Restoration interventions mediate tropical tree recruitment dynamics over time

Forest restoration is increasingly heralded as a global strategy to conserve biodiversity and mitigate climate change, yet long-term studies that compare the effects of different restoration strategies on tree recruit demographics are lacking. We measured tree recruit survival and growth annually in three restoration treatments-natural regeneration, applied nucleation and tree plantations-replicated at 13 sites in southern Costa Rica-and evaluated the changes over a decade. Early-successional seedlings had 14% higher survival probability in the applied nucleation than natural regeneration treatments. Early-successional sapling growth rates were initially 227% faster in natural regeneration and 127% faster in applied nucleation than plantation plots but converged across restoration treatments over time. Later-successional seedling and sapling survival were similar across treatments but later-successional sapling growth rates were 39% faster in applied nucleation than in plantation treatments. Results indicate that applied nucleation was equally or more effective in enhancing survival and growth of naturally recruited trees than the more resource-intensive plantation treatment, highlighting its promise as a restoration strategy. Finally, tree recruit dynamics changed quickly over the 10-year period, underscoring the importance of multi-year studies to compare restoration interventions and guide ambitious forest restoration efforts planned for the coming decades. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.

Multiscale predictors of small tree survival across a heterogeneous tropical landscape

Uncertainties about controls on tree mortality make forest responses to land-use and climate change difficult to predict. We tracked biomass of tree functional groups in tropical forest inventories across Puerto Rico and the U.S. Virgin Islands, and with random forests we ranked 86 potential predictors of small tree survival (young or mature stems 2.5-12.6 cm diameter at breast height). Forests span dry to cloud forests, range in age, geology and past land use and experienced severe drought and storms. When excluding species as a predictor, top predictors are tree crown ratio and height, two to three species traits and stand to regional factors reflecting local disturbance and the system state (widespread recovery, drought, hurricanes). Native species, and species with denser wood, taller maximum height, or medium typical height survive longer, but short trees and species survive hurricanes better. Trees survive longer in older stands and with less disturbed canopies, harsher geoclimates (dry, edaphically dry, e.g., serpentine substrates, and highest-elevation cloud forest), or in intervals removed from hurricanes. Satellite image phenology and bands, even from past decades, are top predictors, being sensitive to vegetation type and disturbance. Covariation between stand-level species traits and geoclimate, disturbance and neighboring species types may explain why most neighbor variables, including introduced vs. native species, had low or no importance, despite univariate correlations with survival. As forests recovered from a hurricane in 1998 and earlier deforestation, small trees of introduced species, which on average have lighter wood, died at twice the rate of natives. After hurricanes in 2017, the total biomass of trees ≥12.7 cm dbh of the introduced species Spathodea campanulata spiked, suggesting that more frequent hurricanes might perpetuate this light-wooded species commonness. If hurricane recovery favors light-wooded species while drought favors others, climate change influences on forest composition and ecosystem services may depend on the frequency and severity of extreme climate events.

Unraveling the wheel of recruitment for salt-marsh seedlings: Resistance to and recovery after dislodgement

Elucidating bottlenecks at critical life stages and quantifying associated resilience (including resistance and recovery) to physical processes are central in inform restoration and attain sustainable development of coastal biogeomorphic ecosystems. Seedling establishment is a key life stage determines saltmarsh restoration potentials. However, the resilience of these recruits, especially through recovery, remains poorly understood. Here, two contrasting globally occurring saltmarsh species, namely Salicornia europaea and Spartina anglica, were employed to generate insights in i) seedling resistance against dislodgement, and ii) seedling recovery potential after dislodgement. Regarding resistance, we found that 1) root-shoot antagonism characterizes the growth rate of seedling resistance to dislodgement through hydraulic disturbance, 2) the root length determines seedling resistance to dislodgement through sheet erosion; 3) a 5 mm sedimentary setting amplifies seedling resistance without inhibiting their morphological evolution. Regarding recovery, we found that 4) dislodged seedlings have a high probability for achieving long-distance dispersal; 5) seedling age and the inundation-free period regulate the re-establishment potential of dislodged seedlings. Overall, S. anglica showed stronger resilience than S. europaea, characterized by stronger seedling resistance against dislodgement and higher re-establishment potential. Our results on seedling resilience suggest that seedling dislodgement is not an end-of-life cycle but a new spin on the "Wheel of Recruitment", a proposed short-term cyclic behavior with alternating phases of seedling dislodgement, dispersal, and (re-)establishment. The Wheel of Recruitment concept is important for forecasting resilience and persistence of biogeomorphic systems such as salt marshes under global change and for guiding life cycle informed restoration.

