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

Abstract

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.

Shrub expansion in maritime forest responding to sea level rise

“…Forest canopy gap portion did not significantly change during the study period; a longer time period is necessary to observe directional change across the site. Other studies of coastal vegetation change that spanned decadal timescales have detected effects of tree canopy cover decline and salinity increase on vegetation change (Taillie et al, 2019;Woods et al, 2021;Anderson et al, 2022). Instead, we observed a positive correlation of tree canopy cover with shrub basal area and shrub canopy area that reflects spatial autocorrelation of greater shrub dominance in the deeper forests levels where canopy cover is also greater.…”

“…Therefore, shrub expansion and inhibition of tree recruitment may play a critical role in forest transition in this region. Shrub dominance has been suggested to be an intermediate or alternative successional pathway during forest transition (Woods et al, 2021). The enhanced growth and colonization of shrubs that we observed and this alternative trajectory may explain the large increase in scrub-shrub dominated areas during forest transition (Ury et al, 2021).…”

“…Its geographic range extends to the North Atlantic Coastal Plain of the United States (Shao and Halpin, 1995) and the Caribbean basin, and it is introduced in Hawaii (GBIF.org, 2023). M. cerifera is an important component of the maritime forest community and increasing in recent decades on barrier islands (Huang et al, 2018;Woods et al, 2021). M. cerifera is an actinorhizal, moderately salt-tolerant evergreen shrub (Young et al, 1994;Tolliver et al, 1997;Naumann et al, 2007;Woods et al, 2020;i.e., more salt tolerant than co-existing glycophytes but less so than several other woody species in the same system; Anderson et al, 2022) and an early successional species that can form dense thickets (Kurten et al, 2008).…”

Characterizing low‐lying coastal upland forests to predict future landward marsh expansion

“…The proliferation of shrubs in the forest understory has also been documented in other areas in the Mid-Atlantic with forest retreat and represents one of the intricate transformations that may be occurring within the complex dynamics of the maritime forest landscape (Sward et al, 2023;Woods et al, 2021). This finding is contrary to the anticipated trend observed in terrestrial temperate forests, where understory density typically diminishes with distance from the forest edge (Decuyper et al, 2018;Meeussen et al, 2020).…”