Planting trees is not the same as restoring a forest. And even while restoration in degraded lands can recover canopy cover and tree diversity, the ecological processes that drive natural regeneration may continue to remain broken. Seed arrival is one such process. Whether active restoration can repair it, and not just the vegetation structure, is a question few studies have followed for long enough to answer.
A new study by Aparna Krishnan and a research team from NCBS and the Nature Conservation Foundation (NCF) set out to answer it. The team examined seed arrival across restored and unrestored rainforest fragments and mature benchmark forests in the Anamalai Hills, Tamil Nadu. The restored sites had been planted with diverse native tree species 13 to 21 years earlier, long enough for some of the planted trees to mature and begin fruiting. Over a year, they monitored 189 seed traps across 63 locations, counting seeds by species every two to three weeks, across three types of sites: actively restored fragments, paired fragments left to regenerate naturally, and mature benchmark rainforests within the Anamalai Tiger Reserve.
The results showed that arrival rates of large seeds in restored sites were comparable to mature benchmark forests. They were also roughly three times higher than in unrestored fragments. Restoration appeared to help in two distinct ways. More fruiting native trees in the canopy increased the local supply of large seeds, and the improved vegetation attracted fruit-eating animals that carried in seeds from trees further away. Restoration, in short, increased both the local availability of large seeds and, to a lesser extent, their dispersal into fragments by animals.
The recovery was not complete, though. The species richness and mix of arriving seeds in restored sites remained similar to unrestored sites, and both stayed distinct from benchmark forests. Benchmark forests received a wider range of large-seeded species, including several wind- and gravity-dispersed trees that were simply absent from the fragments. Many of the planted trees are slow-growing. A number may not yet have reached reproductive maturity within the 13 to 21 years since planting. The gap in species composition may close with time, but the study suggests this could take well beyond two decades.
A closer look at where the large dispersed seeds were coming from revealed a catch. A substantial share of them, around 38%, belonged to non-native species. Maesopsis eminii, a tree widely planted as shade cover in the coffee estates surrounding the fragments, accounted for a significant portion of this. The animals were doing exactly what they should, moving large seeds into areas with suitable habitat. But the seeds most available to them in the surrounding landscape were often non-native ones. When non-native species were excluded from the analysis, the recovery of native large-seed arrival in restored sites was far weaker than the overall numbers suggested.
“This result surprised us, but also pointed out an important lesson for restoration practice. It indicated to us that along with native tree species planting, the recovery of seed arrival in fragments may benefit from promoting the use of native instead of alien species in neighbouring human-use areas,” said Aparna, the lead author of the study.
The study also found that isolation from benchmark forests suppressed large-seed arrival equally in restored and unrestored sites. Arrival rates declined with distance regardless of whether a site had been restored. This suggests that the surrounding human-use landscape, and the distances animals must cross within it, remains a real barrier to seed dispersal into more isolated fragments. Restoration improved conditions inside the fragments. It could not compensate for what was missing beyond their edges.
Prof. Mahesh Sankaran, the principal investigator of the study at NCBS noted, "the study has important implications for how we evaluate and monitor the success of restoration efforts. Importantly, commonly used metrics - such as tree cover and species diversity – only tell us a part of the story when it comes to assessing restoration success. Even if vegetation structure recovers in a couple of decades, the underlying processes that are required to maintain functional ecosystems – such as seed dispersal – can take much longer to recover."
Taken together, the findings make a strong case for diverse native tree planting as a tool for recovering ecological processes, not only forest structure. Within two decades, restoration brought large-seed arrival rates to levels comparable with mature forests, a meaningful result given how central this process is to long-term regeneration. But the study also shows what planting within fragments cannot do alone. It cannot substitute for native tree cover in the surrounding landscape, nor fully overcome the isolation that fragmentation imposes. Longer-term monitoring will be needed to see whether the composition of arriving seeds eventually shifts towards that of mature forests, and whether these gains carry through to seedling establishment and beyond.
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