Spatial and temporal patterns of recent forest encroachment in montane grasslands of the Valles Caldera, New Mexico, USA

Aim Recent forest encroachment into montane and subalpine grasslands has occurred in the Rocky Mountains and many other mountain ranges globally. The timing, rate, and extent of tree invasion can depend on interactions among topography, positive spatial feedbacks, and temporally variable factors (especially climate, grazing, and fire). Here we examine spatial and temporal patterns of tree invasion in the Valles Caldera of the Jemez Mountains. Location This study was conducted in the Valles Caldera (35 degrees 50'-36 degrees 00' N; 106 degrees 24'-106 degrees 37' W), a 24-km-wide volcanic basin in northern New Mexico, USA. Grasslands in this otherwise forested region occur in broad valley bottoms of the caldera floor between 2575 and 2700 m, and on south-facing slopes and mountain tops up to 3300 m. Methods We used a GIS analysis of orthorectified aerial photos taken in 1935 and 1996, covering a 40,000-ha study area, to quantify the extent of tree invasion and to assess its relationship to spatial factors. We obtained dates of establishment from 299 increment cores and basal disks from 50 sites in the Valles Caldera National Preserve (VCNP) to reconstruct temporal patterns of tree invasion. Results The area of grasslands in our study area declined from 11,747 to 9336 ha (nearly 18%) between 1935 and 1996. Tree invasion increased with slope, elevation, and proximity to the previous tree line, but showed no relationship to aspect. Tree invasion was more rapid and continuous on upper mountain slopes, while the invasion of valley-bottom grasslands below reversed tree lines was more episodic, and appeared to track mean summer minimum temperatures. Main conclusions The rapid and continuous invasion of steep, high-elevation slopes suggests that frequent fire was the single most important factor in maintaining grassy communities in these sites. The slower, episodic invasion of valley-bottom grasslands, and the apparent relationship between increased invasion and years of higher summer minimum temperatures are consistent with the hypothesis that these grasslands have been maintained by low temperatures or frosts damaging to tree seedlings. We encourage prescribed fire to restore and maintain grasslands in the VCNP, especially small patches on steep, high-elevation slopes.