Monitoring Change in Conifer Cover Across Biophysical Gradients in National Parks
Funding: National Park Service
Synopsis
The City of Rocks National Reserve (CIRO) is located at the biogeographic crossroads between sagebrush and pinyon-juniper dominated systems. Over at least the past two centuries, the sparse pinyon-juniper woodlands have expanded across the landscape and encroached upon large portions of the sagebrush communities in the reserve. This conifer expansion has similarly occurred in many portions of the sagebrush biome in the Great Basin, raising concern about loss of sagebrush-dependent species and prompting various tree removal strategies. The goals of this study were to quantify the rate and extent of change in pinyon-juniper and sagebrush communities at CIRO between 1950 and 2009 and to evaluate how rates of vegetation change over this time varied by biophysical setting. We estimated vegetation cover from a time-series of aerial photographs based on a stratified random sample of 340 plots distributed across key biophysical gradients of the study area. The results of the analysis revealed that in the absence of disturbance, the overall percent cover of woody, evergreen vegetation increased by nearly 7% across the study area, equating to an annual rate of increase of 0.12% per year over the 59 year time period. Low-elevation and mid-elevation evergreen forests exhibited the highest rates of encroachment at 0.45%/yr. and 0.35%/yr., respectively, with pinyon-juniper forests in particular exhibiting the highest rate of encroachment among vegetation types at 0.37%/yr. Accounting for the effects of the widespread disturbances (namely fires) that occurred in and around CIRO between 1950 and 2009 and were observed in 23% of the sample plots, the overall extent of grassland and shrubland areas increased during the period of observation. However, late-successional woodlands increased in area by over 100% during the 59 year time period. Over the past 4,000 years, the sagebrush and pinyon-juniper ecotone is known to have shifted repeatedly up and down slope in association with climate fluctuations and changes in fire regimes. The relative roles of natural climate factors and human effects of livestock grazing, fire exclusion, woodland clearing, and climate warming on conifer expansion over the last century are not well known. Hence, the concept of “natural range of variation” offers little guidance for management of such biogeographic crossroad syste
Collaborators
- Andy Hansen, Montana State University
- Tom Rodhouse, NPS Upper Columbia Basin Network
- Lisa Garrett, NPS Upper Columbia Basin Network
- Gordon Dicus, NPS Upper Columbia Basin Network
- Meghan Lonneker, NPS Upper Columbia Basin Network
- Kristin Legg, NPS Greater Yellowstone Network
- Wallace Keck, City of Rocks National Reserve