Applied nucleation, mostly based upon planting tree islands, has been proposed as a cost‐effective strategy to meet ambitious global forest and landscape restoration targets.

We review results from a 15‐year study, replicated at 15 sites in southern Costa Rica, that compares applied nucleation to natural regeneration and mixed‐species tree plantations as strategies to restore tropical forest. We have collected data on planted tree survival and growth, woody vegetation recruitment and structure, seed rain, litterfall, epiphytes, birds, bats and leaf litter arthropods.

Our results indicate that applied nucleation and plantation restoration strategies are similarly effective in enhancing the recovery of most floral and faunal groups, vegetation structure and ecosystem functions, as compared to natural regeneration.

Seed dispersal and woody recruitment are higher in applied nucleation and plantation than natural regeneration treatments; canopy cover has increased substantially in both natural regeneration and applied nucleation treatments; and mortality of planted N‐fixing tree species has increased in recent years. These trends have led to rapid changes in vegetation composition and structure and nutrient cycling.

The applied nucleation strategy is cheaper than mixed‐species tree plantations, but there may be social obstacles to implementing this technique in agricultural landscapes, such as perceptions that the land is not being used productively.

Applied nucleation is likely to be most effective in cases where: planted vegetation nuclei enhance seed dispersal and seedling establishment of other species; the spread of nuclei is not strongly inhibited by abiotic or biotic factors; and the approach is compatible with restoration goals and landowner preferences.

Synthesis and applications. Results from our 15‐year, multi‐site study show that applied nucleation can be a cost‐effective strategy for facilitating tropical forest regeneration that holds promise for helping to meet large‐scale international forest restoration commitments.

We sampled bird communities in 11 restoration sites and 5 mature forests in southern Costa Rica (Coto Brus County). Restoration sites were located between Las Cruces and the town of Agua Buena (8° 44' N, 82° 56' W). Study sites were 1100-1400 m a.s.l., and the dominant natural ecosystem was premontane moist forest. Precipitation across the study areas varies with microtopography but is ~4 m y-1 at Las Cruces. Mean annual temperature at Las Cruces is 21° C.

Eleven restoration sites were established on degraded agricultural land in 2004-2006. Each site included three 50 x 50 m plots (N = 33 plots), which were cleared of vegetation and randomly assigned to one of three treatments. Controls were abandoned to regenerate naturally; islands (i.e., applied nucleation treatments) were planted with patches of trees including two each of 4 x 4, 8 x 8, and 12 x 12 m patches; and plantations were planted with rows of seedlings across the entire plot. These local restoration treatments spanned a gradient of intervention intensity; seedling density (seedlings ha-1) was zero for controls, 86 for islands, and 313 for plantations. Seedlings planted in islands and plantations were a mix of two native timber species, Terminalia amazonica (J.F. Gmel.) Exell (Combretaceae) and Vochysia guatemalensis Donn. Sm. (Vochysiaceae), and two naturalized legumes, Erythrina poeppigiana Walp. Skeels and Inga edulis Mart. (Fabaceae). All plots were cleared with machetes for 2.5 years to allow planted seedlings to grow above existing grasses and forbs. Treatments had been in place for 10-12 y when bird counts took place at which time there were already large differences in vegetation structure and composition between treatments. Detailed descriptions of the restoration treatments and tree species selection are provided elsewhere (Cole et al. 2010).

Birds in restoration sites were surveyed by J.A.R. in Mar-Apr, Jul-Aug, and Nov-Dec 2016. Each experimental plot at each site was actively searched in a random order for 20 min per observation, and all frugivorous birds seen or heard within the plot were recorded. Birds flying over the site without using it were excluded from analyses. Bird taxonomy follows the American Ornithologists’ Union Checklist of the Birds of North America and its supplements (American Ornithologists' Union 1998).