Pattern and Process and the Dynamics of Forest Structure: A Simulation Approach

Abstract

A computer model was developed to simulate natural forest dynamics. Relationships of leaf area to sapwood area, stem respiration to sapwood volume and vigour to growth efficiency provide equations for tree growth, leaf-area dynamics and canopy structure. The model simulates mixed-age, mixed-species populations of trees on a 1000-m2 patch. The test site, Fiby urskog (central Sweden), has been unmanaged since a catastrophic storm in 1795. Pinus sylvestris (light-demanding) and Picea abies (shade-bearing) are dominant today. The Pinus trees are > 70 years old with high crowns, the Picea trees are all-aged; single-tree gaps are filled by saplings or small trees of Picea. The model was run for 190 years to simulate the natural succession. Present height and diameter distributions of Pinus and Picea, measured on a 0-5 ha stand, fell mostly within 95% limits from replicate model runs. Simulated basal areas of Pinus and Picea were plotted as a function of variations in the species'' potential growth rates. Picea abundace increased sigmoidally as its growth conditions improves, but Pinus was abundant only under conditions unfavourable to Picea. A wide range of patch sizes produced accurate simulations, but if the patch size was too small (< 50 m2) the model allowed gap-phase replacement of Picea by Pinus. Such replacement was not observed. A simulation with Betula pubescens and Populus tremula included reproduced the approximate composition of the stand. Pinus, Betula and Populus (all light-demanders) were important early in the simulated succession but were eventually replaced by Picea. This result also applied to patch sizes down to c. 50 m2; smaller patch sizes produced cyclical regeneration. Even-aged stands of Pinus were simulated. After an intial phase with low mortality, the stands followed a self-thinning line until they reached a maximum biomass. Some forests show gap-phase dynamics as descibed by Watt; others require disturbances producing multiple-tree gaps to complete the regeneration cycle. Boreal forest are of the latter type. The distinction is an emergent property of vegetation, determinated by the ratio of maximum plant size to the nature scale of interactions between plants.