Species-level tree crown maps improve predictions of tree recruit abundance in a tropical landscape

Predicting forest recovery at landscape scales will aid forest restoration efforts. The first step in successful forest recovery is tree recruitment. Forecasts of tree recruit abundance, derived from the landscape-scale distribution of seed sources (i.e., adult trees), could assist efforts to identify sites with high potential for natural regeneration. However, previous work revealed wide variation in the effect of seed sources on seedling abundance, from positive to no effect. We quantified the relationship between adult tree seed sources and tree recruits and predicted where natural recruitment would occur in a fragmented, tropical, agricultural landscape. We integrated species-specific tree crown maps generated from hyperspectral imagery and property ownership data with field data on the spatial distribution of tree recruits from five species. We then developed hierarchical Bayesian models to predict landscape-scale recruit abundance. Our models revealed that species-specific maps of tree crowns improved recruit abundance predictions. Conspecific crown area had a much stronger impact on recruitment abundance (8.00% increase in recruit abundance when conspecific tree density increases from zero to one tree; 95% credible interval (CI): 0.80% to 11.57%) than heterospecific crown area (0.03% increase with the addition of a single heterospecific tree, 95% CI: -0.60% to 0.68%). Individual property ownership was also an important predictor of recruit abundance: The best performing model had varying effects of conspecific and heterospecific crown area on recruit abundance, depending on individual property ownership. We demonstrate how novel remote sensing approaches and cadastral data can be used to generate high-resolution and landscape-level maps of tree recruit abundance. Spatial models parameterized with field, cadastral, and remote sensing data are poised to assist decision support for forest landscape restoration.

Uniting niche differentiation and dispersal limitation predicts tropical forest succession

Tropical secondary forests are increasingly important for carbon sequestration and biodiversity conservation worldwide; yet, we still cannot accurately predict community turnover during secondary succession. We propose that integrating niche differentiation and dispersal limitation will generate an improved theoretical explanation of tropical forest succession. The interaction between seed sources and dispersers regulates seed movement throughout succession, and recent technological advances in animal tracking and molecular analyses enable us to accurately monitor seed movement as never before. We propose a framework to bridge the gap between niche differentiation and dispersal limitation. The Source-Disperser Limitation Framework (SDLF) provides a way to better predict secondary tropical forest succession across gradients of landscape disturbance by integrating seed sources and frugivore behavior.

Monitoring tropical forest succession at landscape scales despite uncertainty in Landsat time series

Forecasting rates of forest succession at landscape scales will aid global efforts to restore tree cover to millions of hectares of degraded land. While optical satellite remote sensing can detect regional land cover change, quantifying forest structural change is challenging. We developed a state-space modeling framework that applies Landsat satellite data to estimate variability in rates of natural regeneration between sites in a tropical landscape. Our models work by disentangling measurement error in Landsat-derived spectral reflectance from process error related to successional variability. We applied our modeling framework to rank rates of forest succession between 10 naturally regenerating sites in Southwestern Panama from about 2001 to 2015 and tested how different models for measurement error impacted forecast accuracy, ecological inference, and rankings of successional rates between sites. We achieved the greatest increase in forecasting accuracy by adding intra-annual phenological variation to a model based on Landsat-derived normalized difference vegetation index (NDVI). The best-performing model accounted for inter- and intra-annual noise in spectral reflectance and translated NDVI to canopy height via Landsat-lidar fusion. Modeling forest succession as a function of canopy height rather than NDVI also resulted in more realistic estimates of forest state during early succession, including greater confidence in rank order of successional rates between sites. These results establish the viability of state-space models to quantify ecological dynamics from time series of space-borne imagery. State-space models also provide a statistical approach well-suited to fusing high-resolution data, such as airborne lidar, with lower-resolution data that provides better temporal and spatial coverage, such as the Landsat satellite record. Monitoring forest succession using satellite imagery could play a key role in achieving global restoration targets, including identifying sites that will regain tree cover with minimal intervention.

Effects of dispersal- and niche-based factors on tree recruitment in tropical wet forest restoration

Both dispersal- and niche-based factors can impose major barriers on tree establishment. Our understanding of how these factors interact to determine recruitment rates is based primarily on findings from mature tropical forests, despite the fact that a majority of tropical forests are now secondary. Consequently, factors influencing seed limitation and the seed-to-seedling transition (STS) in disturbed landscapes, and how those factors shift during succession, are not well understood. We used a 3.5-yr record of seed rain and seedling establishment to investigate factors influencing tree recruitment after a decade of recovery in a tropical wet forest restoration experiment in southern Costa Rica. We asked (1) how do a range of restoration treatments (natural regeneration, applied nucleation, plantation), canopy cover, and life-history traits influence the STS and (2) how do seed and establishment limitation (lack of seed arrival or lack of seedling recruitment, respectively) influence vegetation recovery within restoration treatments as compared to remnant forest? We did not observe any differences in STS rates across restoration treatments. However, STS rates were lowest in adjacent later successional remnant forests, where seed source availability did not highly limit seed arrival, underscoring that niche-based processes may increasingly limit recruitment as succession unfolds. Additionally, larger-seeded species had consistently higher STS rates across treatments and remnant forests, though establishment limitation for these species was lowest in the remnant forests. Species were generally seed limited and almost all were establishment limited; these patterns were consistent across treatments. However, our results suggest that differences in recruitment rates could be driven by differential dispersal to treatments with higher canopy cover. We found evidence that barriers to recruitment shift during succession, with the influence of seed limitation, mediated by species-level seed deposition rates, giving way to niche-based processes. However, establishment limitation was lowest in the remnant forests for large-seeded and late successional species, highlighting the importance of habitat specialization and life-history traits in dictating recruitment dynamics. Overall, results demonstrate that active restoration approaches such as tree planting catalyze forest recovery, not only by decreasing components of seed limitation, but also by developing canopy cover that increases establishment rates of larger-seeded species.