Trait-based analyses offer the opportunity to evaluate and understand processes driving plant community assembly. This understanding may then be applied to predicting community responses to environmental change or to restoration efforts. Inland salt marshes of the northeastern U.S. are highly imperiled plant communities that are often found within a mosaic of freshwater marshes. They offer the opportunity to examine the sorting of species and traits along broad environmental gradients and to compare trait syndromes of species existing at various levels of soil salinity. In a field study of the four remaining inland salt marshes in New York, we asked the following questions: 1) What are the primary environmental gradients underlying species sorting from freshwater marshes to inland salt marshes? 2) How do the relationships between environment and traits or trait syndromes determine species sorting? 3) Does salinity tolerance constitute an axis of plant trait variation that is distinct from the resource axis? We used NMDS analysis with environmental vector fitting, fourth-corner, and principal components analyses to address each question, respectively.
Results/Conclusions
Multivariate analyses indicate that soil Na:K ratio is the primary variable underlying the sorting of species from freshwater marshes to inland salt marshes. This variable interacted with flooding duration which produced a variety of community types at each end of the soil Na:K gradient. Principal components and fourth-corner analyses demonstrate an axis of plant trait variation (PC2) that is related to salinity tolerance and that is orthogonal to the resource axis. Leaf nitrogen concentration on an area basis (leaf Narea), photosynthetic pathway, leaf size, plant lifespan, and growth form were the traits that loaded most strongly on PC2. Plant species scoring high on this axis had heightened leaf Narea, C4 photosynthesis, small leaves, annual lifespan, non-rhizomatous growth and elevated tolerances to soil salinity and Na:K. Considering the traits loading on PC2, we suggest that the overall tradeoff underlying salinity tolerance is inefficient nitrogen use in exchange for efficient water use and tolerance of potentially toxic soil salinity levels. This overall tradeoff may underlie the often-observed reciprocal relationship between species salinity tolerance and competitive ability. Supporting this hypothesis, fourth-corner analysis suggested that species scoring high on PC2 are competitively inferior in non-saline settings. In sum, this trait-based analysis further elucidates the processes of community assembly along environmental gradients and the tradeoffs associated with salinity tolerance.
