Integrated global assessment of the natural forest carbon potential

Forests are a substantial terrestrial carbon sink, but anthropogenic changes in land use and climate have considerably reduced the scale of this system1. Remote-sensing estimates to quantify carbon losses from global forests2-5 are characterized by considerable uncertainty and we lack a comprehensive ground-sourced evaluation to benchmark these estimates. Here we combine several ground-sourced6 and satellite-derived approaches2,7,8 to evaluate the scale of the global forest carbon potential outside agricultural and urban lands. Despite regional variation, the predictions demonstrated remarkable consistency at a global scale, with only a 12% difference between the ground-sourced and satellite-derived estimates. At present, global forest carbon storage is markedly under the natural potential, with a total deficit of 226 Gt (model range = 151-363 Gt) in areas with low human footprint. Most (61%, 139 Gt C) of this potential is in areas with existing forests, in which ecosystem protection can allow forests to recover to maturity. The remaining 39% (87 Gt C) of potential lies in regions in which forests have been removed or fragmented. Although forests cannot be a substitute for emissions reductions, our results support the idea2,3,9 that the conservation, restoration and sustainable management of diverse forests offer valuable contributions to meeting global climate and biodiversity targets.

The global biogeography of tree leaf form and habit

Understanding what controls global leaf type variation in trees is crucial for comprehending their role in terrestrial ecosystems, including carbon, water and nutrient dynamics. Yet our understanding of the factors influencing forest leaf types remains incomplete, leaving us uncertain about the global proportions of needle-leaved, broadleaved, evergreen and deciduous trees. To address these gaps, we conducted a global, ground-sourced assessment of forest leaf-type variation by integrating forest inventory data with comprehensive leaf form (broadleaf vs needle-leaf) and habit (evergreen vs deciduous) records. We found that global variation in leaf habit is primarily driven by isothermality and soil characteristics, while leaf form is predominantly driven by temperature. Given these relationships, we estimate that 38% of global tree individuals are needle-leaved evergreen, 29% are broadleaved evergreen, 27% are broadleaved deciduous and 5% are needle-leaved deciduous. The aboveground biomass distribution among these tree types is approximately 21% (126.4 Gt), 54% (335.7 Gt), 22% (136.2 Gt) and 3% (18.7 Gt), respectively. We further project that, depending on future emissions pathways, 17-34% of forested areas will experience climate conditions by the end of the century that currently support a different forest type, highlighting the intensification of climatic stress on existing forests. By quantifying the distribution of tree leaf types and their corresponding biomass, and identifying regions where climate change will exert greatest pressure on current leaf types, our results can help improve predictions of future terrestrial ecosystem functioning and carbon cycling.

Asymmetric response of Amazon forest water and energy fluxes to wet and dry hydrological extremes reveals onset of a local drought-induced tipping point

Understanding the effects of intensification of Amazon basin hydrological cycling-manifest as increasingly frequent floods and droughts-on water and energy cycles of tropical forests is essential to meeting the challenge of predicting ecosystem responses to climate change, including forest "tipping points". Here, we investigated the impacts of hydrological extremes on forest function using 12+ years of observations (between 2001-2020) of water and energy fluxes from eddy covariance, along with associated ecological dynamics from biometry, at the Tapajós National Forest. Measurements encompass the strong 2015-2016 El Niño drought and La Niña 2008-2009 wet events. We found that the forest responded strongly to El Niño-Southern Oscillation (ENSO): Drought reduced water availability for evapotranspiration (ET) leading to large increases in sensible heat fluxes (H). Partitioning ET by an approach that assumes transpiration (T) is proportional to photosynthesis, we found that water stress-induced reductions in canopy conductance (Gs ) drove T declines partly compensated by higher evaporation (E). By contrast, the abnormally wet La Niña period gave higher T and lower E, with little change in seasonal ET. Both El Niño-Southern Oscillation (ENSO) events resulted in changes in forest structure, manifested as lower wet-season leaf area index. However, only during El Niño 2015-2016, we observed a breakdown in the strong meteorological control of transpiration fluxes (via energy availability and atmospheric demand) because of slowing vegetation functions (via shutdown of Gs and significant leaf shedding). Drought-reduced T and Gs , higher H and E, amplified by feedbacks with higher temperatures and vapor pressure deficits, signaled that forest function had crossed a threshold, from which it recovered slowly, with delay, post-drought. Identifying such tipping point onsets (beyond which future irreversible processes may occur) at local scale is crucial for predicting basin-scale threshold-crossing changes in forest energy and water cycling, leading to slow-down in forest function, potentially resulting in Amazon forests shifting into alternate degraded states.

