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Although increasing efforts are being made to restore tropical forests, little information is available regarding the time scales required for carbon and plant biodiversity to recover to the values associated with undisturbed forests. To address this knowledge gap, we carried out a meta-analysis comparing data from more than 600 secondary tropical forest sites with nearby undisturbed reference forests. Above-ground biomass approached equivalence to reference values within 80 years since last disturbance, whereas below-ground biomass took longer to recover. Soil carbon content showed little relationship with time since disturbance. Tree species richness recovered after about 50 years. By contrast, epiphyte richness did not reach equivalence to undisturbed forests. The proportion of undisturbed forest trees and epiphyte species found in secondary forests was low and changed little over time. Our results indicate that carbon pools and biodiversity show different recovery rates under passive, secondary succession and that colonization by undisturbed forest plant species is slow. Initiatives such as the Convention on Biological Diversity and REDD+ should therefore encourage active management to help to achieve their aims of restoring both carbon and biodiversity in tropical forests.

We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, species-rich, tropical forests. We also update a key result and present the first species-level analyses of tree growth rates for a 15-yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community-level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low-P soils would respond most slowly. The data did not support this prediction, species-level tree growth responses to P addition were unrelated to species-level soil P associations. The meta-analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta-analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta-analysis found no support for the long-standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species-rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient-poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected.

In the wake of widespread loss of old-growth forests throughout the tropics, secondary forests will likely play a growing role in the conservation of forest biodiversity. We considered a complex hierarchy of factors that interact in space and time to determine the conservation potential of tropical secondary forests. Beyond the characteristics of local forest patches, spatial and temporal landscape dynamics influence the establishment, species composition, and persistence of secondary forests. Prospects for conservation of old-growth species in secondary forests are maximized in regions where the ratio of secondary to old-growth forest area is relatively low, older secondary forests have persisted, anthropogenic disturbance after abandonment is relatively low, seed-dispersing fauna are present, and old-growth forests are close to abandoned sites. The conservation value of a secondary forest is expected to increase over time, as species arriving from remaining old-growth forest patches accumulate. Many studies are poorly replicated, which limits robust assessments of the number and abundance of old-growth species present in secondary forests. Older secondary forests are not often studied and few long-term studies are conducted in secondary forests. Available data indicate that both old-growth and second-growth forests are important to the persistence of forest species in tropical, human-modified landscapes.

Biodiversity loss from deforestation may be partly offset by the expansion of secondary forests and plantation forestry in the tropics. However, our current knowledge of the value of these habitats for biodiversity conservation is limited to very few taxa, and many studies are severely confounded by methodological shortcomings. We examined the conservation value of tropical primary, secondary, and plantation forests for 15 taxonomic groups using a robust and replicated sample design that minimized edge effects. Different taxa varied markedly in their response to patterns of land use in terms of species richness and the percentage of species restricted to primary forest (varying from 5% to 57%), yet almost all between-forest comparisons showed marked differences in community structure and composition. Cross-taxon congruence in response patterns was very weak when evaluated using abundance or species richness data, but much stronger when using metrics based upon community similarity. Our results show that, whereas the biodiversity indicator group concept may hold some validity for several taxa that are frequently sampled (such as birds and fruit-feeding butterflies), it fails for those exhibiting highly idiosyncratic responses to tropical land-use change (including highly vagile species groups such as bats and orchid bees), highlighting the problems associated with quantifying the biodiversity value of anthropogenic habitats. Finally, although we show that areas of native regeneration and exotic tree plantations can provide complementary conservation services, we also provide clear empirical evidence demonstrating the irreplaceable value of primary forests.

