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Funding: Data collection was largely funded by the UK Natural Environment Research Council (NERC) project TREMOR (NE/N004655/1) to D.G., E.G. and O.P., with further funds from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES, finance code 001) to J.V.T. and a University of Leeds Climate Research Bursary Fund to J.V.T. D.G., E.G. and O.P. acknowledge further support from a NERC-funded consortium award (ARBOLES, NE/S011811/1). This paper is an outcome of J.V.T.’s doctoral thesis, which was sponsored by CAPES (GDE 99999.001293/2015-00). J.V.T. was previously supported by the NERC-funded ARBOLES project (NE/S011811/1) and is supported at present by the Swedish Research Council Vetenskapsrådet (grant no. 2019-03758 to R.M.). E.G., O.P. and D.G. acknowledge support from NERC-funded BIORED grant (NE/N012542/1). O.P. acknowledges support from an ERC Advanced Grant and a Royal Society Wolfson Research Merit Award. R.S.O. was supported by a CNPq productivity scholarship, the São Paulo Research Foundation (FAPESP-Microsoft 11/52072-0) and the US Department of Energy, project GoAmazon (FAPESP 2013/50531-2). M.M. acknowledges support from MINECO FUN2FUN (CGL2013-46808-R) and DRESS (CGL2017-89149-C2-1-R). C.S.-M., F.B.V. and P.R.L.B. were financed by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES, finance code 001). C.S.-M. received a scholarship from the Brazilian National Council for Scientific and Technological Development (CNPq 140353/2017-8) and CAPES (science without borders 88881.135316/2016-01). Y.M. acknowledges the Gordon and Betty Moore Foundation and ERC Advanced Investigator Grant (GEM-TRAITS, 321131) for supporting the Global Ecosystems Monitoring (GEM) network (gem.tropicalforests.ox.ac.uk), within which some of the field sites (KEN, TAM and ALP) are nested. The authors thank Brazil–USA Collaborative Research GoAmazon DOE-FAPESP-FAPEAM (FAPESP 2013/50533-5 to L.A.) and National Science Foundation (award DEB-1753973 to L. Alves). They thank Serrapilheira Serra-1709-18983 (to M.H.) and CNPq-PELD/POPA-441443/2016-8 (to L.G.) (P.I. Albertina Lima). They thank all the colleagues and grants mentioned elsewhere [8,36] that established, identified and measured the Amazon forest plots in the RAINFOR network analysed here. The authors particularly thank J. Lyod, S. Almeida, F. Brown, B. Vicenti, N. Silva and L. Alves. This work is an outcome approved Research Project no. 19 from ForestPlots.net, a collaborative initiative developed at the University of Leeds that unites researchers and the monitoring of their permanent plots from the world’s tropical forests [61]. The authros thank A. Levesley, K. Melgaço Ladvocat and G. Pickavance for ForestPlots.net management. They thank Y. Wang and J. Baker, respectively, for their help with the map and with the climatic data. The authors acknowledge the invaluable help of M. Brum for kindly providing the comparison of vulnerability curves based on PAD and on PLC shown in this manuscript. They thank J. Martinez-Vilalta for his comments on an early version of this manuscript. The authors also thank V. Hilares and the Asociación para la Investigación y Desarrollo Integral (AIDER, Puerto Maldonado, Peru); V. Saldaña and Instituto de Investigaciones de la Amazonía Peruana (IIAP) for local field campaign support in Peru; E. Chavez and Noel Kempff Natural History Museum for local field campaign support in Bolivia; ICMBio, INPA/NAPPA/LBA COOMFLONA (Cooperativa mista da Flona Tapajós) and T. I. Bragança-Marituba for the research support. Tropical forests face increasing climate risk1,2, yet our ability to predict their response to climate change is limited by poor understanding of their resistance to water stress. Although xylem embolism resistance thresholds (for example, Ψ50) and hydraulic safety margins (for example, HSM50) are important predictors of drought-induced mortality risk3-5, little is known about how these vary across Earth's largest tropical forest. Here, we present a pan-Amazon, fully standardized hydraulic traits dataset and use it to assess regional variation in drought sensitivity and hydraulic trait ability to predict species distributions and long-term forest biomass accumulation. Parameters Ψ50 and HSM50 vary markedly across the Amazon and are related to average long-term rainfall characteristics. Both Ψ50 and HSM50 influence the biogeographical distribution of Amazon tree species. However, HSM50 was the only significant predictor of observed decadal-scale changes in forest biomass. Old-growth forests with wide HSM50 are gaining more biomass than are low HSM50 forests. We propose that this may be associated with a growth-mortality trade-off whereby trees in forests consisting of fast-growing species take greater hydraulic risks and face greater mortality risk. Moreover, in regions of more pronounced climatic change, we find evidence that forests are losing biomass, suggesting that species in these regions may be operating beyond their hydraulic limits. Continued climate change is likely to further reduce HSM50 in the Amazon6,7, with strong implications for the Amazon carbon sink. Publisher PDF

Key message: Annual stem CO2 efflux increases with stem wood production rates and are inhibited by daily moisture stress. Abstract: Tropical forests cycle a large amount of CO2 between the land and atmosphere, with a substantial portion of the return flux due tree respiratory processes. However, in situ estimates of woody tissue respiratory fluxes and carbon use efficiencies (CUEW) and their dependencies on physiological processes including stem wood production (Pw) and transpiration in tropical forests remain scarce. Here, we synthesize monthly Pw and daytime stem CO2 efflux (ES) measurements over 1 year from 80 trees with variable biomass accumulation rates in the central Amazon. On average, carbon flux to woody tissues, expressed in the same stem area normalized units as ES, averaged 0.90 ± 1.2µmolm−2s−1 for Pw, and 0.55 ± 0.33µmolm−2s−1 for daytime ES. A positive linear correlation was found between stem growth rates and stem CO2 efflux, with respiratory carbon loss equivalent to 15 ± 3% of stem carbon accrual. CUEW of stems was non-linearly correlated with growth and was as high as 77–87% for a fast-growing tree. Diurnal measurements of stem CO2 efflux for three individuals showed a daytime reduction of ES by 15–50% during periods of high sap flow and transpiration. The results demonstrate that high daytime ES fluxes are associated with high CUEW during fast tree growth, reaching higher values than previously observed in the Amazon Basin (e.g., maximum CUEW up to 77–87%, versus 30–56%). The observations are consistent with the emerging view that diurnal dynamics of stem water status influences growth processes and associated respiratory metabolism.

