Drought effects on litterfall, wood production and belowground carbon cycling in an Amazon forest: results of a throughfall reduction experiment

Abstract

The Amazon Basin experiences severe droughts that may become more common in the future. Little is known of the effects of such droughts on Amazon forest productivity and carbon allocation. We tested the prediction that severe drought decreases litterfall and wood production but potentially has multiple cancelling effects on belowground production within a 7-year partial throughfall exclusion experiment. We simulated an approximately 35-41% reduction in effective rainfall from 2000 through 2004 in a 1ha plot and compared forest response with a similar control plot. Wood production was the most sensitive component of above-ground net primary productivity (ANPP) to drought, declining by 13% the first year and up to 62% thereafter. Litterfall declined only in the third year of drought, with a maximum difference of 23% below the control plot. Soil CO2 efflux and its 14C signature showed no significant treatment response, suggesting similar amounts and sources of belowground production. ANPP was similar between plots in 2000 and declined to a low of 41% below the control plot during the subsequent treatment years, rebounding to only a 10% difference during the first post-treatment year. Live aboveground carbon declined by 32.5Mgha-1 through the effects of drought on ANPP and tree mortality. Results of this unreplicated, long-term, large-scale ecosystem manipulation experiment demonstrate that multi-year severe drought can substantially reduce Amazon forest carbon stocks.

Figure 1

Soil VWC integrated from (a) 0 to 200 cm and (b) 200 to 1100 cm and (c) daily precipitation. Measurements were taken from August 1999 to December 2004. The shaded areas represent the periods of throughfall exclusion. The estimated VWC if the entire soil profile had dried to values equal to the θ_r residual soil water van Geneuchten parameters for this soil (Belk et al. 2007) is 430 and 1950 mm for the 0–200 and 200–1000 cm profiles, respectively. Black circles, control; grey circles, exclusion.

Figure 2

(a) Annual per cent of mortality measured on an individual basis for all individuals ≥10 cm dbh in the exclusion and control plots from December 2000 to December 2005. (b) Above-ground standing live biomass of all individuals ≥10 cm dbh for the exclusion and control plots during the same period. Black bars, control; grey bars, exclusion.

Figure 3

Annual trends in the exclusion and control plots of (a) litterfall, (b) wood production, (c) ANPP and (d) necromass production. Measurements were taken from January 2000 to December 2005. Units are Mg ha⁻¹ yr⁻¹. Error bars (±1 s.e.) are presented only for litterfall. Black bars, biomass control; grey bars, biomass exclusion; shaded bars, necromass control; grey shaded bars, necromass exclusion.

Figure 4

Stem growth of individuals in the exclusion and control plots by different size classes: (a) >5 cm dbh (lianas) and >10 cm dbh (trees); (b) between 5 (lianas) or 10 (trees) and 20 cm dbh; and (c) >20 cm dbh. Error bars indicate ±s.e. (N is given by the number of individuals). Measurements were taken from December 1999 to December 2005. Annual stem growth was calculated as the difference in dbh measured between two sampling intervals (December of each year).

Figure 5

Annual trends in the exclusion and control plots of LAI. Error bars indicate ±s.e. In 2005 LAI is represented by 3 months of measurements. Black bars, control; grey bars, exclusion.

Figure 6

Relationships between annual mean VWC measured in the upper soil profile (0–200 cm) and (a) ANPP and (b) LAI. Data represent annual means from January 2000 to December 2004. Black circles, control; grey circles, exclusion.

Figure 7

Measurements of surface flux of (a) Carbon dioxide (CO₂ (gC m⁻² h⁻¹)), (b) δ¹³C and (c) Δ¹⁴C in the exclusion (grey) and control plots (black). The error bars represent standard errors of the mean.