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Aspen Regeneration Across the Southern End of the Cerro Grande Burn
Stephen Fettig, Wildlife Biologist, Bandelier National Monument, Los Alamos, New Mexico
In the spring of 2000, the Cerro Grande fire burned nearly 19,000 ha (48,000 acres) of aspen (Populus tremuloides) and mixed conifer forest on the east side of the Jemez Mountain in northern New Mexico. During winter, the southern part of the burned area can host many of the mountain range's 4,000 elk (Cervus elaphus). Previous work has documented a range of aspen regeneration heights and browse intensities due to this elk population. To quantify the post-fire variation in aspen regeneration and to examine the relationship between site characteristics and the level of browse, we focused on the southern portion of the burned area. In September 2002, we surveyed 58 random 10 m x 10m (33 ft x 33 ft) plots in the study area. We recorded the maximum sprout height; number of sprouts (up to a maximum of 200); sprout condition (browsed or not browsed); elevation, slope, aspect; dominant pre-fire overstory tree species; estimated current percent canopy cover; smallest Diameter at Breast Height (DBH) live tree-size (> 4 cm DBH) aspen; and elk and deer pellet groups. For plots with aspen sprouts, we observed a correlation between the percent browsed shoots and measured maximum height after 3 years (r2 = 0.45, n = 31, F = 23.37, p < 0.0001). Maximum sprout height was correlated with UTM Easting (r2 = 0.21, n = 31, F = 7.66, p = 0.0097), while inversely correlated with UTM Northing (r2 = 0.15, n = 31, F = 5.12, p = 0.0313) and elevation (r2 = 0.15, n = 31, F = 5.29, p = 0.0289). Northing and slope together explained the measured maximum shoot heights after 3 years better than other predictors (r2 = 0.31, n = 31, F = 6.36, p = 0.0053). The impact of elk browsing on aspen regeneration on gentle slopes in the northeastern part of the study area is clearly exemplified by aspen sprouts over 4 m (12 ft) tall within protective exclosures four years after the fire, while outside elk browsing is preventing aspen sprouts from getting much above ankle-level. In one high burn-intensity area, where the Cerro Grande fire killed all trees, aspen spout density has dropped from several thousand sprouts/ha in late 2000 to nearly zero/ha over a 3.1-ha (7.7-acre) area in 2004 years. Considering that annual elk ranges in the eastern Jemez Mountains are most concentrated on the Valles Caldera, our observations suggest slope may be as important as distance from the Caldera in determining levels of browse on aspen sprouts. Because of the highly browsed structure and decreasing spout density of some aspen clones on gentle slopes, future work should examine future sprout density and heights across the range of sites characteristics. Such information may provide the data needed to make longer-term predictions about aspen regeneration in the study area.