Effect of the shape of forest fragments on tree population dynamics

Abstract

A lattice model is used to study the influence of the shape of forest fragments on the augmentation process of a tree population following clear-cutting. Two fragments of different shapes, squarish and linear, were studied without seed immigration from the outside. Both fragments consisted initially of two patches, a 4-ha clear-cut area and a 1-ha remnant forest, mimicking a common forest management regime in Japan. Stochastic simulations using the model reveal several differences in the dynamics and equilibrium structure of the tree population between the fragments. In the squarish fragment, the forest structure reached an equilibrium state in 500–600 years, but the linear fragment took 2100–2400 years. At equilibrium, the proportion of treeless area was significantly larger in the linear fragment than in the squarish fragment. The mean area of gaps was also larger in the linear fragment. The population expansion rate from the remnant forest into the cleared area differed significantly between the two fragments: 0.63 m year−1 in the squarish fragment, and 0.43 m year−1 in the linear fragment. These differences are explained as arising from the spatial configuration of seed dispersal: in the linear fragment, the density of seedlings was lower by half than in the squarish fragment, because a greater proportion of seeds produced within the linear fragment were deposited outside the fragment and were lost. However, the density effect (i.e., one-sided competition among trees) mitigates the negative impact of dispersal loss on the adult population density at equilibrium. Forest fragments with linear shapes should be preferred by fugitive light-demanding species in forested landscapes.