Abstract
The tidal freshwater zone near the estuarine head-of-tide is potentially sensitive to both sea-level rise and associated salinity increases as well as changing watershed inputs of freshwater and nutrients. We evaluated the vegetation response of tidal freshwater forested wetlands (TFFW) to changes in nontidal river versus estuarine controls along the longitudinal gradient of the Mattaponi and Pamunkey rivers in the Mid-Atlantic USA. The gradient included nontidal freshwater floodplain (NT) and upper tidal (UT), lower tidal (LT), and stressed tidal forest transitioning to marsh (ST) TFFW habitats on both rivers. Plot-based vegetation sampling and dendrochronology were employed to examine: (1) downriver shifts in plant community composition and the structure of canopy trees, understory trees/saplings/shrubs and herbs, tree basal-area increment (BAI) and (2) interannual variability in BAI from 2015 dating back as far as 1969 in relation to long-term river and estuary monitoring data. With greater tidal influence downstream, tree species dominance shifted, live basal area generally decreased, long-term mean BAI of individual trees decreased, woody stem mortality increased, and live herbaceous vegetative cover and richness increased. Acer rubrum, Fagus grandifolia, Ilex opaca, and Fraxinus pennsylvanica dominated NT and UT sites, with F. pennsylvanica and Nyssa sylvatica increasingly dominating at more downstream tidal sites. Annual tree BAI growth was positively affected by nontidal river flow at NT and UT sites which were closer to the head-of-tide, positively influenced by small salinity increases at LT and ST sites further downstream, and positively influenced by estuarine water level throughout the gradient; nutrient influence was site specific with both positive and negative influences. The counterintuitive finding of salinity increasing tree growth at sites with low BAI is likely due to either competitive growth release from neighboring tree death or enhanced soil nutrient availability that may temporarily mitigate the negative effects of low-level salinization and sea-level increases on living TFFW canopy trees, even as overall plant community conversion to tidal marsh progresses.
Highlights
Tidal freshwater forested wetlands (TFFW) are an important, yet relatively understudied ecosystem
No clear pattern was found in species evenness in the canopy tree layer at the Pamunkey River sites, though for the understory shrub layer the species evenness generally declined from nontidal freshwater floodplain (NT) to ST—contrary to the trend for the Mattaponi sites
Vegetation communities and structure changed, and long-term tree growth decreased, in forested wetlands along a downriver gradient from nontidal through tidal freshwater forested wetlands to tidal forests transitioning to marsh near the oligohaline zone of two tidal rivers
Summary
Tidal freshwater forested wetlands (TFFW) are an important, yet relatively understudied ecosystem. Tidal freshwater forested wetlands occur in the western United States, primarily limited to Oregon and Washington [4,5,6], where their area has been reduced by ~95% since European settlement, primarily as a result of diking and other land-use conversions [7,8]. These wetlands provide important ecosystem services, with high rates of C storage and sequestration [9], biotic diversity [10], denitrification [11,12], and sediment retention [13]. The importance of these influences may depend on the temporal variability of ecosystem drivers and by location along longitudinal river gradients from head-of-tide to low salinity or oligohaline zones
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