Alessandro C. Araújo

The Amazon basin in transition

Agricultural expansion and climate variability have become important agents of disturbance in the Amazon basin. Recent studies have demonstrated considerable resilience of Amazonian forests to moderate annual drought, but they also show that interactions between deforestation, fire and drought potentially lead to losses of carbon storage and changes in regional precipitation patterns and river discharge. Although the basin-wide impacts of land use and drought may not yet surpass the magnitude of natural variability of hydrologic and biogeochemical cycles, there are some signs of a transition to a disturbance-dominated regime. These signs include changing energy and water cycles in the southern and eastern portions of the Amazon basin.

Respiration from a tropical forest ecosystem: partitioning of sources and low carbon use efficiency

Understanding how tropical forest carbon balance will respond to global change requires knowledge of individual heterotrophic and autotrophic respiratory sources, together with factors that control respiratory variability. We measured leaf, live wood, and soil respiration, along with additional environmental factors over a 1-yr period in a Central Amazon terra firme forest. Scaling these fluxes to the ecosystem, and combining our data with results from other studies, we estimated an average total ecosystem respiration (Reco) of 7.8 μmol·m−2·s−1. Average estimates (per unit ground area) for leaf, wood, soil, total heterotrophic, and total autotrophic respiration were 2.6, 1.1, 3.2, 5.6, and 2.2 μmol·m−2·s−1, respectively. Comparing autotrophic respiration with net primary production (NPP) estimates indicated that only ∼30% of carbon assimilated in photosynthesis was used to construct new tissues, with the remaining 70% being respired back to the atmosphere as autotrophic respiration. This low ecosystem carbon use efficiency (CUE) differs considerably from the relatively constant CUE of ∼0.5 found for temperate forests. Our Reco estimate was comparable to the above-canopy flux (Fac) from eddy covariance during defined sustained high turbulence conditions (when presumably Fac = Reco) of 8.4 (95% ci = 7.5– 9.4). Multiple regression analysis demonstrated that ∼50% of the nighttime variability in Fac was accounted for by friction velocity (u*, a measure of turbulence) variables. After accounting for u* variability, mean Fac varied significantly with seasonal and daily changes in precipitation. A seasonal increase in precipitation resulted in a decrease in Fac, similar to our soil respiration response to moisture. The effect of daily changes in precipitation was complex: precipitation after a dry period resulted in a large increase in Fac, whereas additional precipitation after a rainy period had little effect. This response was similar to that of surface litter (coarse and fine), where respiration is greatly reduced when moisture is limiting, but increases markedly and quickly saturates with an increase in moisture.

THE ROBUSTNESS OF EDDY CORRELATION FLUXES FOR AMAZON RAIN FOREST CONDITIONS

We analyzed errors and uncertainties in time-integrated eddy correlation data for sites in the Amazon. A well-known source of potential error in eddy correlation is through possible advective losses of CO2 emissions during calm nights. There are also questions related to the treatment of low frequencies, non-horizontal flow, and uncertainties in, e.g., corrections for tube delay and frequency loss, as well as the effect of missing data. In this study, we systematically explore these issues for the specific situation of flux mea- surements at two Amazon forest sites. Results indicate that, for this specific environment with tall forest and tall towers, errors and uncertainties caused by data spikes, delay cor- rections, and high-frequency loss are small (,3% on an annual basis). However, sensitivities to the treatment of low frequencies and non-horizontal flow can be large, especially if the landscape is not homogeneous. Given that there is no consensus on methodology here, this represents an uncertainty of 10-25% on annual total carbon uptake. The other large un- certainty is clearly in the nighttime fluxes. Two different ways to evaluate the validity of these fluxes resulted in at least a 100% difference of annual totals. Finally, we show that uncertainty (standard errors) associated with data gaps can be reduced to ,0.5 Mg·ha 21 ·yr 21 if data are covering at least half of the time, with random spread. Overall uncertainty, on annual CO2 fluxes, excluding the nighttime dilemma, is estimated at 612% (central Amazon site) to 632% (southwest Amazon site). Additionally, the nighttime uncertainty is of similar magnitude as the time-integrated fluxes themselves.

