Tolerance of Pinus taeda and Pinus serotina to low salinity and flooding: Implications for equilibrium vegetation dynamics

Questions: 1. Do pine seedlings in estuarine environments display discrete or continuous ranges of physiological tolerance to flooding and salinity? 2. What is the tolerance of Pinus taeda and P. serotina to low salinity and varying hydrologic conditions? 3. Are the assumptions for ecological equilibrium met for modeling plant community migration in response to sea-level rise? Location: Albemarle Peninsula, North Carolina, USA. Methods: In situ observations were made to quantify natural pine regeneration and grass cover along a salinity stress gradient (from marsh, dying or dead forest, to healthy forest). A full-factorial greenhouse experiment was set up to investigate mortality and carbon allocation of Pinus taeda and P. serotina to low-salinity conditions and two hydrology treatments over 6 months. Treatments consisted of freshwater and two salinity levels (4 ppt and 8 ppt) under either permanently flooded or periodically flushed hydrologic conditions. Results: Natural pine regeneration was common (5-12 seedlings per m2) in moderate to well-drained soils where salinity concentrations were below ca. 3.5 ppt. Pine regeneration was generally absent in flooded soils, and cumulative mortality was 100% for 4 and 8 ppt salinity levels under flooded conditions in the greenhouse study. Under weekly flushing conditions, mortality was not significantly different between 0 and 4 ppt, confirming field observations. Biomass accumulation was higher for P. taeda, but for both pine species, the root to shoot ratio was suppressed under the 8 ppt drained treatment, reflecting increased below-ground stress. Conclusions: While Pinus taeda and P. serotina are commonly found in estuarine ecosystems, these species display a range of physiological tolerance to low-salinity conditions. Our results suggest that the rate of forest migration may lag relative to gradual sea-level rise and concomitant alterations in hydrology and salinity. Current bioclimate or landscape simulation models assume discrete thresholds in the range of plant tolerance to stress, especially in coastal environments, and consequently, they may overestimate the rate, extent, and timing of plant community response to sea-level rise.