Journal
RANGELAND ECOLOGY & MANAGEMENT
Volume 62, Issue 3, Pages 203-222Publisher
SOC RANGE MANAGEMENT
DOI: 10.2111/08-188R1.1
Keywords
climate; CO2; drought; fire; grazing; tree infill; tree invasion; range expansion
Categories
Funding
- Bureau of Land Management
- US Dept of Defense
- US Joint Fire Sciences Program
- US National Park Service, the Uncompahgre Plateau Partnership
- US Forest Service
- US Geological Survey
- Nature Conservancy
- Colorado Forest Restoration Institute of Colorado State University (CSU)
- CSU and Universite Laval (Quebec, OC)
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Pinon-juniper is a major vegetation type in western North America. Effective management of these ecosystems has been hindered by inadequate understanding of 1) the variability in ecosystem structure and ecological processes that exists among the diverse combinations of pinons, junipers, and associated shrubs, herbs, and soil organisms; 2) the prehistoric and historic disturbance regimes; and 3) the mechanisms driving changes in vegetation structure and composition during the past 150 yr. This article summarizes what we know (and don't know) about three fundamentally different kinds of pinon-juniper vegetation. Persistent woodlands are found where local soils, climate, and disturbance regimes are favorable for pinon, juniper, or a mix of both; fires have always been infrequent in these woodlands. Pinon juniper savannas are found where local soils and climate are suitable for both trees and grasses; it is logical that low-severity fires may have maintained low tree densities before disruption of fire regimes following Euro-American settlement, but information is insufficient to support any confident statements about historical disturbance regimes in these savannas. Wooded shrublands are found where local soils and climate support a shrub community, but trees can increase during moist climatic conditions and periods without disturbance and decrease during droughts and following disturbance. Dramatic increases in tree density have occurred in portions of all three types of pinon-juniper vegetation, although equally dramatic mortality events have also occurred in some areas. The potential mechanisms driving increases in tree density-such as recovery from past disturbance, natural range expansion, livestock grazing, fire exclusion, climatic variability, and CO, fertilization-generally have not received enough empirical or experimental investigation to predict which is most important in any given location. The intent of this synthesis is 1) to provide a Source of information for managers and policy makers; and 2) to stimulate researchers to address the most important unanswered questions.
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