Watershed and Estuarine Controls Both Influence Plant Community and Tree Growth Changes in Tidal Freshwater Forested Wetlands along Two U.S. Mid-Atlantic Rivers

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.

Biophysical Drivers of Coastal Treeline Elevation

“…White & Kaplan, 2017). Increases in salinity to coastal ecosystems commonly accompany increases in inundation from sea level rise (Williams et al, 1999), although salinization of tidal freshwater forests can independently affect tree mortality (Noe et al, 2021). Consistent with this conceptual framework, we find that threshold elevations increase with salinity (Figure 3b), and that salinity is a key driver of threshold elevation in the multiple linear regression model (Table 1).…”

“…Salinization of freshwater ecosystems is shifting species composition and limiting the extent of freshwater and terrestrial forests (Ensign & Noe, 2018; Noe et al., 2021; Taillie et al., 2019; Tully et al., 2019; E. White & Kaplan, 2017). Increases in salinity to coastal ecosystems commonly accompany increases in inundation from sea level rise (Williams et al., 1999), although salinization of tidal freshwater forests can independently affect tree mortality (Noe et al., 2021). Consistent with this conceptual framework, we find that threshold elevations increase with salinity (Figure 3b), and that salinity is a key driver of threshold elevation in the multiple linear regression model (Table 1).…”

“…Low elevation terrestrial forests adjacent to marshes in this region typically include loblolly pine ( Pinus taeda ) and eastern red cedar ( Juniperus virginiana ) (Perry et al., 2001), with forested wetlands commonly comprised of swamp tupelo ( Nyssa biflora ) and red maple ( Acer rubrum ) (Noe et al., 2021). High marsh is usually composed of saltmeadow cordgrass ( Spartina patens ), saltgrass ( Distichlis spicata ), and black needlerush ( Juncus romerianus ) (Perry et al., 2001; U.S.…”

“…Salinization of freshwater ecosystems is shifting species composition and limiting the extent of freshwater and terrestrial forests (Ensign & Noe, 2018;Noe et al, 2021;Taillie et al, 2019;Tully et al, 2019;E. White & Kaplan, 2017).…”

Modeling impacts of saltwater intrusion on methane and nitrous oxide emissions in tidal forested wetlands

“…TFFW are sensitive to coastal droughts through declines in direct precipitation, lower riverine freshwater discharge, and human alteration of incoming tidal level that affects tidal range and salinity along many rivers (Conrads & Darby, 2017). TFFW are especially vulnerable to saltwater intrusion from SLR, coastal droughts, and heightened frequency and extent of storm surge (Neubauer et al, 2013; Noe et al, 2021; Pierfelice et al, 2015; Thomas et al, 2015).…”