Morphological Seed Traits Structure Relationships Between Biocrusts and Plant Emergence.

Understanding abiotic and biotic drivers of plant emergence and community assembly is a central goal of plant ecology. In drylands, extreme temperatures and water and nutrient limitations play strong roles in determining plant recruitment patterns. Biological soil crusts (biocrusts) modify the physical soil environment in drylands by increasing soil stability, moisture, and nutrient cycling. As such, biocrusts can have substantial effects on plant emergence depending on plant origin, functional group, and biocrust characteristics. However, understanding of possible mechanisms underlying these variable effects remains limited. To explore the possible role of seed traits in determining biocrusts effects on plant emergence, we conducted a meta-analysis of 321 published studies to examine relationships between seed traits (e.g., seed mass, shape, and presence of appendages) and emergence responses to biocrusts. Results showed that (1) morphological seed traits were important predictors of emergence responses to biocrusts. For example, (2) seed mass controlled the effects of biocrusts on the emergence of native and non-native plant species. In native plant species, emergence of large-seeded species was inhibited by biocrusts, while emergence of small-seeded species was slightly increased. (3) Seed mass also interacted with biocrust community composition to control emergence. For example, cyanobacteria biocrusts had no effect on the emergence of small-seeded species but had positive effects on medium- and large-seeded species, while mixed biocrust communities containing lichens and mosses increased the emergence of small-seeded species but decreased emergence of medium and large-seeded species. Seed appendages also mediated biocrust effects on emergence with negative effects on non-native species lacking appendages but neutral effects on native species overall. These results increase our fundamental understanding of the role of plant functional traits in structuring biotic interactions and have implications for understanding biocrust controls on plant recruitment and community assembly processes in drylands.