Forest composition and growth in a freshwater forested wetland community across a salinity gradient in South Carolina, USA

Abstract

Tidal freshwater forested wetlands (TFFW) of the southeastern United States are experiencing increased saltwater intrusion mainly due to sea-level rise. Inter-annual and intra-annual variability in forest productivity along a salinity gradient was studied on established sites.Aboveground net primary productivity (ANPP) of trees was monitored from 2013 to 2015 on three sites within a baldcypress (Taxodium distichum) swamp forest ecosystem in Strawberry Swamp on Hobcaw Barony, Georgetown County, South Carolina. Paired plots (20 × 25-m) were established along a water salinity gradient (0.8, 2.6, 4.6 PSU). Salinity was continuously monitored, litterfall was measured monthly, and growth of overstory trees ≥10 cm diameter at breast height (DBH) was monitored on an annual basis. Annual litterfall and stem wood growth were summed to estimate ANPP. The DBH of live and dead individuals of understory shrubs were measured to calculate density, basal area (BA), and important values (IV). Freshwater forest communities clearly differed in composition, structure, tree size, BA, and productivity across the salinity gradient. The higher salinity plots had decreased numbers of tree species, density, and BA. Higher salinity reduced average ANPP. The dominant tree species and their relative densities did not change along the salinity gradient, but the dominance of the primary tree species differed with increasing salinity. Baldcypress was the predominant tree species with highest density, DBH, BA, IV, and contribution to total ANPP on all sites. Mean growth rate of baldcypress trees decreased with increasing salinity, but exhibited the greatest growth among all tree species. While the overall number of shrub species decreased with increasing salinity, wax myrtle (Morella cerifera) density, DBH, BA, and IV increased with salinity. With rising sea level and increasing salinity levels, low regeneration of baldcypress, and the invasion of wax myrtle, typical successional patterns in TFFW and forest health are likely to change in the future.

A casualty of climate change? Loss of freshwater forest islands on Florida's Gulf Coast

“…In 2014 two previously healthy island stands (H2 and H3) were more similar to intermediate stands in 1992, and intermediate stands in 2014 were generally more degraded than even the decadent stands in 1992. These shifts in forest health are consistent with previously noted impacts of salinity on freshwater forest trees along the Big Bend (Doyle, Krauss, Conner, & From, ; Liu et al., ; Perry & Williams, ; Ross, Sah, Meeder, Ruiz, & Telesnicki, ; Williams et al., ; Williams, Ewel, et al., ; Zhai, Jiang, DeAngelis, & Silveira Lobo Sternberg, ).…”

“…As chronicled in an ever‐growing body of research, climate change is driving spatial and compositional shifts in vegetation at local, regional, and global scales (e.g., Kelly & Goulden, ; Leffler, Klein, Oberbauer, & Welker, ; Svenning & Sandel, ; Walther et al., ). In coastal areas, sea level rise, warming air temperatures, and changing rainfall regimes elicit short‐ and long‐term changes in vegetation by altering physical conditions that affect the survival, distribution, and reproductive success of coastal plants (Gabler et al., ; Kirwan, Guntenspergen, & Morris, ; Liu, Conner, Song, & Jayakaran, ; Osland et al., ; Scavia et al., ). The maintenance of salt marsh, for example, largely depends on feedbacks among sediment availability, plant productivity, and the local rate of sea level rise (e.g., Craft et al., ; Fagherazzi et al., ; Kirwan & Megonigal, ; Reed, ).…”

Constrained tree growth and gas exchange of seawater‐exposed forests in the Pacific Northwest, USA

“…This result suggests P. sitchensis growing under freshwater conditions have similar negative responses to seawater exposure despite different soil salinity, which is consistent with Picea glauca growing at northern Alberta (Lilles, Purdy, Macdonald, & Chang, ). However, other studies at southeastern United States revealed the mean growth rate of Taxodium distichum decreased with increasing salinity (Liu, Conner, Song, & Jayakaran, ; Thomas, Doyle, & Krauss, ). A lower water salinity gradient (0.8 PSU, 2.6 PSU and 4.6 PSU) in the southeastern United States compared to BC (6.0 PSU, 10.1 PSU) would be one possible reason.…”

Why Do We Need to Document and Conserve Foundation Species in Freshwater Wetlands?

“…in Malaysia (a member of the Myrtaceae family [117]) and Pterocarpus officinalis in Puerto Rico (a member of the Fabaceae family [118]), species of palms in Puerto Rico floodplains [119], and hardwood species assemblages [120]. These swamps are increasingly exposed to flooding and saltwater intrusion caused by sea level rise that result in decreased growth and increased mortality due to anoxia and salinity stress [121][122][123]. Bald cypress can tolerate salinity of 3 to 6 ppt through osmotic adjustments [124], but its basal area was halved by a chronic salinity increase from 0.7 to 1.3 ppt over a year (from 87 m 2 ha −1 to 40 m 2 ha −1 [101]).…”