Molecular phylogenies are increasingly used to understand how biotic interactions and environment shape phylogenetic community structure (PCS). However, we do not understand the effects of plant–plant interactions and environment on PCS and phylogenetic diversity across spatial scales, particularly in rangelands. Here, we ask: (1) do plant–plant interactions and environment affect PCS and phylogenetic diversity differently across the three spatial scales of the patch, the community, and the habitat? (2) What are the impacts of dominant cushion-nurse plants on the phylogenetic structure of plant communities? We assessed the PCS of semi-arid plant communities along an elevation gradient at the patch, community and habitat scales. Then, we assessed co-occurrence patterns along two sample slopes. Our results indicated important roles for biotic interactions and environmental filtering in determining phylogenetic diversity, with biotic interactions, in particular, having a stronger tendency to increase phylogenetic diversity. This is most likely due to the asymmetrical effects of nurse plants across the three spatial scales on our two different slopes. The impact of biotic interactions caused non-random phylogenetic patterns in more severe environments. In conclusion, biotic interactions influence phylogenetic diversity by altering PCS across aspects and along elevation gradients.

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