Environmental Filtering of Leaf Traits, Spectral and Genetic Variation in Phragmites australis: Responses and Spatial Patterns

Intraspecific variation in plant phenotypes mediates responses to changing environmental conditions; however, a comprehensive analysis of factors influencing variation in plant species traits is still lacking in freshwaters. Here, we used Phragmites australis, a cosmopolitan foundation wetland species, to understand adaptation by plants to environmental conditions and spatial patterns in local adaptation by combining information on functional traits, including remote sensing-derived spectral traits and genetic diversity. We measured eleven leaf functional traits, including four spectral traits related to leaf structure and pigments content, plus two genetic diversity indices in 48 plots of P. australis from eight wetland systems in Italy. We used GAMMs to model trait responses to water and sediment conditions. We also produced continuous maps of trait variation using very-high-resolution maps obtained by remote sensing and used trait-environment relationships to analyse plant responses at fine and large scales. Ten out of eleven traits and genetic indices were significantly influenced by at least one environmental variable. Eutrophication and N/P ratio limitation led to a decrease in values of traits related to photosynthesis and productivity, especially for water phosphate concentration > 40 μg L−1, and to increased genetic diversity. Extreme conditions at specific sites (like high conductivity above 3000 μS cm−1) also determined a marked increase in traits related to high resource-use efficiency. Stressful conditions determined by P-limited eutrophication induced trait and genetic response of P. australis, shifting to more conservative strategies. Moreover, continuous maps of remotely sensed plant traits that respond strongly to environmental variables highlighted patterns of eutrophication and resource-use at the site scale, providing an innovative approach to monitoring sites densely covered by vegetation. This study demonstrates that coordinating functional, spectral and genetic data can robustly describe factors influencing diversity and responses in plants, and shows that consistent intraspecific variation of a dominant species can influence ecosystem processes at different scales.