ENSO Drives interannual variation of forest woody growth across the tropics

Abstract

Meteorological extreme events such as El Niño events are expected to affect tropical forest net primary production (NPP) and woody growth, but there has been no large-scale empirical validation of this expectation. We collected a large high-temporal resolution dataset (for 1-13 years depending upon location) of more than 172 000 stem growth measurements using dendrometer bands from across 14 regions spanning Amazonia, Africa and Borneo in order to test how much month-to-month variation in stand-level woody growth of adult tree stems (NPP_stem) can be explained by seasonal variation and interannual meteorological anomalies. A key finding is that woody growth responds differently to meteorological variation between tropical forests with a dry season (where monthly rainfall is less than 100 mm), and aseasonal wet forests lacking a consistent dry season. In seasonal tropical forests, a high degree of variation in woody growth can be predicted from seasonal variation in temperature, vapour pressure deficit, in addition to anomalies of soil water deficit and shortwave radiation. The variation of aseasonal wet forest woody growth is best predicted by the anomalies of vapour pressure deficit, water deficit and shortwave radiation. In total, we predict the total live woody production of the global tropical forest biome to be 2.16 Pg C yr^-1, with an interannual range 1.96-2.26 Pg C yr^-1 between 1996-2016, and with the sharpest declines during the strong El Niño events of 1997/8 and 2015/6. There is high geographical variation in hotspots of El Niño-associated impacts, with weak impacts in Africa, and strongly negative impacts in parts of Southeast Asia and extensive regions across central and eastern Amazonia. Overall, there is high correlation (r = -0.75) between the annual anomaly of tropical forest woody growth and the annual mean of the El Niño 3.4 index, driven mainly by strong correlations with anomalies of soil water deficit, vapour pressure deficit and shortwave radiation.This article is part of the discussion meeting issue 'The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.

Keywords: El Niño; drought; meteorological anomalies; tropical forests; woody net primary production.

Figure 1.

The location of the Global Ecosystem Monitoring sites used in this study, overlaid on a map of mean annual precipitation (mm).

Figure 2.

(a,b) Coefficient plots for the seasonal forest NPP_stem and aseasonal wet forest NPP_stem models with 50% and 90% credible intervals for the meteorologically driven statistical model. Abbreviations are as follows: SWmean μ is the long-term monthly mean of shortwave radiation, Tmean μ is the long-term monthly mean of temperature, VPDmean μ is the long-term monthly mean of vapour pressure deficit, VPDmean anom. 3-mo is the moving three-month mean moving anomaly of vapour pressure deficit, SWanom. 3-mo is the three-month moving mean anomaly of shortwave radiation, Wet anom. and Dry anom. are the excessively wet and excessively dry parts of the water deficit anomaly. (c–h) The effect of the model terms are expressed on hypothetical conditional plots with median posterior prediction and 50% and 99% posterior predictive intervals in shaded colours. Apart from the model term that is varied along the x-axis, all other model terms in the conditional plots are set to the mean from the season or aseasonal forest data sets. All panels on the left correspond to the seasonal forest model, while panels on the right correspond to the aseasonal wet forest model.

Figure 3.

Site-level observations (open circles) and predictions (solid circles) with corresponding 50% and 99% prediction intervals of monthly NPP_stem for individual plots located near (a) Kenya, Bolivia, (b) Tambopata, Peru, (c) Santarém, Brazil, (d) Kogyae, Ghana, (e) Bobiri, Ghana, (f,g) regions in the east of Sabah, East Malaysia and (h) Jenaro Herrera, Peru. (Online version in colour.)

Figure 4.

The detrended Pantropical spatial anomalies of predicted NPP_stem during the El Niño events of 1997–1998 and 2015–2016, expressed Mg C ha⁻¹ month ⁻¹.

Figure 5.

(Top) The 12-month detrended and running mean anomaly (expressed in Pg C yr⁻¹) of predicted annual NPP_stem (black) across the tropical regions and the Pantropics. The vertical coloured bars represent corresponding El Niño 3.4 index through time.