Migration dynamics of plant communities in the Texas Nature Conservancy Davis Mountain Preserve

Although considerable research has been conducted on Pinyon-Juniper ecosystems of the western United States, information regarding this ecosystem within west Texas is generally lacking. Anecdotal records suggest an observable expansion of the pinyon-juniper communities in the past 60 years, but no documentation exists to support this claim. The objectives of this study were to examine changes in spatial distribution of plant communities and associated ecotones in the Texas Nature Conservancy (TNC) Davis Mountain Preserve through the assessment of cover types using historic aerial photography. In addition, the rate of spread and rate of loss (change in areahime) were determined for the expanding plant communities and retracting plant communities, respectively. Panchromatic aerial photos for 1982, as well as color infrared DOQQs (Digital Orthorectified Quarter Quadrangle) for 1996 and 2004 were visually assessed for cover type by defining each 50 m by 50 m square based on density classes. For all three imagery sets, the cover types were assessed as openings, open/woodland ecotones, or woodland. In addition, cover types were more narrowly defined as either openings, open/woodland ecotone, open woodlands, dense woodlands, pine/woodland ecotone, and pine for the 1996 and 2004 image sets. The vegetation distribution of the Davis Mountains Preserve has progressively shifted in favor of Mexican pinyon (Pinus cembroides Zucc.) and Alligator juniper (Juniperus deppeana Steud.) at the expense of Ponderosa pine (Pinus ponderosa P. & C. Lawson) and grasslands. The openings and the open/woodland ecotones decreased by 68% and 88% from 1982 to 2004, respectively. The pine/woodland ecotone and the pine forests declined 36% and 46% between 1996 and 2004, respectively. The expansion of the woodlands is causing the openings and pine forests to become more fragmented. Some of the changes reported here can be partially attributed to a wildfire in 1988, a western pine bark beetle (Dendroctonus brevicomis LeConte) infestation in 1996, and droughty conditions within the 1990's.