Tropical Carbon Research Articles

Faster dieback of rainforests altering tropical carbon sinks under climate change

Carbon sinks in the tropical rainforests are restricting the global warming to attain unprecedented heights. However, deforestation and climate change is switching them to a net carbon source at some of the deforested patches. Using machine learning algorithm we predict that more than 50% of the tropical rainforests will undergo rapid “Savannisation”/transformation by the end of 21st century under high emission scenarios. Climate change projects ‘El Niño-like’ warming condition, which decreases precipitation in the rainforests and favors atmospheric dryness. In Central Amazonia vegetation degradation saturates the carbon sink and more than 25% of the rainforests will transform into a net carbon source due to increase in soil microbial respiration. This transition will accelerate if Eastern Pacific/Global temperature warms beyond 1.5◦K/2.3◦K (by 2050’s) and will undergo a steeper transit by ~2075 (2.45◦K/3.8◦K warming). This alteration will exacerbate global warming and has consequences for policies that are intended to stabilize Earth’s climate.

Quantifying the Carbon Stocks in Urban Trees: The Rio de Janeiro Botanical Garden as an Important Tropical Carbon Sink

Quantifying the Carbon Stocks in Urban Trees: The Rio de Janeiro Botanical Garden as an Important Tropical Carbon Sink

Negative Asymmetric Response of Pantropical Gross Primary Productivity to Precipitation Anomalies

AbstractThe carbon sink in pantropical biomes play a crucial role in modulating the inter‐annual variations of global terrestrial carbon balance and is threatened by extreme climate events. However, it has not been carefully examined whether an increase in tropical gross primary productivity (GPP) can compensate the decrease during precipitation anomalies. Using the asymmetry index (AI) and multiple GPP products, we assessed responses of pantropical GPP to precipitation anomalies during 2001–2022. Positive AI indicates that GPP increases are greater than GPP decreases during precipitation anomalies, and vice versa. Our results showed an average negative pantropical GPP asymmetry, that is, GPP decreases exceeded the GPP increases during precipitation anomalies. In addition, a positive AI was found in tropical hyper‐arid and arid regions, which is opposite to the negative AI observed in tropical semi‐arid, sub‐humid, and humid regions. This suggest that tropical GPP asymmetry changes from positive to negative as the moisture increases. Notably, a significant decreasing trend of negative AI was observed over the entire tropical region, indicating that the negative effect of inter‐annual precipitation variations on pantropical vegetation productivity has enhanced. Considering the model predicted increasing climate variability and extremes, the negative impact of precipitation variability on tropical carbon cycle may continue to intensify. Lastly, the divergence in AI estimates among multiple GPP products highlight the need to further improve our understanding of the response of tropical carbon cycle to climate changes, especially for the tropical humid regions.

Dominant role of the non-forest woody vegetation in the post 2015/16 El Niño tropical carbon recovery.

The extreme dry and hot 2015/16 El Niño episode caused large losses in tropical live aboveground carbon (AGC) stocks. Followed by climatic conditions conducive to high vegetation productivity since 2016, tropical AGC are expected to recover from large losses during the El Niño episode; however, the recovery rate and its spatial distribution remain unknown. Here, we used low-frequency microwave satellite data to track AGC changes, and showed that tropical AGC stocks returned to pre-El Niño levels by the end of 2020, resulting in an AGC sink of Pg C year-1 during 2014-2020. This sink was dominated by strong AGC increases ( Pg C year-1) in non-forest woody vegetation during 2016-2020, compensating the forest AGC losses attributed to the El Niño event, forest loss, and degradation. Our findings highlight that non-forest woody vegetation is an increasingly important contributor to interannual to decadal variability in the global carbon cycle.

Constraining long-term model predictions for woody growth using tropical tree rings.

