Epsilon Archive for Student Projects

Abstract

Tropical forest degradation threatens biodiversity and carbon recovery across Southeast Asia, highlighting the need for effective restoration strategies. A key response to this challenge is assisted natural regeneration (ANR) — an active restoration approach for degraded tropical landscapes — which combines selective enrichment planting of native seedlings with maintenance activities to enable and enhance natural regeneration. Yet long-term field evidence on the efficacy of specific enrichment planting strategies within ANR remains scarce, particularly from large-scale restoration projects in degraded tropical forests.
This thesis evaluates 25-year outcomes of gap-cluster and line enrichment planting against untreated control plots at the INIKEA restoration project in Sabah, Malaysian Borneo, using linear mixed-effects models across a gradient of initial site degradation to assess treatment effects on aboveground biomass, seedling performance, and planted stem contribution to stand-level recovery.
Gap-cluster planting produced significantly greater total aboveground biomass than untreated controls (p = 0.019; difference: 53.9 ± 18.4 Mg/ha). The line treatment showed a consistent positive trend that did not reach statistical significance, and no significant difference in planted seedling survival or establishment was detected between treatments. Initial site condition, measured as the basal area of pre-disturbance remnant stems exceeding 60 cm DBH, was the dominant predictor of total and natural biomass (p < 0.001) but showed no association with post-restoration biomass accumulation, while planted stem contribution to total stand biomass declined significantly with increasing remnant basal area (p < 0.001). Biomass differences between treatments were concentrated in larger stem size classes, in line with either the direct contribution of planted material or the growth of naturally regenerating stems benefiting from maintenance activities, or a combination of both. These patterns are consistent with a compensatory dynamic in which natural regeneration offsets reduced planted stem contribution at structurally intact sites, producing post-restoration biomass accumulation broadly comparable across the degradation gradient.
These findings suggest that gap-cluster enrichment planting can meaningfully accelerate biomass recovery in degraded dipterocarp forest, and that initial site degradation level is at least as important a determinant of restoration outcomes as the choice of planting format — affecting not only how enrichment planting is executed but also whether active seedling planting is warranted at all, at least within the 25-year timeframe examined here.