Riparian Dependence Increases with Atmospheric Demand Among Species of Willows (Salix L.) In Western North America
Brad Butterfield*1 and Kevin Hultine2
1Center for Ecosystem Science and Society (ECOSS), Northern Arizona University, Flagstaff, AZ, USA; Bradley.Butterfield@nau.edu
2Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix, AZ, USA; khultine@dbg.org
Plants vary in both their hydrological and climatic niches. The manner in which these niche dimensions covary among closely related species can help to identify potential co-adaptation to hydrological and climatic factors, as well as predict ecological and evolutionary responses to environmental change. We assessed relationships between riparian-dependence and climate niches among species of willows in the western United States, incorporating phylogenetic correlations and functional trait information to better understand the adaptive nature of variation and covariation in hydrological and climatic niches. The riparian-dependence niche, estimated as the mean distance from georeferenced occurrence records to the nearest stream based on the National Hydrography Database, indicated that willows generally occurred closer to streams than expected by chance, but that riparian-dependence varied substantially among species. Riparian-dependence was positively correlated with mean annual temperature and diurnal temperature range (vapor pressure deficit), both strong drivers of atmospheric demand on evapotranspiration. Phylogenetic independent contrast (PIC) correlations for these relationships were significant as well, and the high degree of niche convergence among species indicated evolutionarily labile co-adaptations to riparian-dependence and atmospheric demand. Plant height increased with mean annual temperature niche, with and without PICs, and was positively correlated with specific leaf area, indicating underlying physiological tradeoffs associated with these niche dimensions. More generally, these results suggest that the reliable water supply afforded by riparian zones is necessary to compensate for increasing atmospheric demand of hotter leaves and drier air. This relationship suggests that increases in atmospheric demand and declining water tables across this and other dryland regions are likely to exact synergistic negative effects on woody riparian plants.