Abstract
The study of species coexistence and community assembly has been a hot topic in ecology for decades. Disentangling the hierarchical role of abiotic and biotic filters is crucial to understand community assembly processes. The most critical environmental factor in semi-arid environments is known to be water availability, and perennials are usually described as nurses that create milder local conditions and expand the niche range of several species. We aimed to broaden this view by jointly evaluating how biological soil crusts (BSCs), water availability, perennial species (presence/absence of Stipa tenacissima) and plant-plant interactions shape a semi-arid annual plant community. The presence and cover of annual species was monitored during three years of contrasting climate. Water stress acted as the primary filter determining the species pool available for plant community assembly. Stipa and BSCs acted as secondary filters by modulating the effects of water availability. At extremely harsh environmental conditions, Stipa exerted a negative effect on the annual plant community, while at more benign conditions it increased annual community richness. Biological soil crusts exerted a contradictory effect depending on climate and on the presence of Stipa, favoring annuals in the most adverse conditions but showing repulsion at higher water availability conditions. Finally, interactions among co-occurring annuals shaped species richness and diversity of the final annual plant assembly. This study sheds light on the processes determining the assembly of annual communities and highlights the importance of Biological Soil Crusts and of interactions among annual plants on the final outcome of the species assembly.
Highlights
Disentangling the mechanisms promoting plant coexistence has been a hot topic in ecology for the last decades [1,2,3]
According to a hierarchical assembly process (Fig. 1), the effects of this primary abiotic filter on semiarid annual communities are modulated at small spatial scales by biotic filters such as perennial neighbors [17,18] and biological soil crusts
Our results suggest that plant-plant interactions should be considered during species assembly in a hierarchical filtering context if we want to scale up the role of plant-plant interactions to the community level [8]; interactions with perennials, which are the norm in these systems, are concurrent with interactions at a smaller spatial scale to conform the assembly of annual communities
Summary
Disentangling the mechanisms promoting plant coexistence has been a hot topic in ecology for the last decades [1,2,3]. According to a hierarchical assembly process (Fig. 1), the effects of this primary abiotic filter on semiarid annual communities are modulated at small spatial scales by biotic filters such as perennial neighbors [17,18] and biological soil crusts These crusts strongly affect ecosystem functions such as hydrology and nutrient cycling [20,21], as well as the establishment and performance of vascular plants [22]
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