Tropical forests are increasingly being subjected to hotter, drier conditions as a result of global climate change. The effects of drought on forests along successional gradients remain poorly understood. We took advantage of the 2015–2016 El Niño event to test for differences in drought response along a successional gradient by measuring the sap flow in 76 trees, representing 42 different species, in 8-, 25- and 80-yr-old secondary forests in the 15-km2 ‘Agua Salud Project’ study area, located in central Panama. Average sap velocities and sapwood-specific hydraulic conductivities were highest in the youngest forest. During the dry season drought, sap velocities increased significantly in the 80-yr-old forest as a result of higher evaporative demand, but not in younger forests. The main drivers of transpiration shifted from radiation to vapor pressure deficit with progressing forest succession. Soil volumetric water content was a limiting factor only in the youngest forest during the dry season, probably as a result of less root exploration in the soil. Trees in early-successional forests displayed stronger signs of regulatory responses to the 2015–2016 El Niño drought, and the limiting physiological processes for transpiration shifted from operating at the plant–soil interface to the plant–atmosphere interface with progressing forest succession.

Environmental selection and dispersal limitation are two of the primary processes structuring biotic communities in ecosystems, but little is known about these processes in shaping soil microbial communities during secondary forest succession. We examined the communities of ectomycorrhizal (EM) fungi in young, intermediate and old forests in a Chinese subtropical ecosystem, using 454 pyrosequencing. The EM fungal community consisted of 393 operational taxonomic units (OTUs), belonging to 21 EM fungal lineages, in which three EM fungal lineages and 11 EM fungal OTUs showed significantly biased occurrence among the young, intermediate and old forests. The EM fungal community was structured by environmental selection and dispersal limitation in old forest, but only by environmental selection in young, intermediate, and whole forests. Furthermore, the EM fungal community was affected by different factors in the different forest successional stages, and the importance of these factors in structuring EM fungal community dramatically decreased along the secondary forest succession series. This study suggests that different assembly mechanisms operate on the EM fungal community at different stages in secondary subtropical forest succession.

Despite extensive development of successional theory, few empirical studies have evaluated whether existing models are applicable to human‐modified landscapes. Seasonally dry tropical forests are experiencing widespread transformation, and represent a critical system to assess in a successional framework to infer the mechanisms that shape assembly of secondary forests post‐management. We used a functional trait‐based approach to assess changes in community assembly mechanisms along a successional gradient in secondary dry forests of varying stages following abandonment of cattle activity. Nearby old‐growth forests served as an undisturbed comparison. We evaluated whether re‐assembly proceeded consistent with existing theory, in early successional stages via habitat filtering, leading to reduced trait range, and via competitive exclusion in later stages, leading to increased trait evenness. Using three orthogonal traits and two functional diversity indices we tested for evidence of assembly mechanisms along resource axes that may be particularly important to dry forest systems, dynamic changes in habitat filtering and competitive exclusion, and potential legacy effects of management. We found little support of a progression from habitat filtering to competitive exclusion, with the exception of dispersion patterns for specific leaf area and multi‐variate functional richness. Water availability may underlie species sorting in most stages, as evidenced by range reduction in wood density, and might be a resource particularly important to dry forest assembly. Early successional stages displayed an unexpected pattern of low evenness, which may be due to initial stand composition including remnants and resprouts, a regeneration strategy typical to dry forest species. Beyond influencing initial conditions, management legacy was apparent in advanced secondary forests, which differed in range, even dispersion, and functional richness from old‐growth forests, suggesting the operation of different assembly mechanisms. The departures from expectations suggest the need to re‐assess successional models to include effects of management legacies on the operation of community assembly mechanisms in human‐modified landscapes, as well as characteristics that distinguish wet and dry forest dynamics.

AimsThe few remaining old-growth forests in the northeastern United States are often comprised of ectomycorrhizal (EM) tree-dominated patches surrounded by arbuscular mycorrhizal (AM) tree-dominated secondary forests. We examined how (1) distance from old growth and tree neighborhood composition influenced EM colonization, fungal richness, and fungal community composition of Betula lenta L. (black birch) seedlings, a common EM tree that colonizes abandoned agricultural fields, and (2) potential effects of EM fungal genera on seedling physiological performance.MethodsWe sampled soils and tree composition from the edge of an EM-dominated old-growth forest into an adjacent AM-dominated secondary forest. We used soils to grow black birch seedlings in a growth chamber bioassay. We measured seedling EM colonization and investigated effects of EM fungi and soil characteristics on seedling physiological performance.ResultsWe identified 20 EM fungal species and found decreases in EM colonization and fungal richness with distance from old growth, with many taxa present only near the edge. Neighborhood EM tree abundance best explained EM colonization while distance interacted with EM tree basal area to best explain EM fungal richness of seedlings. Soils from neighborhoods lacking EM trees resulted in sparse EM colonization of seedlings. We found no clear effects of EM fungal genera on seedling performance, but we detected a slight decrease in seedling photosynthetic rate with distance from old growth.ConclusionsOld-growth forests can be reservoirs of EM fungi, and EM tree patches can function as localized inoculum sources in AM-dominated secondary forests, potentially facilitating EM tree establishment.

