COS 130-2
Dead wood change in a dry coniferous forest over time: Wildlife habitat and fuel management considerations
Dead trees play an important role in forests, with snags and coarse woody debris used by many bird and mammal species for nesting, resting, or foraging. However, wood is also combustible and too much dead wood contributes to extreme fire behavior. This tension between dead wood as habitat, and dead wood as fuel, has raised the question of how much is appropriate in fire-dependent forested ecosystems. Three plots installed in mixed conifer forest in the Stanislaus-Tuolumne Experimental Forest of the central Sierra Nevada in 1929 provide a valuable historical perspective about how amounts and sizes of dead wood have changed through time as a result of logging and fire exclusion. Diameter of snags was measured and coarse woody debris (CWD) mapped in the old-growth condition, prior to logging using three different methods. Snags were re-measured in 2007 or 2008, and CWD was re-mapped in 2012. Median historical fire return interval was 6 years, with the last fire occurring in 1889.
Results/Conclusions
Density of snags increased from 15.2 ha-1 in 1929 to 141.2 ha-1 in 2007/08. However, average snag size declined, with 72% and 22% of snags classified as medium or large (>38.1 cm diameter) in 1929 and 2007/08, respectively. The plot logged the lightest contained the most large snags. Mechanisms of tree mortality appear to have changed with more dying small, possibly as a result of competition and excessive live tree density. CWD mass did not differ significantly between 1929 and 2012, with increased mortality and lack of periodic consumption by fire apparently compensating for the reduced inputs of large wood due to past logging. Number of logs increased from 28 to 76 per ha and average size declined. Because larger-sized dead wood is preferred by many wildlife species, the current configuration of more, smaller, and more decayed woody pieces likely has a lower ratio of habitat value relative to potential fire hazard than it once did. To restore dead wood to conditions more like those found historically will require growing larger trees and reducing the rate of small tree death. Fire, which preferentially consumes smaller and more rotten wood, may also help shift the balance to larger pieces.