Fire History, Woodland Structure, and Mortality in a Piñon-Juniper Woodland in the Colorado National Monument

Abstract

The Colorado National Monument (COLM), on the northeastern edge of the Uncompahgre Plateau, supports a persistent Piñon (Pinus edulis Engelm.) — juniper (Juniperus osteosperma (Torr.) Little) woodland, which has not been disturbed by large stand-replacing fires since modern fire records began. We examined the fire history of large (> 100 ha) stand-replacing fires, documented tree population structures, and characterized tree density, quadratic mean diameter (QMD), relative composition, and cumulative mortality using 431 -0.1-haplots distributed over 1600 ha of the Monument. We found no evidence of large stand-replacing fires (charred wood or truncated stand structures) in the study area. Stand ages inferred from size structures suggest that large stand-replacing fires have been absent for possibly a millennia. Tree population structures show a more stable stand structure for juniper; Piñon pine population structures show a more recent and sustained regeneration pulse. Cumulative mortality of Piñon pines was 18%, peaking at 47% in trees 20 – 24.5 cm diameter. Spatial patterns of juniper density, QMD, and mortality were more homogeneous than those of Piñon pine. Results suggest temporal dynamics and spatial patterns of the COLM woodland are more influenced by drought and small fires (< 10 ha) than large fires (> 100 ha). This study provides important baseline data for changes that may be brought about by climate change in coming decades. It also stresses the importance of controlling cheatgrass (Bromus tectorum) and other invasive species to increase resistance of these persistent Piñon-juniper woodlands to future fires.