Coastal shoreline forests are vulnerable to seawater exposure, the impacts of which will increase due to sea-level rise, but the long-term adaptation strategies and vulnerability of coastal forests are not well understood. We used whole-tree transpiration, leaf water potential, tree-ring width, and tree-ring δ13C (a proxy for intrinsic water use efficiency, iWUE) to examine the long-term adaptation strategies of red maple (Acer rubrum) trees at the coastal interface (i.e., shoreline) and nearby upland in Maryland, USA. Red maple trees that grew along the shoreline and were exposed to slightly saline water (up to two PSU) had higher transpiration rates than those growing in the nearby upland forest during a wet year, but these differences disappeared during a normal precipitation year. Shoreline trees grew more slowly than upland trees over the last four decades, but these growth differences have disappeared in the last six years. Shoreline and upland red maple trees had similar variation in iWUE, indicating that higher transpiration rates of the seawater-exposed trees did not translate into differences in water use efficiency. There were no differences in predawn and midday water potential between upland and shoreline trees, suggesting no additional water stress occurs in shoreline trees. These findings indicate that mature red maple in our coastal study site maintains gas exchange and growth at a consistent or homeostatic level under slight soil salinity.