The evolutionary loss of aerenchyma limits both realized and fundamental ecohydrological niches in the Cape reeds (Restionaceae)

Abstract

The basic physiological mechanisms that determine the niche limits, and so the local ranges, of species are often poorly understood. Here, we assessed whether the presence of root aerenchyma can account for both the fundamental and realized ecohydrological niche in the Restionaceae.
We documented the ability to make aerenchyma of almost 300 of the c. 350 South African Restionaceae species with the help of field observations, cultivation experiments and herbarium collections, and correlated this information with the ecohydrological niche of the species. We used a diversity of sampling approaches to account for variation in both aerenchyma and habitat at individual, population and species level. Tolerance of waterlogging in species with and without aerenchyma was investigated experimentally.
To account for phylogenetic relatedness, the evolution of both aerenchyma and the ecohydrological niche was explored in the context of an almost complete species-level phylogeny using both parsimony and maximum likelihood optimization methods.
We showed that the presence of aerenchyma was correlated with the ecohydrological niche at population and at species level, and after phylogenetic correction. Most individuals of non-aerenchymatous species died when waterlogged, whereas most species with aerenchyma were waterlogging tolerant. This indicates that waterlogging acts as an environmental filter that excludes non-aerenchymatous species from these conditions. The absence, and to a certain degree also the presence, of aerenchyma predicts both the field-observed realized and experimentally determined fundamental ecohydrological niche.
Aerenchyma was ancestrally present as inferred by both parsimony and maximum likelihood reconstruction and showed a strong phylogenetic signal with frequent losses but few gains. The ecohydrological niche was evolutionary more labile, with niche changes in many evolutionary lineages. Changes from wetland to dry habitats were reconstructed in species with aerenchyma, whereas the reverse change was never inferred for species that had lost aerenchyma.
Synthesis. Our study provides evidence that functional traits can effectively predict species niches, and evolution of these may constrain the habitats available to the clades. We underpin the importance of understanding the causal driver of the local distribution of species for making robust predictions of species range shifts under climate change.