QUESTIONS: Natural regeneration is increasingly recognized as a potentially cost‐effective strategy to reach ambitious forest landscape restoration targets, but rates of recovery are notoriously variable. We asked how well initial habitat conditions after cessation of agriculture predict forest recovery after nearly a decade. We aimed to provide land managers with general rules of thumb to assess when it is necessary to invest resources in active restoration, such as tree planting, to accelerate forest recovery. LOCATION: Coto Brus County, Puntarenas, Costa Rica. METHODS: We compiled data on initial vegetation structure, soil nutrients, prior land‐use history and surrounding forest cover at 13 sites. After 8.5 years, we measured vegetation indicators commonly used to assess forest recovery, namely amount of canopy closure and number and diversity of woody recruits. RESULTS: Two variables, grass cover and canopy closure, measured 1.5 years after site abandonment, explained 47–87% of five of the six response variables after 8.5 years; recovery was faster in sites with lower grass cover and higher canopy closure initially. Waiting an additional year to measure initial vegetation variables did not improve model fit. Time since the original forest was cleared explained 62% of change in canopy cover, whereas percentage of surrounding forest cover, length of pasture use and soil variables explained minimal additional variation. CONCLUSIONS: Our results suggest that two easily measurable vegetation variables can provide guidance to land managers and policy makers about where to invest scarce restoration resources to facilitate forest recovery.

Abstract: In degraded tropical landscapes, lack of seed dispersal can strongly limit recovery, and restoration interventions can overcome this barrier by attracting dispersers. However, seed dispersal patterns are typically studied over short time periods, thus the influences of temporal and spatial variability on seed arrival cannot be teased apart. The choice of management approach can have important implications for restoration‐mediated community reassembly. Accordingly, we used a 3.5‐year record of seed deposition in pre‐montane tropical wet forest in southern Costa Rica to examine how seed arrival differed between passive (natural regeneration) and active (applied nucleation, plantation) restoration after a decade of recovery, compared to remnant forest. We investigated: (a) how restoration treatments affected seed deposition rates and community composition; (b) how within‐plot heterogeneity of animal‐dispersed seed deposition varied by intervention; and (c) how interannual variation influenced animal‐dispersed seed arrival across treatments. Overall seed rain composition and diversity in restoration treatments was converging towards, but still differed substantially from, remnant forest (89.7%, 86.6% and 76.3% Shannon diversity recovered in applied nucleation, plantation and natural regeneration respectively). Within‐plot animal‐dispersed seed heterogeneity was similar in applied nucleation and remnant forest, 27.0% more heterogeneous in applied nucleation than plantation, and equivalent when comparing natural regeneration to either applied nucleation or plantation. In contrast to active interventions, animal‐dispersed tree and shrub communities did not differ year to year in natural regeneration, which may promote the assembly of relatively homogeneous plant communities at this successional stage. Synthesis and applications. Compared to natural regeneration, active restoration interventions: (a) catalysed the recovery of seed diversity (overall Shannon diversity 17.5% and 13.4% higher in applied nucleation and plantation respectively), (b) shifted seed community composition towards remnant forest more rapidly (overall Shannon diversity 13.4% and 10.2% closer), (c) almost doubled the proportion of later‐successional tree species arriving, and (d) had seed communities that differed year to year—a pattern not observed in natural regeneration. Finally, applied nucleation was the only intervention where seed arrival was as spatially heterogeneous as remnant forest, highlighting that this approach may facilitate the recovery of specific natural dispersal processes.

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.

Choosing appropriate forest restoration interventions is challenging. Natural regeneration can rapidly facilitate forest recovery in many situations. However, barriers such as dispersal limitation and competition with non-native species can require assisted restoration approaches to facilitate plant community recovery. We used a study that has directly compared the outcomes of tropical wet forest restoration interventions across 11 replicate sites in southern Costa Rica. Within this framework, we examined the functional recovery trajectories of recruiting tree sapling communities across a gradient of restoration interventions including low (natural regeneration), intermediate (applied nucleation), and high (plantation) initial resource-investment, which we compared to remnant reference forest. We collated leaf and stem functional traits for tree species that comprised the bulk of recruiting saplings, then determined how community-weighted trait means and functional diversity metrics changed over a decade across treatments. Results show that assisted restoration approaches (applied nucleation, plantation) sped the development of more functionally diverse tree communities, more than tripling the functional richness (FRic) of recruiting communities when compared to natural regeneration. However, functional dispersion (i.e., the trait range of dominant species) was equivalent across interventions, and between 28 and 44% lower than remnant forest, indicating that increases in FRic under assisted restoration were driven by species recruiting in low abundances (<10 individuals across treatments). Recruits in assisted restoration treatments also had 10–15% tougher, less-palatable leaves, and leaves were even tougher in reference forest, which could be driven by increasing herbivory pressure along the gradient of interventions. Results show that tracking simple metrics such as species richness can mask a more mechanistic understanding of ecosystem recovery that is elucidated by taking a functional trait-driven approach toward evaluating outcomes. For example, our work identified a paucity of dense-wooded species recruiting across restoration interventions, wood density was 11–13% lower in restoration treatments than reference forests, underscoring such species as prime targets for enrichment planting. Overall, findings suggest that assisted restoration can catalyze the functional recovery of naturally recruiting tree communities in landscapes that are slow to recover naturally and highlight the importance of evaluating how different components of functional diversity shift over time to fully understand restoration outcomes.