Geomorphic controls on shrub canopy volume and spacing of creosote bush in northern Mojave Desert, USA

Context Studies on the role of geomorphology on vegetation structure at the basin scale are rarely available and less likely in the future due to access, funding, and potential health risks. Objectives Our goal is to determine the primary abiotic controlling factor(s) on shrub canopy structure using a dataset of approximately 23 million individual shrubs, generated using remote sensing and ground-truthing by Young et al. (in Remote Sens 9(5):16, https://doi.org/10.3390/rs9050458, 2017). We posit that landscape position and local-scale geomorphic features in a desert alluvial fan environment will influence canopy volume and shrub spacing of creosote bush Larrea tridentata. Methods We relate selected characteristics (canopy volume and spatial distribution) of identified L. tridentata to aspect and surface geomorphology at each shrub location. Statistical analyses included K-S testing, distribution fitting, and several generalized linear models (GLMs). The study was located in Eldorado Valley, Nevada, USA. Results Aspect and surface age have demonstrable influences on both shrub canopy volume and shrub spacing for all 5 geomorphic surfaces studied, with the highest median canopy volumes on east-facing surface (0.758 m(3)) almost 5 x larger than the lowest median volumes (0.152 m(3)) on the WNW-facing surfaces; variability in shrub volume was also higher on east-facing than west-facing surfaces. Shrub spacing on alluvial flat and fan skirt surfaces (2.418 and 2.333 m, respectively) were larger than older alluvial fan, fan piedmont and fan remnant surfaces (1.776, 1.837 and 1.892 m, respectively). Conclusions Results show a significant relationship between shrub spacing and canopy volume by aspect and geomorphic surface, indicating a threshold at which biotic effects on canopy structure from intra-species competition transition to abiotic effects governed by geomorphic and climatological factors.