Disturbance and succession in a coniferous forest of northwestern North America: simulations with dryades, a spatial gap model

In this paper we describe a model of forest development that has been adapted for use in a coniferous forest of northwestern North America. The simulator, dryades, is a spatial gap model used to examine the effects of disturbances of different types, sizes, and frequencies in mature Douglas fir dominated forests on zonal sites of the Coastal Western Hemlock drier maritime (CWHdm) subzone of British Columbia. Simulation exercises were carried out to test the following (not mutually exclusive) hypotheses in relation to forest compositional variations observed within the study area today: (1) disturbance-mediated succession can accelerate the conversion of early successional forest communities dominated by pioneer tree species (e.g. Douglas fir) to later successional associations; (2) when present in the canopy, long-lived, tall, pioneer species such as Douglas fir strongly influence stand dynamics, regardless of the type of canopy disturbance; (3) silvicultural practices that emulate the autogenic natural disturbance regime of small canopy gaps will maintain a late-successional character at the stand level. Model simulations of forest successional dynamics suggested that: (1) repeated, small-scale disturbances such as light windstorms or small patch harvesting can accelerate the rate of tree species replacement by accelerating forest succession; (2) large-scale disturbances such as infrequent severe fires set back succession to an earlier seral stage dominated by Douglas fir; (3) clear-cutting without Douglas fir planting accelerates forest succession towards a western hemlock/western redcedar forest; (4) on mesic sites in the CWHdm subzone, Douglas fir will not dominate stand dynamics, unless there are infrequent, severe fire disturbances; (5) the creation of small openings through partial harvesting did not allow shade intolerant species to re-establish naturally and dominate the forest stand dynamics. It was concluded that modelled successional dynamics and composition of these forests are largely a function of the initial competitive interactions and the relative shade tolerance of species involved.