Damage to tropical forests caused by cyclones is driven by wind speed but mediated by topographical exposure and tree characteristics.

Each year, an average of 45 tropical cyclones affect coastal areas and potentially impact forests. The proportion of the most intense cyclones has increased over the past four decades and is predicted to continue to do so. Yet, it remains uncertain how topographical exposure and tree characteristics can mediate the damage caused by increasing wind speed. Here, we compiled empirical data on the damage caused by 11 cyclones occurring over the past 40 years, from 74 forest plots representing tropical regions worldwide, encompassing field data for 22,176 trees and 815 species. We reconstructed the wind structure of those tropical cyclones to estimate the maximum sustained wind speed (MSW) and wind direction at the studied plots. Then, we used a causal inference framework combined with Bayesian generalised linear mixed models to understand and quantify the causal effects of MSW, topographical exposure to wind (EXP), tree size (DBH) and species wood density (ρ) on the proportion of damaged trees at the community level, and on the probability of snapping or uprooting at the tree level. The probability of snapping or uprooting at the tree level and, hence, the proportion of damaged trees at the community level, increased with increasing MSW, and with increasing EXP accentuating the damaging effects of cyclones, in particular at higher wind speeds. Higher ρ decreased the probability of snapping and to a lesser extent of uprooting. Larger trees tended to have lower probabilities of snapping but increased probabilities of uprooting. Importantly, the effect of ρ decreasing the probabilities of snapping was more marked for smaller than larger trees and was further accentuated at higher MSW. Our work emphasises how local topography, tree size and species wood density together mediate cyclone damage to tropical forests, facilitating better predictions of the impacts of such disturbances in an increasingly windier world.

Tree demographic strategies largely overlap across succession in Neotropical wet and dry forest communities.

Secondary tropical forests play an increasingly important role in carbon budgets and biodiversity conservation. Understanding successional trajectories is therefore imperative for guiding forest restoration and climate change mitigation efforts. Forest succession is driven by the demographic strategies-combinations of growth, mortality and recruitment rates-of the tree species in the community. However, our understanding of demographic diversity in tropical tree species stems almost exclusively from old-growth forests. Here, we assembled demographic information from repeated forest inventories along chronosequences in two wet (Costa Rica, Panama) and two dry (Mexico) Neotropical forests to assess whether the ranges of demographic strategies present in a community shift across succession. We calculated demographic rates for >500 tree species while controlling for canopy status to compare demographic diversity (i.e., the ranges of demographic strategies) in early successional (0-30 years), late successional (30-120 years) and old-growth forests using two-dimensional hypervolumes of pairs of demographic rates. Ranges of demographic strategies largely overlapped across successional stages, and early successional stages already covered the full spectrum of demographic strategies found in old-growth forests. An exception was a group of species characterized by exceptionally high mortality rates that was confined to early successional stages in the two wet forests. The range of demographic strategies did not expand with succession. Our results suggest that studies of long-term forest monitoring plots in old-growth forests, from which most of our current understanding of demographic strategies of tropical tree species is derived, are surprisingly representative of demographic diversity in general, but do not replace the need for further studies in secondary forests.

Incomplete recovery of tree community composition and rare species after 120 years of tropical forest succession in Panama.

Determining how fully tropical forests regenerating on abandoned land recover characteristics of old-growth forests is increasingly important for understanding their role in conserving rare species and maintaining ecosystem services. Despite this, our understanding of forest structure and community composition recovery throughout succession is incomplete, as many tropical chronosequences do not extend beyond the first 50 years of succession. Here, we examined trajectories of forest recovery across eight 1-hectare plots in middle and later stages of forest succession (40-120 years) and five 1-hectare old-growth plots, in the Barro Colorado Nature Monument (BCNM), Panama. We first verified that forest age had a greater effect than edaphic or topographic variation on forest structure, diversity and composition and then corroborated results from smaller plots censused 20 years previously. Tree species diversity (but not species richness) and forest structure had fully recovered to old-growth levels by 40 and 90 years, respectively. However, rare species were missing, and old-growth specialists were in low abundance, in the mid- and late secondary forest plots, leading to incomplete recovery of species composition even by 120 years into succession. We also found evidence that dominance early in succession by a long-lived pioneer led to altered forest structure and delayed recovery of species diversity and composition well past a century after land abandonment. Our results illustrate the critical importance of old-growth and old secondary forests for biodiversity conservation, given that recovery of community composition may take several centuries, particularly when a long-lived pioneer dominates in early succession. Abstract in Spanish is available with online material.

