EFFECTS OF MULTIPLE FIRES ON THE STRUCTURE OF SOUTHWESTERN WASHINGTONFORESTS

Fire frequency, intensity, and size can influence the nature of forest development, with potentially profound effects on ecosystem processes and the abundance of native species. The effect of an intense wildfir e and subsequent severe fires within a short period (reburns) on fores t establishment, composition, and structure was examined in the 16,000 ha Siouxon Creek watershed in the western Cascades of southern Washin gton. Evidence of large intense fires and small patchy fires was found in the watershed, with 4 fires occurring since 1900, Tree establishme nt was rapid and abundant throughout most of the area burned in the 19 02 fire, possibly from survival of on-site seed sources. Tree establis hment was delayed on most reburns and corresponded with years of abund ant regional Douglas-fir (Pseudotsuga menziesii) cone production, indi cating off-site sources of seed. Western hemlock (Tsuga heterophylla) was less abundant on reburns than on the 1902 burn. The ranges of Doug las-fir ages within stands were greater on reburns than on single burn s, but fire frequency effects could not be distinguished from potentia l stand age and composition effects. There were no clear patterns of t ree species abundance related to presumed dispersal distances of up to 3 km from unburned forest. Fire frequency, topography, and seed sourc e had important effects on forest structure and composition across the watershed.