Native diversity buffers against severity of non-native tree invasions

Determining the drivers of non-native plant invasions is critical for managing native ecosystems and limiting the spread of invasive species1,2. Tree invasions in particular have been relatively overlooked, even though they have the potential to transform ecosystems and economies3,4. Here, leveraging global tree databases5-7, we explore how the phylogenetic and functional diversity of native tree communities, human pressure and the environment influence the establishment of non-native tree species and the subsequent invasion severity. We find that anthropogenic factors are key to predicting whether a location is invaded, but that invasion severity is underpinned by native diversity, with higher diversity predicting lower invasion severity. Temperature and precipitation emerge as strong predictors of invasion strategy, with non-native species invading successfully when they are similar to the native community in cold or dry extremes. Yet, despite the influence of these ecological forces in determining invasion strategy, we find evidence that these patterns can be obscured by human activity, with lower ecological signal in areas with higher proximity to shipping ports. Our global perspective of non-native tree invasion highlights that human drivers influence non-native tree presence, and that native phylogenetic and functional diversity have a critical role in the establishment and spread of subsequent invasions.

Co-limitation towards lower latitudes shapes global forest diversity gradients

The latitudinal diversity gradient (LDG) is one of the most recognized global patterns of species richness exhibited across a wide range of taxa. Numerous hypotheses have been proposed in the past two centuries to explain LDG, but rigorous tests of the drivers of LDGs have been limited by a lack of high-quality global species richness data. Here we produce a high-resolution (0.025° × 0.025°) map of local tree species richness using a global forest inventory database with individual tree information and local biophysical characteristics from ~1.3 million sample plots. We then quantify drivers of local tree species richness patterns across latitudes. Generally, annual mean temperature was a dominant predictor of tree species richness, which is most consistent with the metabolic theory of biodiversity (MTB). However, MTB underestimated LDG in the tropics, where high species richness was also moderated by topographic, soil and anthropogenic factors operating at local scales. Given that local landscape variables operate synergistically with bioclimatic factors in shaping the global LDG pattern, we suggest that MTB be extended to account for co-limitation by subordinate drivers.

Tallo: A global tree allometry and crown architecture database

Data capturing multiple axes of tree size and shape, such as a tree's stem diameter, height and crown size, underpin a wide range of ecological research-from developing and testing theory on forest structure and dynamics, to estimating forest carbon stocks and their uncertainties, and integrating remote sensing imagery into forest monitoring programmes. However, these data can be surprisingly hard to come by, particularly for certain regions of the world and for specific taxonomic groups, posing a real barrier to progress in these fields. To overcome this challenge, we developed the Tallo database, a collection of 498,838 georeferenced and taxonomically standardized records of individual trees for which stem diameter, height and/or crown radius have been measured. These data were collected at 61,856 globally distributed sites, spanning all major forested and non-forested biomes. The majority of trees in the database are identified to species (88%), and collectively Tallo includes data for 5163 species distributed across 1453 genera and 187 plant families. The database is publicly archived under a CC-BY 4.0 licence and can be access from: https://doi.org/10.5281/zenodo.6637599. To demonstrate its value, here we present three case studies that highlight how the Tallo database can be used to address a range of theoretical and applied questions in ecology-from testing the predictions of metabolic scaling theory, to exploring the limits of tree allometric plasticity along environmental gradients and modelling global variation in maximum attainable tree height. In doing so, we provide a key resource for field ecologists, remote sensing researchers and the modelling community working together to better understand the role that trees play in regulating the terrestrial carbon cycle.