Determinar en que medida los bosques tropicales que se regeneran en tierras abandonadas recuperan las características de los bosques primarios es cada vez más importante para comprender su papel en la conservación de especies raras y el mantenimiento de los servicios ecosistémicos. A pesar de ello, nuestro entendimiento sobre la recuperación de la estructura del bosque y la composición de la comunidad a lo largo de la sucesión es incompleta, ya que muchas cronosecuencias tropicales no van más allá de los primeros 50 años de sucesión. En este estudio, investigamos las trayectorias de recuperación del bosque en ocho parcelas de 1 hectárea en estadíos medios y tardíos de la sucesión forestal (40­120 años) y cinco parcelas de 1 hectárea de bosque primario, en el Monumento Natural Barro Colorado (MNBC), Panamá. En primer lugar, verificamos que la edad del bosque tenía un mayor efecto que la variación edáfica o topográfica en la estructura, diversidad y composición del bosque y luego corroboramos los resultados de parcelas más pequeñas estudiadas 20 años antes. La diversidad de especies arbóreas, pero no la riqueza de especies, y la estructura forestal se habían recuperado completamente hasta alcanzar los niveles de bosque primario a los 40 y 90 años, respectivamente. Sin embargo, los bosques secundarios carecían de especies raras y presentaban una escasa abundancia de especies especialistas del bosque antiguo, lo que condujo a una recuperación incompleta de la composición de especies, incluso a 120 años de sucesión. También encontramos pruebas de que el predominio de un pionero longevo en las primeras etapas de la sucesión provocó una alteración de la estructura forestal y retrasó la recuperación de la diversidad y composición de especies más allá de un siglo después el abandono de las tierras. Nuestros resultados ilustran la importancia crítica de los bosques primarios y secundarios más antiguos para la conservación de la biodiversidad, dado que la recuperación de la composición de la comunidad puede llevar varios siglos, especialmente cuando un pionero longevo domina en la sucesión temprana.

Animal seed dispersal recovery during passive restoration in a forested landscape.

Seed dispersal by animals is key for restoration of tropical forests because it maintains plant diversity and accelerates community turnover. Therefore, changes in seed dispersal during forest restoration can indicate the recovery of species interactions, and yet these changes are rarely considered in forest restoration planning. In this study, we examined shifts in the importance of different seed dispersal modes during passive restoration in a tropical chronosequence spanning more than 100 years, by modelling the proportion of trees dispersed by bats, small birds, large birds, flightless mammals and abiotic means as a function of forest age. Contrary to expectations, tree species dispersed by flightless mammals dominated after 20 years of regeneration, and tree richness and abundance dispersed by each mode mostly recovered to old growth levels between 40 and 70 years post-abandonment. Seed dispersal by small birds declined over time during regeneration, while bat dispersal played a minor role throughout all stages of succession. Results suggest that proximity to old growth forests, coupled with low hunting, explained the prevalence of seed dispersal by animals, especially by flightless mammals at this site. We suggest that aspects of seed dispersal should be monitored when restoring forest ecosystems to evaluate the reestablishment of species interactions. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.

Observations on the Population Genetic Structure of the Leaf Galling Nematode, Ditylenchus gallaeformans.

Ditylenchus gallaeformans is a plant parasitic nematode that induces galls on aboveground parts of Melastomataceae plants. It differs from most gall-inducing nematodes in that it is not an endoparasite and has been considered as a possible biological control agent against invasive species of Miconia. Little is known about D. gallaeformans biology, genetic differences among populations, and host preferences. This study examined the genetic differences among D. gallaeformans populations from different locations and host species and the phylogenetic relationships among them. Nematodes were collected from galls in plants from Costa Rica, Dominica, and Trinidad. The Cytochrome c oxidase 1 (cox1) region was sequenced from a total of 33 individual nematodes isolated from 33 different plant individuals, representing 21 species of Melastomataceae. Phylogenetic reconstructions, haplotype networks, and analysis of molecular variance showed that the species is monophyletic and has three major clades, which were mostly consistent with geographic location but not with host species. The first clade was composed by two subclades, one with individuals from Costa Rica and one with individuals from Dominica. The second and third clades comprised nematodes only from Trinidad. Overall, there is no evidence of host-species specialization in D. gallaeformans. Biocontrol efforts using the nematode against invasive Miconia could focus on geographical location matching but likely will not need to match host species.

