Characterize Southwestern U.S. Pinon-Juniper Woodlands Final Report USGS-BRD/NPS NRPP-Research (CSU-87/106)

Jacobs, Brian; Romme, William; Allen, Craig

Abstract Pinon-juniper woodlands are widely distributed on millions of acres across the southwestern U.S. Recent, historical changes in woodland occurrence and stand density have been variously interpreted as continued adjustment to Holocene climate, recovery from harvest (historic and pre-historic), succession after fire, drought, insect, and/ or disease induced mortality events, or response to grazing practices, altered fire regimes, elevated temperatures and CO2 levels. The broad ecological amplitude of southwestern pifion-juniper woodland tree species is demonstrated by the wide range of sites on which these trees can grow. Conversely, these species are extremely sensitive to fire disturbance and easily killed; drought, insect, and disease mortality can also have large effects on woodland stand dynamics, structure, and local distribution patterns. We suggest that while ecological amplitude and regional climate set broad limits on potential woodland species distribution, environmental factors and associated disturbance regimes formerly constrained the local extent and expression of woodland species occurrence on the landscape. If competitive interactions with understory, and disturbances like periodic surface fire, formerly restricted woodlands to a more limited range of sites (i.e., through competition with grasses and shrubs or fire avoidance) we could expect to detect meaningful relationships between "old" or persistent woodland stands (i.e., pre-dating Euro-American settlement) and suites of environmental site parameters. Delineating "old" or persistent stands, on the basis of semi-quantitative and qualitative characters using a diagnostic key, and relating the occurrence of these aged stands to environmental site conditions through spatial modeling efforts, would potentially provide a means for predictively mapping "older" (i.e., pre-settlement) stands, and thereby delineate "younger" (i.e., post-settlement) woodlands to facilitate appropriate and targeted management actions.