The strength and persistence of the tropical carbon sink hinges on the long-term responses of woody growth to climatic variations and increasing CO2 . However, the sensitivity of tropical woody growth to these environmental changes is poorly understood, leading to large uncertainties in growth predictions. Here, we used tree ring records from a Southeast Asian tropical forest to constrain ED2.2-hydro, a terrestrial biosphere model with explicit vegetation demography. Specifically, we assessed individual-level woody growth responses to historical climate variability and increases in atmospheric CO2 (Ca ). When forced with historical Ca , ED2.2-hydro reproduced the magnitude of increases in intercellular CO2 concentration (a major determinant of photosynthesis) estimated from tree ring carbon isotope records. In contrast, simulated growth trends were considerably larger than those obtained from tree rings, suggesting that woody biomass production efficiency (WBPE = woody biomass production:gross primary productivity) was overestimated by the model. The estimated WBPE decline under increasing Ca based on model-data discrepancy was comparable to or stronger than (depending on tree species and size) the observed WBPE changes from a multi-year mature-forest CO2 fertilization experiment. In addition, we found that ED2.2-hydro generally overestimated climatic sensitivity of woody growth, especially for late-successional plant functional types. The model-data discrepancy in growth sensitivity to climate was likely caused by underestimating WBPE in hot and dry years due to commonly used model assumptions on carbon use efficiency and allocation. To our knowledge, this is the first study to constrain model predictions of individual tree-level growth sensitivity to Ca and climate against tropical tree-ring data. Our results suggest that improving model processes related to WBPE is crucial to obtain better predictions of tropical forest responses to droughts and increasing Ca . More accurate parameterization of WBPE will likely reduce the stimulation of woody growth by Ca rise predicted by biosphere models.

A dataset of soil carbon flux in Xishuangbanna tropical rainforest ecosystem from 2003 to 2008

Long-term and large-scale carbon flux data supports research on key processes of the global carbon and water cycle. Soil is the largest carbon pool in the terrestrial ecosystem. Small variations in soil carbon can significantly impact the carbon budgets of terrestrial ecosystems. Tropical forest is a crucial carbon sink in the world. However, there have been some reports indicating a decrease in soli carbon levels, potentially approaching neutrality, as a result of soil carbon degradation caused by global changing. Thus, the long-term and continuous observation of soil carbon flux is of great significance for evaluating the role of soil carbon in the tropical carbon cycle. ChinaFLUX Observation and Research Network (ChinaFLUX) has not only conducted the observation and research on carbon flux by using vorticity correlation technology, but also carried out the long-term observation and research on surface CO2 flux by using the box method. In this dataset, we collected and sorted out the soil CO2 emission flux data and conventional meteorological data of Xishuangbanna tropical rain forest ecosystem from January 2003 to August 2008 according to the static box observation method and data quality control management of greenhouse gas emissions. The dataset consists of three types of files at daily, monthly and yearly scales, covering surface CO2 flux, pressure, atmospheric temperature, soil temperature (5 cm) and soil water content (5 cm). It is expected to provide essential data for estimating carbon balance and understanding carbon cycle processes at both tropical and global scales.

A Shift From Temperature to Water as the Primary Driver for Interannual Variability of the Tropical Carbon Cycle

AbstractThe relative importance of temperature and water constraints on the tropical carbon cycle is an ongoing debate. Here we combine multiple independent tropical ecosystem carbon exchange simulations with satellite observations and show that the primary control for the tropical carbon cycle has shifted recently from temperature to water. We examined the response of tropical ecosystem processes to climate change and suggest that the emerging role of water as the dominant driver for the tropical carbon cycle is attributable to increased precipitation variability, which is associated with intensified extreme drought and wet conditions. Extreme conditions strengthen water's regulation of ecosystem carbon exchange. The weakening influence of temperature is explained by the previously additive effects of temperature on ecosystem productivity and respiration now becoming opposing effects owing to amplified drought conditions, resulting in a reduction to the net effect of temperature on ecosystem carbon exchange.

Natural Capital, Institutional Quality and SDG Progress in Emerging Market and Developing Economies

Whether environmental impacts, natural capital depreciation, and effective governance have impacted progress in emerging market and developing economies (EMDEs) to achieving the 17 Sustainable Development Goals (SDGs) of the UN Agenda 2030 has become a significant policy topic. We determine estimates of the changes in net welfare that indicate progress over 2000–2019 in terms of meeting the 17 SDGs for 99 EMDEs. We compare these estimates with the environmental impacts associated with SDGs 11–15, natural resource depletion as a share of national income, and average institutional quality. Net SDG welfare gains over 2000–2019 were often accompanied by adverse environmental impacts, natural capital depletion, and poor-quality institutions. This is especially the case for low- and lower-middle-income economies. Higher net SDG welfare gains are correlated with reduced losses associated with SDGS 11–15, and larger welfare gains from attaining SDG 1 No Poverty are associated with better governance. These results suggest that long-term progress towards the SDGs in EMDEs hinges on improved management of natural capital and the environment, as well as more effective governance. Three policies can be implemented to achieve these objectives without additional financing or imposing fiscal burdens: fossil fuel subsidy swaps, tropical carbon taxes, and improved management and distribution of resource revenues.

