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USGS Navajo Land Use Planning Project (NLUPP)

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Vegetation investigations include:

  • Plant communities, distribution
  • Surface erosion
  • Medicinal plants
  • Invasive species
Part of our research objectives is to monitor landscape and vegetation changes resulting from current drought conditions, and to evaluate landscape and vegetation changes and their timing relative to changes in climate and land use practices.


Project volunteers examining plant cover on the Navajo Nation
Bimodal precipitation in the southwest is characterized by winter snowfall and summer monsoons that have different relative effects on the survival and germination of shrubs vs. C4 grasses (Neilson, R. P., 1986). Yet, the general shift in landscape conditions, from grass dominated to shrub dominated that has been well documented in the southwest, is often attributed to desertification and overgrazing (Humphrey, 1958; Savage and Swetnam, 1990). In addition, changes in land use practices on the Navajo Nation that began in the mid- 1970’s, from sheep (that forage on shrubs and grass) to cattle (that graze on grass almost exclusively) may contribute to a decrease in grass vs. shrub vegetation cover. Shrub dominated surfaces with a low relative percentage of surface cover provide less surface stabilization, and are more vulnerable to erosion by wind and water (Schlesinger and others, 1990). A comparison of satellite imagery from wet years in the early 80’s, and successive drought years will help to delineate trends in vegetation change and landscape change related to drought. Recent environmental changes that could contribute to increased drought severity and desertification include 1) changes in the types of precipitation, and timing and intensity of events 2) resulting changes in groundwater and surface water interactions and recharge, and 3) landscape and vegetation response to these changes. The current drought, as well as the droughts of 1908-1913, 1934-1935, 1951-1956, 1963-1965, were generally dry in both winter and summer, but also accompanied by short bursts of intense summer rain (Neilson, 1986). These traits, characteristic of drought in the past century, may become a more general characteristic of southwest precipitation with Global Warming (Trenberth, 1998). Although local climatic conditions are usually determined by temporally and spatially averaging precipitation patterns, sessile organisms, and the ecosystem in which they inhabit respond to the timing and sequences of weather events. Plants that provide surface stabilization require the successful completion of every stage in their life history, each of which is sensitive to different frequencies of meteorological variability (Neilson, 1986). The current drought in northeastern Arizona is characterized by less overall precipitation, but is accompanied by increased temperatures beginning in 2000, and a relatively late monsoon compared to earlier observations by Humphrey (1958) and Douglas and others, (1992). These current environmental trends, resulting in both drought and intermittent floods, will be examined in order to understand climate-landscape interactions that contribute to drought severity in the southwest.
Grasslands on the Navajo Nation
Grasslands on managed lands of the Navajo Reservation.
(images source: Margaret Hiza-Redsteer, USGS Flagstaff, AZ)

Douglas, M. W., Maddox, R. A. Howard, K., and Reyes, S., 1992, The Mexican Monsoon: Journal of Climate, v. 6, p. 1665-1677.

Humphrey, Robert R., 1958, The desert grassland: A history of vegetational change and analyses of causes: Botanical Review, v. 24, p. 193-252.

Neilson, R. P., 1986, High-resolution climatic analysis and southwest biogeography: Science, v. 232, p.27-34.

Savage, M. and Swetnam, T. W., 1990, Early 19th century fire decline following sheep pasturing in a Navajo ponderosa pine forest: Ecology, v. 71, p. 2374-2378.

Schlesinger, W. H., Reynolds, J. F., Cunningham, G. L. Huenneke, L. F., Jarrel, W. M., Virginia, R. A., and Whitford, W. G., 1990, Biological feedbacks of global desertification: Science v. 247, p.1043-1048.

Trenberth, K. E., 1998, Atmospheric moisture residence times and cycling: Implications for rainfall rates and climate change: Climate Change v. 39, p. 667-694.

 

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