Aboveground Biomass Accumulation Over Two Decades Across a Gradient of Tropical Forest Restoration Interventions.

Tropical forest restoration is an important nature-based solution that can sequester carbon as a means to mitigate climate change. Restoration approaches range from natural regeneration to intensive tree planting, but few studies have compared aboveground biomass (AGB) stocks over decadal time scales across multiple restoration treatments implemented at the same sites. Here we leverage data from a two-decade restoration experiment in southern Costa Rica; in 2004-2006, we established restoration plots representing a gradient of intervention intensity: natural regeneration (no planting), applied nucleation (planting tree clusters), and plantation (full planting). We compare 18-20 years of AGB across treatments of the four planted tree species and naturally recruited trees. We also examine the relationships between AGB pools and structural metrics derived from UAV-borne LiDAR data collected after 16-18 years. Because most AGB was in planted trees, AGB was about 6.5 times greater in plantations compared to natural regeneration, and double in plantations compared to applied nucleation after two decades, despite substantial mortality of planted trees. However, the plantation treatment suppressed naturally recruited AGB and accumulated only half the amount of the natural regeneration treatment. Natural recruits comprised four times the proportion of AGB in applied nucleation compared to plantation. LiDAR-quantified LAI was more tightly correlated with total AGB in natural regeneration plots, whereas canopy height was more strongly correlated with planted biomass in plantation and applied nucleation. Our results highlight that tree planting accelerates AGB accumulation at degraded sites and illustrate that practitioners should select species with complementary life history strategies that enable carbon to be sequestered beyond the first decade. Given the tradeoff between planted tree biomass and naturally recruited biomass, spatially patterned methods that plant fewer trees may better balance restoration goals beyond carbon accumulation, leading to more structurally and biologically diverse reforested systems.