COS 67-6 - Tropical forest restoration strategies show tradeoffs across recruitment stages during later succession

Abstract: In the wake of commitments by world governments and private initiatives to address global climate threats, forest restoration has been increasingly prioritized for the potential win-win of conserving biodiversity and sequestering carbon. Nevertheless, long-term data from tropical forest restoration projects needed to improve management strategies and practices are rare and further limited by a lack of spatial and temporal replication. We evaluated the effects of three restoration approaches on tree recruitment and community succession across a fragmented tropical forest landscape in southern Costa Rica. Plots (50 × 50 m) were established in 2004-2006 with scaled planting intensity: natural regeneration (no planting), applied nucleation (planted tree clusters), and plantation (full planting), and were compared to reference forest plots. We surveyed all tree taxa >20 cm at 10 replicate restoration sites during 2022 to determine treatment effects on the community composition and demographic recruitment patterns of seedlings, saplings, and trees. We recorded 242 tree species from 63 families. Both active restoration treatments (applied nucleation and plantations) resulted in significantly elevated seedling and sapling establishment compared to natural regeneration and had comparable tree richness to reference forests. Non-metric multidimensional scaling of community dissimilarity showed an increasing convergence of species composition that scaled positively with treatment intensity. Similarly, later-successional species that colonized seedling banks were significantly more likely to reach the sapling layer in active restoration treatments. However, this treatment effect was not sustained with trees >5 cm stem diameter, with natural regeneration and applied nucleation plots producing greater abundances and diversity of small and large trees. Our results show that active tree planting accelerates recovery even after nearly two decades, but that the differential between active restoration and natural regeneration is decreasing over time. We found notable trade-offs in initial restoration practice on mid-to-long-term results. Traditional and widely applied plantation arrays provide accelerated early establishment of a diverse seedling bank and understory communities but delay the progression of recruits into the forest canopy. Whereas applied nucleation planting techniques provide similar increases to overall diversity and early recruitment of later-successional, dispersal-limited taxa as more intensive planting arrays, they effectively mitigate the inhibiting effects of planted vegetation on tree recruitment through the development of a more heterogenous and open canopy. Results provide increasing empirical support for the practical and cost advantages of nucleated planting strategies on biodiversity outcomes.