Restoration opportunities beyond highly degraded tropical forests: insights from India's Western Ghats

Tropical rainforest remnants in human-modified landscapes exhibit varying levels of degradation, from highly degraded open-canopied and invasive plant-invaded habitats to closed-canopy forests that appear structurally intact. The former are frequently identified as being in a state of arrested recovery, and targeted for restoration, but restoration needs and opportunities in the latter remain underexplored. Using tree and seedling data from 105 plots spanning a canopy cover gradient in rainforest fragments and 19 plots in relatively intact "reference" rainforests in India's Western Ghats mountains, we show that the floristic composition, conservation significance, and carbon stocks of closed-canopy fragments (CC) more closely resemble open-canopy fragments (OC) than reference rainforests (RR). For example, densities of old-growth forest species, endemic/threatened species, and carbon stocks, increased from 15%, 28%, and 22% of reference values in OC to 32%, 46%, and 35% in CC, respectively, while tree community similarity to RR showed no increase from OC to CC (15%). Seedlings mirrored this pattern, offering little indication of natural recovery in closed-canopy fragments. Further, we show using simulations that seedling enrichment in closed-canopy fragments can initiate varying levels of floristic and functional recovery towards reference targets. Our findings illustrate that seemingly structurally intact tropical rainforest fragments can remain arrested in a floristically degraded condition, and represent worthwhile targets for ecological restoration. Such forests expand opportunities for restoring conservation-priority and high carbon-storing species using relatively low-cost methods (e.g., enrichment planting), which can complement intensive restoration of highly degraded forests and minimally-assisted natural recovery of less-fragmented forests.