Spatial Patterns of Pinyon–Juniper Woodland Expansion in Central Nevada

The expansion of the pinyon–juniper (Pinus monophylla Torr. & Frém.–Juniperus osteosperma Torr.) woodland type in the Great Basin has been widely documented, but little is known concerning how topographic heterogeneity influences the temporal development of such vegetation changes. The goals of this study were to quantify the overall rates of pinyon–juniper expansion over the past 3 decades, and determine the landscape factors influencing patterns of expansion in central Nevada. Aerial panchromatic photos (1966–1995) were used to quantify changing distribution of pinyon–juniper woodland, over multiple spatial scales (0.002-, 0.02-, and 0.4-ha median patch sizes), and for discrete categories of elevation, slope aspect, slope steepness, hillslope position, and prior canopy cover class. An object-oriented multiscale segmentation and classification scheme, based on attributes of brightness, shape, homogeneity, and texture, was applied to classify vegetation. Over the 30-year period, the area of woodland has increased by 11% over coarse, ecotonal scales (0.4-ha scale) but by 33% over single-tree scales (20-m2 scale). Woodland expansion has been dominated by infilling processes where small tree patches have established in openings between larger, denser patches. Infilling rates have been greatest at lower elevations, whereas migration of the woodland belt over coarser scales has proceeded in both upslope and downslope directions. Increases in woodland area were several times greater where terrain variables indicated more mesic conditions. Management treatments involving removal of trees should be viewed in a long-term context, because tree invasion is likely to proceed rapidly on productive sites.