Quantifying gap dynamics at the patch mosaic level using a spatially-explicit model of a northern hardwood forest ecosystem

Small-scale disturbances that create canopy gaps play a key role in regulating successional pathways in temperate forests. SORTIE, a spatially-explicit, individual-based model, allows the modelling of temperate forest succession by means of gap dynamics. The aim of this paper is to evaluate, using a spatio-temporal sensitivity analysis, the spatial cohesiveness of gaps generated by SORTIE. Northern hardwood forest succession, based on local interactions of three shade tolerant and three shade intolerant species, was simulated over 1000 years. To investigate the effects of initial conditions, two seedling densities (low and high) and two spatial configurations of new seedlings (random and aggregated) were replicated five times. Statistical analyses were then performed to detect the presence of significant global spatio-ternporal gap structures. These analyses were followed by a more detailed characterisation of the spatio-temporal behaviour of the model using three different spatial statistics (spatial join-count, nearest neighbour and gap size) at each time step. The cohesive temporal periodicity of these statistics was assessed using phase coherence (PC) analysis. Results exhibit consistent global spatial dynamics, with subtle differences between replicates and between initial conditions. PC analysis reveals the presence of important cyclic behaviour around 200-250 years. Finally, our results provide useful insights about the intrinsic factors regulating long-term spatial gap dynamics, as well as the effects of shade tolerance on the immediate responses of tree species to disturbance in temperate forests. (C) 2001 Elsevier Science B.V. All rights reserved.