1. Nitrogen-fixing trees (N₂ fixers) provide new nitrogen critical for rapid biomass accumulation of tropical forests during early secondary succession, but it remains unclear how the abundance of N₂ fixers in the forest community affects the growth of non-fixers or the primary productivity of the whole forest. 2. On the one hand, N₂ fixers may enhance forest productivity by providing a facultative effect through the provision of plant-available nitrogen to non-fixing trees. On the other hand, N₂ fixers may suppress the growth of non-fixers by growing faster and competing more vigorously for light and other resources. A third alternative is that the growth of N₂ fixers themselves accumulate biomass rapidly, while having a neutral effect on non-fixers, leading to an overall increase in forest biomass. 3. We examine these alternative hypotheses using 5-year tree census data from 88 plots in 44 seasonal tropical moist secondary forests (3-32 years old) across a human-modified landscape in central Panama. We examined whether N 2 fixers accumulated biomass more rapidly than non-fixers, and how relative biomass of N₂ fixers as a functional group and as individual species influenced the growth of non-fixer and whole stand primary productivity. 4. Surprisingly, we found no evidence for either a net competitive or a facilitative effect of N₂ fixers as a functional group or individual species on the biomass recovery in these young forests. N₂ fixers did not grow faster than non-fixers. Individual mortality rates were lower among N₂ fixers, but biomass losses due to mortality were similar between the two groups. Overall, we found no relationship between the relative abundance of N₂ fixers and stand primary productivity during succession. 5. Synthesis. Nitrogen-fixing trees may be critical for reducing nitrogen limitation and accelerating biomass growth during tropical secondary forest succession, thereby impacting the global carbon cycle. However, our findings indicate that, in early successional seasonal tropical moist forests, nitrogen fixers provide neither a net competitive nor a facilitative effect on non-fixing trees or the whole forest stand, likely because tropical nitrogen fixers utilize facultative fixation and hence abundance poorly approximates the ecosystem function of fixation. Our results indicate that models should not simply scale symbiotic fixation and its effects from nitrogen-fixing tree abundance.

Tropical forest regeneration after abandonment of former agricultural land depends critically on the input of tree seeds, yet seed dispersal is increasingly disrupted in contemporary human-modified landscapes. Here, we introduce the concept of seed-rain–successional feedbacks as a deterministic process in which seed rain is shaped by successional dynamics internal to a forest site and that acts to reinforce priority effects. We used a combination of time series and chronosequence approaches to investigate how the quantity and taxonomic and functional composition of seed rain change during succession and to evaluate the strength of seed-rain–successional feedbacks, relative to other deterministic and stochastic mechanisms, in secondary wet forests of Costa Rica. We found that both successional niches and seed-rain–successional feedbacks shaped successional trajectories in the seed rain. Determinism due to successional niche assembly was supported by the increasing convergence of community structure to that of a mature forest, in terms of both functional and taxonomic composition. With successional age, the proportions of large-seeded, shade-tolerant species in the seed rain increased, whereas the proportion of animal-dispersed species did not change significantly. Seed-rain–successional feedbacks increased in strength with successional age, as the proportion of immigrant seeds (species not locally represented in the site) decreased with successional age, and the composition of the seed rain became more similar to that of the adult trees at the forest site. The deterministic assembly generated by seed-rain–successional feedback likely contributed to the increasing divergence of secondary forest sites from each other during succession. To the extent that human modification of tropical forest landscapes reduces connectivity via factors such as forest cover loss, our results suggest that seed-rain–successional feedbacks are likely to increasingly shape regeneration trajectories in and amplify floristic heterogeneity among tropical secondary forests.