Interacting Effects of Fire Activity, Climate, and Habitat Diversity on Forest Dynamics, El Malpais National Monument, New Mexico, USA

The historic effects of fire and climate on stand dynamics in the ponderosa pine-dominated forests of the American Southwest are of increasing concern to land management agencies. Using present forest stand structure, the mixed-conifer forests of the volcanic features in El Malpais National Monument were analyzed at three separate sites: a cinder cone, an ancient basalt flow, and an isolated “island” (kipuka) completely surrounded by basalt flows. Increment cores were collected from 632 trees in 19 plots within the monument to obtain dates of establishment. These dates were compared with historic fire histories and precipitation records to analyze the effects of both fire and climate-forcing mechanisms on establishment of several tree species within each forest. Results show an increase in tree establishment that coincides with increased precipitation in the 1800s. Fire regimes were also altered, shifting from a predominance of small, patchy fires to that of larger, more widespread fires. Over the past 350 years, rates of tree establishment have responded positively to increased precipitation and longer fire intervals, and negatively to periods of anthropogenic disturbance (i.e. intense livestock grazing and logging). The fire suppression era (beginning ca. 1940) caused large shifts in species composition, with influxes of fire-intolerant species, such as pinyon pine, juniper species, and Gamble’s oak. Variability was seen in species establishment at spatial, temporal, and inter-habitat levels in response to these external forcing mechanisms, as the cinder cone site appeared to be more reactive to shifting fire regimes and the basalt flow site appeared to be more reactive to precipitation levels over time. This study supports a shift in forest management to promote ecosystem resilience within present stands, rather than attempting to mirror the conditions of previous analogous periods, as current forest structure is now non-analogous and will require unique management practices.