Friday's Findings - September 10 2021

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Summary:  Salinity is a pervasive stress for forested wetlands located in coastal regions of the US. In many areas, increases in salinity have caused shifts in forested wetland distribution, and how those wetlands function. Forested wetlands are not always lost immediately as salinity increases, but rather they undergo transitions to more salt tolerant tree species and then to marsh. Similar transitions have occurred for millennia, but the vast majority of our scientific understanding of salinity thresholds and tolerance mechanisms is related to seedlings, and most often in controlled settings offering short-term exposure. Millions of dollars have been invested in research to determine not only the salinity thresholds for seedlings of specific tree species, but also in the physiological mechanisms used in salinity tolerance. As a result, we have an exceptional understanding of how seedlings of certain tree species respond to salinity, but as ecologists asked to develop models and make predictions of ecosystem shifts involving mature trees, we must ask whether these insights alone are adequate? This question transcends forested wetland type (e.g., Carolina Bays, mangroves, tidal freshwater forested wetlands). Here, using measured transpiration and tree water use as evidence, we explored why a renewed focus on mature tree responses to salinity might help to advance coastal stress ecology. We used sap flow data from multiple studies to show that freshwater forested wetland tree species respond to salinity in ways that seedlings cannot; by re-structuring both the spatial distribution of trees within stands and the partitioning of water use within tree stems. Counterintuitively, trees exposed to long-term salinity concentrations often have lower thresholds of mortality than seedlings exposed to short-term salinity concentrations. These differences influence the way stands use water and, hence, how we may need to model transitions and understand thresholds of mortality in the future.