Small gradients in salinity have large effects on stand water use in freshwater wetland forests

Krauss, Ken W.; Baldwin, Michael J.; Allen, Scott T.; Conner, William H.; Salter, John S.; Miloshis, Michael

doi:10.1016/j.foreco.2020.118308

Cited by 24 publications

(15 citation statements)

References 61 publications

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“…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.…”

Section: Introductionmentioning

confidence: 66%

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

Noe

Bourg

Krauss

et al. 2021

Forests

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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.

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Section: Introductionmentioning

confidence: 66%

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

Noe

Bourg

Krauss

et al. 2021

Forests

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“…With the expansion of M. cerifera, typical successional patterns are interrupted. Conversion to shrubs may be accelerated as documented elsewhere along the Atlantic coast following salt intrusion due to coastal storms and sea-level rise [60,61]. Additional climate change factors, such as warming winter temperatures, favor M. cerifera expansion along with microclimate modification, creating a positive feedback to maintain a shrub state [62,63].…”

Section: Discussionmentioning

confidence: 97%

Long-Term Community Dynamics Reveal Different Trajectories for Two Mid-Atlantic Maritime Forests

Woods

Tuley

Zinnert

2021

Forests

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Maritime forests are threatened by sea-level rise, storm surge and encroachment of salt-tolerant species. On barrier islands, these forested communities must withstand the full force of tropical storms, hurricanes and nor’easters while the impact is reduced for mainland forests protected by barrier islands. Geographic position may account for differences in maritime forest resilience to disturbance. In this study, we quantify two geographically distinct maritime forests protected by dunes on Virginia’s Eastern Shore (i.e., mainland and barrier island) at two time points (15 and 21 years apart, respectively) to determine whether the trajectory is successional or presenting evidence of disassembly with sea-level rise and storm exposure. We hypothesize that due to position on the landscape, forest disassembly will be higher on the barrier island than mainland as evidenced by reduction in tree basal area and decreased species richness. Rate of relative sea-level rise in the region was 5.9 ± 0.7 mm yr−1 based on monthly mean sea-level data from 1975 to 2017. Savage Neck Dunes Natural Area Preserve maritime forest was surveyed using the point quarter method in 2003 and 2018. Parramore Island maritime forest was surveyed in 1997 using 32 m diameter circular plots. As the island has been eroding over the past two decades, 2016 Landsat imagery was used to identify remaining forested plots prior to resurveying. In 2018, only plots that remained forested were resurveyed. Lidar was used to quantify elevation of each point/plot surveyed in 2018. Plot elevation at Savage Neck was 1.93 ± 0.02 m above sea level, whereas at Parramore Island, elevation was lower at 1.04 ± 0.08 m. Mainland dominant species, Acer rubrum, Pinus taeda, and Liquidambar styraciflua, remained dominant over the study period, with a 14% reduction in the total number of individuals recorded. Basal area increased by 11%. Conversely, on Parramore Island, 33% of the former forested plots converted to grassland and 33% were lost to erosion and occur as ghost forest on the shore or were lost to the ocean. Of the remaining forested plots surveyed in 2018, dominance switched from Persea palustris and Juniperus virginiana to the shrub Morella cerifera. Only 46% of trees/shrubs remained and basal area was reduced by 84%. Shrub basal area accounted for 66% of the total recorded in 2018. There are alternative paths to maritime forest trajectory which differ for barrier island and mainland. Geographic position relative to disturbance and elevation likely explain the changes in forest community composition over the timeframes studied. Protected mainland forest at Savage Neck occurs at higher mean elevation and indicates natural succession to larger and fewer individuals, with little change in mixed hardwood-pine dominance. The fronting barrier island maritime forest on Parramore Island has undergone rapid change in 21 years, with complete loss of forested communities to ocean or conversion to mesic grassland. Of the forests remaining, dominant evergreen trees are now being replaced with the expanding evergreen shrub, Morella cerifera. Loss of biomass and basal area has been documented in other low elevation coastal forests. Our results indicate that an intermediate shrub state may precede complete loss of woody communities in some coastal communities, providing an alternative mechanism of resilience.

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“…Salinity enters the site from periodic storm surges or by backflooding from the estuary during severe events. This salinity is associated with widespread tree mortality and replacement by shrub cover (Williams et al, 2014), as well as reduced biomass (Liu et al, 2017) and transpiration (Duberstein et al, 2020).…”

Section: Site Descriptionmentioning

confidence: 99%

“…LAI values ranged from one, in the mesohaline zone, to over five, in the freshwater zone (Figure 1). Stand basal areas were quantified across this gradient in previous studies (Liu et al, 2017;Duberstein et al, 2020) that discretized the site into zones classified as freshwater, intermediate, and mesohaline; those studies measured trees and shrubs with diameters at breast height (dbh, at 1.3 m) over 10 cm in two 20 × 25 m plots in each zone. In this study, we use site-averaged basal areas (BA), using the average of all six plots from these previous studies.…”

Section: Site Descriptionmentioning

confidence: 99%

“…TABLE 1 | Forest characteristics-salinity, basal area (BA), dominant species (>15% of BA), density, and size-for the overstory trees by zone, and shrub characteristics across the whole site (Liu et al, 2017;Duberstein et al, 2020 Despite similarities in basal areas among zones (Table 1), the freshwater and mesohaline zones differed substantially in ecosystem structure: baldcypress (Taxodium distichum var. distichum) and water tupelo (Nyssa aquatica) occurred in both zones, but swamp tupelo (Nyssa biflora) and sweetgum (Liquidambar styraciflua) only occurred in the freshwater zone.…”

Section: Site Descriptionmentioning

confidence: 99%

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Localized Augmentation of Net Precipitation to Shrubs: A Case Study of Stemflow Funneling to Hummocks in a Salinity-Intruded Swamp

Allen

Conner

2021

Front. For. Glob. Change

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The interception of precipitation by plant canopies can alter the amount and spatial distribution of water inputs to ecosystems. We asked whether canopy interception could locally augment water inputs to shrubs by their crowns funneling (freshwater) precipitation as stemflow to their bases, in a wetland where relict overstory trees are dying and persisting shrubs only grow on small hummocks that sit above mesohaline floodwaters. Precipitation, throughfall, and stemflow were measured across 69 events over a 15-months period in a salinity-degraded freshwater swamp in coastal South Carolina, United States. Evaporation of intercepted water from the overstory and shrub canopies reduced net precipitation (stemflow plus throughfall) across the site to 91% of gross (open) precipitation amounts. However, interception by the shrub layer resulted in increased routing of precipitation down the shrub stems to hummocks – this stemflow yielded depths that were over 14 times larger than that of gross precipitation across an area equal to the shrub stem cross-sectional areas. Through dimensional analysis, we inferred that stemflow resulted in local augmentation of net precipitation, with effective precipitation inputs to hummocks equaling 100–135% of gross precipitation. Given that these shrubs (wax myrtle, Morella cerifera) are sensitive to mesohaline salinities, our novel findings prompt the hypothesis that stemflow funneling is an ecophysiologically important mechanism that increases freshwater availability and facilitates shrub persistence in this otherwise stressful environment.

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Small gradients in salinity have large effects on stand water use in freshwater wetland forests

Cited by 24 publications

References 61 publications

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

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

Long-Term Community Dynamics Reveal Different Trajectories for Two Mid-Atlantic Maritime Forests

Localized Augmentation of Net Precipitation to Shrubs: A Case Study of Stemflow Funneling to Hummocks in a Salinity-Intruded Swamp

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