Testing applied nucleation as a strategy to facilitate tropical forest recovery.
Active forest restoration typically involves planting trees over large areas; this practice is costly, however, and establishing homogeneous plantations may favour the recruitment of a particular suite of species and strongly influence the successional trajectory. An alternative approach is to plant nuclei (islands) of trees to simulate the nucleation model of succession and accelerate natural recovery. We evaluated natural tree recruitment over 4 years in a restoration study replicated at eight former pasture sites in the tropical premontane forest zone of southern Costa Rica. At each site, two active restoration strategies were established in 50×50 m plots: planting trees throughout, and planting different-sized tree islands (4×4, 8×8, 12×12 m) within the plot. Restoration plots were compared to similar-sized controls undergoing passive restoration. Sites were spread across c. 100 km2 and distributed along a gradient of surrounding forest, allowing us to compare the relative importance of adjacent forest to that of within-site treatment on tree recruitment. Recruitment of animal-dispersed tree species was more than twofold higher in active (μ=0.45 recruits m-2) as compared to passive restoration; recruitment was equivalent in plantation and island treatments, even though only 20% of the area in island plots was planted originally. The majority of recruits (>90%) represented early successional species (n=54 species total). Density of animal-dispersed recruits was greater in large (0.80±0.66 m-2) than small (0.28±0.36 m-2) islands and intermediate in medium-sized islands. Seedling recruitment (<1 m tall) was greater in the interior of islands as compared to plantations, whereas sapling recruitment was similar, suggesting that island interiors may develop greater density of woody recruits as succession proceeds. Surrounding forest cover did not influence density or species richness of recruits among sites, although this factor may become more important over time. Synthesis and applications. Applied nucleation is a promising restoration strategy that can accelerate forest recovery to a similar degree as plantation-style restoration but is more economical. Appropriate island size is on the order of c. 100 m2. Practitioners should consider the methodology as an alternative to large-scale plantings.