Multidimensional tropical forest recovery.

Tropical forests disappear rapidly because of deforestation, yet they have the potential to regrow naturally on abandoned lands. We analyze how 12 forest attributes recover during secondary succession and how their recovery is interrelated using 77 sites across the tropics. Tropical forests are highly resilient to low-intensity land use; after 20 years, forest attributes attain 78% (33 to 100%) of their old-growth values. Recovery to 90% of old-growth values is fastest for soil (<1 decade) and plant functioning (<2.5 decades), intermediate for structure and species diversity (2.5 to 6 decades), and slowest for biomass and species composition (>12 decades). Network analysis shows three independent clusters of attribute recovery, related to structure, species diversity, and species composition. Secondary forests should be embraced as a low-cost, natural solution for ecosystem restoration, climate change mitigation, and biodiversity conservation.

Functional recovery of secondary tropical forests.

One-third of all Neotropical forests are secondary forests that regrow naturally after agricultural use through secondary succession. We need to understand better how and why succession varies across environmental gradients and broad geographic scales. Here, we analyze functional recovery using community data on seven plant characteristics (traits) of 1,016 forest plots from 30 chronosequence sites across the Neotropics. By analyzing communities in terms of their traits, we enhance understanding of the mechanisms of succession, assess ecosystem recovery, and use these insights to propose successful forest restoration strategies. Wet and dry forests diverged markedly for several traits that increase growth rate in wet forests but come at the expense of reduced drought tolerance, delay, or avoidance, which is important in seasonally dry forests. Dry and wet forests showed different successional pathways for several traits. In dry forests, species turnover is driven by drought tolerance traits that are important early in succession and in wet forests by shade tolerance traits that are important later in succession. In both forests, deciduous and compound-leaved trees decreased with forest age, probably because microclimatic conditions became less hot and dry. Our results suggest that climatic water availability drives functional recovery by influencing the start and trajectory of succession, resulting in a convergence of community trait values with forest age when vegetation cover builds up. Within plots, the range in functional trait values increased with age. Based on the observed successional trait changes, we indicate the consequences for carbon and nutrient cycling and propose an ecologically sound strategy to improve forest restoration success.

Demographic trade-offs predict tropical forest dynamics.

Understanding tropical forest dynamics and planning for their sustainable management require efficient, yet accurate, predictions of the joint dynamics of hundreds of tree species. With increasing information on tropical tree life histories, our predictive understanding is no longer limited by species data but by the ability of existing models to make use of it. Using a demographic forest model, we show that the basal area and compositional changes during forest succession in a neotropical forest can be accurately predicted by representing tropical tree diversity (hundreds of species) with only five functional groups spanning two essential trade-offs-the growth-survival and stature-recruitment trade-offs. This data-driven modeling framework substantially improves our ability to predict consequences of anthropogenic impacts on tropical forests.

Above- and belowground carbon stocks are decoupled in secondary tropical forests and are positively related to forest age and soil nutrients respectively.