ACRIDICON–CHUVA Campaign: Studying Tropical Deep Convective Clouds and Precipitation over Amazonia Using the New German Research Aircraft HALO

AbstractBetween 1 September and 4 October 2014, a combined airborne and ground-based measurement campaign was conducted to study tropical deep convective clouds over the Brazilian Amazon rain forest. The new German research aircraft, High Altitude and Long Range Research Aircraft (HALO), a modified Gulfstream G550, and extensive ground-based instrumentation were deployed in and near Manaus (State of Amazonas). The campaign was part of the German–Brazilian Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems–Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (Global Precipitation Measurement) (ACRIDICON– CHUVA) venture to quantify aerosol–cloud–precipitation interactions and their thermodynamic, dynamic, and radiative effects by in situ and remote sensing measurements over Amazonia. The ACRIDICON–CHUVA field observations were carried out in cooperation with the second intensive operating period...

NOCTURNAL ACCUMULATION OF CO2 UNDERNEATH A TROPICAL FOREST CANOPY ALONG A TOPOGRAPHICAL GRADIENT

Flux measurements of carbon dioxide and water vapor above tropical rain forests are often difficult to interpret because the terrain is usually complex. This complexity induces heterogeneity in the surface but also affects lateral movement of carbon dioxide (CO2) not readily detected by the eddy covariance systems. This study describes such variability using measurements of CO2 along vertical profiles and along a toposequence in a tropical rain forest near Manaus, Brazil. Seasonal and diurnal variation was recorded, with atmospheric CO2 concentration maxima around dawn, generally higher CO2 build-up in the dry season and stronger daytime CO2 drawdown in the wet season. This variation was reflected all along the toposequence, but the slope and valley bottom accumulated clearly more CO2 than the plateaus, depending on atmospheric stability. Particularly during stable nights, accumulation was along lines of equal altitude, suggesting that large amounts of CO2 are stored in the valleys of the landscape. Flushing of this store only occurs during mid-morning, when stored CO2 may well be partly transported back to the plateaus. It is clear that, for proper interpretation of tower fluxes in such complex and actively respiring terrain, the horizontal variability of storage needs to be taken into account not only during the night but also during the mornings.

