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 forests 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 carbonstoring 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.