Understanding mechanisms of tree mortality and geometric patterns of canopy gaps is relevant for robust estimates of carbon stocks and balance in tropical forests, and for assessing how they are responding to climate change. We combined monthly RGB images acquired from an unmanned aerial vehicle with field surveys to identify gaps in an 18-ha permanent plot in an old-growth Central Amazon forest over a period of 28 months. In addition to detecting, we measured the size and shape of gaps, and analyzed their temporal variation and correlation with rainfall. We further described associated modes of tree mortality or branch fall and quantified associated losses of biomass. Overall, the sensitivity of gap detection differed between field surveys and imagery data. In total, we detected 32 gaps either in the images and field, ranging in area from 9 m2 to 835 m2. Relatively small gaps (< 39 m2) associated with branch fall were the most frequent (11 gaps). Out of 18 gaps for which both field and imagery data were available, three could not be detected remotely. This result shows that a considerable fraction of tree mortality and branch-fall events (~ 17 %) affect only the lower canopy and the understory of the forest and thus, are likely neglected by assessments of top of the canopy. Regardless the detection method, the size distribution of gaps in our study region was better captured by a Weibull function. As confirmed by our detailed field surveys, we believe that this pattern was not biased by gaps possibly undetected from image data. Although not related to differences in gap size, the main modes of tree mortality partially explained associated losses of biomass. The rate of gap area formation expressed as the percent per month was positively correlated with the frequency of extreme rainfall events, which may be related to a higher frequency of storms propagating destructive wind gusts. Our results demonstrate the importance of combining field observations with remote sensing methods for monitoring gap dynamics in dense forests. The correlation between modes of tree mortality and gap geometry with associated losses of biomass provide evidence on the importance of small-scale events of tree mortality and branch fall as processes that contribute to landscape patterns of carbon balance and species diversity in Amazon forests. Regional assessments of the dynamics and geometry of canopy gaps formed from branch fall and individual tree-mortality (e.g., from few to hundreds of m2) up to catastrophic blowdowns associated with extreme rain and wind (e.g., from hundreds of m2 to thousands of ha) can reduce the uncertainty of landscape assessments of carbon balance, especially as the frequency and intensity of storms causing these events is likely to change with future Amazon climate.

The use of wood from small-diameter species is still restricted, and knowledge of its characteristics is limited. In this context, the objective of this study was to characterize the technological properties of eight species of small diameters of greater occurrence in the Central Amazon to indicate sustainable use. Samples were obtained from a managed area of secondary forest (Amazonas/Brazil). 24 trees (diameter ≤ 50 cm) were selected for the determination of chemical and physicomechanical properties. The highest concentrations of extractives and total polyphenols were detected for Eschweilera odora (7.08 and 2.63%), and lignin and cellulose were detected for Micrandropsis scleroxylon (34.80%) and Byrsonima crispa (55.62%). For the physical-mechanical properties, the average moisture content was 12.84%. For density, the species were classified in the medium to high range (0.56-0.93 g/cm3). In general, the studied species presented a high calorific value (~4,907 cal/g), and Eschweilera truncata presented higher mechanical strength (modulus of elasticity 17,350 MPa; modulus of rupture 173.93 MPa). The multivariate analysis using the K-means algorithm, based on the centroid of the data, indicated the formation of five groups, where the group of Eschweilera truncata was represented by higher values of MOE, MOR, and ash, while the group of Inga alba had characteristics of low-strength wood. The quality of the small-diameter wood studied here has the potential to be indicated for management since the technological characterization is a fundamental tool to assist decision-making in management plans that may indicate the use of new species in the forestry sector.

In recent years, the study of skid trail recovery processes has gained momentum. In this review, 121 studies on various aspects of skid trail recovery were evaluated to determine when, where, and how the dominant factors that influence the process of recuperation occur. These studies were located proportionally in the following forest biomes: temperate (60%), tropical (31%), and boreal (9%). Research focused mainly on soil physical properties to ascertain if there had been evidence of recovery. The majority of studies of a decade or less after abandonment demonstrated that heavily used skid trails had not recovered. On the contrary, lightly used skid trails did present full recoveries over the same time span. Soil recovery tended to occur in medium- to coarse-textured soils in temperate and boreal forests. Considering all forest biomes, the impacts of compaction persisted at least two to five decades after logging operations. The impacts were evident in diminished tree heights and volumes from trees growing on skid trails. The last 50 years of research indicates that skid trails, globally, do recover from compaction, albeit slowly.

The objective of this study was to assess the quality and potential of wood from naturally fallen trees in the forest for product development and to provide subsidies for the use of raw material. The inventory of fallen trees was carried out along the road from the Experimental Station of Tropical Silviculture of the National Institute for Research in the Amazon (EEST / INPA) - Nucleus ZF-2, at km 23 of the ZF-2 road that starts to the left of km 50 of Highway BR-174 (Manaus-Boa Vista). Only trees that had fall characteristics due to natural factors, that is, that had exposed roots, were considered for the inventory. It was also stipulated as a requirement for measurement the diameter class of trees of 25 cm ≤ DBH ≤ 45 cm. From each naturally fallen tree, the diameters (largest and smallest) and their length were measured. 5cm thick discs were removed from the trees to obtain samples for scientific identification of the wood and determination of physical properties. Based on the inventory, it was identified that many of the trees naturally fallen in the forest are in good conditions of use, considering their woody material and their volume. The Alexa grandiflora species presented the highest volume with 2,788 m³ for a single tree, followed by the species Ormósia sp. with 2,287 m³ and Protium tenuifolium Engl with a volume of 1,269 m³. Regarding the health of the inventoried trees, all from the species Byrsonima crispaJuss. had no sign of degradation, followed by the species Croton lanjouwensis and Ingá sp. The most frequent class was medium density with 9 species with a variation of 0.47g/cm3 to 0.62g /cm3. Eperua schomburgkiana Benth was the most dense with 0.78g /cm3.The levels of degradation found and the intrinsic characteristics of the species did not compromise the possibilities of using this wooden product, and can represent an excellent opportunity for economic return, contributing to minimize the pressures exerted around the living forest.