Widespread herbivory cost in tropical nitrogen-fixing tree species.

Recent observations suggest that the large carbon sink in mature and recovering forests may bestrongly limited by nitrogen1-3. Nitrogen-fixing trees (fixers) in symbiosis with bacteria provide the main natural source of new nitrogen to tropical forests3,4. However, abundances of fixers are tightly constrained5-7, highlighting the fundamental unanswered question of what limits new nitrogen entering tropical ecosystems. Here we examine whether herbivory by animals is responsible for limiting symbiotic nitrogen fixation in tropical forests. We evaluate whether nitrogen-fixing trees experience more herbivory than other trees, whether herbivory carries a substantial carbon cost, and whether high herbivory is a result of herbivores targeting the nitrogen-rich leaves of fixers8,9. We analysed 1,626 leaves from 350 seedlings of 43 tropical tree species in Panama and found that: (1) although herbivory reduces the growth and survival ofall seedlings, nitrogen-fixing trees undergo 26% more herbivory than non-fixers; (2) fixers have 34% higher carbon opportunity costs owing to herbivory than non-fixers, exceeding the metabolic cost of fixing nitrogen; and (3) the high herbivory of fixers is not driven by high leaf nitrogen. Our findings reveal that herbivory may be sufficient to limit tropical symbiotic nitrogen fixation and could constrain its role in alleviating nitrogen limitation on the tropical carbon sink.

Estimation of carbon pool at mangrove forest of Kudat, Sabah, Malaysia

Abstract. Hatta SM, Salleh E, Suhaili NS, Besar NA. 2022. Estimation of carbon pool at mangrove forest of Kudat, Sabah, Malaysia. Biodiversitas 23: 4601-4608. Mangroves play a significant role in reducing tropical carbon emissions and preventing climate change. This study was carried out in Kudat's Tun Mustapha Park mangrove forest. This research aims to quantify the aboveground, belowground, and soil carbon pools. Nine 125-meter-long transect lines were set up, and every 25 meters, a 7-meter-diameter circle was placed. A forest inventory was conducted to determine the diameter at the breast height of standing trees. For soil analysis and bulk density, soil samples were collected at four different depths (0-15 cm, 15-30 cm, 30-50 cm, and 50-100 cm). An ICP-OES analyzer was used to determine the value of soil nutrients, and a CHNS analyzer was used to determine the soil carbon concentration. The aboveground and belowground biomass was calculated using the allometric equation, and the carbon stock was estimated at 50% of the total biomass. The outcome showed a 455.87 MgCha-1 total carbon pool. The soil carbon has the highest value with 273.76 MgCha-1, followed by aboveground carbon (living trees) with 136.58 MgCha-1 and belowground carbon (roots) with 45.53 MgCha-1. This study found that soil carbon stock made up almost 60% of the total carbon stock in the mangrove forest.

Estimates of soil nutrient limitation on the CO2 fertilization effect for tropical vegetation.

The CO2 fertilization effect in tropical forests is a key factor for the global land carbon sink. We show that the normalized CO2 effect on tropical vegetation carbon was c. 70% lower in seedling CO2 experiments without nutrient fertilizers and c. 50% and 70% lower in models that consider nitrogen and phosphorus cycles, based on two model ensembles. The inadequate representation or lack of nutrient cycles in Earth System models likely leads to overestimating future tropical carbon gains.

Human impacts as the main driver of tropical forest carbon.

