Evaluating Juniper Cover Change from 1936-1997 in the Wupatki Area using Repeat Aerial Photography

Parker, Cynthia

There is general consensus among scientists that there has been an increase in the range and density of pinyon/juniper (PJ) woodlands across the west over the last 150 years. In many locations, the pinyon and juniper trees have been recorded as moving into areas that were historically open grasslands. As trees move into open grassland areas, important ecological changes begin to occur. For example, while elk and mule deer are commonly found in wooded areas, pronghorn show a distinct preference for open grassland areas. In Wupatki National Monument (NM), a unit of the National Park Service (NPS) located in north central Arizona, pronghorn are considered a species of concern. Recent studies in Wupatki NM, and the adjacent areas of the Coconino National Forest (NF), generally agree that there has been an increase in the range and density of regional PJ woodlands. Quantifying the change that has occurred would be useful to the National Park Service natural resource managers. Continuous quadrats with a one hectare grid size, and a classification system that defined six levels of canopy cover, were applied to two sets of aerial photographs to quantify the change. The first set of air photos was taken in 1936 by the Soil Conservation Survey (SCS); the second was taken in 1997 by the United States Geological Survey (USGS). This effort was designed to be a landscape-scale study, and examined 13,490 hectares. Approximately half of this area is within Wupatki NM, while the other half is within the Coconino NF. The results of this study show that approximately 27% of the 1936 landscape (3,695 ha) was open, treeless grassland. By 1997, this area had been reduced to 9.5% of the landscape (1,290 ha). A full two-thirds of the previously treeless grasslands in 1936 had obtained some number of trees by 1997. The rest of the landscape showed various amounts of canopy cover increase. The greatest increase occurred in the closed woodland designation, which revealed an increase in area of just under 7%. This study suggests there is marked instability at the grassland end of the ecological spectrum in the study area, while there is considerable stability at the wooded end. Information of such historical trends are useful to land managers charged with ecological preservation. Decisions on issues such as grazing and fire management may be informed by the results of this study.

Evaluating Juniper Cover Change from 1936-1997 in the Wupatki Area using Repeat Aerial Photography

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