Hydraulic traits explain differential responses of Amazonian forests to the 2015 El Niño-induced drought

Reducing uncertainties in the response of tropical forests to global change requires understanding how intra- and interannual climatic variability selects for different species, community functional composition and ecosystem functioning, so that the response to climatic events of differing frequency and severity can be predicted. Here we present an extensive dataset of hydraulic traits of dominant species in two tropical Amazon forests with contrasting precipitation regimes - low seasonality forest (LSF) and high seasonality forest (HSF) - and relate them to community and ecosystem response to the El Niño-Southern Oscillation (ENSO) of 2015. Hydraulic traits indicated higher drought tolerance in the HSF than in the LSF. Despite more intense drought and lower plant water potentials in HSF during the 2015-ENSO, greater xylem embolism resistance maintained similar hydraulic safety margin as in LSF. This likely explains how ecosystem-scale whole-forest canopy conductance at HSF maintained a similar response to atmospheric drought as at LSF, despite their water transport systems operating at different water potentials. Our results indicate that contrasting precipitation regimes (at seasonal and interannual time scales) select for assemblies of hydraulic traits and taxa at the community level, which may have a significant role in modulating forest drought response at ecosystem scales.

Seasonal and drought-related changes in leaf area profiles depend on height and light environment in an Amazon forest

Seasonal dynamics in the vertical distribution of leaf area index (LAI) may impact the seasonality of forest productivity in Amazonian forests. However, until recently, fine-scale observations critical to revealing ecological mechanisms underlying these changes have been lacking. To investigate fine-scale variation in leaf area with seasonality and drought we conducted monthly ground-based LiDAR surveys over 4 yr at an Amazon forest site. We analysed temporal changes in vertically structured LAI along axes of both canopy height and light environments. Upper canopy LAI increased during the dry season, whereas lower canopy LAI decreased. The low canopy decrease was driven by highly illuminated leaves of smaller trees in gaps. By contrast, understory LAI increased concurrently with the upper canopy. Hence, tree phenological strategies were stratified by height and light environments. Trends were amplified during a 2015-2016 severe El Niño drought. Leaf area low in the canopy exhibited behaviour consistent with water limitation. Leaf loss from short trees in high light during drought may be associated with strategies to tolerate limited access to deep soil water and stressful leaf environments. Vertically and environmentally structured phenological processes suggest a critical role of canopy structural heterogeneity in seasonal changes in Amazon ecosystem function.

Ecosystem heterogeneity and diversity mitigate Amazon forest resilience to frequent extreme droughts

The impact of increases in drought frequency on the Amazon forest's composition, structure and functioning remain uncertain. We used a process- and individual-based ecosystem model (ED2) to quantify the forest's vulnerability to increased drought recurrence. We generated meteorologically realistic, drier-than-observed rainfall scenarios for two Amazon forest sites, Paracou (wetter) and Tapajós (drier), to evaluate the impacts of more frequent droughts on forest biomass, structure and composition. The wet site was insensitive to the tested scenarios, whereas at the dry site biomass declined when average rainfall reduction exceeded 15%, due to high mortality of large-sized evergreen trees. Biomass losses persisted when year-long drought recurrence was shorter than 2-7 yr, depending upon soil texture and leaf phenology. From the site-level scenario results, we developed regionally applicable metrics to quantify the Amazon forest's climatological proximity to rainfall regimes likely to cause biomass loss > 20% in 50 yr according to ED2 predictions. Nearly 25% (1.8 million km² ) of the Amazon forests could experience frequent droughts and biomass loss if mean annual rainfall or interannual variability changed by 2σ. At least 10% of the high-emission climate projections (CMIP5/RCP8.5 models) predict critically dry regimes over 25% of the Amazon forest area by 2100.