Understanding the mechanisms controlling forest carbon storage is crucial to support “nature-based” solutions for climate change mitigation. We used a dataset of 892 Atlantic Forest inventories to assess the direct and indirect effects of environmental conditions, human impacts, tree community proprieties, and sampling methods on tree above-ground carbon stocks. We showed that the widely accepted drivers of carbon stocks, such as climate, soil, topography, and forest fragmentation, have a much smaller role than the forest disturbance history and functional proprieties of the Atlantic Forest. Specifically, within-forest disturbance level was the most important driver, with effect at least 30% higher than any of the environmental conditions individually. Thus, our findings suggest that the conservation of tropical carbon stocks may be dependable on, principally, avoiding forest degradation and that conservation policies focusing only on carbon may fail to protect tropical biodiversity.

Tropical tree mortality has increased with rising atmospheric water stress.

Evidence exists that tree mortality is accelerating in some regions of the tropics1,2, with profound consequences for the future of the tropical carbon sink and the global anthropogenic carbon budget left to limit peak global warming below 2 °C. However, the mechanisms that may be driving such mortality changes and whether particular species are especially vulnerable remain unclear3-8. Here we analyse a 49-year record of tree dynamics from 24 old-growth forest plots encompassing a broad climatic gradient across the Australian moist tropics and find that annual tree mortality risk has, on average, doubled across all plots and species over the last 35years, indicating a potential halving in life expectancy and carbon residence time. Associated losses in biomass were not offset by gains from growth and recruitment. Plots in less moist local climates presented higher average mortality risk, but local mean climate did not predict the pace of temporal increase in mortality risk. Species varied in the trajectories of their mortality risk, with the highest average risk found nearer to the upper end of the atmospheric vapour pressure deficit niches of species. A long-term increase in vapour pressure deficit was evident across the region, suggesting that thresholds involving atmospheric water stress, driven by global warming, may be a primary cause of increasing tree mortality in moist tropical forests.

Exploration of a novel geoengineering solution: lighting up tropical forests at night

Abstract. Plants primarily conduct photosynthesis in the daytime, offering an opportunity to increase photosynthesis and carbon sink by providing light at night. We used a fully coupled Earth system model to quantify the carbon sequestration and climate effects of a novel carbon removal proposal: lighting up tropical forests at night via lamp networks above the forest canopy. Simulation results show that additional light increased the tropical forest carbon sink by 10.4±0.05 Pg of carbon per year during a 16-year lighting experiment, resulting in a decrease in atmospheric CO2 and suppression of global warming. In addition, local temperature and precipitation increased. The energy requirement for capturing 1 t of carbon is lower than that of direct air carbon capture. When the lighting experiment was terminated, tropical forests started to release carbon slowly. This study suggests that lighting up tropical forests at night could be an emergency solution to climate change, and carbon removal actions focused on enhancing ecosystem productivity by altering environmental factors in the short term could induce post-action CO2 outgassing.

Exploring Dissolved Organic Carbon Variations in a High Elevation Tropical Peatland Ecosystem: Cerro de la Muerte, Costa Rica

Tropical peatlands are distributed mainly in coastal lowlands; however high elevation regions exhibit a large prevalence of small and fragmented peatlands that are mostly understudied. Artificial drainage of peatlands to expand the area of cattle farming, horticulture, and urbanization is increasing carbon losses to the atmosphere and streams worldwide. Here, we present an exploratory characterization of dissolved carbon optical properties in ombrotrophic peat bogs of the Talamanca range of Costa Rica, across an altitudinal gradient (2,400–3,100 m a.s.l.) during the rainy season. Dissolved organic matter (DOM) sources and decomposition processes were evaluated in the light of dissolved organic and inorganic carbon (DOC and DIC), optical properties, and major water chemistry. DOC concentrations ranged from 0.2 up to 47.0 mg/L. DIC concentrations were below 2 mg/L and δ13CDIC values indicated a mixture between soil organic matter, CO2 in soil water, and to a lesser degree DIC derived from bacterial CO2. Absolute fluorescence intensity of humic-like peaks was 6–7 times greater than fresh-like peaks across all sites. Fluorescence peak ratios coupled with the biological and humification indexes point to a greater relative contribution of recalcitrant soil-derived DOM. Excitation/Emission matrices denoted a high prevalence of humic and fulvic acids in the peat bogs, with moderate intensities in soluble microbial by-products-like and aromatic protein regions at three sites. Our data provides a baseline to underpin tropical carbon dynamics across high elevation peatlands.

