GCE IV - Key Finding in 2022

Synchrony among salt marsh sites: does one size fit all?

The GCE program has monitored 10 core sites since 2000. An obvious question is whether variables at these sites rise and fall together, in synchrony, or whether individual sites show unique temporal patterns. The extent to which sites behave synchronously is important, because it affects both our ability to generalize from well-studied sites to the landscape, and the extent to which multiple sites compensate at the scale of the landscape through a portfolio effect (one site doing well in a year when another site does poorly). In addition, to the extent that sites behave asynchronously, vulnerability to global change may be greatest in particular locations that could be the focus of research and management attention.

We have found that water temperature and salinity are highly correlated among the GCE core sites (Fig. 1). When we turn to biotic variables, however, the results are more complex. Plant end-of-year biomass is synchronous among sites, but only if we compare within a species and within a marsh zone—for example, creekbank biomass of Spartina alterniflora (Fig. 2). The correlation is much weaker if we compare creekbank biomass with mid-marsh biomass within a site, even though these plots are quite close together, or if we compare among plant species (Liu and Pennings 2021). This suggests that the important abiotic drivers of end-of-year biomass differ between marsh zones, and that different plant species vary in their sensitivity to these drivers. We found even less synchrony when we compared salt marsh katydid (“grasshopper”) abundance among the five sites at which they were common, indicating that results from any one site should be interpreted with caution. We don’t yet have a rigorous understanding of what affects salt marsh katydid abundance—relationships with potential drivers of abundance had low predictive performance and were different among sites (Adams et al. 2022).

Taken together, these results indicate that the abiotic drivers that affect marsh function mostly operate synchronously across the GCE landscape, especially when evaluated using annual averages, but that biological responses are more complicated. Future studies will evaluate synchrony in other biotic variables, such as snail and crab abundance.