Swamp Reforestation in Coastal Louisiana, USA Exposes Landscape Scale Differences in Survival and Growth Across Two Hydrologically Restored Regions

ABSTRACT

Coastal swamp forests fill ecological niches that provide valuable environmental services, yet their extent across coastal Louisiana has diminished from logging and saltwater intrusion. Previous reforestation attempts yielded mixed results, attributed to hydrological disturbances leaving regions of the coast vulnerable to environmental stress. Environmental conditions may be improving. In the Pontchartrain Estuary, the Caernarvon Freshwater Diversion (CFD) has pulsed Mississippi River water into lower estuary wetlands since 1991, and the Mississippi River Gulf Outlet Canal closure in 2009 lowered soil salinity further upstream in the estuary on the Maurepas Landbridge (MLB). However, how hydrological restoration impacts habitat restoration remains undocumented. The Pontchartrain Conservancy planted ~80,000 saplings from 2011–2021 in two regions (CFD, MLB) within the estuary. Species included Taxodium distichum (L.) Rich, Nyssa aquatica L., Nyssa sylvatica Marshal var. biflora, Acer rubrum L. var. drummondii (Hook. & Arn. ex Nutt.) Sarg., and Fraxinus pennsylvanica, though > 80% of the saplings planted were T. distichum. We assessed survival and growth for 7.3% of saplings. Survival differed between regions; survival was lower around the CFD (63%) and higher on the MLB (82%). Growth rates also differed; growth was higher in CFD (height: 0.18–0.69 m/yr; diameter 0.66–1.62 cm/yr) and lower in MLB (height: 0.00–0.44 m/yr; diameter 0.05–0.18 cm/yr). Growth varied temporally between areas, but trends were similar. Results indicate 1) hydrological restoration benefits coastal swamp reforestation, and 2) river water increased growth. As habitat restoration, including swamp reforestation, scales-up in Louisiana, these outcomes help resource managers and planners refine restoration goals and target areas to maximize restoration success.