Water related stresses drive distributions and fitness differences among closely-related Salicaceae species along a hydrological gradient
The distributions of terrestrial plants are strongly constrained by water-related stresses. The Salicaceae species in Central Minnesota have differential but phylogenetically conserved distributions along a hydrological gradient, where both drought and flooding can be drivers of abiotic filtering. We conducted a field-based reciprocal transplant experiment and an artificial flooding experiment under controlled conditions to address two questions: 1. How does the fitness of coexisting species respond to a hydrologic gradient and do fitness responses predict species natural distributions? 2. What are the functional mechanisms that underlie differences in species distributions and fitness across the gradient?
First, we established 40 common gardens along a natural water table depth gradient in the field. In each garden we planted two cuttings from each of the 14 Salicaceae species that occur at the study site. We measured survival and basal area for two years and estimated individual fitness, incorporating both survival and growth, using Aster models. We collected data on 14 water stress or leaf economic spectrum traits and performed principle component analysis. We then examined statistical associations between species distributions, their fitness response along the hydrologic gradient and the principal components of plant functional traits; and tested the phylogenetic signals of the principal components.
Second, we subjected seven species, which together span the full hydrological gradient, to experimental water logging treatments. We exposed cuttings of these species to a four-week long flooding treatment and let them recover for another four weeks. We measured gas-exchange, chlorophyll fluorescence, predawn water potential, stomata density, lenticel density, and above-ground growth before, during, and after the flooding treatment. We also measured the biomass of a subset of plants, and collected stem and root samples to examine the development of aerenchyma, at the ends of the flooding and recovery periods.
Fitness response to water table depth is best modeled by a quadratic function, indicating fitness is limited by both drought and flooding. Using the quadratic model, the water table depth at which each species had its highest fitness explained 72% of the variation in species natural distributions.
The first principal component of functional traits, which includes both water stress and leaf economic spectrum traits, is phylogenetically conserved compared to a random null model and is significantly correlated with species distributions and with their fitness response along the hydrological gradient.
These results highlight the importance of abiotic filtering and functional differences among species in community assembly along hydrological gradients.