Tree recruitment following wildfire in a mountain hemlock forest, Oregon Cascades

Background/Question/Methods

Wildfire is the predominant large-scale disturbance of mountain hemlock forests in the Pacific Northwest; fires tend to be infrequent and of high severity. Seedling establishment following fire is a critical phase in the dynamics of these forests, since mature trees and advanced regeneration usually succumb to fire. We studied recruitment of tree seedlings (stems < 1.37 m in height) and saplings (stems at least 1.37 m in height and < 5 cm diameter at breast height) in 15 years following a 1996 wildfire in the Cascade Range of Oregon, comparing unburned areas to areas with varying fire severity. The three levels of fire severity were high mortality (<10% surviving trees) with tree crowns consumed by the fire, high mortality with tree crowns scorched by the fire, and partial mortality (>10% and <90% surviving trees). In addition to seedlings and saplings, we tracked numbers of trees (stems ≥5 cm diameter at breast height, including survivors and post-fire recruits). We observed three, 0.1 ha plots for each level of fire severity (including unburned). All results are in stems per ha.

Results/Conclusions

One year after the fire, live trees were absent for high mortality--crowns consumed and sparse for high mortality--crowns scorched (0 to 50), variable for partial mortality (50 to 830) and abundant on unburned plots (320 to 2050). Two years after the fire, seedlings were sparse where there was high mortality (0 to 500 for crowns consumed and 700 to 1600 for crowns scorched) compared to partial- mortality (> 1900) and unburned (>3500) plots. We detected no saplings two years after the fire on high-mortality plots, and few on partial-mortality (0 to 700) and unburned (0 to 500) plots. Fifteen years after the fire, seedling density was not statistically distinguishable between levels of fire severity (overall median 5100). Sapling density was also not distinguishable between levels of fire severity (overall median 70). Live trees were sparse on high-mortality plots (0 to 10 for crowns consumed, 0 to 50 for crowns scorched), variable for partial mortality (40 to 850) and abundant on unburned plots (290 to 2040). Though recruitment of seedlings was abundant at all levels of fire severity 15 years after the fire, there has been little recruitment of saplings or trees.