Reducing atmospheric CO2 is an international priority. One way to assist stabilising and reducing CO2 is to promote secondary tropical forest regrowth on abandoned agricultural land. However, relationships between above- and belowground carbon stocks with secondary forest age and specific soil nutrients remain unclear. Current global estimates for CO2 uptake and sequestration in secondary tropical forests focus on aboveground biomass and are parameterised using relatively coarse metrics of soil fertility. Here, we estimate total carbon stocks across a chronosequence of regenerating secondary forest stands (40-120 years old) in Panama, and assess the relationships between both above- and belowground carbon stocks with stand age and specific soil nutrients. We estimated carbon stocks in aboveground biomass, necromass, root biomass, and soil. We found that the two largest carbon pools - aboveground biomass and soil - have distinct relationships with stand age and soil fertility. Aboveground biomass contained ~61-97 Mg C ha-1 (24-39% total carbon stocks) and significantly increased with stand age, but showed no relationship with soil nutrients. Soil carbon stocks contained ~128-206 Mg C ha-1 (52-70% total stocks) and were unrelated to stand age, but were positively related to soil nitrogen. Root biomass carbon stocks tracked patterns exhibited by aboveground biomass. Necromass carbon stocks did not increase with stand age, but stocks were held in larger pieces of deadwood in older stands. Comparing our estimates to published data from younger and older secondary forests in the surrounding landscape, we show that soil carbon recovers within 40 years of forest regeneration, but aboveground biomass carbon stocks continue to increase past 100 years. Above- and belowground carbon stocks appear to be decoupled in secondary tropical forests. Paired measures of above- and belowground carbon stocks are necessary to reduce uncertainty in large-scale models of atmospheric CO2 uptake and storage by secondary forests.

Wet and dry tropical forests show opposite successional pathways in wood density but converge over time.

Tropical forests are converted at an alarming rate for agricultural use and pastureland, but also regrow naturally through secondary succession. For successful forest restoration, it is essential to understand the mechanisms of secondary succession. These mechanisms may vary across forest types, but analyses across broad spatial scales are lacking. Here, we analyse forest recovery using 1,403 plots that differ in age since agricultural abandonment from 50 sites across the Neotropics. We analyse changes in community composition using species-specific stem wood density (WD), which is a key trait for plant growth, survival and forest carbon storage. In wet forest, succession proceeds from low towards high community WD (acquisitive towards conservative trait values), in line with standard successional theory. However, in dry forest, succession proceeds from high towards low community WD (conservative towards acquisitive trait values), probably because high WD reflects drought tolerance in harsh early successional environments. Dry season intensity drives WD recovery by influencing the start and trajectory of succession, resulting in convergence of the community WD over time as vegetation cover builds up. These ecological insights can be used to improve species selection for reforestation. Reforestation species selected to establish a first protective canopy layer should, among other criteria, ideally have a similar WD to the early successional communities that dominate under the prevailing macroclimatic conditions.

Biodiversity recovery of Neotropical secondary forests.

Old-growth tropical forests harbor an immense diversity of tree species but are rapidly being cleared, while secondary forests that regrow on abandoned agricultural lands increase in extent. We assess how tree species richness and composition recover during secondary succession across gradients in environmental conditions and anthropogenic disturbance in an unprecedented multisite analysis for the Neotropics. Secondary forests recover remarkably fast in species richness but slowly in species composition. Secondary forests take a median time of five decades to recover the species richness of old-growth forest (80% recovery after 20 years) based on rarefaction analysis. Full recovery of species composition takes centuries (only 34% recovery after 20 years). A dual strategy that maintains both old-growth forests and species-rich secondary forests is therefore crucial for biodiversity conservation in human-modified tropical landscapes.

Legume abundance along successional and rainfall gradients in Neotropical forests.

The nutrient demands of regrowing tropical forests are partly satisfied by nitrogen-fixing legume trees, but our understanding of the abundance of those species is biased towards wet tropical regions. Here we show how the abundance of Leguminosae is affected by both recovery from disturbance and large-scale rainfall gradients through a synthesis of forest inventory plots from a network of 42 Neotropical forest chronosequences. During the first three decades of natural forest regeneration, legume basal area is twice as high in dry compared with wet secondary forests. The tremendous ecological success of legumes in recently disturbed, water-limited forests is likely to be related to both their reduced leaflet size and ability to fix N2, which together enhance legume drought tolerance and water-use efficiency. Earth system models should incorporate these large-scale successional and climatic patterns of legume dominance to provide more accurate estimates of the maximum potential for natural nitrogen fixation across tropical forests.

Phylogenetic classification of the world's tropical forests.

