Vivien Michel

Master thesis

Authot: Vivien Michel

Supervisors: Gustavo Brant Paterno & Holger Kreft

Title: Natural regeneration in tree islands drives longterm recovery of native forest-associated woody species in oil palm landscapes (Sumatra, Indonesia)

PDF: Master thesis

Summary

1. Large-scale conversion of tropical rainforest to oil palm plantations is a major driver of biodiversity loss in Southeast Asia, particularly threatening native forest-associated woody species and their associated biota. Integrating tree islands within monocultural oil palm landscapes has emerged as a promising approach to enhance native biodiversity. However, the effectiveness of such interventions in sustaining natural regeneration and long-term recovery of native woody species remains unclear.
2. Using a large-scale biodiversity enrichment experiment of 52 tree islands varying in size (25 m² to 1600 m²) and planted tree diversity (0, 1, 2, 3, and 6 species) embedded within a conventional oil palm plantation in Sumatra since 2013, I analyzed temporal trajectories in the diversity and abundance of naturally recruiting woody species based on four complete censuses conducted between four and ten years (2018 – 2024) after natural regeneration began.
3. Across all censuses, 82 recruiting woody species (trees and shrubs) were recorded, representing 64 genera and 34 families. At the landscape (gamma) scale, species richness and abundance increased by ~42% and ~69%, respectively, from 2018 to 2024, accompanied by shifts in community evenness and continued arrival of locally rare species. After ten years, neither richness nor abundance showed signs of saturation, indicating ongoing accumulation of recruiting species.
4. Tree islands consistently favored natural regeneration of native diversity, with 90 % of recruiting species and individuals being native. Native forest-associated species (N=35 across years) increased steadily at landscape scale, while localized dominance of alien species did not indicate successional arrest.
5. At the local (alpha) scale, larger tree island area exerted and sustained a positive effect on recruiting diversity, and higher initial planted tree diversity promoted native recruitment, and in particular native forest-associated recruiting species, at earlier stages, although this effect diminished over time.
6. Synthesis and applications. Tree islands enhance natural regeneration and accelerate the recovery of native forest-associated woody diversity over extended time periods, providing an effective strategy for conserving tropical forest biodiversity within oil palm landscapes. However, preserving primary forests, irreplaceable reservoirs of native forest-associated species, remain essential for supporting continued recruitment into tree islands and maintaining their function as stepping-stone habitats.