Canopy cover and ecological restoration increase natural regeneration of rainforest trees in the Western Ghats, India

Abstract

Restoration of canopy cover through tree planting can assist in overcoming barriers to natural regeneration and catalyze recovery of degraded tropical forests. India has made international pledges to restore millions of hectares of degraded forests by 2030, but lacks empirical research on regeneration under different types of planted and natural overstories to guide this mission. We conducted a field study (65 plots of 25 m2) to examine the influence of overstory type and canopy cover on naturally regenerating tree seedlings across degraded rainforests (DRs), mixed‐native species ecological restoration (ER) sites, monoculture eucalypt plantations (MP), and mature “benchmark” rainforests (BR) in the Western Ghats mountains of peninsular India. ER had higher native tree seedling densities and recovered community composition toward BR levels compared to DR, while communities in MP shifted in the opposite direction. Densities of native late‐successional species increased with canopy cover (particularly in ER), but greater canopy cover was also associated with increases in alien species, a few of which are shade‐tolerant. Further, in a nursery experiment comprising four rainforest species, seed germination and early survival increased with shade, but did not vary across soils originating from DR, ER, and MP. Our findings show that while improving canopy cover is important, doing so by planting diverse native species, and controlling invasive alien species, can benefit rainforest recovery in DR fragments. Conversely, planting non‐native monocultures in degraded forests, which is a prevalent practice in India, could prove counterproductive for forest recovery in the long term.

Bornean tropical forests recovering from logging at risk of regeneration failure

“…While the co‐benefits of biodiversity recovery during silvicultural approaches for biomass recovery are often presumed (Crouzeilles et al., 2017; Hariharan & Shankar Raman, 2022; Osuri et al., 2022), it remains unknown whether active restoration through these means delivers recovery of the reference community assemblage. Our study shows that 15–27 years after implementation of active restoration via rehabilitative silvicultural treatments in the logged forests, the richness and evenness of seedling communities were higher than those in adjacent NR forests and more closely resembled those of the seedling communities in UL forest, while nonetheless remaining distinct from these, in the initial 2 months after masting.…”

“…While the co-benefits of biodiversity recovery during silvicultural approaches for biomass recovery are often presumed (Crouzeilles et al, 2017;Hariharan & Shankar Raman, 2022;Osuri et al, 2022), it remains unknown whether active restoration through these means delivers recovery of the reference community assemblage.…”

Diversity stabilizes but does not increase sapling survival in a tree diversity experiment

“…While we did not deliberately manipulate shade-tolerance in our planting, other restoration projects have found that it is possible to facilitate forest recovery by planting shade-intolerant species initially and then allowing natural recruitment of shade-tolerant species in the understory (Osuri et al 2022). This can increase planting survival and reduce the costs of planting by requiring the planting of fewer species (Brancalion et al 2019).…”

Ecosystem restoration in India during the United Nations decade on ecosystem restoration: the way forward

“…This paper provides brief information on the rehabilitation of highly degraded lands with trees, bioenergy crops, diversified cropping systems, man-made forests, and agroforestry models investigated as ecological restoration models to be adopted for various socioeconomic goals. Osuri et al (2022), on the restoration of degraded rainforests in the Western Ghats of India, show that restoration through diverse native species can help recovery to a very close state to the reference ecosystem compared to monoculture plantations. The authors conducted field studies in a large area within a tropical rainforest ecosystem to investigate the effect of top story canopy cover (which plays a vital role in shaping ground story vegetation) on regeneration of native tree seedlings in degraded rainforests, forest under ecological restoration with mixed-native species, single-species forest stands, and a reference mature rainforest.…”