Knowledge about the biogeographic affinities of the world's tropical forests helps to better understand regional differences in forest structure, diversity, composition, and dynamics. Such understanding will enable anticipation of region-specific responses to global environmental change. Modern phylogenies, in combination with broad coverage of species inventory data, now allow for global biogeographic analyses that take species evolutionary distance into account. Here we present a classification of the world's tropical forests based on their phylogenetic similarity. We identify five principal floristic regions and their floristic relationships: (i) Indo-Pacific, (ii) Subtropical, (iii) African, (iv) American, and (v) Dry forests. Our results do not support the traditional neo- versus paleotropical forest division but instead separate the combined American and African forests from their Indo-Pacific counterparts. We also find indications for the existence of a global dry forest region, with representatives in America, Africa, Madagascar, and India. Additionally, a northern-hemisphere Subtropical forest region was identified with representatives in Asia and America, providing support for a link between Asian and American northern-hemisphere forests.

Carbon sequestration potential of second-growth forest regeneration in the Latin American tropics.

Regrowth of tropical secondary forests following complete or nearly complete removal of forest vegetation actively stores carbon in aboveground biomass, partially counterbalancing carbon emissions from deforestation, forest degradation, burning of fossil fuels, and other anthropogenic sources. We estimate the age and spatial extent of lowland second-growth forests in the Latin American tropics and model their potential aboveground carbon accumulation over four decades. Our model shows that, in 2008, second-growth forests (1 to 60 years old) covered 2.4 million km(2) of land (28.1% of the total study area). Over 40 years, these lands can potentially accumulate a total aboveground carbon stock of 8.48 Pg C (petagrams of carbon) in aboveground biomass via low-cost natural regeneration or assisted regeneration, corresponding to a total CO2 sequestration of 31.09 Pg CO2. This total is equivalent to carbon emissions from fossil fuel use and industrial processes in all of Latin America and the Caribbean from 1993 to 2014. Ten countries account for 95% of this carbon storage potential, led by Brazil, Colombia, Mexico, and Venezuela. We model future land-use scenarios to guide national carbon mitigation policies. Permitting natural regeneration on 40% of lowland pastures potentially stores an additional 2.0 Pg C over 40 years. Our study provides information and maps to guide national-level forest-based carbon mitigation plans on the basis of estimated rates of natural regeneration and pasture abandonment. Coupled with avoided deforestation and sustainable forest management, natural regeneration of second-growth forests provides a low-cost mechanism that yields a high carbon sequestration potential with multiple benefits for biodiversity and ecosystem services.

Biomass resilience of Neotropical secondary forests.

Land-use change occurs nowhere more rapidly than in the tropics, where the imbalance between deforestation and forest regrowth has large consequences for the global carbon cycle. However, considerable uncertainty remains about the rate of biomass recovery in secondary forests, and how these rates are influenced by climate, landscape, and prior land use. Here we analyse aboveground biomass recovery during secondary succession in 45 forest sites and about 1,500 forest plots covering the major environmental gradients in the Neotropics. The studied secondary forests are highly productive and resilient. Aboveground biomass recovery after 20 years was on average 122 megagrams per hectare (Mg ha(-1)), corresponding to a net carbon uptake of 3.05 Mg C ha(-1) yr(-1), 11 times the uptake rate of old-growth forests. Aboveground biomass stocks took a median time of 66 years to recover to 90% of old-growth values. Aboveground biomass recovery after 20 years varied 11.3-fold (from 20 to 225 Mg ha(-1)) across sites, and this recovery increased with water availability (higher local rainfall and lower climatic water deficit). We present a biomass recovery map of Latin America, which illustrates geographical and climatic variation in carbon sequestration potential during forest regrowth. The map will support policies to minimize forest loss in areas where biomass resilience is naturally low (such as seasonally dry forest regions) and promote forest regeneration and restoration in humid tropical lowland areas with high biomass resilience.

An experimental test of the EICA Hypothesis in multiple ranges: invasive populations outperform those from the native range independent of insect herbivore suppression.

