GEOSTATISTICAL ANALYSIS OF RESOURCE ISLANDS UNDER ARTEMISIA-TRIDENTATA IN THE SHRUB-STEPPE

Desert plants can influence the pattern of resources in soil resulting in small-scale enriched zones. Although conceptually simple, the shap e, size, and orientation of these ''resource islands'' are difficult t o study in detail using conventional sampling regimes. To demonstrate an alternative approach, we sampled soil under and around individual A rtemisia tridentata (sagebrush), a dominant shrub of cool desert envir onments, and analyzed the samples with univariate statistics and geost atistics. Univariate statistics revealed that soil variables like tota l inorganic-N, soluble-C, and microbial biomass-C were distributed wit h highest mean values within about 25 cm of the plant axis and signifi cantly lower mean values at distances beyond 60 cm. However, such simp le analyses restricted our view of resource islands to identically siz ed, symmetrical accumulations of soil resources under each plant. Geos tatistics provided additional information about spatial characteristic s of soil variables. Variography revealed that samples separated by a distance of less than about 70 cm were correlated spatially. Over 75% of the sample variance was attributable to spatial variability. We mod eled these spatial relationships and used kriging to predict values fo r unsampled locations. Resulting maps indicated that magnitude, size, and spatial distribution of soil resource islands vary between individ ual plants and for different soil properties. Maps, together with cros s-variography, further indicate that resource islands under A. trident ata are not always distinguishable from the surrounding soil by sharp transition boundaries and may be asymmetrically distributed around the plant axis.