In 2005 and 2010 the Amazon basin experienced two strong droughts1, driven by shifts in the tropical hydrological regime2 possibly associated with global climate change3, as predicted by some global models3. Tree mortality increased after the 2005 drought4, and regional atmospheric inversion modelling showed basin-wide decreases in CO2 uptake in 2010 compared with 2011 (ref. 5). But the response of tropical forest carbon cycling to these droughts is not fully understood and there has been no detailed multi-site investigation in situ. Here we use several years of data from a network of thirteen 1-ha forest plots spread throughout South America, where each component of net primary production (NPP), autotrophic respiration and heterotrophic respiration is measured separately, to develop a better mechanistic understanding of the impact of the 2010 drought on the Amazon forest. We find that total NPP remained constant throughout the drought. However, towards the end of the drought, autotrophic respiration, especially in roots and stems, declined significantly compared with measurements in 2009 made in the absence of drought, with extended decreases in autotrophic respiration in the three driest plots. In the year after the drought, total NPP remained constant but the allocation of carbon shifted towards canopy NPP and away from fine-root NPP. Both leaf-level and plot-level measurements indicate that severe drought suppresses photosynthesis. Scaling these measurements to the entire Amazon basin with rainfall data, we estimate that drought suppressed Amazon-wide photosynthesis in 2010 by 0.38 petagrams of carbon (0.23–0.53 petagrams of carbon). Overall, we find that during this drought, instead of reducing total NPP, trees prioritized growth by reducing autotrophic respiration that was unrelated to growth. This suggests that trees decrease investment in tissue maintenance and defence, in line with eco-evolutionary theories that trees are competitively disadvantaged in the absence of growth6. We propose that weakened maintenance and defence investment may, in turn, cause the increase in post-drought tree mortality observed at our plots.
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We thank P. Brando and Tanguro partners for logistical support and advice. This work is a product of the Global Ecosystems Monitoring (GEM) network (http://gem.tropicalforests.ox.ac.uk) and the RAINFOR and ABERG research consortia, and was funded by grants to Y.M. and O.L.P. from the Gordon and Betty Moore Foundation to the Amazon Forest Inventory Network (RAINFOR) and the Andes Biodiversity and Ecosystems Research Group (ABERG), and grants from the UK Natural Environment Research Council (NE/D01025X/1, NE/D014174/1, NE/F002149/1 and NE/J011002/1), the NERC AMAZONICA consortium grant (NE/F005776/1) and the EU FP7 Amazalert (282664) GEOCARBON (283080) projects. Some data in this publication were provided by the Tropical Ecology Assessment and Monitoring (TEAM) Network, a collaboration between Conservation International, the Missouri Botanical Garden, the Smithsonian Institution and the Wildlife Conservation Society, and partly funded by these institutions, the Gordon and Betty Moore Foundation, and other donors. T.R.F. is supported by a National Council for Scientific and Technological Development (CNPq, Brazil) award. P.M. is supported by an ARC fellowship award FT110100457; O.L.P. is supported by an ERC Advanced Investigator Award and a Royal Society Wolfson Research Merit Award; Y.M. is supported by an ERC Advanced Investigator Award and by the Jackson Foundation. C.E.D. acknowledges funding from the John Fell Fund.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Comparison of plot-based and flux-based estimates of gross primary productivity (GPP) at six different sites worldwide.
The black line represents 1:1; the dashed line represents a linear fit with a y intercept of 0. Slope = 0.97 ± 0.04, coefficient of determination = 0.61. If the Caxiuanã tower site is removed, then slope = 1.01 ± 0.03 and coefficient of determination = 0.87. Data points are from Manaus, Tapajos, Caxiuanã (Brazil), Wytham Woods (UK) and Lambir Hills (Malaysia). For further details see ref. 39.
a, Cumulative water deficit (CWD) anomaly for six droughted plots (red) and the remaining seven non-droughted plots (black) in 2010, on the basis of data from Skye instruments meteorological stations near each plot. Meteorology stations were set up in either ∼2005 (n = 4) or ∼2009 (n = 4). b, MCWD anomaly for 2010 (the minimum of CWDmean minus CWD2010) for the entire Amazon basin based on TRMM version 7 data (1998–2012). For clarity we do not show MCWD for non-droughted sites for which MCWDanom = 0; this is the maximum potential value because by definition the wettest average month has a CWD value of 0). The arrows depict the site-specific MCWD anomaly for each drought area and average for all drought plots. c, e, Data from Skye instruments meteorological stations from January 2009 to December 2011 near our drought plots for cumulative water deficit (mm per month) (c) and air temperature (°C) (e). d, f, Anomalies for the same variables (mean values are average of all data from ∼2005, ∼2009–2011 or ∼2009–2012). The drought period in our drought sites had a slightly lower average temperature during the drought than during the equivalent months of 2009 (24.6 °C versus 24.7 °C). The bar highlights the approximate period of the 2010 drought in the region based on CWD anomaly. To calculate CWD see ref. 40. Error bars are standard error differences between plots.