The success of invasive plants may reflect environmental differences in their native and introduced ranges including both abiotic and biotic conditions, such as release from aboveground herbivory. However, in response to these novel conditions, plants from invasive populations may have higher growth rates and lower defense levels compared to those in the native range. This may contribute to their success in the introduced range but perhaps not in the native range. Here, we grew 1000 Triadica sebifera plants from 14 native and introduced populations in seven common gardens with unmanaged background vegetation for three growing seasons in three geographic venues that varied in T. sebifera status and insect herbivore communities: Texas -T. sebifera is invasive, low levels of generalist herbivory; Hawaii - T. sebifera introduced but not invasive, high levels of generalist herbivory from exotic herbivores; China - native range, both generalist and specialist herbivores. We suppressed aboveground insects with insecticide on half the plants. Aboveground damage in the first growing season was lowest in Texas and insecticide sprays reduced damage in China. At the end of the first growing season, plants were tallest on average in China and shortest in Hawaii. However, height in later years and mass were highest on average in Texas and lowest in Hawaii. However, there was large variation in damage and plant performance among gardens within venues. Our results suggest that more rapid aboveground growth rates contribute to T. sebifera's success in both the invasive and native ranges independent of aboveground herbivory. However, strong variation among sites indicates that T. sebifera plants from invasive populations only have a strong advantage in a subset of sites in Texas.

An estimate of the number of tropical tree species.

The high species richness of tropical forests has long been recognized, yet there remains substantial uncertainty regarding the actual number of tropical tree species. Using a pantropical tree inventory database from closed canopy forests, consisting of 657,630 trees belonging to 11,371 species, we use a fitted value of Fisher's alpha and an approximate pantropical stem total to estimate the minimum number of tropical forest tree species to fall between ∼ 40,000 and ∼ 53,000, i.e., at the high end of previous estimates. Contrary to common assumption, the Indo-Pacific region was found to be as species-rich as the Neotropics, with both regions having a minimum of ∼ 19,000-25,000 tree species. Continental Africa is relatively depauperate with a minimum of ∼ 4,500-6,000 tree species. Very few species are shared among the African, American, and the Indo-Pacific regions. We provide a methodological framework for estimating species richness in trees that may help refine species richness estimates of tree-dependent taxa.

Geographic distribution of genetic variation among native and introduced populations of Chinese tallow tree, Triadica sebifera (Euphorbiaceae).

PREMISE OF THE STUDY: Invasive plants often display genetically determined variation in patterns of growth and resource allocation between native and introduced genotypes, as well as among genotypes within different regions of the introduced range. We examined patterns of genetic variation within and among native and introduced populations of the tetraploid Chinese tallow tree (Triadica sebifera, Euphorbiaceae) to determine whether nonselective evolutionary processes or the introduction history could contribute to previously observed phenotypic differences between native and introduced populations as well as among introduced populations. METHODS: We used six microsatellite markers to study 12 native populations in China, 51 introduced populations in the southeastern USA, and one introduced population in Australia. KEY RESULTS: Genetic diversity was greater within and among native populations than introduced populations. Within the southeastern USA, populations in Georgia and South Carolina differed substantially in their genetic composition and had greater genetic diversity than the rest of the southeastern USA. Greater genetic similarity between some populations in the native range and introduced range indicate a common provenance for Georgia and South Carolina populations that could have come from any of several western or southern Chinese populations and a different provenance for other southeastern USA populations and the Australian population, which were most similar to more northeastern Chinese populations. CONCLUSIONS: Differences among introduced populations in potentially adaptive traits (e.g., herbivore tolerance, herbivore resistance, growth rates) may result in part from the introduction history, in particular from differences present among source populations in the native range.

Rapid adaptation of insect herbivores to an invasive plant.

Introduced plant success often is attributed to release from natural enemies in their new ranges. However, herbivores may accumulate over time and reduce invasiveness but evidence for this process to date is weak. We report here that enemy release is indeed limited to the early stages of introduction of the Chinese tallow tree (Sapium sebiferum). In bioassays and gardens along a geographical gradient of time since tallow tree introduction, herbivory was highest and tree performance was poorest where tallow tree has been present longer (i.e. introduced earlier). Additionally, Asian ecotypes (grown from seeds collected in Asia) had lower survival than North American ecotypes (seeds collected in North America), which is consistent with genetic responses to low herbivory in the introduced range (EICA Hypothesis). Release from insect herbivores appears to contribute to early success of the tallow tree, but accumulation of insect herbivores has apparently reduced this benefit over time.