Stress Physiology, Scaling, and Water Use of Forested Wetland Trees and Stands

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USGS investigates the eco-physiological responses of coastal forested wetland vegetation to envrionmental stressors, and what role vegetation may have in affecting local hydrological cycling as a result of these stressors. 

Strawberry Swamp, Hobcaw Barony, SC, USA
Strawberry Swamp, Hobcaw Barony, South Carolina

The Science Issue and Relevance: Our studies investigate seedling, sapling, and stand-level eco-physiological responses of coastal forested wetland vegetation to environmental stressors, and in determining what role vegetation may have in affecting local hydrological cycling as a consequence of those stressors. Much of the past literature does not focus on scaling responses beyond the seedling stage, yet so much of what occurs at that level might be decidedly less important at higher organizational scales. Some effects, however, become more important. We are as interested in determining what effect flooding has on trees, for example, as we are in determining the effect that vegetation has on water use in a particular wetland. How do trees apportion their water use to reduce overall wetland water losses to evapotranspiration? Many coastal forests seem to be very conservative in water use; defining this potential may define a relatively unrevealed impetus for conservation of coastal forest communities and allow us to link carbon and nutrient budgets with vegetation use of water.

Methodologies for Addressing the Issue: Historically, we relied heavily on portable infrared gas analyzers to study the impact of environmental stress on seedlings and saplings. While we still rely on these techniques at times, we became increasing more intrigued by whether those responses translated to the trees and stands being described. We currently rely heavily on sap flow techniques (via heat dissipation), which allow us to measure water use by trees in different settings in-situ. Very few forested wetland research programs use sap flow techniques, yet the techniques are well developed from other terrestrial ecosystems. We currently work in tidal freshwater and oligohaline forested wetlands, bottomland hardwood forests, mangroves, and coastal strand vegetation (e.g., Pacific atolls) to investigate specific questions related to water use. Sap flow studies are particularly useful for scaling water use to the stand, and we are increasingly finding more opportunities to pair sap flow studies with estimation of evapotranspiration or net ecosystem exchange of carbon from similar areas.

Palmyra Atoll NWR, Line Islands, Central Pacific
Palmyra Atoll NWR, Line Islands, Central Pacific

Future Steps: We would like to focus more attention on well-instrumented forested wetland watersheds which is only possible through collaborations. One such study was recently initiated in coastal South Carolina that will allow us to link stand water use to shifts in the energy budget along a salinity gradient with the hope of learning more about how stands (in lieu of leaves) respond to chronic stress.