Early-stage restoration outcomes under active and passive rehabilitation interventions in a humid tropical forest

Abstract

Restoration of degraded forests and landscapes is emerging as a global conservation priority. Opinion is divided, however, whether active restoration is a better approach than passive intervention in forest ecosystem rehabilitation. We assessed the variation in woody species composition and stand structure during the first five years of both active and passive restoration interventions in a degraded humid forest in western Kenya. Passive restoration entailed natural regeneration, while active restoration comprised planting at 5m, 1m and 0.3m spacing. The plots were protected from repeat incidences of disturbance using enclosures. Natural regeneration registered a significantly higher woody species richness (36.75±6.97) than active restoration (12.75±1.75). It had a higher Shannon-Wiener diversity index (3.05) than active restoration (2.32). Despite up to 10,000 seedlings per ha being planted under active restoration, woody stem density was significantly higher under natural regeneration (21,789±7,087 stems ha-1) than active restoration (13,118±1,857 stems ha-1). Mean sapling height was higher under natural regeneration (2.60±0.31 m) than active restoration (1.38±0.30 m). The results suggest that passive restoration interventions may be superior to active restoration approaches in rehabilitating degraded forest landscapes in the humid tropics if repeat incidences of disturbance are controlled. The long-held view that active restoration leads to greater woody species diversity and stand structure may have been spurred by the fact that ecological restoration interventions are often carried out in open sites that are exposed to continual incidences of disturbance, which tends to hamper the recruitment and survival of natural regrowth.