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Relative nutrient uptake efficiency was measured using a dual-radiotracer ((super 32)P and (super 33)P) technique for Agropyron desertorum, Artemisia tridentata, and Pseudoroegneria spicata, species from the arid shrub-steppe of the Intermountain Region in the western U.S. These species differed in the timing and magnitude of their response to experimentally enriched soil microsites. The most rapid and significant increase in the uptake of phosphorus tracer from enriched microsites was exhibited by Agropyron, followed by Artemisia, while there was only a trend toward an increase in uptake for Pseudoroegneria. The volume of soil enriched was only 4% of the total pot volume yet Agropyron extracted more phosphorus from the enriched microsite than was removed from the remainder (96%) of the soil volume based on tracer uptake from enriched and unenriched microsites. For Agropyron and Pseudoroegneria, plant uptake of phosphorus from microsites was approximately proportional to the soil microsite enrichment and all three species exhibited increased root length density within the microsites in proportion to the concentration of available nutrients. Integrated physiological and morphological response of the plants to nutrient enrichment increased phosphorus uptake, normalized to the amount of phosphorus available in the soil microsites, by 19.7 to 97.5 x compared to uptake from a similar volume of unenriched soil. Preferential allocation of recently assimilated carbon to roots within enriched soil microsites was also demonstrated. The most important differences detected among species were associated with the temporal flexibility they exhibited in response to enrichment of soil microsites. Differences in flexibility and response among these species have been reported for other aspects of nutrient uptake, suggestive of species-specific differences contributing to demonstrated differences in productivity, persistence, and competitive ability.

ANTONIO M. MANGIONE [1] M. DENISE DEARING [2] Abstract. Plant secondary metabolites (PSMs) can affect survival, reproduction, and distribution of herbivores. Individuals with a high capacity to tolerate PSMs will [...]

Soils that develop on areas of hydrothermally altered rock in the western Great Basin of Nevada support Sierran conifers, such as Pinus ponderosa, in a desert climate. These soils have low pH ([is nearly equal] 3.7) and low concentrations of [HCO.sub.3.sup.-]-extractable P (5.6 [microgram]/g), compared to adjacent brown desert soils supporting sagebrush (Artemisia tridentata) vegetation. The occurrence of Sierran conifers appears to be related to their tolerance of the unusual soil chemistry, high nutrient-use efficiency in growth, high reabsorption of foliar nutrients before leaf abscission, and slow growth rates. Most of these traits appear to be inherent characteristics of evergreen vegetation, and show little acclimation to the nutrient-poor conditions on hydrothermally altered rock. In contrast, plants of the Great Basin sagebrush vegetation are physiologically intolerant of the unusual soils that develop from hydrothermally altered rock and are excluded from such sites.