Abstract Many coastal wetlands are threatened by rapid sea level rise (SLR). One approach to determine if wetlands are vulnerable to drowning is to investigate plant traits that are plastic and can respond rapidly to changing environmental conditions. Using 25 years of data on the dominant grass in US southeast and Gulf coast salt marshes, we tested for temporal trends in six traits across eight marsh sites in a region experiencing rapid SLR. We expected increases in some traits (mean and maximum height, biomass, size at flowering and proportion flowering) and decreases in others (stem density) at mid‐marsh locations and no change in plant traits along creekbanks due to the strong relationship between inundation and plant growth, a factor that co‐varies with other traits. Plants in both creekbank and mid‐marsh zones largely followed the expected patterns in trait trends in response to increased inundation due to SLR. However, trends were strong at some sites and weak at others. This site‐level variation obscured temporal changes leading to weak overall evidence for temporal trends in plant traits. When considering traits simultaneously, plants in both zones exhibited weak trade‐offs through time between shoot height and stem density, again with substantial site‐level variation. None of the four abiotic conditions we investigated (elevation, porewater salinity, soil N and soil N:P) were effective predictors of differences in site‐ and zone‐level trends in plant height or biomass. Across years, freshwater input to the estuary was the key abiotic variable predicting interannual variability in mean shoot height for both creekbank and mid‐marsh plants, but responses to sea level varied among sites. Synthesis . Substantial spatial variability in salt marsh plant trait responses to changing abiotic conditions emphasizes the value of taking a landscape perspective when investigating trends in plant traits. At sites where plants are responding to SLR, a suite of traits co‐vary in response to water levels in ways that are consistent with expectations based on the canonical growth–inundation relationship. Sites which are not responding as expected are likely affected more strongly by other abiotic factors known to modulate plant growth.
Dunn et al. (Fri,) studied this question.