Tree recruitment in relation to climate and disturbance in northern Mexico

Background/Question/Methods

A documented shift away from historical pre-Euro-American settlement structure provides the scientific justification for manipulating forest structure in extensive areas of montane forests throughout the U.S. Southwest. These changes are a product of confounding effects of disturbance, climate, species competition, and modern land-use history. The single greatest forest reproduction event in the Southwest occurred in the early 20^th century, a climatically wet period following a period of widespread fire exclusion. This pronounced reproduction event is at the heart of increased forest density and current restoration efforts. Yet understanding of the ecological processes driving that event and others like it remains difficult due to confounding factors of climate variability, land use changes, and fire suppression policies in nearly all U.S. forests. Separation of these interacting factors was possible in the Sierra San Luis of northern Mexico. We investigated climate, fire, and tree recruitment in this area with a spectrum of land-use changes from areas with little or no history of grazing or logging, to sites that were logged (early 1950s) and/or grazed (beginning in the early 1930s). Fire regimes and tree recruitment clearly reflected these differences.

Results/Conclusions

Fires were strongly tied to interannual wet-dry cycles of climate, whereas recruitment peaks were more closely tied to local processes than broad-scale climatically wet conditions. The greatest pulse of tree recruitment coincided with a pronounced mid-century drought (1942-57) and a period of reduced fire frequency. The second largest pulse of recruitment (ca. 1900) preceded a well documented period of recruitment (and an anomalously wet period) elsewhere across the Southwest in the 1910s-20s. This early 20^th century recruitment event in the Sierra San Luis coincided with specific fire-free periods during periods of below average precipitation. We also found greater spatial dependence and clustering in older age-classes of trees. This spatial pattern indicates a legacy of fire-induced mortality in shaping stand structure, underscoring the importance of frequent fire effects on spatial variability in forests. Stand structure was largely driven by fire-induced mortality of new recruits, with current even-age patches of pines reflecting survival of seedlings rather than tree recruitment following rare stand-replacing fires.