Mixed conifer forests in the Sierra San Pedro Martir, Mexico: What makes them so resilient to drought and fire

Background/Question/Methods

Knowledge of the ecological effect of wildfire is important to ecologists and resource managers, especially from forests in which past anthropogenic influences, e.g., fire suppression and timber harvesting, have been limited. Changes to forest structure and regeneration patterns were documented in a relatively unique old-growth Jeffrey pine-mixed conifer forest in northwestern Mexico in the Sierra San Pedro Martir (SSPM) after a July 2003 wildfire. This forested area has never been harvested and fire suppression did not begin until the 1970s.

Results/Conclusions

Fire effects were moderate especially considering that the wildfire occurred at the end of a severe, multi-year (1999-2003) drought. Why were the forests in the SSPM able to incorporate drought, insects, and wildfire without producing catastrophic mortality? Research suggests that heterogeneity in spatial patterns of forest structure and fuels are critical for a resilient forest. High variability characterizes all live tree, snag, fuel, and coarse woody debris structures in the forests of the SSPM. Spatial forest structure was random to slightly clumped pre and post wildfire and Jeffrey pine dominance increased after fire. Seedlings were spatially clumped pre-fire and this was maintained after fire and most surviving seedlings were isolated from overstory cover and shrubs where fuel loads would be relatively low. This wildfire enhanced or maintained a patchy forest structure which lead to high resiliency; similar spatial heterogeneity should be included in US forest restoration plans. Most US forest restoration plans include thinning from below to separate tree crowns and attain a narrow range for residual basal area/ha. This essentially produces uniform forest conditions over broad areas that are in strong contrast to  resilient SSPM forests. In addition to producing more spatial heterogeneity in restoration plans of forests that once experienced frequent, low-moderate intensity fire regimes, increased use of managed wildfire could also be implemented at broader spatial scales to increase forest resiliency. Federal policy allows wildfires to be managed for multiple objectives. Managed wildfire allows a fire to incorporates a full range of responses, from aggressive suppression to less intense responses to minimize suppression costs and increase forest resiliency.