Tropical forests absorb large amounts of atmospheric CO2 through photosynthesis, but high surface temperatures suppress this absorption while promoting isoprene emissions. While mechanistic isoprene emission models predict a tight coupling to photosynthetic electron transport (ETR) as a function of temperature, direct field observations of this phenomenon are lacking in the tropics and are necessary to assess the impact of a warming climate on global isoprene emissions. Here we demonstrate that in the early successional species Vismia guianensis in the central Amazon, ETR rates increased with temperature in concert with isoprene emissions, even as stomatal conductance (gs ) and net photosynthetic carbon fixation (Pn ) declined. We observed the highest temperatures of continually increasing isoprene emissions yet reported (50°C). While Pn showed an optimum value of 32.6 ± 0.4°C, isoprene emissions, ETR, and the oxidation state of PSII reaction centers (qL ) increased with leaf temperature with strong linear correlations for ETR (ƿ = 0.98) and qL (ƿ = 0.99) with leaf isoprene emissions. In contrast, other photoprotective mechanisms, such as non-photochemical quenching, were not activated at elevated temperatures. Inhibition of isoprenoid biosynthesis repressed Pn at high temperatures through a mechanism that was independent of stomatal closure. While extreme warming will decrease gs and Pn in tropical species, our observations support a thermal tolerance mechanism where the maintenance of high photosynthetic capacity under extreme warming is assisted by the simultaneous stimulation of ETR and metabolic pathways that consume the direct products of ETR including photorespiration and the biosynthesis of thermoprotective isoprenoids. Our results confirm that models which link isoprene emissions to the rate of ETR hold true in tropical species and provide necessary "ground-truthing" for simulations of the large predicted increases in tropical isoprene emissions with climate warming.

Amazonian droughts are increasing in frequency and severity. However, little is known about how this may influence species-specific vulnerability to drought across different ecosystem types. We measured 16 functional traits for 16 congeneric species from six families and eight genera restricted to floodplain, swamp, white-sand or plateau forests of Central Amazonia. We investigated whether habitat distributions can be explained by species hydraulic strategies, and if habitat specialists differ in their vulnerability to embolism that would make water transport difficult during drought periods. We found strong functional differences among species. Nonflooded species had higher wood specific gravity and lower stomatal density, whereas flooded species had wider vessels, and higher leaf and xylem hydraulic conductivity. The P50 values (water potential at 50% loss of hydraulic conductivity) of nonflooded species were significantly more negative than flooded species. However, we found no differences in hydraulic safety margin among species, suggesting that all trees may be equally likely to experience hydraulic failure during severe droughts. Water availability imposes a strong selection leading to differentiation of plant hydraulic strategies among species and may underlie patterns of adaptive radiation in many tropical tree genera. Our results have important implications for modeling species distribution and resilience under future climate scenarios.

In the forests of the Amazon Basin, there are still few studies on soil compaction caused by logging activities. This study evaluated an operation located on the property of a timber company in Central Amazonia, which uses harvesting techniques based on the harvesting system developed by the Centre for Agricultural Research in Suriname. The timber-extraction method employed by the timber company consists of three activities: (1) skid trail construction with a track-type tractor, (2) log winching with a winch attached to the tractor, and (3) log skidding with a rubber-tired skidder. Soil bulk density and soil penetration resistance were quantified to 20 cm in depth. After a single tractor ingress and egress for trail construction, the soil incurred an increased root growth-limiting bulk density and penetration resistance. However, log winching did not cause significant soil compaction. The conclusions of the study were: (1) the greatest impact from this harvesting system came from the skid trail construction, (2) the technique of log winching limited forest floor compaction and disturbance that damages forest regeneration, and (3) log skidding during dry soil conditions limited increases in compaction and track formation.

Leaf respiration in the dark (Rdark) and light (Rday) is poorly characterized in diverse tropical ecosystems, and little to no information exists on the degree of light suppression in common tree species within the Amazon basin, and their dependences upon plant functional traits and position within the canopy. We quantified Rdark and apparent Rday using the Kok method and measured key leaf traits in 26 tree individuals of different species distributed in three different canopy positions: canopy, lower canopy, and understory. To explore the relationships between the leaf traits we used the standardized major axis (SMA). We found that canopy trees had significantly higher rates of Rdark and Rday than trees in the understory. The difference between Rdark and Rday (the light suppression of respiration) was greatest in the understory (68 ± 9%, 95% CI) and lower canopy (49 ± 9%, 95% CI) when compared to the canopy (37 ± 10%, 95% CI). We also found that Rday was significantly and strongly correlated with Rdark (p < 0.001) for all the canopy positions. Also, leaf mass per area (LMA) and leaf Phosphorus concentration (P) had a significant relationship with Rdark (p < 0.001; p = 0.003), respectively. In addition, a significant relationship was found for LMA in the canopy and lower canopy positions (p = 0.009; p = 0.048) while P was only significant in the canopy (p = 0.044). Finally, no significant relationship was found between Rdark and nitrogen, sugars, and starch. Our results highlight the importance of including representation of the light suppression of leaf respiration in terrestrial biosphere models and also of accounting for vertical gradients within forest canopies and connections with functional traits.

