How low can you go? Soil depth gradients contribute to the elevational creosotebush-to-blackbrush transition in the Mojave Desert

Background/Question/Methods

Regional warming trends are expected to alter species ranges, shifting species assemblages upward and northward. Models primarily use temperature and precipitation to predict future vegetation distributions with little consideration of edaphic factors. In the Mojave Desert, shrub dominance changes along an elevational gradient from Larrea tridentata (creosotebush) at low elevations (<750 m) to Coleogyne ramosissima(blackbrush) at mid-elevations (750-1,900 m). We hypothesized that this vegetation transition is partially facilitated by concomitant decreases in soil depth. Specifically, we expected that access to deep soil moisture favored creosotebush dominance at lower elevation, while tolerance to shallow soils favored blackbrush dominance at higher elevation. To test this hypothesis, we conducted an experiment at three locations along an elevational gradient, encompassing the vegetation transition, in southern California between May and August 2010. Soil depths range from >100 cm at the low elevation site, 60 cm at the mid-elevation site, and 30 cm at the high elevation site. We determined the depth of water uptake for both species by collecting soil and plant stem samples for stable isotope analysis of water. Further, we evaluated physiological stress levels by measuring leaf gas exchange with a LiCor 6400 portable gas exchange system.

Results/Conclusions

Based on δ¹⁸O data, creosotebush took up water from more shallow soil layers than blackbrush across all sites in May. Subsequently, depth of water uptake for creosotebush increased, while it remained more stable for blackbrush. By August, at low and mid-elevation, creosotebush used water from deeper soil layers than blackbrush. However, at the highest elevation, the depth of water uptake was identical for the two species in August. At that time, creosotebush had the lowest rates of photosynthesis at mid-elevation and rates for blackbrush were not significantly different between sites. Our results indicate that blackbrush has a more fixed pattern of water uptake compared to creosotebush and limitations of soil depth do not affect physiological stress in the summer. By contrast, creosotebush accesses water at depths greater than 30 cm, if possible, to maintain photosynthesis rates. While future warmer temperatures will likely limit the productivity of blackbrush at higher elevation, it is unlikely that creosotebush will expand into the current blackbrush range due to limitations of soil depth.