Daniel C. Nepstad

Large-scale impoverishment of Amazonian forests by logging and fire

Amazonian deforestation rates are used to determine human effects on the global carbon cycle and to measure Brazils progress in curbing forest impoverishment,,. But this widely used measure of tropical land use tells only part of the story. Here we present field surveys of wood mills and forest burning across Brazilian Amazonia which show that logging crews severely damage 10,000 to 15,000 km2 yr−1 of forest that are not included in deforestation mapping programmes. Moreover, we find that surface fires burn additional large areas of standing forest, the destruction of which is normally not documented. Forest impoverishment due to such fires may increase dramatically when severe droughts provoke forest leaf-shedding and greater flammability; our regional water-balance model indicates that an estimated 270,000 km2 of forest became vulnerable to fire in the 1998 dry season. Overall, we find that present estimates of annual deforestation for Brazilian Amazonia capture less than half of the forest area that is impoverished each year, and even less during years of severe drought. Both logging and fire increase forest vulnerability to future burning, and release forest carbon stocks to the atmosphere, potentially doubling net carbon emissions from regional land-use during severe El Niño episodes. If this forest impoverishment is to be controlled, then logging activities need to be restricted or replaced with low-impact timber harvest techniques, and more effective strategies to prevent accidental forest fires need to be implemented.

Drought sensitivity of the Amazon rainforest

Amazon forests are a key but poorly understood component of the global carbon cycle. If, as anticipated, they dry this century, they might accelerate climate change through carbon losses and changed surface energy balances. We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 × 1015 to 1.6 × 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.

Modelling conservation in the Amazon basin

Expansion of the cattle and soy industries in the Amazon basin has increased deforestation rates and will soon push all-weather highways into the regions core. In the face of this growing pressure, a comprehensive conservation strategy for the Amazon basin should protect its watersheds, the full range of species and ecosystem diversity, and the stability of regional climates. Here we report that protected areas in the Amazon basin—the central feature of prevailing conservation approaches—are an important but insufficient component of this strategy, based on policy-sensitive simulations of future deforestation. By 2050, current trends in agricultural expansion will eliminate a total of 40% of Amazon forests, including at least two-thirds of the forest cover of six major watersheds and 12 ecoregions, releasing 32 ± 8 Pg of carbon to the atmosphere. One-quarter of the 382 mammalian species examined will lose more than 40% of the forest within their Amazon ranges. Although an expanded and enforced network of protected areas could avoid as much as one-third of this projected forest loss, conservation on private lands is also essential. Expanding market pressures for sound land management and prevention of forest clearing on lands unsuitable for agriculture are critical ingredients of a strategy for comprehensive conservation.

The 2010 Amazon Drought

Amazonia experienced lower rainfall and higher calculated carbon emissions from tree deaths than in the 2005 drought. In 2010, dry-season rainfall was low across Amazonia, with apparent similarities to the major 2005 drought. We analyzed a decade of satellite-derived rainfall data to compare both events. Standardized anomalies of dry-season rainfall showed that 57% of Amazonia had low rainfall in 2010 as compared with 37% in 2005 (≤–1 standard deviation from long-term mean). By using relationships between drying and forest biomass responses measured for 2005, we predict the impact of the 2010 drought as 2.2 × 1015 grams of carbon [95% confidence intervals (CIs) are 1.2 and 3.4], largely longer-term committed emissions from drought-induced tree deaths, compared with 1.6 ×1015 grams of carbon (CIs 0.8 and 2.6) for the 2005 event.

Belowground cycling of carbon in forests and pastures of eastern Amazonia

Forests in seasonally dry areas of eastern Amazonia near Paragominas, Para, Brazil, maintain an evergreen forest canopy through an extended dry season by taking up soil water through deep (>1 m) roots. Belowground allocation of C in these deep-rooting forests is very large (1900 g C m−2 yr−1) relative to litterfall (460 g C m−2 yr−1). The presence of live roots drives an active carbon cycle deeper than l m in the soil. Although bulk C concentrations and 14C contents of soil organic matter at >l-m depths are low, estimates of turnover from fine-root inputs, CO2 production, and the 14C content of CO2 produced at depth show that up to 15% of the carbon inventory in the deep soil has turnover times of decades or less. Thus the amount of fast-cycling soil carbon between 1 and 8-m depths (2–3 kg C m−2, out of 17–18 kg C m−2) is significant compared to the amount present in the upper meter of soil (3–4 kg C m−2 out of 10–11 kg C m−2). A model of belowground carbon cycling derived from measurements of carbon stocks and fluxes, and constrained using carbon isotopes, is used to predict C fluxes associated with conversion of deep-rooting forests to pasture and subsequent pasture management. The relative proportions and turnover times of active (including detrital plant material; 1–3 year turnover), slow (decadal and shorter turnover), and passive (centennial to millennial turnover) soil organic matter pools are determined by depth for the forest soil, using constraints from measurements of C stocks, fluxes, and isotopic content. Reduced carbon inputs to the soil in degraded pastures, which are less productive than the forests they replace, lead to a reduction in soil carbon inventory and Δ14C, in accord with observations. Managed pastures, which have been fertilized with phosphorous and planted with more productive grasses, show increases in C and 14C over forest values. Carbon inventory increases in the upper meter of managed pasture soils are partially offset by predicted carbon losses due to death and decomposition of fine forest roots at depths >1 m in the soil. The major adjustments in soil carbon inventory in response to land management changes occur within the first decade after conversion. Carbon isotopes are shown to be more sensitive indicators of recent accumulation or loss of soil organic matter than direct measurement of soil C inventories.

