Biological soil crusts (biocrusts) occur on and in the soil surface between plants in arid and semi-arid lands. Communities of cyanobacteria, bryophytes, lichens, and microbes, biocrusts provide a variety of ecosystem services and interact with vascular plants. Literature indicates biocrust effects on grass recruitment may be positive, negative, or neutral, but reasons for this range of variation in outcomes are not readily apparent. We are working to understand the extent to which biocrust characteristics (community composition, microtopography, continuity) might interact with seed characteristics (e.g. size, shape, appendage size) to influence grass emergence. In support of a larger project, we quantified the emergence of purple threeawn (Aristida purpurea; awn length 35-45 mm) and blue grama (Bouteloua gracilis; awn length 1-3 mm) on different biocrusts and bare soils in a greenhouse. We utilized pinnacled biocrusts from the Colorado Plateau and relatively smooth biocrusts from the Sonoran Desert to determine the role of biocrust microtopography. Biocrusts were either cyanobacteria-dominated or were dark biocrusts with some lichens and were left intact or were broken to simulate trampling. Awns of grass seeds were left intact or removed to ascertain the influence of appendages on emergence outcomes on intact biocrusts.
Results/Conclusions
Emergence of A. purpurea (mean ± SE, standardized for seed viability) was significantly lower on intact Sonoran Desert cyanobacteria (34% ± 5%) and dark (38% ± 7%) biocrusts compared to intact Colorado Plateau cyanobacteria (66% ± 4%) and dark (54% ± 6%) biocrusts and was generally lower on intact biocrusts than on broken biocrusts (81% ± 2%) and bare soil (85% ± 2%). Time to 50% of final emergence (T50m) was longest on Sonoran Desert dark biocrusts (9 days). Contrary to expectations, awn removal decreased emergence on intact biocrusts. B. gracilis emergence followed similar patterns, being lower on intact Sonoran Desert cyanobacteria (40% ± 9%) and dark (38% ± 6%) biocrusts compared to intact Colorado Plateau cyanobacteria (73% ± 6%) and dark (70% ± 6%) biocrusts. T50m was longest on intact Sonoran desert biocrusts (7 days). Awn removal did not affect emergence on intact biocrusts. Overall, emergence was greatest on bare soil or broken biocrusts and was greater on intact biocrusts from the Colorado Plateau. Seeds moved within the pots, with germination occurring on bare soil micro-patches and biocrust aberrations. We speculate that emergence patterns may be related to biocrust abundance, microtopography and seed dispersal within the pots.