Non-hazardous seed transport by runoff on slopes determines composition and ecological strategies of plants in patches and interpatches in drylands: A trait-based approach

• Seed dispersal by runoff shapes species composition of patches and interpatches. • Seed trait interaction with runoff is crucial for seed fate in patch- or interpatches. • Trait distribution differs between communities of plants in patches and interpatches. • In interpatches seeds avoid removal by runoff flow and plants withstand stress. • In patches seeds are highly transportable by runoff and plants are competitive. In drylands, feedbacks between vegetation and water erosion play a major role in the spatial patchy organization and functionning of these ecosystems on slopes. Although seed dispersal and fate play a crucial role in these feedbacks, the influence of seed interactions with runoff flow in the origin, maintenance and functioning of patchy ecosystems on slopes has been poorly documented. In this study, we aimed at investigating how secondary seed dispersal by runoff flow, through its interactions with seed traits, influences composition and ecological strategies of plant communities in patches and interpatches in drylands. We also explored whether seed traits, relevant at early stages of plant life, are associated with other plant traits, relevant at further stages when seedlings start to compete with other plants. Based on community assembly rule theory, we compared floristic composition and trait distribution of plant communities assembling in patches and interpatches on natural hillslopes and man-made roadslopes. Studied traits were seed susceptibility to removal by runoff (SSR), seed anchoring to the soil (antitelechory), and specific leaf area (SLA) as a proxy of plant resource-acquisition and competitive ability. Communities in interpatches had lower mean SSR and SLA values than those assembling in patches on hillslopes. Moreover, antitelechory was associated with low mean SLA values in plant communities assembling on bare areas. We identified two ecological strategies that provide insight into the most constraining filters that shape plant communities in patches and interpatches, namely erosion and resource availability. In bare interpatches, plants evolved strategies to avoid seed removal and withstand harsh conditions. Conversely, plants in patch communties were characterized by easily transportable seeds and resource acquisitive and competitive abilities. We discuss implications of our results for the functioning of patchy ecosystems, consequences of climate change on their stability and applications to restoration of degraded slopes.