Made available in DSpace on 2021-06-25T10:27:12Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-01 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) The forests of Amazonia are among the most biodiverse plant communities on Earth. Given the immediate threats posed by climate and land-use change, an improved understanding of how this extraordinary biodiversity is spatially organized is urgently required to develop effective conservation strategies. Most Amazonian tree species are extremely rare but a few are common across the region. Indeed, just 227 ‘hyperdominant’ species account for >50% of all individuals >10 cm diameter at 1.3 m in height. Yet, the degree to which the phenomenon of hyperdominance is sensitive to tree size, the extent to which the composition of dominant species changes with size class and how evolutionary history constrains tree hyperdominance, all remain unknown. Here, we use a large floristic dataset to show that, while hyperdominance is a universal phenomenon across forest strata, different species dominate the forest understory, midstory and canopy. We further find that, although species belonging to a range of phylogenetically dispersed lineages have become hyperdominant in small size classes, hyperdominants in large size classes are restricted to a few lineages. Our results demonstrate that it is essential to consider all forest strata to understand regional patterns of dominance and composition in Amazonia. More generally, through the lens of 654 hyperdominant species, we outline a tractable pathway for understanding the functioning of half of Amazonian forests across vertical strata and geographical locations. Institute of Environment Department of Biological Sciences Florida International University School of Geography University of Leeds Center for Global Discovery and Conservation Science Arizona State University Instituto Nacional de Pesquisas da Amazônia (INPA) Ecology and Evolutionary Biology University of Michigan Departamento de Ciencias Forestales Universidad Nacional de Colombia Departamento de Biología Universidad Autónoma de Madrid Centro de Investigación en Biodiversidad y Cambio Global (CIBC-UAM) Universidad Autónoma de Madrid Naturalis Biodiversity Center Systems Ecology Vrije Universiteit Lancaster Environment Centre Lancaster University Environmental Change Institute University of Oxford Faculty of Environmental Sciences and Natural Resource Management Norwegian University of Life Sciences School of Geosciences University of Edinburgh Missouri Botanical Garden Brazilian Agricultural Research Corporation (Embrapa) Universidad Nacional de San Antonio Abad del Cusco Department of Intergrative Biology University of California Berkeley Department of Biology University of Turku Instituto de Investigaciones de la Amazonía Peruana UNELLEZ-Guanare Programa de Ciencias del Agro y el Mar Herbario Universitario (PORT) Compensation International Progress S. A.—Ciprogress Greenlife AMAP Université de Montpellier CIRAD CNRS INRAE IRD Environmental and Rural Science University of New England INRA UMR EcoFoG AgroParisTech CNRS CIRAD Université des Antilles Université de Guyane CIRAD UMR EcoFoG BIOMAS Universidad de Las Américas Instituto de Ecología Herbario Nacional de Bolivia Departamento de Biologia Universidade Federal de Rondônia Laboratoire Evolution et Diversité Biologique (EDB) CNRS/UPS School of Geography Earth and Environmental Sciences University of Plymouth Department of Ecology and Evolutionary Biology Cornell University Department of Geography University of Exeter Facultad de Ciencias Biológicas Universidad Nacional de la Amazonía Peruana Estación Biológica del Jardín Botánico de Missouri Institut de Ciència i Tecnologia Ambientals Universitat Autònoma de Barcelona School of Geography & Sustainable Development University of St Andrews Department of Environment and Development Federal University of Amapá Centre for Tropical Environmental and Sustainability Science (TESS) and College of Marine and Environmental Sciences James Cook University Department of Environmental Science and Policy George Mason University Inventory and Monitoring Program National Park Service Smithsonian Institution Department of Plant Sciences University of Cambridge Living with Lakes Centre Laurentian University DRGB Instituto Nacional de Innovación Agraria (INIA) Herbarium Amazonense (AMAZ) Universidad Nacional de la Amazonia Peruana Department of Ecology Universidade de São Paulo Institute for Biodiversity and Ecosystem Dynamics University of Amsterdam Centro de Estudos da Biodiversidade Universidade Federal de Roraima Museo de Historia Natural Noel Kempff Mercado Universidad Autónoma Gabriel Rene Moreno Faculdade de Ciências Agrárias Biológicas e Sociais Aplicadas Universidad do Estado de Mato Grosso Centro de Biociências Universidade Federal do Rio Grande do Norte Department of Ecology Evolution and Behaviour University of Minnesota Department of Geography University College London Departamento de Ciencias Biológicas Universidad de Los Andes (Colombia) Centro de Ciências Biológicas e da Natureza Universidade Federal do Acre Nicholas School of the Environment Duke University Iwokrama International Centre for Rainforest Conservation and Development Smithsonian’s National Zoo & Conservation Biology Institute Instituto de Ciencias Naturales Universidad Nacional de Colombia Universidade Federal Rural da Amazônia—UFRA/CAPES Museu Paraense Emílio Goeldi Laboratorio de Ecología de Bosques Tropicales y Primatología Fundación Natura Colombia Universidad de Los Andes Facultad de Forestales Universidad Nacional de la Amazonía Peruana Institute of Research for Forestry Development Universidad de los Andes School of Environmental and Forest Sciences (SEFS) University of Washington Universidad Regional Amazónica Ikiam Agteca-Amazonica Universidad Autónoma del Beni Instituto Amazónico de Investigaciones (IMANI) Universidad Nacional de Colombia Broward County Parks and Recreation Biological Sciences Florida Atlantic University-Davie Museu Universitário Universidade Federal do Acre Facultad de Ingeniería Ambiental Universidad Estatal Amazónica Department of Biology Washington University in St Louis National Institute for Space Research (INPE) Geoinformática & Sistemas (GeoIS) School of Earth Sciences and Environmental Sustainability Northern Arizona University Department of Geography and the Environment University of Texas at Austin Instituto de Ciência e Tecnologia São Paulo State University (UNESP) School of Anthropology and Conservation University of Kent Universidade Federal do Amazonas Herbario Alfredo Paredes (QAP) Universidad Central del Ecuador School of Geography Earth and Environmental Sciences University of Birmingham Department of Life Sciences Imperial College London Science and Education The Field Museum Universidad Tecnica del Norte Herbario Nacional del Ecuador Research Institute Alexander von Humboldt Instituto de Ciência e Tecnologia São Paulo State University (UNESP)

This study was conducted in a humid tropical forest located in Brazil within the Amazon Basin on a heavy clay Ferralsol 24 and 30 years after logging to evaluate the recovery process of skid trails. All timber was extracted with a D6 track-type tractor. In total, four skid trails were evaluated, all logged in 1987 and two of them again in 1993. Systematic sampling was applied to the four trails and two controls. The data collected were rut depth, soil bulk density (BD), soil penetration resistance (PR) and seedling and sapling density. Soil physical properties were determined in three layers from 0 to 15 cm in depth, and seedling density was determined from 36 plots of 4m² in size at the soil sample locations. Rut depths were significantly different (p

Selective logging is one of the most prominent land uses in tropical forests and although it involves harvesting a limited number of trees, the impact on forest structure, composition and aboveground biomass can be significant. Although these impacts are well documented, what is little known is the extent to which selective logging affects tree floristic composition and its recovery process. Understanding how floristic composition is affected by logging activities is essential for determining subsequent cutting cycles, for the maintenance of carbon stocks and for biodiversity conservation. This research investigates the effect of logging on long-term trends on the recovery of species composition in a tropical forest using a unique logging experiment where measurements have been taken annually over a period of 25 years. Changes in 12 long-term 1-hectare (ha) permanent plots were assessed where different selective logging intensities occurred. In the first years after logging, floristic composition differed widely between intact and selectively logged forests, with exploited areas deviating from pre-logged composition. Over time, exploited areas shifted towards the original composition, with more pronounced changes in this trend after ∼13 years. Shifts in floristic composition were caused mainly by a significant increase in light-demanding fast-growing pioneer species and their subsequent continuous high mortality rates after 13 years of the recovery process. In contrast, the control plots showed similar shifts in composition over time, suggesting external factors such as long-term climate changes may be driving these shifts. The results suggest that 25 years after an experimental selective logging has taken place, floristic composition tends to recover closer to the pre-logged status. Thus, in the absence of further human disturbances, experimental selectively logged forests in low to moderate intensities are compatible with biodiversity conservation, at least during the first cycle of exploitation. Reconciling conservation strategies with the recovery of stocks of commercial timber species would be greatly improved by using these results and lead towards more sustainable forest management plans.

