Above- and belowground microenvironmental attributes cluster to varying degrees around Wyoming big sagebrush canopies

Abstract

Microsites beneath shrub canopies in dryland landscapes characterized by a mosaic of bare ground and patchy vegetation are often considered critical for the establishment of understory species. “Shrub islands” can create more favorable environmental conditions for both early plant establishment and longer term persistence near the shrub canopy relative to the interspace area between shrubs. However, the degree to which favorable conditions cluster around canopies and whether specific environmental attributes remain beneficial at intermediate distances between the canopy maximum and interspace microsites is understudied. If environmental attributes do not strongly cluster around shrub canopies, the interspace region beyond canopies may provide at least partially favorable conditions for establishment or persistence of understory plants. We hypothesized that favorable aboveground environmental attributes would more tightly cluster around sagebrush canopies than would favorable belowground attributes. At four sites across the Intermountain West, USA we sampled aboveground (i.e., solar radiation and vapor pressure deficit) and belowground (i.e., soil organic matter, moisture content, potassium, phosphorus) attributes at four microsites associated with big sagebrush (Artemisia tridentata Nutt.): canopy edge, maximum interspace distance between neighboring sagebrush canopy edges, and two intermediate distances between these extremes (25% and 50% of the intervening distance). Despite high site-to-site variation of all attributes, solar radiation (aboveground attribute) and soil potassium (belowground attribute) levels generally changed abruptly between the canopy and nearest (25%) interspace microsite, indicative of clustering around the canopy. Interspace levels for the belowground soil attributes, organic matter and soil moisture, as well as the above-ground attribute vapor pressure deficit, tended to change more gradually with distance from canopy, whereas phosphorus did not differ significantly across microsites. Overall, our results reveal that some elements of the favorable shrub canopy microenvironment are concentrated near the canopy, whereas levels of other attributes attenuate more gradually with distance from canopy. Together these results suggest that moving beyond a “canopy versus interspace” dichotomy and independently considering the spatial distributions of different attributes could improve our understanding of shrub island dynamics.