Shrubs create heterogeneity in resource availability, yet the influences of shrub age and size on these conditions in semiarid ecosystems is largely unknown. In order to inform restoration and conservation efforts in global shrub-steppe ecosystems that are currently imperiled, we assessed plant age-size relationships within an Artemisia tridentata stand in southern Idaho, USA, and examined the dependence of 2 resources, soil water content (SWC) and light at the soil surface (photosynthetically active radiation [PAR]), on size of individual plants in understory and interspace microsites. Results included a positive relationship between shrub age and size, a median shrub age of 19 years old, and shrub sizes that varied by more than 3 orders of magnitude (i.e., 0.001 m3 to 1 m3). Across this broad range in stand structure, PAR was significantly lower in understory than in interspace microsites, and it declined slightly with increasing shrub volume. Similarly, SWC declined faintly with shrub volume, but understory and interspace microsites did not differ. These findings indicate that resource heterogeneity created by shrubs is potentially dependent on shrub size within this ecosystem, and that variation in stand structure directly influenced resource heterogeneity between understory and interspace microsites. We suggest that routine monitoring of heterogeneity in stand structure could serve as a valuable indicator to assess site suitability for restoration activities and to make quantitative comparisons among sites to prioritize conservation efforts.
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Vol. 80 • No. 1