The factors shaping the distribution of giant tropical trees are poorly understood, despite its importance as a link between evolutionary biology and ecosystem biogeochemistry. The recent discovery of clusters of trees over 80 metres tall in the Guiana Shield region of the Amazon rainforest challenges the current understanding of the factors controlling the growth and survival of giant trees. The new discovery led us to revisit the question: what determines the distribution of the tallest trees of the Amazon?Here, we used high-resolution airborne LiDAR (Light Detection and Ranging) surveys to measure canopy height across 282,750 ha of primary old-growth and secondary forests throughout the entire Brazilian Amazon to investigate the relationship between the occurrence of giant trees and the environmental factors that influence their growth and survival. Our results suggest that the factors controlling where trees grow extremely tall are distinct from those controlling their longevity. Trees grow taller in areas with high soil clay content (> 42%), lower radiation (< 130 clear days per year) and wind speeds, avoiding alluvial areas (elevations higher than 40 m a.s.l), and with an optimal precipitation range of 1,500 to 2,500 mm yr-1. We then used an envelope model to determine the environmental conditions that support the very tallest trees (i.e. over 70 m height). We found that, as opposed to the myriad of interacting factors that control the maximum height at a large scale, wind speed had by far the largest influence on the distribution of these sentinel trees, and explained 67% of the probability of finding trees over 70 m in the Brazilian Amazon forest.The high-resolution pan-Amazon LiDAR data showed that environmental variables that drive growth in height are fundamentally different from environmental variables that support their survival. While precipitation and temperature seem to have lower importance for their survival than expected from previous studies, changes in wind and radiation regimes could reshape our forested biomes. This should be carefully considered by policy-makers when identifying important hotspots for the conservation of biodiversity in the Amazon.

The Amazon rainforest covers an area of about 50% of the Brazilian national territory, which consists mainly of upland rainforests that are generally poorly managed, because of low investment in technology, planning, operations and manufacturing. Logging activities require a large contingent of heavy machinery and an intense physical workload from the operators and crews. The thermal comfort at work in tropical regions is between 20 and 24°C; however, in the Central Amazon the daily average temperature exceeds 28°C. The sum of these adverse factors leads to a common denominator: low quality logging operations that lead to unsustainable forest management. The objective of this study was to investigate the influence of environmental thermal conditions on performance, rest breaks, work dynamics and safety of workers involved in the logging operations, as well as to understand better their interactions. The data was collected from the following logging activities: felling, pre-skidding, skidding and landing operations. These variables were analyzed using PCA analysis, MANOVA and multiple linear regression. The variables of productivity and rest breaks were strongly influenced by mechanical interruption and time of the workday. We concluded that mechanical availability was the most influential factor in the performance of logging operations. In addition, environmental thermal conditions, bonus payments and work dynamics showed an influence. To a less extent, there was an influence of safety and physical comfort of workers, which resulted in higher rest breaks, depending on the operation involved. This influence was observed in operations with a higher physical workload (felling and pre-skidding). Moreover, the tree volume had a significant impact on the productivity of the chainsaw operator, which was also influenced by the species factor, as in the species Hymenolobium modestum. Lastly, improvements in working conditions such as appropriate clothing, job rotation and scheduled breaks would lead to a greater worker well-being with increased labor productivity and safety. In turn, this would greatly contribute to the quality and performance of overall forest management and sustainability in the economic development of the Amazon region.

International audience

The Amazon rainforest covers an area of about 50% of the Brazilian national territory, which consists mainly of upland rainforests that are generally poorly managed, because of low investment in technology, planning, operations and manufacturing. Logging activities require a large contingent of heavy machinery and an intense physical workload from the operators and crews. The thermal comfort at work in tropical regions is between 20 and 24°C; however, in the Central Amazon the daily average temperature exceeds 28°C. The sum of these adverse factors leads to a common denominator: low quality logging operations that lead to unsustainable forest management. The objective of this study was to investigate the influence of environmental thermal conditions on performance, rest breaks, work dynamics and safety of workers involved in the logging operations, as well as to understand better their interactions. The data was collected from the following logging activities: felling, pre-skidding, skidding and landing operations. These variables were analyzed using PCA analysis, MANOVA and multiple linear regression. The variables of productivity and rest breaks were strongly influenced by mechanical interruption and time of the workday. We concluded that mechanical availability was the most influential factor in the performance of logging operations. In addition, environmental thermal conditions, bonus payments and work dynamics showed an influence. To a less extent, there was an influence of safety and physical comfort of workers, which resulted in higher rest breaks, depending on the operation involved. This influence was observed in operations with a higher physical workload (felling and pre-skidding). Moreover, the tree volume had a significant impact on the productivity of the chainsaw operator, which was also influenced by the species factor, as in the species Hymenolobium modestum. Lastly, improvements in working conditions such as appropriate clothing, job rotation and scheduled breaks would lead to a greater worker well-being with increased labor productivity and safety. In turn, this would greatly contribute to the quality and performance of overall forest management and sustainability in the economic development of the Amazon region.

Forest biomass is an essential indicator for monitoring the Earth’s ecosystems and climate. It is a critical input to greenhouse gas accounting, estimation of carbon losses and forest degradation, assessment of renewable energy potential, and for developing climate change mitigation policies such as REDD+, among others. Wall-to-wall mapping of aboveground biomass (AGB) is now possible with satellite remote sensing (RS). However, RS methods require extant, up-to-date, reliable, representative and comparable in situ data for calibration and validation. Here, we present the Forest Observation System (FOS) initiative, an international cooperation to establish and maintain a global in situ forest biomass database. AGB and canopy height estimates with their associated uncertainties are derived at a 0.25 ha scale from field measurements made in permanent research plots across the world’s forests. All plot estimates are geolocated and have a size that allows for direct comparison with many RS measurements. The FOS offers the potential to improve the accuracy of RS-based biomass products while developing new synergies between the RS and ground-based ecosystem research communities., Measurement(s)above-ground biomass • organic materialTechnology Type(s)tree species census • measurement methodFactor Type(s)geographic location • tree speciesSample Characteristic - Environmentforest biomeSample Characteristic - LocationEarth (planet) Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.9850571

Azteca ants are widely distributed in the neotropics and have been utilized as natural insect repellent for centuries. Azteca oils provide natural defense against herbivores in mutualistic interactions between ants and their host trees. While chemical characterization of oil secretions revealed a composition dominated by iridoids and ketones, the volatile emissions from Azteca ants, and therefore the active gas-phase semiochemicals, remain uncharacterized. In this study, we determined the composition of volatile emissions from a sample of an Azteca ant nest near the Rio Negro in the central Amazon. We found Azteca emissions were composed of a blend of methyl cyclopentyl and methyl cyclopentenyl based volatiles previously identified in Azteca oil extracts as potent alarm pheromones. The ketone 6-methyl-5-hepten-2-one, which also waspreviously identified as a major component of the Azteca oil, was found to be the dominant volatile emitted. For the first time, we report emissions of the highly volatile ketones 2,3-butadione and acetoin from the Azteca nest. Our study has important implications for the better understanding of the ecology and defense strategies of Azteca ants in herbivore defense and provides a base for future commercial applications involving Azteca ant essential oils as natural insect repellents.

