Effects of multiple fires on the structure of southwestern Washington forests
Gray, Andrew N.
Franklin, Jerry F.
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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 wildfire and subsequent severe fires within a short period (reburns) on forest establishment, composition, and structure was examined in the 16,000 ha Siouxon Creek watershed in the western Cascades of southern Washington. Evidence of large intense fires and small patchy fires was found in the watershed, with 4 fires occurring since 1900. Tree establishment was rapid and abundant throughout most of the area burned in the 1902 fire, possibly from survival of on-site seed sources. Tree establishment was delayed on most reburns and corresponded with years of abundant regional Douglas-fir (Pseudotsuga menziesii) cone production, indicating off-site sources of seed. Western hemlock (Tsuga heterophylla) was less abundant on reburns than on the 1902 burn. The ranges of Douglas-fir ages within stands were greater on reburns than on single burns, but fire frequency effects could not be distinguished from potential stand age and composition effects. There were no clear patterns of tree species abundance related to presumed dispersal distances of up to 3 km from unburned forest. Fire frequency, topography, and seed source had important effects on forest structure and composition across the watershed