Salinity tolerance and the decoupling of resource axis plant traits

Questions: What ecophysiological relationships underlie the sorting of species from freshwater to inland salt marsh communities? Which plant functional trade-offs are present along a gradient of low to high salinity? Location: New York State, USA. Methods: We evaluated changes in species composition and plant functional traits along resource and salinity gradients at three sites using principal components, correlation and fourth-corner analyses. Results: Analyses of plant traits suggested an axis of specialization that was related to salinity tolerance and distinct from a resource axis describing a spectrum of plant traits associated with resource acquisition and retention. Plant species with low scores on the salinity axis had highest abundance in the most saline plots and tended to have small leaf size, high leaf nitrogen per area (N_area), C₄ photosynthesis and succulence. Whereas C₄ photosynthesis, leaf size and leaf succulence were each linked to patterns of species presence and relative cover along soil Na:K and electrical conductivity (EC) gradients, leaf N_area was most positively correlated with salinity tolerance. Leaf size also played a key role in species sorting along each of the environmental gradients examined in this study (soil EC, Na:K, flooding duration, soil extractable calcium and magnesium and soil N concentrations). Conclusions: Salinity tolerance is an alternative axis of plant specialization. The overall trade-off underlying salinity tolerance is inefficient N use in exchange for efficient water use and tolerance of toxic soil salinity levels. This research further elucidates the processes of community assembly along environmental gradients, trade-offs associated with salinity tolerance, and provides baseline information for wetland restoration. Elevated soil salinity favors species with high leaf nitrogen per area, and this trait is principal on a salinity axis of specialization that also includes as key traits: leaf size, photosynthetic pathway, and leaf succulence. Location on the salinity axis is a strong predictor of species presence and relative abundance in inland salt and freshwater marsh communities.