Demographic effects of experimental shading and microtopography on desert annual plant performance
Spatial and temporal conditions, operating on multiple scales, determine where and when a given species occupies and persists in suitable habitat. Plant performance can fluctuate with variations in resource availability even at highly local scales, and performance tends to be more variable in uncertain environments. California’s Mojave Desert supports many special-status plant species as well as a booming renewable energy industry. Solar energy development alters the local topography and drainage patterns at multiple scales, and permanent structures impose novel shade and moisture gradients on the ground surface. We assess how populations of desert annuals utilize variations in natural and modified landscapes to persist in a region with great environmental uncertainty. Our four-year study combines an observational and experimental approach, comparing demography of two related annual sunflowers in unmanipulated populations and an experimental shading study. We collected survivorship and fecundity data in four growing seasons, then constructed matrix projection models to compare population performance across a range of topographic variation and experimental shade and water regime change treatments that mimic the effects of a photovoltaic array.
Despite their highly similar life history, growth form, and taxonomic relatedness, the two species had very different responses to variations in landscape and experimental shading. Compared to the common species, E. wallacei, the rare E. mohavense showed a more sensitive trigger for emergence and much higher reliance on microsite refuges in marginal rainfall years. Favorable microsites allowed some rare plants to reproduce and disperse seed into the seed bank, but this replenishment did not offset the overall negative effect of low rainfall years. Stochastic projections of population growth (λ) for E. mohavense rarely attained stable or increasing growth, and were always lower than projected growth rates for E. wallacei, which was able to maintain more consistent performance across years. Experimental shading had a negative effect on density and fitness of E. wallacei, and projections of population growth were lower in shade. In summary, our results show that desert annuals can respond to subtle variations in the landscape and that changes to shading and water runoff regimes can have a strong effect on population performance and persistence. Effective management of these communities will require consideration of the variable response species exhibit to both natural and anthropogenically modified landscapes.