Author(s): Holm, JA; Knox, RG; Zhu, Q; Fisher, RA; Koven, CD; Nogueira Lima, AJ; Riley, WJ; Longo, M; Negron-Juarez, RI; de Araujo, AC; Kueppers, LM; Moorcroft, PR; Higuchi, N; Chambers, JQ | Abstract: There is large uncertainty whether Amazon forests will remain a carbon sink as atmospheric CO2 increases. Hence, we simulated an old-growth tropical forest using six versions of four terrestrial models differing in scale of vegetation structure and representation of biogeochemical (BGC) cycling, all driven with CO2 forcing from the preindustrial period to 2100. The models were benchmarked against tree inventory and eddy covariance data from a Brazilian site for present-day predictions. All models predicted positive vegetation growth that outpaced mortality, leading to continual increases in present-day biomass accumulation. Notably, the two vegetation demographic models (VDMs) (ED2 and ELM-FATES) always predicted positive stem diameter growth in all size classes. The field data, however, indicated that a quarter of canopy trees didn't grow over the 15-year period, and while high interannual variation existed, biomass change was near neutral. With a doubling of CO2, three of the four models predicted an appreciable biomass sink (0.77 to 1.24 Mg ha−1 year−1). ELMv1-ECA, the only model used here that includes phosphorus constraints, predicted the lowest biomass sink relative to initial biomass stocks (+21%), lower than the other BGC model, CLM5 (+48%). Models projections differed primarily through variations in nutrient constraints, then carbon allocation, initial biomass, and density-dependent mortality. The VDM's performance was similar or better than the BGC models run in carbon-only mode, suggesting that nutrient competition in VDMs will improve predictions. We demonstrate that VDMs are comparable to nondemographic (i.e., “big-leaf”) models but also include finer scale demography and competition that can be evaluated against field observations.

Author(s): Negron-Juarez, R; Ferreira, SJF; Mota, MC; Faybishenko, B; Monteiro, MTF; Candido, LA; Ribeiro, RP; de Oliveira, RC; de Araujo, AC; Warren, JM; Newman, BD; Gimenez, BO; Varadharajan, C; Agarwal, D; Borma, L; Tomasella, J; Higuchi, N; Chambers, JQ | Abstract: Soil moisture plays a key role in hydrological, biogeochemical, and energy budgets of terrestrial ecosystems. Accurate soil moisture measurements in remote ecosystems such as the Amazon are difficult and limited because of logistical constraints. Time domain reflectometry (TDR) sensors are widely used to monitor soil moisture and require calibration to convert the TDR's dielectric permittivity measurement (Ka) to volumetric water content (θv). In this study, our objectives were to develop a field-based calibration of TDR sensors in an old-growth upland forest in the central Amazon, to evaluate the performance of the calibration, and then to apply the calibration to determine the dynamics of soil moisture content within a 14.2-m-deep vertical soil profile. Depth-specific TDR calibration using local soils in a controlled laboratory setting yielded a novel Ka–θv third-degree polynomial calibration. The sensors were later installed to their specific calibration depth in a 14.2-m pit. The widely used Ka–θv relationship (Topp model) underestimated the site-specific θv by 22–42%, indicating significant error in the model when applied to these well-structured, clay-rich tropical forest soils. The calibrated wet- and dry-season θv data showed a variety of depth and temporal variations highlighting the importance of soil textural differentiation, root uptake depths, as well as event to seasonal precipitation effects. Data such as these are greatly needed for improving our understanding of ecohydrological processes within tropical forests and for improving models of these systems in the face of changing environmental conditions.

The sensitivity of tropical forest carbon to climate is a key uncertainty in predicting global climate change. Although short-term drying and warming are known to affect forests, it is unknown if such effects translate into long-term responses. Here, we analyze 590 permanent plots measured across the tropics to derive the equilibrium climate controls on forest carbon. Maximum temperature is the most important predictor of aboveground biomass (-9.1 megagrams of carbon per hectare per degree Celsius), primarily by reducing woody productivity, and has a greater impact per °C in the hottest forests (>32.2°C). Our results nevertheless reveal greater thermal resilience than observations of short-term variation imply. To realize the long-term climate adaptation potential of tropical forests requires both protecting them and stabilizing Earth's climate., Our plot monitoring networks havebeen supported by multiple grants from a large number of funding bodies: European Research Council; Natural Environment Research Council; European Union’s Fifth, Sixth, and Seventh Framework Programme; Royal Society, Leverhulme Trust; Gordon and Betty Moore Foundation; David and Lucile Packard Foundation; State of São Paulo Research Foundation (FAPESP); National Council for Science and Technology Development of Brazil (CNPq); Agence Nationale de la Recherche; Conservation International; Missouri Botanical Garden; Smithsonian Institution; Wildlife Conservation Society; National Geographic Society; Centre for International Forestry; and Gabon’s National Park Agency.

Tree growth and survival differ strongly between canopy trees (those directly exposed to overhead light), and understory trees. However, the structural complexity of many tropical forests makes it difficult to determine canopy positions. The integration of remote sensing and ground-based data enables this determination and measurements of how canopy and understory trees differ in structure and dynamics. Here we analyzed 2 cm resolution RGB imagery collected by a Remotely Piloted Aircraft System (RPAS), also known as drone, together with two decades of bi-annual tree censuses for 2 ha of old growth forest in the Central Amazon. We delineated all crowns visible in the imagery and linked each crown to a tagged stem through field work. Canopy trees constituted 40% of the 1244 inventoried trees with diameter at breast height (DBH) > 10 cm, and accounted for ~70% of aboveground carbon stocks and wood productivity. The probability of being in the canopy increased logistically with tree diameter, passing through 50% at 23.5 cm DBH. Diameter growth was on average twice as large in canopy trees as in understory trees. Growth rates were unrelated to diameter in canopy trees and positively related to diameter in understory trees, consistent with the idea that light availability increases with diameter in the understory but not the canopy. The whole stand size distribution was best fit by a Weibull distribution, whereas the separate size distributions of understory trees or canopy trees > 25 cm DBH were equally well fit by exponential and Weibull distributions, consistent with mechanistic forest models. The identification and field mapping of crowns seen in a high resolution orthomosaic revealed new patterns in the structure and dynamics of trees of canopy vs. understory at this site, demonstrating the value of traditional tree censuses with drone remote sensing.