Amazon forest carbon dynamics predicted by profiles of canopy leaf area and light environment

Tropical forest structural variation across heterogeneous landscapes may control above-ground carbon dynamics. We tested the hypothesis that canopy structure (leaf area and light availability) - remotely estimated from LiDAR - control variation in above-ground coarse wood production (biomass growth). Using a statistical model, these factors predicted biomass growth across tree size classes in forest near Manaus, Brazil. The same statistical model, with no parameterisation change but driven by different observed canopy structure, predicted the higher productivity of a site 500 km east. Gap fraction and a metric of vegetation vertical extent and evenness also predicted biomass gains and losses for one-hectare plots. Despite significant site differences in canopy structure and carbon dynamics, the relation between biomass growth and light fell on a unifying curve. This supported our hypothesis, suggesting that knowledge of canopy structure can explain variation in biomass growth over tropical landscapes and improve understanding of ecosystem function.

Stocks of carbon and nitrogen and partitioning between above- and belowground pools in the Brazilian coastal Atlantic Forest elevation range

We estimated carbon and nitrogen stocks in aboveground biomass (AGB) and belowground biomass (BGB) along an elevation range in forest sites located on the steep slopes of the Serra do Mar on the north coast of the State of São Paulo, southeast Brazil. In elevations of 100 m (lowland), 400 m (submontane), and 1000 m (montane) four 1-ha plots were established, and above- (live and dead) and belowground (live and dead) biomass were determined. Carbon and nitrogen concentrations in each compartment were determined and used to convert biomass into carbon and nitrogen stocks. The carbon aboveground stock (C_AGB) varied along the elevation range from approximately 110 to 150 Mg·ha⁻¹, and nitrogen aboveground stock (N_AGB), varied from approximately 1.0 to 1.9 Mg·ha⁻¹. The carbon belowground stock (C_BGB) and the nitrogen belowground stock (N_BGB) were significantly higher than the AGB and varied along the elevation range from approximately 200–300 Mg·ha⁻¹, and from 14 to 20 Mg·ha⁻¹, respectively. Finally, the total carbon stock (C_TOTAL) varied from approximately 320 to 460 Mg·ha⁻¹, and the nitrogen total stock (N_TOTAL) from approximately 15 to 22 Mg·ha⁻¹. Most of the carbon and nitrogen stocks were found belowground and not aboveground as normally found in lowland tropical forests. The above- and belowground stocks, and consequently, the total stocks of carbon and nitrogen increased significantly with elevation. As the soil and air temperature also decreased significantly with elevation, we found a significantly inverse relationship between carbon and nitrogen stocks and temperature. Using this inverse relationship, we made a first approach estimate that an increase of 1°C in soil temperature would decrease the carbon and nitrogen stocks in approximately 17 Mg·ha⁻¹ and 1 Mg·ha⁻¹ of carbon and nitrogen, respectively.

Características ecológicas e implicações para a conservação da Reserva Florestal do Morro Grande

Os levantamentos biológicos realizados nos últimos cinco anos na Reserva Florestal do Morro Grande (RFMG) permitiram inventariar mais de 13.000 indivíduos pertencentes a 673 espécies de árvores, mamíferos não-voadores, aves, répteis, anuros e aranhas orbitelas. A comparação dos dados obtidos revela padrões biológicos consistentes que levam a quatro conclusões principais: i) a RFMG se situa numa condição de transição, recebendo influências das florestas ombrófilas densas e mistas, das estacionais e até do cerrado; ii) todos os grupos taxonômicos estudados apresentam diferenças significativas na riqueza e/ou na composição de espécies quando comparadas as florestas em estádios sucessionais intermediários/avançados de sucessão (80-90 anos após corte raso) e a floresta madura, mais antiga (onde houve apenas corte seletivo), o que deve contribuir para um aumento da diversidade gama da RFMG, uma vez que esta é um complexo mosaico de vegetação em diferentes estádios sucessionais; iii) dado, em parte, aos dois fatores anteriores, a RFMG apresenta um padrão, consistente para diferentes grupos taxonômicos, de elevada riqueza de espécies quando comparada com outras áreas de Floresta Atlântica; iv) por sua localização numa matriz de alta densidade populacional humana, a RFMG vem sofrendo fortes perturbações, em particular devido à caça, extrativismo vegetal predatório e entrada de espécies exóticas e invasoras. A conservação da singular biota da RFMG dependerá do estabelecimento de um plano de manejo que consiga tirar vantagens da proximidade à cidade de São Paulo, em termos de pesquisa científica e projetos educacionais/turísticos, e estabeleça o controle das espécies exóticas, da caça, do extrativismo vegetal e da ocupação inadequada do seu entorno imediato.

