Description and simulation of tree-layer composition and size distributions in a primaeval Picea-Pinus forest

A structural description of a 0.5 ha stand in the primaeval coniferous forest of Fiby in central Sweden is reported, and a simulation model is used to test the hypothesis that the diameter and height distributions of the two most important tree species Pinus sylvestris L. (shade-intolerant) and Picea abies (L.) Karst. (shade-tolerant), are the outcome of simple successional processes following storm-felling 190 yr previously. The simulation model, FORSKA, is a 'gap model' treating light and other resource availability as homogeneous within patches. Simple formulations of dimensional relationships, vertical leaf area distributions within trees, light attenuation through the canopy and net assimilation as a function of ligh intensity allow FORSKA to simulate the species mixture, size distributions and vertical canopy structure of mixed-age, mixed-species forests. Parameters of the model were estimated from literature and from field data on height-diameter relationships, establishment rates and maximum ring-widths. The simulation generated a stand description similar to the real one in all essential characteristics. FORSKA allows vertical overlap between the crowns of nearby trees. The present simulation also differed from many gap model simulations in that the patch size was much greater than the maximum crown size, consistent with field observations that single treefall gaps have little effect; the persistence of shade-intolerant species in boreal forests generally depends on larger-scale disturbance events, such as large storm-gaps and forest fires.