Impacts of fire exclusion and recent managed fire on forest structure in old growth Sierra Nevada mixed-conifer forests: Resampling of 1911 timber inventories

Background/Question/Methods

Past harvesting practices and livestock grazing, coupled with over a century of fire suppression have shifted the structure and composition of many dry, mid- to low-elevation forests throughout the western U. S. This shift is generally characterized by increased tree densities, smaller average tree diameters, increased proportions of shade-tolerant tree species, and elevated surface fuel loads relative to historical or pre-European settlement forest conditions. In this study we resampled areas included in systematic early U.S. Forest Service timber inventories that were conducted on what was then part of the Stanislaus National Forest, but subsequently became incorporated into Yosemite National Park. Our objectives were to: 1) identify usable historical timber inventory records, 2) locate historical inventory areas ‘on the ground’ and sample current forest stand conditions, and 3) perform statistical analyses comparing current to historical forest stand conditions, both overall and partitioned based on recent fire activity.

Results/Conclusions

Our results indicate substantially altered present forest conditions, relative to the 1911 data, and can largely be attributed to the disruption of the key ecosystem process for these forests, fire. For areas that burned recently there was a noticeable difference in forest structure based on fire severity. Current tree density and canopy cover in areas burned recently with moderate severity did not differ from 1911 estimates, while areas that burned recently with low severity or were unburned had higher tree density and canopy cover relative to the 1911estimates. This emphasizes an important distinction with regard to using fire to restore forests, resting primarily on whether fires kill trees in the lower and intermediate canopy strata. Our results also demonstrate nearly a doubling of live tree carbon stocks in the present forest compared to the historical forest. The findings presented here can be used by managers and ecologists interested in Sierra Nevada mixed conifer systems.