Abstract
1. We introduce a novel method that analyses environmental filtering of plant species in a geographic and phylogenetic context. By connecting species traits with phylogeny, traits with environment, and environment with geography, this comprehensive approach partitions the ecological and evolutionary processes that influence community assembly. 2. Our analysis extends RLQ ordination, which connects site attributes in matrix R (here environmental variables and spatial positions) with species attributes in matrix Q (here biological traits and phylogenetic positions), through the composition of sites in terms of species presences or abundances (matrix L). This methodology, which explores and identifies environmental filters that organize communities, was developed to answer four questions: which combinations of trait states are filtered by the environment, which lineages are affected by these filters, which environmental variables contribute to the assemblage of local communities and where do these filters act? 3. At La Mafragh in north-eastern Algeria, our approach shows that plant species traits were distributed according to environmental filters associated with a salinity gradient. Traits associated with the salinity gradient were convergent among Juncaceae, Cyperaceae and Amaranthaceae. The observed phylogenetic and trait patterns were related to how species survived the xeric season. Juncaceae and Cyperaceae, being perennials and anemogamous, tolerate the xeric hot season by restricting their range to the humid centre of the study area (where conditions are close to a subtropical climate). Several Amaranthaceae species co-occur with the Juncaceae and Cyperaceae in two areas with the highest salinity. Most dicots were observed at higher elevations (up to 7 m a.s.l.), had hairy structures that can retain water and reflect solar radiation and were mostly annual or biennial, completing their life cycle before the onset of the xeric season. 4. Synthesis. Our methodology describes environmental filters in terms of identified combinations of traits and environmental factors. It allows spatial and phylogenetic signals to be determined by identifying convergent and conserved patterns in the evolution of traits and spatial scales that structured the environment. Our statistical framework is generic and can be readily extended to a wide range of exciting issues, such as host-parasite, plant-pollinator and predator-prey interactions.