The effects of climate and disturbance on Madrean pine-oak forests in Mexico's Sierra Madre Occidental

In this study, the fire regime within the Las Bayas Forestry Reserve was described and patterns of tree regeneration in relation to variability in climate, the fire regime, and human land-use were identified. Fire frequencies varied temporally for all fires, with the largest changes occurring in the 20th century. Significantly more fires with shorter fire return intervals occurred within the Reserve from 1900 to 1950 than from 1950 to 2001. However, the frequency of widespread fire years (25% filter) was unchanged over time. Fires were synchronized in widespread fire years (25% filter) by regional climate. Widespread fire years occurred during dry years that lagged wet years. Widespread fire years lagged the negative El Nino phase (wet winters) of the Southern Oscillation by one year, but were not synchronized by the positive, La Nina phase (dry winters) of the Southern Oscillation. The smaller, more localized fires that occurred during the first 50 years of the 20th Century were attributed to changes in land tenure with the introduction of the ejido system in the early 1950s. Ejido management strategies lowered fire frequencies by suppressing fires and lowering anthropic fire ignitions.; Conifer tree regeneration occurred in three regeneration pulses during the time period from 1750-2001 (mid-20th century, late 1800s, and early 1800s). In the past, conifer seedlings established during periods of favorable climate that coincided with fire free (fire safe) periods that apparently followed a severe fire that killed many overstory trees and removed forest floor competition creating suitable conditions for seedling establishment. Currently, new seedlings are not establishing under the canopy of forests within the Las Bayas Forestry Reserve although many areas have not burned for two decades or more. Climate appears to be conducive as seedlings (both conifers and angiosperms) are regenerating in sufficient numbers in many recently disturbed areas, just not under the canopy of overstory trees. I suggest that a disturbance such as a severe fire that removes resource competition is required in combination with favorable climate for multi-year periods to successfully reproduce Madrean pine-oak forests. Climate alone does not appear to stimulate tree reproduction in these forests.