Shading patterns along a low- to high-density saguaro gradient: an ecohydrological perspective

Background/Question/Methods In arid and semiarid ecosystems, woody plants that comprise the overstory produce shading patterns that can have important ecohydrological consequences for both understory vegetation and water balance dynamics. Several recent studies highlight how trends into increasing woody plant cover translate in trends in near-ground solar radiation as a function of woody plant cover, including trends in the mean and in the spatial variance. In Sonoran desert upland, saguaro cacti (Carnegiea gigantea) can be the largest stature overstory plant and can be present in a range of densities from low to high. The architecture of these cacti, of course, differs from that of shrubs or trees, but nonetheless provides shading patterns. Although shading by saguaro is likely to be less than that from many shrubs or trees, the shading that they provide may still be important in an ecohydrological context, especially as saguaro density increases. Our objective was to address this issue by quantifying how shading patterns vary along a gradient from low to high saguaro cover. We established 10 m transects in each of 4 different densities of saguaro cactus and obtained hemispherical photos 1 meter intervals, which we then analyzed to assess trends in near ground solar radiation and how it varies spatially across a gradient.

Results/Conclusions We initially focused on mean annual near-ground solar radiation, expressed as the Direct Site Factor (DSF), where 1.0 corresponds to a completely open location with no shading and 0.0 corresponds to a completely shaded location. At this level of temporal aggregation, mean DSF did not exhibit a pronounced decrease with increasing saguaro density; all 4 sites had a mean DSF of greater than 0.9. The largest variance was associated with an intermediate density, as expected and similar to findings in other systems. For some estimates, neighboring woody plants such as mesquite and palo verde may have confounded results to some degree. The effects of increasing density, of course, become more pronounced for periods with lower sun angles, both seasonally and within the diurnal cycle. Our results quantify the degree to which saguaro cacti effect ecohydrological microclimate and highlight a similarity effect between shading patterns along gradients of sagauaro density and along those for other woody plants.