Emergent constraints on tropical atmospheric aridity—carbon feedbacks and the future of carbon sequestration

Carbon-climate feedbacks, which amplifies or attenuates atmospheric CO2 from fossil fuel emissions, are one of the largest sources of uncertainty in climate projections. However, these feedbacks depend both on temperature and its coupling to water and energy cycles, especially in the tropics. We show that atmospheric aridity—quantified as vapor pressure deficit (VPD)—is a good proxy for this coupling. Tropical VPD is strongly correlated to the global CO2 growth rate (CGR) with observed present-day sensitivities of −2.5 ± 0.4 GtC mb−1 yr−1. The sensitivity of CGR to tropical VPD interannual variability has increased by a factor of 1.7 ± 0.3 in the 21st century. A combination of causality and statistical analysis point to mechanistic moisture drivers of the VPD–CGR sensitivities, independent of temperature. Observational records provide evidence that tropical atmospheric aridity is linked to both water deficit and spatially correlated with evaporative fraction suggesting that CGR variability is indirectly driven by land—atmosphere coupling (compound soil and atmospheric drought). This coupling is manifest as a kind of carbon-climate feedback in CMIP6 Earth System Models where long–term increases in tropical VPD reduce tropical carbon storage but with a substantial inter-model range [−1.4 to −59.4 GtC mb−1]. However, by employing a hierarchical emergent constraint, the best estimate of atmospheric aridity—carbon cycle feedback (φ TL ) is 10 GtC mb−1, which is 28% lower than model estimates with an uncertainty reduction of 50%. Our results bridge the role of moisture and land–atmosphere coupling on net carbon variability to the vulnerability of carbon storage in a changing climate.

The declining tropical carbon sink

The declining tropical carbon sink

Detecting vulnerability of humid tropical forests to multiple stressors

Detecting vulnerability of humid tropical forests to multiple stressors

Tropical Carbon and Water Observed from Above

Satellite observations of carbon balance and water fluxes are changing the role of tropical forests in local and global climate.

Lagged effects regulate the inter-annual variability of the tropical carbon balance

Abstract. Inter-annual variations in the tropical land carbon (C) balance are a dominant component of the global atmospheric CO2 growth rate. Currently, the lack of quantitative knowledge on processes controlling net tropical ecosystem C balance on inter-annual timescales inhibits accurate understanding and projections of land–atmosphere C exchanges. In particular, uncertainty on the relative contribution of ecosystem C fluxes attributable to concurrent forcing anomalies (concurrent effects) and those attributable to the continuing influence of past phenomena (lagged effects) stifles efforts to explicitly understand the integrated sensitivity of a tropical ecosystem to climatic variability. Here we present a conceptual framework – applicable in principle to any land biosphere model – to explicitly quantify net biospheric exchange (NBE) as the sum of anomaly-induced concurrent changes and climatology-induced lagged changes to terrestrial ecosystem C states (NBE = NBECON+NBELAG). We apply this framework to an observation-constrained analysis of the 2001–2015 tropical C balance: we use a data–model integration approach (CARbon DAta-MOdel fraMework – CARDAMOM) to merge satellite-retrieved land-surface C observations (leaf area, biomass, solar-induced fluorescence), soil C inventory data and satellite-based atmospheric inversion estimates of CO2 and CO fluxes to produce a data-constrained analysis of the 2001–2015 tropical C cycle. We find that the inter-annual variability of both concurrent and lagged effects substantially contributes to the 2001–2015 NBE inter-annual variability throughout 2001–2015 across the tropics (NBECON IAV = 80 % of total NBE IAV, r = 0.76; NBELAG IAV = 64 % of NBE IAV, r = 0.61), and the prominence of NBELAG IAV persists across both wet and dry tropical ecosystems. The magnitude of lagged effect variations on NBE across the tropics is largely attributable to lagged effects on net primary productivity (NPP; NPPLAG IAV 113 % of NBELAG IAV, r = −0.93, p value < 0.05), which emerge due to the dependence of NPP on inter-annual variations in foliar C and plant-available H2O states. We conclude that concurrent and lagged effects need to be explicitly and jointly resolved to retrieve an accurate understanding of the processes regulating the present-day and future trajectory of the terrestrial land C sink.