Ecological restoration success is higher for natural regeneration than for active restoration in tropical forests

Abstract

Is active restoration the best approach to achieve ecological restoration success (the return to a reference condition, that is, old-growth forest) when compared to natural regeneration in tropical forests? Our meta-analysis of 133 studies demonstrated that natural regeneration surpasses active restoration in achieving tropical forest restoration success for all three biodiversity groups (plants, birds, and invertebrates) and five measures of vegetation structure (cover, density, litter, biomass, and height) tested. Restoration success for biodiversity and vegetation structure was 34 to 56% and 19 to 56% higher in natural regeneration than in active restoration systems, respectively, after controlling for key biotic and abiotic factors (forest cover, precipitation, time elapsed since restoration started, and past disturbance). Biodiversity responses were based primarily on ecological metrics of abundance and species richness (74%), both of which take far less time to achieve restoration success than similarity and composition. This finding challenges the widely held notion that natural forest regeneration has limited conservation value and that active restoration should be the default ecological restoration strategy. The proposition that active restoration achieves greater restoration success than natural regeneration may have arisen because previous comparisons lacked controls for biotic and abiotic factors; we also did not find any difference between active restoration and natural regeneration outcomes for vegetation structure when we did not control for these factors. Future policy priorities should align the identified patterns of biophysical and ecological conditions where each or both restoration approaches are more successful, cost-effective, and compatible with socioeconomic incentives for tropical forest restoration.

Fig. 1. Meta-analysis controlling for the biotic and abiotic factors.

Bootstrapped mean response ratio for biodiversity (plants, invertebrates, birds, mammals, and herpetofauna) and vegetation structure (cover, density, biomass, height, and litter) in natural regeneration or active restoration systems compared with reference systems controlled for the four biotic and abiotic factors (amount of forest cover at the landscape scale, total annual precipitation, past disturbance, and the time elapsed since restoration started). n, total sample size; sl, number of study landscapes (sample size used in each resampling to avoid spatial pseudoreplication). Each box plot shows the median value (central solid line) and first and third quartile ranges (left and right outer borders of the box) of 1000 resampled (with replacement) mean response ratios. Dashed lines indicate a response ratio of 0, that is, no difference to reference systems. Notches in boxes represent 95% confidence intervals, and thus, nonoverlapping notches between boxes imply a significant difference (66). *, not controlled for forest cover (always significantly different between natural regeneration and active restoration systems). For mammals and herpetofauna, restoration success was not estimated in active restoration systems because of the small number of study landscapes.