Top: light-saturated (1,000 μmol m−2 s−1 irradiance, 25 °C, ambient CO2) leaf gas exchange (μmol m−2 s−1) (means ± s.e.m.) for a drought period (November 2010) and a non-drought period (June 2011) for sunlit branches (cut and rehydrated) on the same ∼20 trees each season distributed evenly through the two (Kenia-A and Kenia-B) 1-ha plots. Asterisks indicate significant differences between the plots: P < 0.05; P < 0.001. Bottom: weekly averaged leaf-level photosynthesis for eight species from three canopy walk-up towers measured at 1,000 μmol m−2 s−1 light and 30 °C between July (the start of the dry season) and November from the Tapajós, Brazil (see ref. 41 for further details and methodology). In the Tapajós the average dry season lasts from about July to about November. Note the lack of a decrease in photosynthesis during the dry season. Over this period, soil moisture decreases from ∼0.45 to 0.40 m3 m−3, most of the decrease that occurs during the dry season42. These data suggest that a large (that is, 50%) sharp decline in leaf-level photosynthesis is not typical during an average dry season and that the declines shown in the table above are probably due to the 2010 drought.
Extended Data Figure 4 A conceptual model with simulated data of the impact of drought on the study sites.
Total photosynthesis (grey dashed line; 100% represents average photosynthesis) decreased during the drought period (vertical bar). Total NPP (grey line, shown as a percentage of total photosynthesis) and growth respiration (Rgrowth) (black dotted line) remained constant, whereas maintenance respiration (Rmaintain) (black line) decreased after NSC stores were depleted. Total NSC stores decreased (we define a negative value as NSC storage) during the drought period (the red line indicates a NSC storage of 0) and then increased at the end of the drought. Red arrows represent the timing of when the basin was a source (up) or a sink (down) of CO2 to the atmosphere based on atmospheric inversion measurements from ref. 5.
This figure shows similar trends to those in Fig. 2, but with all the plots separated and extended for a further year for Tambopata and Kenia. a–d, Four years of total NPP (Mg C ha−1 per month) (a), percentage allocation to canopy (b), percentage allocation to wood (c) and percentage allocation to fine roots (d) for the two plots in Kenia, Bolivia (black line, Kenia-A; grey line, Kenia-B). e–h, Seasonally detrended anomaly data for the same variables. This data set is explored in detail in ref. 26. i, j, Comparison of four years of wood (i) and root (j) allocation data for two plots in Tambopata, Peru (black lines). Canopy NPP is not shown because LAI data were not processed for the entire 4-year period. k, Three years of woody (brown), fine-root (black) and canopy (red) allocation data for two plots in Tanguro, Brazil (solid line, Tanguro A; dashed line, Tanguro C ). The bar indicates the approximate drought period.
a, Respiration from deadwood over a 4-year period from Kenia-A (grey) and Kenia-B (black). b, Branch fall over a 4-year period from Kenia-A (grey) and Kenia-B (black); smoothed values are shown in bold lines; actual values are shown in dashed lines. c, Per-stem mortality rates for Peruvian drought plots (grey line, n = 3; error bars indicate standard errors), Bolivian drought plots (black line, n = 2) and the control plot in Caxiuanã (red line, n = 1). We do not show mortality for the Brazilian drought plots, but a recent paper43 has shown an increase in mortality after drought at these sites. Mortality was marginally significantly higher (P = 0.06; paired 1-tailed t-test, n = 5) during the 2-year period after the drought than in other periods. The bar indicates the approximate period of the drought.
Total PCE (grey solid line), wood and rhizosphere respiration (black solid line), and canopy respiration (red solid line) for the six droughted plots and smoothed 2009 equivalents (stippled lines). This figure shows that the decline in Ra was due to the components measured monthly (wood and rhizosphere respiration) and not to canopy respiration (which was measured only once or twice a year). This does not mean that canopy respiration did not decrease during the drought, only that we did not track canopy respiration sufficiently to measure changes. The bar indicates the approximate period of the drought.
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Doughty, C., Metcalfe, D., Girardin, C. et al. Drought impact on forest carbon dynamics and fluxes in Amazonia. Nature 519, 78–82 (2015). https://doi.org/10.1038/nature14213
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