Toward extension of a single tree functional-structural model of Scots pine to stand level: effect of the canopy of randomly distributed, identical trees on development of tree structure.

Functional-structural plant growth models (FSPMs) combine the description of the structure of plants and the resource acquisition and partitioning at a detailed architectural level. They offer a means to study tree and stand development on the basis of a structurally accurate description that combines resource capture at the same level of detail. We describe here how a 'shoot-based' individual tree model, LIGNUM of Scots pine (Pinus sylvestris L.) has been applied to a group of identical trees (forest). The model has been applied to isolated trees and saplings growing in forest gaps. First, we present the LIGNUM model and the changes necessary for simulation of a forest instead of individual trees. LIGNUM derives tree growth on the basis of a process-based model of tree carbon balance and the architectural development of the 3-D tree crown. The time step is 1 year. We realised the forest as consisting of individual Scots pine trees on a plot 17 × 17 m, but simplified the stand description by simulating the growth of only one tree in the middle of the plot and assumed that the other trees were identical to it at all times. The model produced results that are comparable with observations made in real Scots pine trees and tree stands in Finland. The simulations with variable values of the parameters controlling the foliage-sapwood relationship, amount of sapwood required below a point in a branch or a stem, and the senescence of sapwood showed how growth declines when the sapwood requirement in the branches and stem was high. In this case, the proportion of resources allocated to the needles became small and the needle mass was low.