Dry-season greening of Amazon forests

Evidence from ecological studies1,2, eddy flux towers3–5, and satellites3,6 shows that many tropical forests ‘green up’ during higher sunlight annual dry seasons, suggesting they are more limited by light than water. Morton et al.7 reported that satellite-observed dry-season green up in Amazon forests is an artefact of seasonal variations in sunsensor geometry. However, here we argue that even after artefact correction, data from Morton et al. show statistically significant increases in canopy greenness during the dry season. Integrating corrected satellite with ground observations indicates that dry-season forest greening is prevalent in Amazonia, probably reflecting large-scale seasonal upregulation of photosynthesis by canopy leaf dynamics. There is a reply to this Brief Communication Arising by Morton, D. C. et al. Nature 531, http://dx.doi.org/10.1038/nature16458 (2016). Variations in sun-sensor geometry induce artefacts in remotely sensed vegetated surfaces8. Satellite studies thus typically use models to correct artefacts (for example, Moderate Resolution Imaging Spectroradiometer (MODIS) leaf area index9, and multiangle implementation of atmospheric correction (MAIAC) enhanced vegetation index10 (EVI)) or compositing algorithms designed to minimize artefacts (standard MODIS EVI11). Morton et al.7 used a modelling approach to correct MODIS satellite data, which they state removed seasonal changes in surface reflectance, and redefined debates over how climate controls forest productivity in the Amazon. Setting aside arguments that the remote sensing analysis by Morton et al. is faulty12, we take their correction7 at face value, and ask two questions. First, we ask whether the corrected results support their core conclusion that dry-season green up, previously observed by MODIS EVI, is eliminated. The hypothesis that Amazon forests green up in the dry season3 can be rigorously evaluated by formal statistical tests. Morton et al.7 showed that their correction reduces estimated dry season green up, Δ EVI (the EVI change during the dry season, Δ EVI = October EVI − June EVI; figure 3 in ref. 7 and Fig. 1). As the corrected mean Δ EVI was smaller than an a priori estimate of error for individual EVI observations, they concluded that the corrected mean Δ EVI was indistinguishable from zero. We find that this comparison, however, is not appropriate for assessing whether corrected EVI can resolve a basin-wide green up. The correct comparison, of mean Δ EVI to the error of the mean of the whole population of observations, is accomplished with standard statistical tests that lever the probability theory ‘law of large numbers’13. For example, the 95% confidence interval13 for basin-wide mean of corrected Δ EVI significantly excludes zero (Fig. 1). Alternatively, the corrected Δ EVI distribution7 can be compared to the binomial distribution generated by the null hypothesis that pixels are equally likely to exhibit positive or negative Δ EVI (Fig. 1), which is analogous to treating ‘green up’ or ‘brown down’ as the outcome of the flip of a fair coin. These standard tests show that corrected Δ EVI7, though substantially smaller in magnitude than uncorrected, nonetheless shows a highly significant increase in forest greenness. Second, we ask whether the smaller, but statistically significant, green up seen in the data from Morton et al. (Fig. 1) is biologically meaningful in terms of consistency with mechanisms and magnitude of seasonal changes in canopy-scale biophysics observed on the ground. We find that at an intensively measured site, significant dry-season increases in leaf area index are driven by coordinated flushing of new leaves, which have higher near-infrared reflectance (Fig. 2a) (mechanisms that Morton et al.7 hypothesized could drive true increases in satelliteobserved EVI). Leaf flushing is followed, after 1 to 2 months, by increases in photosynthetic capacity derived from CO2 fluxes measured at eddy flux towers (Fig. 2a). This correlation—1-month-lagged photosynthetic capacity with leaf area index, r = + 0.90, and with MAIAC EVI, r = + 0.89, where r is Pearson’s correlation coefficient, and the time lag is for new leaves to develop their photosynthetic capacity14—establishes a link between eddy flux measurements and biophysical properties observable from satellites. On the basis of this link, we find that increases in dry-season greenness seen by corrected EVI products (whether those of ref. 7 or the MAIAC EVI of Lyapustin et al.10; Fig. 2b) are real and consistently correlated with photosynthetic capacity increases seen at towers within the region analysed by Morton et al. (including adjustment for possible sun-angle effects on canopy illumination). This suggests that even the smaller corrected Δ EVI7 reflects mechanisms of canopy changes actually observed on the ground, and is therefore biologically meaningful. The analysis in Morton et al.7 is, notably, stimulating a productive re-examination of the methodology, meaning and magnitude of remote sensing indices, their artefacts, and their relation to field studies on the ground6,12. However, we believe that the primary substantive finding of Morton et al. of consistent canopy structure and greenness is incorrect. Both satellite remote sensing and ground-based observations show dry-season increases in greenness and biophysical properties associated with canopy photosynthesis across scales, from individual leaves to ecosystems to regions, in support of the conclusion that Amazon forests green up with sunlight in the dry season3,14.

