct fun ctional models of forest tree populations. The size-structure-based mo del requires far less memory and steps of calculation compared with in dividual-based models, and it is easy to incorporate the dimension of geographic locations into the model to describe large-scale dynamics o f forest-type distributions. The effect of increasing size growth rate , expected from increasing atmospheric carbon dioxide, was diminished at the stand-level basal area density, because of regulation by one-si ded competition. Model simulations of a century-long global warming at around 3 degrees C predicted that (1) biomass changed in resident for ests rather simultaneously in response to warming, and that (2) there was a considerable time lag in movement at the boundaries of different forest types, particularly under the existence of resident forest typ es that would be finally replaced. It required several thousand years after a century-long warming spell for forest types to attain new stea dy-state distributions after shifting. As a consequence, global warmin g created a zigzag pattern of biomass distribution along a latitudinal gradient, i.e., an increase in the cooler-side boundary of forest typ es and a decrease in the warmer-side boundary.