Abstract
Flood frequencies in coastal forests are increasing as sea level rise accelerates from 3–4 mm year−1 to possibly more than 10 mm year−1 by the end of this century. As flooding increases, coastal forests retreat, ghost forests form, and coastal marshes migrate inland. The existence of ghost forests makes the mechanism of forest retreat clear: low-lying trees become more exposed to coastal flooding until they ultimately die. Variability in these retreat rates, however, makes it difficult to predict where and when retreat will continue to occur. Understanding tree growth responses to tidal water levels relative to other environmental factors is a critical step in elucidating the factors that influence retreat variability. Here, dendrochronology was used to study factors that contribute to variations in growth patterns in four coastal forests fringing the Delaware and Barnegat Bays. Species chosen for study included loblolly pine (Pinus taeda), pitch pine (Pinus rigida), and American holly (Ilex opaca). Pearson’s and partial correlation tests showed that growth relationships with monthly environmental conditions varied across sites and were moderate in strength (generally R < 0.5), but each site had at least one significant growth-water level correlation. As coastal flooding exposure is spatially dependent, tree chronologies were also separated into high and low elevation groups. Pearson’s and partial correlation tests of the mean differences between elevation groups showed that at some sites, low elevation trees grew less than high elevation trees when water levels were high, as might be expected. At one site, however, lower elevation trees grew more when water levels were higher, which suggests that other interacting factors—regardless of current flood exposure—potentially have positive, yet likely temporary, influence over tree growth in these low-lying areas.
Highlights
Low-lying coastal forests adjacent to intertidal areas are being transformed by rapidly rising tidal water levels
Dendrochronology was used to study factors that contribute to variations in growth patterns of different tree species (i.e., Pinus taeda, Pinus rigida, Ilex opaca) of coastal forests within two different estuarine settings (i.e., Delaware Bay and Barnegat Bay)
Relationships between the NOAA Atlantic City gauge and Mantoloking, the local USGS gage used for Cattus Island, had the lowest correlation coefficient (R = 0.560) but the highest concordance coefficient (W = 0.75)
Summary
Low-lying coastal forests adjacent to intertidal areas are being transformed by rapidly rising tidal water levels. In the Mid Atlantic, sea level is rising 3–4 mm year−1 and will likely surpass 10 mm year−1 within the 21st century [1,2]. Mean high water has risen even faster over the last 19 years, exceeding 8 mm·year−1 in some Mid Atlantic locations [3]. Higher water levels lead to higher flood frequencies and/or durations in these low-lying forests. As flood heights reach critical thresholds, trees die, and the forest retreats. Ghost forests, or the remnants of forests that have succumbed to rising sea levels, have expanded over the last several decades in Florida, North Carolina, Virginia, Maryland, and New
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