This meta-analysis comparing active restoration to natural ecosystem regeneration found the latter to be more effective. The authors conclude that “lower-cost natural regeneration surpasses active restoration in achieving tropical forest restoration success for biodiversity and vegetation structure” [Crouzeilles 2017: 4]. This conclusion runs counter to conventional wisdom that active restoration is preferable despite being more expensive.
Natural forest regeneration is the spontaneous recovery of native tree species that colonize and establish in abandoned fields or natural disturbances; this process can also be assisted through human interventions such as fencing to control livestock grazing, weed control, and fire protection. In contrast, active restoration requires planting of nursery-grown seedlings, direct seeding, and/or the manipulation of disturbance regimes (for example, thinning and burning) to speed up the recovery process, often at a high cost to establish structural features of the vegetation (hereafter termed vegetation structure), reassemble local species composition, and/or catalyze ecological succession [Crouzeilles 2017: 1].
However, “restoration success for biodiversity and vegetation structure was significantly lower in both natural regeneration and active restoration than in reference systems” [Crouzeilles 2017: 2], underscoring the importance of conserving existing intact ecosystems.
“Restoration success for biodiversity and vegetation structure was significantly lower in both natural regeneration and active restoration than in reference systems” [Crouzeilles 2017: 2], underscoring the importance of conserving existing intact ecosystems.
Part of the explanation for the lower success of active restoration compared to natural regeneration is that the composition and/or diversity of species chosen for planting in active restoration may be inappropriate, while the species that colonize abandoned land are likely to be diverse and locally adapted.
Natural regeneration is initiated through the colonization of opportunistic and locally adapted species, resulting in a stochastic dynamic process of forest restoration that ultimately leads to higher diversity of native, locally adapted plant species than in tree planting schemes (that is, active restoration). Active restoration also can create a highly diverse habitat through human introduction of up to 6000 seedlings/ha, but tree species used in plantings often lack the full range of functional traits found in natural regrowth forests. In addition, most tropical forest plantings for restoration or forest plantations use relatively few species, that is, these plantations may not be planted primarily for biodiversity outcomes. Thus, the higher plant biodiversity in naturally regenerated systems creates more habitats and resources, which provide additional sources of food, shelter, nesting, and breeding sites, to support higher animal biodiversity [Crouzeilles 2017: 2].
Crouzeilles, Renato, et al., 2017, Ecological restoration success is higher for natural regeneration than for active restoration in tropical forests, Science Advances 3, https://advances.sciencemag.org/content/3/11/e1701345?intcmp=trendmd-adv.
 Vegetation structure was determined by measuring the number of individuals per unit area, the amount of leaf litter, the area covered by vegetation (measured in three forest strata - floor, understory, and canopy), the amount of below- and above-ground biomass produced, and the aboveground height of vegetation [Crouzeilles 2017: 4].