Natural regeneration in tree islands drives long-term recovery of native forest-associated woody species in oil palm landscapes
1. Large-scale conversion of tropical rainforests to oil palm plantations is a major driver of biodiversity loss in Southeast Asia, particularly threatening native forest-associated plant species and their associated biota. Integrating tree islands within monocultural oil palm landscapes has emerged as a promising approach to mitigate biodiversity loss. However, their effectiveness in sustaining natural regeneration and long-term recovery of native forest-associated plant species remains unclear. 2. We analyzed temporal trajectories of diversity, abundance, and composition of naturally recruiting woody species in tree islands embedded in a monoculture-dominated landscape. We used a large-scale biodiversity enrichment experiment with 52 tree islands, varying in area (25 - 1600 m2) and planted tree diversity (0, 1, 2, 3, or 6 species), established in a conventional oil palm plantation in Sumatra, Indonesia, in 2013. 3. Across four censuses conducted between 2 and 8 years after the onset of natural regeneration, we found 82 species of recruiting woody plants (trees and shrubs) belonging to 64 genera and 34 families. At the landscape (gamma) scale, species richness and abundance increased by ~42% and ~69%, respectively, from 2018 to 2024, with shifts in community evenness and continued species arrival. After 8 years, neither richness nor abundance saturated, indicating continued accumulation of recruiting plant species. 4. Tree islands promoted natural regeneration of native diversity, with 90% of species and individuals being native. Native forest-associated species (N = 35) increased steadily at the landscape scale, while localized dominance of alien species did not indicate arrested succession. 5. At the local (alpha) scale, larger island area exerted a sustained positive effect on recruiting diversity. Higher initial planted tree diversity promoted native recruitment, particularly of forest-associated species, at earlier stages, an effect that diminished over time. 6. Synthesis and applications. Tree islands promote natural regeneration and accelerate recovery of native forest-associated diversity over extended periods, providing an effective strategy to enhance forest-associated biodiversity within oil palm landscapes. Preserving the last remaining lowland rainforests, irreplaceable reservoirs of native forest-associated species, remains essential to support continued recruitment into tree islands that can create stepping-stone habitats and a more wildlife-friendly agricultural matrix.