The carbon sink capacity of tropical forests is substantially affected by tree mortality. However, the main drivers of tropical tree death remain largely unknown. Here we present a pan-Amazonian assessment of how and why trees die, analysing over 120,000 trees representing > 3800 species from 189 long-term RAINFOR forest plots. While tree mortality rates vary greatly Amazon-wide, on average trees are as likely to die standing as they are broken or uprooted—modes of death with different ecological consequences. Species-level growth rate is the single most important predictor of tree death in Amazonia, with faster-growing species being at higher risk. Within species, however, the slowest-growing trees are at greatest risk while the effect of tree size varies across the basin. In the driest Amazonian region species-level bioclimatic distributional patterns also predict the risk of death, suggesting that these forests are experiencing climatic conditions beyond their adaptative limits. These results provide not only a holistic pan-Amazonian picture of tree death but large-scale evidence for the overarching importance of the growth–survival trade-off in driving tropical tree mortality., Tree mortality has been shown to be the dominant control on carbon storage in Amazon forests, but little is known of how and why Amazon forest trees die. Here the authors analyse a large Amazon-wide dataset, finding that fast-growing species face greater mortality risk, but that slower-growing individuals within a species are more likely to die, regardless of size.

Tropical forests are acknowledged to be the largest global source of isoprene (C5H8) and monoterpenes (C10H16) emissions, with current synthesis studies suggesting few tropical species emit isoprenoids (20-38%) and do so with highly variable emission capacities, including within the same genera. This apparent lack of a clear phylogenetic thread has created difficulties both in linking isoprenoid function with evolution and for the development of accurate biosphere-atmosphere models. Here, we present a systematic emission study of "hyperdominant" tree species in the Amazon Basin. Across 162 individuals, distributed among 25 botanical families and 113 species, isoprenoid emissions were widespread among both early and late successional species (isoprene: 61.9% of the species; monoterpenes: 15.0%; both isoprene and monoterpenes: 9.7%). The hyperdominant species (69) across the top five most abundant genera, which make up about 50% of all individuals in the Basin, had a similar abundance of isoprenoid emitters (isoprene: 63.8%; monoterpenes: 17.4%; both 11.6%). Among the abundant genera, only Pouteria had a low frequency of isoprene emitting species (15.8% of 19 species). In contrast, Protium, Licania, Inga, and Eschweilera were rich in isoprene emitting species (83.3% of 12 species, 61.1% of 18 species, 100% of 8 species, and 100% of 12 species, respectively). Light response curves of individuals in each of the five genera showed light-dependent, photosynthesis-linked emission rates of isoprene and monoterpenes. Importantly, in every genus, we observed species with light-dependent isoprene emissions together with monoterpenes including β-ocimene. These observations support the emerging view of the evolution of isoprene synthases from β-ocimene synthases. Our results have important implications for understanding isoprenoid function-evolution relationships and the development of more accurate Earth System Models.

Higher levels of taxonomic and evolutionary diversity are expected to maximize ecosystem function, yet their relative importance in driving variation in ecosystem function at large scales in diverse forests is unknown. Using 90 inventory plots across intact, lowland, terra firme, Amazonian forests and a new phylogeny including 526 angiosperm genera, we investigated the association between taxonomic and evolutionary metrics of diversity and two key measures of ecosystem function: aboveground wood productivity and biomass storage. While taxonomic and phylogenetic diversity were not important predictors of variation in biomass, both emerged as independent predictors of wood productivity. Amazon forests that contain greater evolutionary diversity and a higher proportion of rare species have higher productivity. While climatic and edaphic variables are together the strongest predictors of productivity, our results show that the evolutionary diversity of tree species in diverse forest stands also influences productivity. As our models accounted for wood density and tree size, they also suggest that additional, unstudied, evolutionarily correlated traits have significant effects on ecosystem function in tropical forests. Overall, our pan-Amazonian analysis shows that greater phylogenetic diversity translates into higher levels of ecosystem function: tropical forest communities with more distantly related taxa have greater wood productivity.

Growth rings are investigated in trees harvested in the second cutting cycle in southern Amazonia and have important implications for dendrochronological studies and for forest management. In the southern Brazilian Amazon, upland moist forests have been managed based on a polycyclic system, which cutting cycle (CC) varies from 25 to 35 years, and the minimum logging diameter (MLD) is 50 cm for all species. Many forests logged during the 1970s are being prepared for the second cycle. However, without growth and yield rates information on the remaining forests as well as for individual species, the principles of sustainable management will be jeopardized. For species with annual growth rings, such information can be obtained using dendrochronological techniques. This study investigated the periodicity of rings in Qualea paraensis and Parkia pendula in a forest that had already experienced one cutting cycle. This information was used to estimate growth and yield rates, and adjusting to equations to estimate individual species MLD and CC. Dendrochronological techniques were combined with radiocarbon analyses to confirm whether rings were annual. Rings of Q. paraensis were confirmed to be annual without radiocarbon analysis. However, P. pendula rings were poorly distinguishable; therefore, delimitation and ring counting were systematically underestimated by 10%. Growth and yield rates of managed forests were favored by logging. The MLD should be 53 cm for Q. paraensis, and 42 cm for P. pendula; and the CC must be 11 and 17 years, respectively. It is concluded that MLD and CC legally defined by the Brazilian laws are not adequate for the studied species; in addition, the use of radiocarbon-calibrated dendrochronology technique is essential to produce robust and unbiased estimates of growth and yield rates.

Plant-derived latex is widely used in rubber production and plays important roles in ecological processes in the tropics. Although latex from the commercially important tree Hevea brasiliensis regulates latex oxidation resulting in browning, little is known about latex oxidation in highly diverse tropical ecosystems. Here we show that upon physical trunk damage, yellow latex released from the canopy tree Muiratinga (Maquira sclerophylla (Ducke) C.C. Berg) is rapidly and extensively oxidized to a black resin in the presence of air within 15-30 min. In a nitrogen atmosphere, latex oxidation was inhibited, but was immediately activated upon exposure to air. The results suggest the occurrence of O2-dependent oxidative enzymes including polyphenol oxidase (PPO) within the latex of Muiratinga and supports previous findings of a key role of oxidation during latex coagulation. The findings therefore have important implications for understanding the mechanisms of latex coagulation and its ecological role(s) including wound sealing and defense against herbivores.

The original version of this Article contained an error in Table 2, where the number of individuals in the “All Amazonia” row was reported as 11,6431 instead of 116,431. Also, the original version of this Article contained an error in the Methods, where the R2 for the proportion of broken/uprooted dead trees increase per year was reported as 0.12, the correct value being 0.06. The original version of this Article contained errors in the author affiliations. The affiliation of Gerardo A. Aymard C. with UNELLEZGuanare, Herbario Universitario (PORT), Portuguesa, Venezuela Compensation International Progress S.A. Ciprogress–Greenlife.