A regeneração florestal em áreas de floresta secundária na Reserva Florestal do Morro Grande, Cotia, SP

Apesar de sua complexidade, é necessário um conhecimento mais sintético sobre o processo de regeneração em florestas tropicais sujeitas a diferentes regimes de perturbação, tanto natural como antrópico. Dada a grande extensão de florestas secundárias na região Neotropical, um importante tópico a ser abordado é o potencial dessas manchas de floresta na conservação da diversidade biológica do componente arbóreo típico de manchas de floresta mais antiga, menos perturbada. O objetivo principal deste estudo foi avaliar a estrutura da comunidade de plântulas de espécies arbóreas e arbustivas e seu potencial como fonte de regeneração em áreas de floresta secundária na Reserva Florestal do Morro Grande (RFMG), uma das porções mais significativas de Mata Atlântica no Planalto Atlântico. A estrutura e composição da comunidade foram amostradas através de parcelas de 1 m² alocadas em três áreas de floresta secundária localizadas no interior da RFMG. Foram identificadas 106 espécies dentre as 742 plântulas amostradas, distribuídas em 35 famílias e 69 gêneros. As famílias mais ricas (Myrtaceae, Fabaceae, Rubiaceae e Lauraceae) representaram 47,2% do total de espécies, e as cinco espécies mais abundantes foram Myrcia fallax, Ocotea dispersa, Psychotria vellosiana, Psychotria suterella e Matayba elaeagnoides. O estrato de regeneração apresentou-se predominantemente constituído de espécies zoocóricas e tolerantes à sombra, características de estádios mais avançados da sucessão florestal da Mata Atlântica. A comunidade de plântulas não apresentou variação em sua abundância e estrutura de tamanho, mas a composição de espécies e a riqueza variaram significativamente entre as áreas. Os resultados sugerem que o estrato de regeneração nas áreas de floresta secundária da RFMG tem um alto valor potencial na conservação da diversidade biológica do componente arbóreo típico de manchas de floresta mais antiga e, consequentemente, na restauração florestal em escala regional. As diferenças espaciais observadas na composição florística e na riqueza da chuva de sementes e da comunidade de plântulas em áreas de floresta secundária no interior da RFMG, representam componentes importantes que influenciam o processo de sucessão e a manutenção da diversidade de espécies, pois podem funcionar como fontes de recrutamento de novos indivíduos e de espécies.

Uma área de relevante interesse biológico, porém pouco conhecida: a Reserva Florestal do Morro Grande

A Reserva Florestal do Morro Grande (RFMG, 10.870 ha) é um dos maiores remanescentes florestais do Planalto Atlântico paulista, região que foi submetida a fortes pressões de desmatamento tanto para a agricultura quanto para a exploração da lenha e do carvão e, mais recentemente, para a expansão imobiliária. A RFMG situa-se sobre embasamento cristalino, justo acima da Serra de Paranapiacaba, em altitudes que variam de 860 a 1075 m, numa região de transição entre o clima pluvial tropical do litoral e da encosta Atlântica, com o clima estacional do interior do estado. Por conseguinte, suas florestas acabam tendo uma composição de espécies particular, com elementos das florestas ombrófila densa e mesófila semi-decidual. Apesar desta particularidade, não houve até recentemente praticamente nenhum estudo nas florestas do Morro Grande. Foi apenas a partir de 2000 que um grupo de pesquisadores, no contexto de um projeto temático vinculado ao programa BIOTA/FAPESP, procurou analisar de forma sistemática a composição de diferentes grupos taxonômicos, assim como caracterizar processos ecológicos associados com a regeneração florestal. Este artigo tem por objetivo introduzir uma série de nove artigos com os principais resultados dos levantamentos biológicos na RFMG, obtidos nos últimos quatro anos, assim como fazer uma caracterização do histórico e das condições abióticas na Reserva, em termos de embasamento geológico, pedológico, clima, hidrografia, e do uso e cobertura do território.