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.

覆盖地球广袤干旱与半干旱区域土壤的生物土壤结皮（Biological soil crusts），可对维管植物产生显著的正负双向调控作用，进而能够塑造植物群落结构。目前学界对这类调控效应的认知，大多由生物土壤结皮对单种或多种植物的直接效应外推而来，且通常仅聚焦于短期萌发与早期建植阶段。然而，若生物结皮在干旱生态系统中充当“组织原则”，那么其对群落的效应可能表现为链式或表观级联效应——即结皮对植物的影响会延伸至其如何调控受结皮影响的植物物种间的相互作用。在斑块尺度（通常直径大于10米）的野外实地观测中，优势灌木三齿蒿（Artemisia tridentata）与生物土壤结皮呈正相关，而与丛生禾草物种呈负相关。在单株灌木及灌木间开阔基质的尺度上，生物土壤结皮与丛生禾草则均与三齿蒿呈负相关。温室实验表明，在一定湿度与温度范围内，生物土壤结皮可固持大量氮素，并对植物萌发与建植产生正向效应。在为期两年的野外实验中，生物土壤结皮可提升羊茅属（Festuca）植物的生长速率，以及三齿蒿的光合速率。在表观级联效应的第二个环节中，种植于三齿蒿冠层下的羊茅属植物，其生物量是种植于开阔生境或三齿蒿移除处理组的两倍以上。生物土壤结皮可在长期尺度上促进维管植物生长，并促成一条促进链：生物土壤结皮促进三齿蒿灌木与羊茅属植物，三齿蒿则促进羊茅属植物，同时可抑制其他丛生禾草物种；在大尺度下，其可能会形成斑块状生境，在灌木间相对无草的区域提升生物土壤结皮的覆盖度。