Percolation theory provides a formalism under which to identify critical densities of tree cover. Below the critical density, trees are distributed sparsely across the landscape. As the density approaches the critical density, larger and larger clusters of trees should be found, until massive clusters of trees dominate the landscape. Although the critical density is well known for simple random maps, it is not known for real landscapes created by non-random processes.
We at the Milne Landscape Ecology Lab are applying the formalisms of percolation theory to Pinyon-Juniper woodlands and their interface with grasslands. We are developing a spatially-explicit database of woodlands, from USGS digital elevation models and aerial photographs. A sample extract from an aerial photo is shown above. We have a list of study sites for which we have obtained DEMs and aerial photos. In time we will make these data available to interested researchers. Please send requests for access to these data to pj@algodones.unm.edu.
B. Modeling growth response
Zelig is an individual-based simulation of forest growth. We are using results from Zelig to make predictions about critical sets of environmental factors, such as terrain and available soil moisture, on locations of ecotones. At this point, we're just calibrating the model to provide reliable predictions of growth response. In time, we will use the results of the model to generate artificial terrain maps and test the hypothesis that location of ecotones is regulated by environmental conditions.
Native American peoples, including Navajo, Ute, Shoshone, and Paiute tribes, made extensive use of P. edulis and J. monosperma for at least two millenia (Leitch 1975). Plant fibers were used as fuelwood, in baskets, dyes, textiles, and ceremonies (Ebeling 1986). The edible fruits were used for cosmetic and medicinal purposes. Pine-nut soup was used to nourish motherless infants. The harvest of pine nuts was an occassion for festivity. Excesses were buried to ensure an adequate winter food supply. Pine pitch was chewed to cure sore throats, and was also used as an adhesive (Murphey 1990).
Many other animal species find refuge in Pinyon-Juniper woodlands, including mule deer, white-tailed deer, elk, the desert cottontail (Burns and Honkola 1990), the Pinyon jay, and Clark's nutcracker (Balda 1987).
Burns, R.M. and B.M. Honkola, eds. Silvics of North America: Volume 1, Conifers. U.S. Department of Agriculture, Forest Service: Washington, DC.
Ebeling, W. 1986. Handbook of Indian Foods and Fibers of Arid America. UCLA Press: Berkeley, CA.
Leitch, B.A. 1975. Chronology of the American Indian. Scholarly Press: St. Clair Shores, MI.
Murphey, E.V.A. 1990. Indian Uses of Native Plants. Meyerbooks: Glenwood, IL.
Tausch, R.J., N.E. West, and A.A. Nabi. 1981. Tree age and dominance patterns in Great Basin Pinyon/Juniper woodlands. J. Range Man. 34:259-264.
West, N.E., K.H. Rea, and R.J. Tausch. 1975. Basic synecological relationships in pinon-juniper woodlands. in G.F. Gifford and F.E. Busby, eds. The Pinyon-Juniper Ecosystem: A Symposium. Utah State University, Logan UT.