Biological soil crusts and plant-plant interactions in a sagebrush steppe community

Biological soil crusts, which cover the soil over extensive portions of the arid and semi-arid regions of Earth, can have both strong positive and negative effects on vascular plants, and thus can organize plant community structure. Our understanding of these effects has generally been extrapolated from the direct effects of biological soil crusts on one or more plant species, and usually on short term germination and early establishment. However, if biocrusts provide an “organizing principle” in arid systems, the effects of crusts on communities might be expressed as “chains” or “cascades” in which the effects of crusts on plants extend to how they influence interactions among crust-affected plant species. In field measurements, at the scale of patches (generally > 10 m diameter) the dominant shrub, Artemisia tridentata, was positively associated with biocrusts and negatively associated with bunchgrass species. At the scale of individual shrubs and the open matrix among shrubs, biocrusts and bunchgrasses were negatively associated with Artemisia. In the greenhouse, biocrusts fixed large amounts of nitrogen over a ranges of moisture and temperature levels, and had positive effects on germination and establishment. In two-year field experiments, biocrusts increased the growth of Festuca and the photosynthetic rates of Artemisia. In a second element of an apparent cascade, Festuca planted under Artemisia canopies were over twice as large as those planted in open sites or in Artemisia removal treatments. Biological soil crusts can facilitate vascular plants over a long time frame and contribute to a facilitation chain in which biocrusts facilitate Artemisia shrubs and Festuca, and Artemisia facilitates Festuca, appears to exclude other bunchgrass species, and at large scales may create patchy habitat that increases biocrust cover in relatively grass-free spaces among shrubs.