Biological soil crusts (biocrusts) modify soil physical structure, surface temperature, and hydrology. These modifications may have critical implications for seed emergence in dryland ecosystems. However, past studies of biocrust impacts on seed emergence have yielded mixed results, whereby biocrusts have been shown to have, positive, neutral, or negative impacts on emergence. Variation in emergence responses to biocrusts may be partially explained by differences in species seed size and external morphology. However, of yet we have a limited understanding of how physical seed traits impact these interactions. To address this knowledge gap, in a greenhouse setting, we compared the emergence rates of three native perennial bunchgrass species, of differing seed sizes, common in the cool Colorado Plateau Desert: Bouteloua gracilis (~5mm), Pleuraphis jamesii (~10mm), and Aristida purpurea (~40mm) on pinnacled, taxonomically-mixed biocrusts relative to a bare soil control. These study species have differing external seed morphologies which may play a role in their ability to germinate on biocrusted soils. Specifically, we investigated the following research hypothesis:
1) Biocrusts decrease emergence of native perennial grasses; and
2) physical seed traits influence the magnitude of biocrust inhibition of seed emergence with large-seeded species experiencing greater inhibition than small-seeded species.
Results/Conclusions
In support of our first hypothesis, biocrusts negatively affected plant species emergence, decreasing emergence in B. gracilis and P. jamesii. the severity of these effects differed between species. The smallest seeded species, B. gracilis, experienced a 55.9% decrease in emergence on biocrusted soil relative to the bare soil control. While P. jamesii, experienced 90.3% decrease in emergence on biocrust. In contrast to our predictions emergence of A. purpurea, the largest seeded species in our study, was not inhibited on biocrust suggesting this species may possess morphological adaptations to overcome biocrust physical barriers to emergence. Overall, our results suggest biocrusts may act as differential biotic filters to seed emergence, and may thus mediate plant recruitment and community assembly in dryland ecosystems.