Examining large infrequent disturbances in the context of the subsidy-stress paradigm: How will coastal forested wetlands respond to increased frequency of strong storms and accelerated sea level rise?

Ecosystem processes and properties in periodically flooded bottomland hardwood forests are exquisitely sensitive to variation in soil water content and hydroperiod. Consistent with the predictions of the subsidy-stress hypothesis, measurements of aboveground and belowground ephemeral carbon pools along a hydrologic gradient in a mature coastal bottomland hardwood forest in the Mississippi Delta indicate that foliage and fine root production are limited by both low and high soil water content and production peaks in the zone of periodic short-term flooding. Soil respiration carbon flux follows the same pattern. Interannual variation in precipitation, and its impacts on soil water content and hydroperiod, are reflected in changes in the relative magnitude of ephemeral carbon pools and fluxes along this hydrologic gradient, and as differences in overall magnitude among years. Affirming a second prediction of the subsidy-stress hypothesis, interannual variation is most pronounced as flooding "stress" increases along this hydrologic gradient. Disturbance caused by a strong hurricane has shifted patterns of foliar production across this gradient, reducing the variation attributed to position along the hydrologic subsidy-stress continuum. Contrary to a third prediction of the subsidy-stress hypothesis, the former most productive zone of forest carbon assimilation experienced the greatest depression in foliar productivity immediately following the storm, while the most flooded zone experienced the least change. Using these data I evaluate the utility of the subsidy-stress hypothesis for understanding long-term patterns of coastal forest productivity in the context of multiple system perturbations. More generally, I inquire in what ways may the subsidy-stress paradigm and related conceptual frameworks inform our understanding of coastal forest responses to concomitant increases in hydroperiod and in the frequency of strong storms?