Variability of Carbon and Water Fluxes Following Climate Extremes over a Tropical Forest in Southwestern Amazonia

The carbon and water cycles for a southwestern Amazonian forest site were investigated using the longest time series of fluxes of CO2 and water vapor ever reported for this site. The period from 2004 to 2010 included two severe droughts (2005 and 2010) and a flooding year (2009). The effects of such climate extremes were detected in annual sums of fluxes as well as in other components of the carbon and water cycles, such as gross primary production and water use efficiency. Gap-filling and flux-partitioning were applied in order to fill gaps due to missing data, and errors analysis made it possible to infer the uncertainty on the carbon balance. Overall, the site was found to have a net carbon uptake of ≈5 t C ha−1 year−1, but the effects of the drought of 2005 were still noticed in 2006, when the climate disturbance caused the site to become a net source of carbon to the atmosphere. Different regions of the Amazon forest might respond differently to climate extremes due to differences in dry season length, annual precipitation, species compositions, albedo and soil type. Longer time series of fluxes measured over several locations are required to better characterize the effects of climate anomalies on the carbon and water balances for the whole Amazon region. Such valuable datasets can also be used to calibrate biogeochemical models and infer on future scenarios of the Amazon forest carbon balance under the influence of climate change.

Modeling Carbon Sequestration over the Large-Scale Amazon Basin, Aided by Satellite Observations. Part I: Wet- and Dry-Season Surface Radiation Budget Flux and Precipitation Variability Based on GOES Retrievals

Abstract In this first part of a two-part investigation, large-scale Geostationary Operational Environmental Satellite (GOES) analyses over the Amazonia region have been carried out for March and October of 1999 to provide detailed information on surface radiation budget (SRB) and precipitation variability. SRB fluxes and rainfall are the two foremost cloud-modulated control variables that affect land surface processes, and they require specification at space–time resolutions concomitant with the changing cloud field to represent adequately the complex coupling of energy, water, and carbon budgets. These processes ultimately determine the relative variations in carbon sequestration and carbon dioxide release within a forest ecosystem. SRB and precipitation retrieval algorithms using GOES imager measurements are used to retrieve surface downward radiation and surface rain rates at high space–time resolutions for large-scale carbon budget modeling applications in conjunction with the Large-Scale Biosphere–A...

Widespread reduction in sun-induced fluorescence from the Amazon during the 2015/2016 El Niño

The tropical carbon balance dominates year-to-year variations in the CO2 exchange with the atmosphere through photosynthesis, respiration and fires. Because of its high correlation with gross primary productivity (GPP), observations of sun-induced fluorescence (SIF) are of great interest. We developed a new remotely sensed SIF product with improved signal-to-noise in the tropics, and use it here to quantify the impact of the 2015/2016 El Niño Amazon drought. We find that SIF was strongly suppressed over areas with anomalously high temperatures and decreased levels of water in the soil. SIF went below its climatological range starting from the end of the 2015 dry season (October) and returned to normal levels by February 2016 when atmospheric conditions returned to normal, but well before the end of anomalously low precipitation that persisted through June 2016. Impacts were not uniform across the Amazon basin, with the eastern part experiencing much larger (10–15%) SIF reductions than the western part of the basin (2–5%). We estimate the integrated loss of GPP relative to eight previous years to be 0.34–0.48 PgC in the three-month period October–November–December 2015. This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.

Strong sesquiterpene emissions from Amazonian soils

The Amazon rainforest is the world’s largest source of reactive volatile isoprenoids to the atmosphere. It is generally assumed that these emissions are products of photosynthetically driven secondary metabolism and released from the rainforest canopy from where they influence the oxidative capacity of the atmosphere. However, recent measurements indicate that further sources of volatiles are present. Here we show that soil microorganisms are a strong, unaccounted source of highly reactive and previously unreported sesquiterpenes (C15H24; SQT). The emission rate and chemical speciation of soil SQTs were determined as a function of soil moisture, oxygen, and rRNA transcript abundance in the laboratory. Based on these results, a model was developed to predict soil–atmosphere SQT fluxes. It was found SQT emissions from a Terra Firme soil in the dry season were in comparable magnitude to current global model canopy emissions, establishing an important ecological connection between soil microbes and atmospherically relevant SQTs.Recent measurements in the Amazon rainforest indicate missing sources of volatile organic compounds (VOCs). Here the authors show that soil microorganisms are a strong, unaccounted source of highly reactive sesquiterpenes, a class of VOCs that can regulate ozone chemistry within the forest canopy.