Interactions among Amazon land use, forests and climate: prospects for a near-term forest tipping point

Some model experiments predict a large-scale substitution of Amazon forest by savannah-like vegetation by the end of the twenty-first century. Expanding global demands for biofuels and grains, positive feedbacks in the Amazon forest fire regime and drought may drive a faster process of forest degradation that could lead to a near-term forest dieback. Rising worldwide demands for biofuel and meat are creating powerful new incentives for agro-industrial expansion into Amazon forest regions. Forest fires, drought and logging increase susceptibility to further burning while deforestation and smoke can inhibit rainfall, exacerbating fire risk. If sea surface temperature anomalies (such as El Niño episodes) and associated Amazon droughts of the last decade continue into the future, approximately 55% of the forests of the Amazon will be cleared, logged, damaged by drought or burned over the next 20 years, emitting 15–26 Pg of carbon to the atmosphere. Several important trends could prevent a near-term dieback. As fire-sensitive investments accumulate in the landscape, property holders use less fire and invest more in fire control. Commodity markets are demanding higher environmental performance from farmers and cattle ranchers. Protected areas have been established in the pathway of expanding agricultural frontiers. Finally, emerging carbon market incentives for reductions in deforestation could support these trends.

MORTALITY OF LARGE TREES AND LIANAS FOLLOWING EXPERIMENTAL DROUGHT IN AN AMAZON FOREST

Severe drought episodes such as those associated with El Niño Southern Oscillation (ENSO) events influence large areas of tropical forest and may become more frequent in the future. One of the most important forest responses to severe drought is tree mortality, which alters forest structure, composition, carbon content, and flammability, and which varies widely. This study tests the hypothesis that tree mortality increases abruptly during drought episodes when plant-available soil water (PAW) declines below a critical minimum threshold. It also examines the effect of tree size, plant life form (palm, liana, tree) and potential canopy position (understory, midcanopy, overstory) on drought-induced plant mortality. A severe, four-year drought episode was simulated by excluding 60% of incoming throughfall during each wet season using plastic panels installed in the understory of a 1-ha forest treatment plot, while a 1-ha control plot received normal rainfall. After 3.2 years, the treatment resulted in a 38% increase in mortality rates across all stems >2 cm dbh. Mortality rates increased 4.5-fold among large trees (>30 cm dbh) and twofold among medium trees (10-30 cm dbh) in response to the treatment, whereas the smallest stems were less responsive. Recruitment rates did not compensate for the elevated mortality of larger-diameter stems in the treatment plot. Overall, lianas proved more susceptible to drought-induced mortality than trees or palms, and potential overstory tree species were more vulnerable than midcanopy and understory species. Large stems contributed to 90% of the pretreatment live aboveground biomass in both plots. Large-tree mortality resulting from the treatment generated 3.4 times more dead biomass than the control plot. The dramatic mortality response suggests significant, adverse impacts on the global carbon cycle if climatic changes follow current trends.

The End of Deforestation in the Brazilian Amazon

Government commitments and market transitions lay the foundation for an effort to save the forest and reduce carbon emission. Brazil has two major opportunities to end the clearing of its Amazon forest and to reduce global greenhouse gas emissions substantially. The first is its formal announcement within United Nations climate treaty negotiations in 2008 of an Amazon deforestation reduction target, which prompted Norway to commit

Slowing Amazon deforestation through public policy and interventions in beef and soy supply chains.

1 billion if it sustains progress toward this target (1). The second is a widespread marketplace transition within the beef and soy industries, the main drivers of deforestation, to exclude Amazon deforesters from their supply chains (2) [supplementary online material (SOM), section (§) 4]. According to our analysis, these recent developments finally make feasible the end of deforestation in the Brazilian Amazon, which could result in a 2 to 5% reduction in global carbon emissions. The

Role of Brazilian Amazon protected areas in climate change mitigation

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