Appendix A. Site description.
The Bighorn Basin is enclosed by mountain ranges on all sides, with the Bighorn Mountains on the east, the Absaroka and Beartooth Ranges on the west, and the Owl Creek and Pryor Mountains forming the south and north ends, respectively (Table1, Fig. 1). Collective rainshadow effects from these ranges persist throughout the year, creating semi-arid conditions. Mean annual precipitation is 25.7 cm, but at lower-elevation interior locations, is as low as 16.1 cm (Gray et al. 2004). Driese et al. (1997) classified vegetation over most of the basin as greasewood shrubland characteristic of an arid and saline environment. Along with high salinity, many parts of the basin are underlain by bentonitic clays, which hinder the establishment of young plants (Knight 1994) and provide a relatively unfavorable environment for tree recruitment. However, limber pines (Pinus flexilis) and Rocky Mountain junipers (Juniperus scopulorum) occur in scattered patches throughout the basin. These species are found on steep, thin-soiled slopes with eroding sandstone or limestone exposures. Ponderosa pine co-occurs at low densities with these two other tree species at our four study sites (Fig. A1). All four sites have poorly developed rocky soils, with trees growing predominantly on sandstone or shale exposures (Table A1, Fig. A1) (Love and Christiansen 1985), on slopes ranging from 10 to >45 degrees. Ponderosa pine grows on all aspects in relatively equal proportions with the exception of Anchor Dam, where no north-or east-facing slopes occur within the area occupied by the population. The combination of low tree densities along with low densities of shrubs (mainly scattered Artemisia tridentata) and bunch-grasses creates a high proportion of bare rock and/or soil at the sites (Fig. A1).
Regionally, ponderosa pine occurs across much of north-eastern Wyoming (Fig. 1). Ponderosa pine grows in large continuous forests in the Black Hills and along the eastern slope of the Bighorn Mountains, with small scattered populations occurring throughout the intervening basins. Ponderosa pine is extensive in the southern portion of the western slope of the Bighorn Mountains, but becomes less frequent and occurs only in small scattered populations to the north.
TABLE A1. Geological characteristics of the four study sites (Love and Christiansen 1985).
|FIG. A1. Representative photographs of each study site. (a) Castle Garden. Note that ponderosa pine are restricted to the sandstone ridges and that the majority of the trees in view are limber pine or juniper. (b) Grass Creek. Note the scattered ponderosa pine along with limber pine and juniper all occurring at low densities. (c) Cottonwood Creek. Note the scattered ponderosa pine along with juniper and limber pine along the near hillsides. There is no ponderosa pine on the far hillsides, only juniper and limber pine. (d) Anchor Dam. The entire population of ponderosa pine is growing on the single hillside in view. The majority of trees in view, however, are limber pine or juniper. Note the sparse ground cover present in all the photos, and the complete absence of trees on most of the landscape. Photo credits: M. R. Lesser.|
|FIG. A2. (a) Boxplots of all distances between trees at each study population. (b) Boxplots of minimum distance between trees at each study population. For both (a) and (b) the box represents the 1st and 3rd quartiles. The whiskers extend to the largest/smallest data point that falls within 1.5 times the box length from the nearest edge. The thick black bar is the median point of the data. Outliers are represented by black squares.|
Driese, K. L., W. A. Reiners, E. H. Merrill, and K. G. Gerow. 1997. A digital land cover map of Wyoming, USA: a tool for vegetation analysis. Journal of Vegetation Science. 8:133146.
Gray, S. T., C. L. Fastie, S. T. Jackson, and J. L. Betancourt. 2004. Tree-ring based reconstructions of precipitation in the Bighorn Basin, Wyoming since 1260 A.D. Journal of Climate 17:38553865.
Knight, D. H. 1994. Mountains and plains: the ecology of Wyoming landscapes. Yale University Press, New Haven, Connecticut, USA. 338pp.
Little, E. L., Jr. 1971. Atlas of United States trees, volume 1, conifers and important hardwoods. U.S. Dept. of Agriculture Miscellaneous Publication 1146, Washington D.C., USA.
Love, J. D., and A. C. Christiansen. 1985. Geologic Map of Wyoming: U.S. Geological Survey Special Geologic Map, 3 sheets, scale 1:500000. USGS.
Rocky Mountain Herbarium Specimen Database. 2011. Rocky Mountain Herbarium, Dept. of Botany, University of Wyoming, Laramie, Wyoming, USA.