Article Watershed and Estuarine Controls Both Influence Plant Community and Tree Growth Changes in Tidal Freshwater Forested Wetlands along Two U.S. Mid-Atlantic Rivers Gregory B. Noe 1,* , Norman A. Bourg 1, Ken W. Krauss 2 , Jamie A. Duberstein 3 and Cliff R. Hupp 1 1 U.S. Geological Survey, Florence Bascom Geoscience Center, Reston, VA 20192, USA; n_bourg@hotmail.com (N.A.B.); crhupp@usgs.gov (C.R.H.) 2 U.S. Geological Survey, Wetland and Aquatic Center, Lafayette, LA 70506, USA; kraussk@usgs.gov 3 Institute of Coastal Ecology and Forest Science, Clemson University, Georgetown, SC 29442, USA; jdubers@clemson.edu * Correspondence: gnoe@usgs.gov 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 Mat- taponi 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 Citation: Noe, G.B.; Bourg, N.A.; were employed to examine: (1) downriver shifts in plant community composition and the structure Krauss, K.W.; Duberstein, J.A.; Hupp, of canopy trees, understory trees/saplings/shrubs and herbs, tree basal-area increment (BAI) and C.R. Watershed and Estuarine (2) interannual variability in BAI from 2015 dating back as far as 1969 in relation to long-term river Controls Both Influence Plant and estuary monitoring data. With greater tidal influence downstream, tree species dominance Community and Tree Growth shifted, live basal area generally decreased, long-term mean BAI of individual trees decreased, Changes in Tidal Freshwater Forested woody stem mortality increased, and live herbaceous vegetative cover and richness increased. Wetlands along Two U.S. Mid-Atlantic Acer rubrum, Fagus grandifolia, Ilex opaca, and Fraxinus pennsylvanica dominated NT and UT sites, Rivers. Forests 2021, 12, 1182. with F. pennsylvanica and Nyssa sylvatica increasingly dominating at more downstream tidal sites. https://doi.org/10.3390/ f12091182 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 Academic Editor: Thomas J. Dean 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 Received: 29 June 2021 salinity increasing tree growth at sites with low BAI is likely due to either competitive growth release Accepted: 25 August 2021 from neighboring tree death or enhanced soil nutrient availability that may temporarily mitigate the Published: 31 August 2021 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. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: tidal freshwater wetland; tidal marsh; tidal forest; watershed; estuary; salinization; published maps and institutional affil- sea-level rise iations. 1. Introduction Copyright: © 2021 by the authors. Tidal freshwater forested wetlands (TFFW) are an important, yet relatively under- Licensee MDPI, Basel, Switzerland. studied ecosystem. In the United States, they predominantly occur in low-lying coastal This article is an open access article areas along the Atlantic and Gulf coasts where they occupy at least 200,000 ha, likely more distributed under the terms and area than tidal freshwater marshes [1,2]. Ensign and Noe [3] recently calculated that tidal conditions of the Creative Commons freshwater rivers, which support most TFFW, extend 2850 km along the U.S. Atlantic Coast, Attribution (CC BY) license (https:// with a median length of 20 km among 127 rivers. Tidal freshwater forested wetlands also creativecommons.org/licenses/by/ occur in the western United States, primarily limited to Oregon and Washington [4–6], 4.0/). Forests 2021, 12, 1182. https://doi.org/10.3390/f12091182 https://www.mdpi.com/journal/forests Forests 2021, 12, 1182 2 of 22 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]. Given their hydrogeomorphic land- scape position downstream of the head-of-tide along rivers, tidal freshwater wetlands are likely influenced by both watershed (e.g., nontidal river discharge and availability of nutrients and sediment) and estuarine controls (e.g., salinity and tidal inundation) [14,15]. Indeed, TFFW ecosystems are especially susceptible to sea-level rise, flood or drought stress, and saltwater intrusion [1,16,17]. However, 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. The vegetation communities of TFFW are known to change along hydrologic gra- dients of water depth and salinity along tidal rivers. The first published works on the vegetation of a tidal freshwater swamp did not start appearing until the mid-1980s [18,19]. Doumlele et al. [19] examined only a single site on the Pamunkey River in Virginia and de- scribed the dominant vegetation in TFFW and a nearby marsh community. Brinson et al. [18] was more descriptive of multiple sites and focused on change along a freshwater-to-low salinity transition within the Pamlico River Estuary, North Carolina. Stands with higher porewater salinity had reduced basal area and a higher proportion of dead trees along with dominant species shifts [18]. These initial studies began to identify that the stressors to TFFW composition and growth vary longitudinally downstream along rivers. Vegetation studies of TFFW have increased in recent years. Rheinhardt [20] conducted a much more extensive survey of 23 tidal freshwater swamps along the Pamunkey River and found that they harbored two primary plant communities—downriver, wetter sites dominated by Nyssa biflora Walter and Fraxinus spp., and upriver, drier Acer rubrum L.- Liquidambar styraciflua L.-N. biflora dominated swamps. Subsequent studies of TFFW vegetation, primarily in Maryland [21,22], North Carolina [23], and South Carolina, Geor- gia, Louisiana, and Florida [24–29] expanded upon this initial work, culminating in the definition of four general community types ranging the southeastern United States by Duberstein et al. [30]: Water Tupelo (dominated by Nyssa aquatica L.); Swamp Tupelo (dom- inated by N. biflora); Dwarf Palmetto (dominated by Sabal minor (Jacq.) Pers.); and Cabbage Palm (with notable S. palmetto (Walter) Lodd. ex Schult. and Schult. f.) communities. In the Pacific Northwest of the United States, TFFW are often brackish (~8–10 ppt) during the summer dry season, resulting in a much less diverse tree community dominated by Picea sitchensis (Bongard) Carrière with the shrubs Lonicera involucrata (Richards) Banks ex. Spreng and Malus fusca (Raf.) C. K. Schneid. in the understory [4]. In addition to plant community studies, a few studies have focused on the effects of salinity and sea level rise on tree growth in TFFW. Some have examined the effects of these factors on growth indirectly, including sapflow and water use [31,32], chloride concentrations in stem tissue [33], and nutrient and water stress [34]. Other studies have assessed salinity and inundation impacts directly by measuring annual tree growth rings [27,35–38] or stem wood growth over time [29], but these studies were limited to investigation of a single species, Taxodium distichum (L.) Rich. In general, these and other findings from nontidal lowland habitats have shown that forest primary productivity exhibits a positive growth response to intermediate levels of flood duration but growth declines under prolonged or permanently flooded conditions [39]. Field studies have also identified the tolerance of T. distichum to low, chronic, or short acute salinization, but not to sustained levels beyond 2 ppt, and chronic or elevated saltwater presence inevitably results in decreased tree growth and ecophysiological stress [23,35,40]. Studies on tree growth in TFFW in relation to salinity and water level have, to our knowledge, not been undertaken on species other than T. distichum. The objective of this study was to examine the extent to which watershed vs. estuarine controls influence TFFW vegetation along a longitudinal river gradient spanning from Forests 2021, 12, 1182 3 of 22 nontidal through the tidal freshwater zone on both the Mattaponi and Pamunkey rivers in eastern Virginia. We made the following predictions: 1. Estuarine water level and salinity negatively affect annual tree growth, and generate shifts in plant community composition and structure, with estuarine water level and salinity becoming increasingly important at driving vegetation assemblage in downriver locations within the tidal freshwater zone. 2. Watershed nutrient inputs and