Distance to forest, mammal and bird dispersal drive natural regeneration on degraded tropical peatland

Restoration of peat swamp forest (PSF) on degraded Southeast Asian peatlands could reduce global carbon emissions and biodiversity loss. However, multiple ecological barriers are believed to hinder natural regeneration of native trees on degraded peatland and make restoration expensive. We evaluated if natural PSF regeneration occurs and what factors may influence it on eight different land use and land cover (LULC) classes with different types of disturbance, including drainage and fire, in a retired Acacia crassicarpa Benth. (Acacia) plantation landscape. The study involved 42 plots inside five PSF LULCs - intact, logged, burnt (1997, 2015), remnant and 212 plots at distances up to 2 km from the PSF edge in three Acacia plantation LULCs - unharvested, harvested, and burnt. The number of species per plot were similar between intact PSF (25 +/- 6 (SD) per 20 m x 10 m plot), logged forest (30 +/- 6) and 1997 burnt forest (30 +/- 13) but lower in 2015 burnt forest (11 +/- 10) and remnant forest (18 +/- 11). Regeneration away from the PSF across all degraded LULCs varied from fern dominated areas with no regeneration to clusters with high stem densities. The plantation LULCs, unharvested (94 species) and harvested Acacia (71 species), had similar overall species diversity after 3-4 years of regeneration to the intact and logged PSF (90 species). In unharvested Acacia, total species diversity, species per plot and stem density decreased with distance from forest edge (1-300 m - 87 species; 9 +/- 6 (SD) species per 20 m x 10 m plot; 1,056 stems/ha; 301-500 m - 33; 5 +/- 2; 511 and > 500 m - 38; 6 +/- 3; 683). In harvested Acacia, there was low plot species diversity irrespective of distance from the forest (1-300 m - 51; 4 +/- 2; 578; 301-500 m - 17; 4 +/- 2; 1,100; > 500 m - 48; 4 +/- 2; 780). Factors which may influence regeneration differed between different LULCs, but there was a clear influence of distance from forest edge and dispersal mechanism - i.e. whether a tree was bird or mammal dispersed and the interaction between these two factors. While our study suggests that if not further disturbed by logging, drainage and/or fire, degraded PSF could regenerate naturally to a similar species diversity as intact PSF, the lower levels of natural regeneration further away from the forest may warrant selective planting of species which do not disperse over long distances. More study is needed on the factors facilitating natural regeneration, whether it leads to restoration of PSF ecosystem functioning and the role of Acacia as a potential regeneration catalyst.