Explore the vast range of titles available.

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.

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.

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.

Tropical forests account for more than half of the global carbon forest stock and much of the biological diversity on Earth. However, disturbances such as deforestation and forest degradation threaten the maintenance of these ecosystem services. This study aimed to understand how different disturbance histories affect the forest stand biomass, as well as species and functional diversity, and to what extent these differences can change the relationships between biomass and their drivers. We used data from forests with clear-cut and selectively logged disturbance histories, and from old-growth forests, situated in the Brazilian Atlantic forest. Forests with logging disturbances showed significant losses in their aboveground biomass compared to those of old-growth forests (50% loss in selectively logged forests and 80% loss in clear-cut forests). Interestingly, only clear-cut secondary forests showed differences in species and functional diversity, and were dominated by species with acquisitive trait values, commonly found early in succession. Shifts in stand biomass drivers were observed in selectively logged forests. The mass-ratio hypothesis (mainly through the functional trait of maximum height) was the most important biomass driver in clear-cut secondary and old-growth forests, whereas the importance of the niche complementarity hypothesis (through functional richness and dispersion) was higher in selectively logged forests. Our study highlights that disturbance histories can affect forest aboveground biomass and its drivers. Moreover, our results reinforce the need for conservation of intact forests but highlight the importance of including degraded forests in conservation mechanisms based in carbon stocks, as these forests retain high values of species and functional diversities that are crucial to biomass and consequently carbon stock acquisition.

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.

Subtropical broad-leaved forests in southeastern China support a high diversity of woody plants. Using a comparative study design with 30 ×× 30 m plots ( n == 27) from five successional stages (<20, <40, <60, <80, and ≥≥80 yr), we investigated how the gradient in species composition reflects underlying processes of community assembly. In particular, we tested whether species richness of adult trees and shrubs decreased or increased and assessed to which degree this pattern was caused by negative density dependence or continuous immigration over time. Furthermore, we tested whether rare species were increasingly enriched and the species composition of adult trees and shrubs became more similar to species composition of seedlings during the course of succession. We counted the individuals of all adult species and shrubs >1 m in height in each plot and counted all woody recruits (bank of all seedlings ≤≤1 m in height) in each central 10 ×× 10 m quadrant of each plot. In addition, we measured a number of environmental variables (elevation, slope, aspect, soil moisture, pH, C, N, and C/N ratio) and biotic structural variables (height and cover of layers). Adult species richness varied from 25 to 69 species per plot, and in total 148 woody species from 46 families were recorded. There was a clear successional gradient in species composition as revealed by nonmetric multidimensional scaling (NMDS), but only a poor differentiation of different successional stages with respect to particular species. Adult richness per 100 individuals (rarefaction method) increased with successional stage. None of the measured abiotic variables were significantly correlated with adult species richness. We found no evidence that rare species were responsible for the increasing adult species richness, as richness of rare species among both adults and recruits was independent of the successional stage. Furthermore, the similarity between established adults and recruits did not increase with successional stage. There was a constant number of recruit species and also of exclusive recruit species, i.e., those that had not been present as adult individuals, across all successional stages, suggesting a continuous random immigration over time.