A utilização de novas tecnologias para caracterizar madeiras tropicais é essencial para reduzir desperdícios na cadeia produtiva da indústria madeireira, aumentando o rendimento e produtividade do setor. Objetivou-se neste trabalho avaliar a utilização da tomografia de impulso na identificação do estado de sanidade e rendimento do lenho de árvores na floresta. O estudo foi conduzido em 01 hectare na Estação Experimental de Silvicultura Tropical – Núcleo ZF-2 - INPA. Foram selecionadas quatro árvores de Micrandopsis scleroxylon W. Rodr. para a utilização da tomografia de impulso; técnica essa que se baseia no cálculo da velocidade de propagação das ondas mecânicas no interior do lenho, originando uma imagem tomográfica. Utilizou-se a fórmula de Smalian para determinação dos volumes de madeira total e do oco, sendo sua diferença, o volume de madeira útil. Também foi estimado o volume de madeira útil a partir dos dados da tomografia de impulso. As árvores foram desdobradas na serraria para o cálculo do rendimento. Estes dados foram comparados com a classificação feita no software Envi 4.8, verificando a relação entre essas variáveis. A tomografia de impulso mostrou-se eficiente para estimar o estado de sanidade da árvore na floresta, demonstrando ser uma ferramenta útil no planejamento da exploração florestal.Palavra-chave: metodologia não destrutiva, manejo florestal, madeiras da Amazônia. IMPULSE TOMOGRAPHY IN THE EVALUATION SANITY AND PERFOMANCE OF \Micrandopsis scleroxylon W. Rodr. ABSTRACT:The use of new technologies to characterize tropical Wood is essential to reduce waste in wood industry productive chain, increasing the performance and productivity of the sector. The goal of this work was to access the use of the non-destructive method of impulse tomography in the identification of sanity state and wood performance of standing trees in the forest. The study was led in 01 hectare in Experimental Station of Tropical Silviculture – Campus ZF-2 - INPA. Four trees of the Micrandopsis scleroxylon W. Rodr. were selected for the impulse tomography application, technique based on the calculation of the wave propagation speed in the interior of the wood, giving rise to a tomographic image. It was used the Smalian formula to determine the total wood volume taking away the hollow volume equals the volume of useful wood. It was also estimated the useful volume using the data of the impulse tomography. The trees were unfolded in the sawmill for performance calculation. These data were compared to the classification done in the Envi 4.8 software, verifying the relation between the variables. The impulse tomography proved to be efficient to estimate the health status of the tree in the forest, showing to be a useful tool the forest exploration planning.Keywords: non-destructive methodology, forest management, Amazon wood. DOI: http://dx.doi.org/10.5935/2318-7670.v05nespa28

The Amazon forest is integral to the global climate system in part because of the high rate of rainfall recycling through tree transpiration and biodiversity (size and species composition). However, the partitioning of precipitation into evaporation, transpiration and runoff, has been quantified at only a few sites. At our study site in the central Amazon, annual rainfall in 2013 was 2302 mm and latent heat flux measurements made using eddy covariance revealed that 1360 mm (59%) was returned to the atmosphere through evaporation and transpiration. Runoff accounted for 41% of the net ecosystem water loss. Combining annual xylem sap flux estimates with total stand sap wood area, we estimated annual stand transpiration rate to be 851 mm (36% of annual rainfall). Emergent canopy trees (diameter >30 cm; average height of 28 m) were responsible for the majority (71%) of the transpired water flux, recycling potentially 26% of the rainfall back to the atmosphere. By difference, we estimate that 510 mm of intercepted rainwater (22% of rainfall) was evaporated directly back to atmosphere from the canopy. Highest stand transpiration rates occurred during the dryer months due to both increased water vapor pressure deficit and the onset of new leaf flush. This study provides further evidence for convergent water use characteristics of tropical trees and highlights the importance of large trees in tropical moist forests. Large trees have been demonstrated to be vulnerable to drought-related mortality, and thus potentially will make up a critical component of the response of tropical forests to climate change.

Tropical forests absorb large amounts of atmospheric CO2 through photosynthesis but elevated temperatures suppress this absorption and promote monoterpene emissions. Using 13 CO2 labeling, here we show that monoterpene emissions from tropical leaves derive from recent photosynthesis and demonstrate distinct temperature optima for five groups (Groups 1-5), potentially corresponding to different enzymatic temperature-dependent reaction mechanisms within β-ocimene synthases. As diurnal and seasonal leaf temperatures increased during the Amazonian 2015 El Nino event, leaf and landscape monoterpene emissions showed strong linear enrichments of β-ocimenes (+4.4% °C-1 ) at the expense of other monoterpene isomers. The observed inverse temperature response of α-pinene (-0.8% °C-1 ), typically assumed to be the dominant monoterpene with moderate reactivity, was not accurately simulated by current global emission models. Given that β-ocimenes are highly reactive with respect to both atmospheric and biological oxidants, the results suggest that highly reactive β-ocimenes may play important roles in the thermotolerance of photosynthesis by functioning as effective antioxidants within plants and as efficient atmospheric precursors of secondary organic aerosols. Thus, monoterpene composition may represent a new sensitive 'thermometer' of leaf oxidative stress and atmospheric reactivity, and therefore a new tool in future studies of warming impacts on tropical biosphere-atmosphere carbon-cycle feedbacks.

High descending winds generated by convective storms are a frequent and a major source of tree mortality disturbance events in the Amazon, affecting forest structure and diversity across a variety of scales, and more frequently observed in western and central portions of the basin. Soil texture in the Central Amazon also varies significantly with elevation along a topographic gradient, with decreasing clay content on plateaus, slopes and valleys respectively. In this study we investigated the critical turning moments (Mcrit - rotational force at the moment of tree failure, an indicator of tree stability or wind resistance) of 60 trees, ranging from 19.0 to 41.1 cm in diameter at breast height (DBH) and located in different topographic positions, and for different species, using a cable-winch load-cell system. Our approach used torque as a measure of tree failure to the point of snapping or uprooting. This approach provides a better understanding of the mechanical forces required to topple trees in tropical forests, and will inform models of wind throw disturbance. Across the topographic positions, size controlled variation in Mcrit was quantified for cardeiro (Scleronema mincranthum (Ducke) Ducke), mata-mata (Eschweilera spp.), and a random selection of trees from 19 other species. Our analysis of Mcrit revealed that tree resistance to failure increased with size (DBH and ABG) and differed among species. No effects of topography or failure mode were found for the species either separately or pooled. For the random species, total variance in Mcrit explained by tree size metrics increased from an R2 of 0.49 for DBH alone, to 0.68 when both DBH and stem fresh wood density (SWD) were included in a multiple regression model. This mechanistic approach allows the comparison of tree vulnerability induced by wind damage across ecosystems, and facilitates the use of forest structural information in ecosystem models that include variable resistance of trees to mortality inducing factors. Our results indicate that observed topographic differences in windthrow vulnerability are likely due to elevational differences in wind velocities, rather than by differences in soil-related factors that might effect Mcrit.