Approximately 2.5 × 10 ⁶ square kilometers of the Amazon forest are currently degraded by fire, edge effects, timber extraction, and/or extreme drought, representing 38% of all remaining forests in the region. Carbon emissions from this degradation total up to 0.2 petagrams of carbon per year (Pg C year ⁻¹ ), which is equivalent to, if not greater than, the emissions from Amazon deforestation (0.06 to 0.21 Pg C year ⁻¹ ). Amazon forest degradation can reduce dry-season evapotranspiration by up to 34% and cause as much biodiversity loss as deforestation in human-modified landscapes, generating uneven socioeconomic burdens, mainly to forest dwellers. Projections indicate that degradation will remain a dominant source of carbon emissions independent of deforestation rates. Policies to tackle degradation should be integrated with efforts to curb deforestation and complemented with innovative measures addressing the disturbances that degrade the Amazon forest.

We investigated the seasonal patterns of Amazonian forest photosynthetic activity, and the effects\ud thereon of variations in climate and land-use, by integrating data from a network of ground-based eddy\ud flux towers in Brazil established as part of the ‘Large-Scale Biosphere Atmosphere Experiment in Amazonia’\ud project. We found that degree of water limitation, as indicated by the seasonality of the ratio\ud of sensible to latent heat flux (Bowen ratio) predicts seasonal patterns of photosynthesis. In equatorial\ud Amazonian forests (5◦ N–5◦ S), water limitation is absent, and photosynthetic fluxes (or gross ecosystem\ud productivity, GEP) exhibit high or increasing levels of photosynthetic activity as the dry season\ud progresses, likely a consequence of allocation to growth of new leaves. In contrast, forests along the\ud southern flank of the Amazon, pastures converted from forest, and mixed forest-grass savanna, exhibit\ud dry-season declines in GEP, consistent with increasing degrees of water limitation. Although previous\ud work showed tropical ecosystem evapotranspiration (ET) is driven by incoming radiation, GEP observations\ud reported here surprisingly show no or negative relationships with photosynthetically active\ud radiation (PAR). Instead, GEP fluxes largely followed the phenology of canopy photosynthetic capacity (Pc),\ud with only deviations from this primary pattern driven by variations in PAR. Estimates of leaf flush at threeNASA (LBA investigation CD-32 and the LBA-DMIP project, award #NNX09AL52G)National Science Foundation (Amazon-PIRE, NSF award #OISE-0730305)Gordon and Betty Moore Foundation's Andes-Amazon InitiativeFAPESP (02/09289-9)CNPq (Instituto do Milênio-LAB, Ed. Universal-01, Ed. CT-Hidro 03

Forests around Manaus have staged the oldest and the longest forest‐atmosphere CO₂ exchange studies made anywhere in the Amazon. Since July 1999 the exchange of CO₂, water, and energy, as well as weather variables, have been measured almost continuously over two forests, 11 km apart, in the Cuieiras reserve near Manaus, Brazil. This paper presents the sites and climatology of the region based upon the new data sets. The landscape consists of plateaus dissected by often waterlogged valleys, and the two sites differ in terms of the relative areas of those two landscape components represented in the tower footprints. The radiation and wind climate was similar to both towers. Generally, both the long‐wave and short‐wave radiation input was less in the wet than in the dry season. The energy balance closure was imperfect (on average 80%) in both towers, with little variation in energy partitioning between the wet and dry seasons; likely a result of anomalously high rainfall in the 1999 dry season. Fluxes of CO₂ also showed little seasonal variation except for a slightly shorter daytime uptake duration and somewhat lower respiratory fluxes in the dry season. The net effect is one of lower daily net ecosystem exchange (NEE) in the dry season. The tower, which has less waterlogged valley areas in its footprint, measured a higher overall CO₂ uptake rate. We found that on first sight, NEE is underestimated during calm nights, as was observed in many other tower sites before. However, a closer inspection of the diurnal variation of CO₂ storage fluxes and NEE suggests that at least part of the nighttime deficits is recovered from either lateral influx of CO₂ from valleys or outgassing of soil storage. Therefore there is a high uncertainty in the magnitude of nocturnal NEE, and consequently preliminary estimates of annual carbon uptake reflecting this range from 1 to 8 T ha⁻¹ y⁻¹, with an even higher upper range for the less waterlogged area. The high uptake rates are clearly unsustainable and call for further investigations into the integral carbon balance of Amazon landscapes.

Global terrestrial models currently predict that the Amazon rainforest will continue to act as a carbon sink in the future, primarily owing to the rising atmospheric carbon dioxide (CO₂) concentration. Soil phosphorus impoverishment in parts of the Amazon basin largely controls its functioning, but the role of phosphorus availability has not been considered in global model ensembles—for example, during the Fifth Climate Model Intercomparison Project. Here we simulate the planned free-air CO₂ enrichment experiment AmazonFACE with an ensemble of 14 terrestrial ecosystem models. We show that phosphorus availability reduces the projected CO₂-induced biomass carbon growth by about 50% to 79 ± 63 g C m⁻² yr⁻¹ over 15 years compared to estimates from carbon and carbon–nitrogen models. Our results suggest that the resilience of the region to climate change may be much less than previously assumed. Variation in the biomass carbon response among the phosphorus-enabled models is considerable, ranging from 5 to 140 g C m⁻² yr⁻¹, owing to the contrasting plant phosphorus use and acquisition strategies considered among the models. The Amazon forest response thus depends on the interactions and relative contributions of the phosphorus acquisition and use strategies across individuals, and to what extent these processes can be upregulated under elevated CO₂.