After more than a decade of soil moisture deficit, tropical rainforest trees maintain photosynthetic capacity, despite increased leaf respiration

Abstract

Determining climate change feedbacks from tropical rainforests requires an understanding of how carbon gain through photosynthesis and loss through respiration will be altered. One of the key changes that tropical rainforests may experience under future climate change scenarios is reduced soil moisture availability. In this study we examine if and how both leaf photosynthesis and leaf dark respiration acclimate following more than 12 years of experimental soil moisture deficit, via a through-fall exclusion experiment (TFE) in an eastern Amazonian rainforest. We find that experimentally drought-stressed trees and taxa maintain the same maximum leaf photosynthetic capacity as trees in corresponding control forest, independent of their susceptibility to drought-induced mortality. We hypothesize that photosynthetic capacity is maintained across all treatments and taxa to take advantage of short-lived periods of high moisture availability, when stomatal conductance (gs ) and photosynthesis can increase rapidly, potentially compensating for reduced assimilate supply at other times. Average leaf dark respiration (Rd ) was elevated in the TFE-treated forest trees relative to the control by 28.2 ± 2.8% (mean ± one standard error). This mean Rd value was dominated by a 48.5 ± 3.6% increase in the Rd of drought-sensitive taxa, and likely reflects the need for additional metabolic support required for stress-related repair, and hydraulic or osmotic maintenance processes. Following soil moisture deficit that is maintained for several years, our data suggest that changes in respiration drive greater shifts in the canopy carbon balance, than changes in photosynthetic capacity.

Keywords: drought; leaf dark respiration; photosynthetic capacity; through-fall exclusion; tropical rainforest.

Figure 1

Average V _cmax (a), Jmax (b) and R _d (c) expressed in μmol m⁻² s⁻¹ in the control (C; white) and TFE (grey) plots, in peak dry season 2013 (November) and peak wet season 2014 (June). Error bars show the standard error.

Figure 2

R _d for the resistant and vulnerable tree taxa in the control (white) and TFE (grey) plot in peak dry season of 2013. Columns with a * indicates significant difference with P < 0.05. Error bars show standard error.

Figure 3

Relationships between V _cmax and R _d (a–d) and J _max and R _d (e–h) during the wet and dry season in the control and TFE plot. If the linear line is significant (P < 0.05), the linear relationship, correlation coefficient (r ²) and significance value (P) are shown.

Figure 4

Box plots of water use efficiency (WUE; a), stomatal conductance (g _s; b) and light‐saturated photosynthesis at 400 ppm of CO ₂ (A _sat) during peak dry season 2013 and peak wet season 2014 for vulnerable and resistant tree taxa in the control (white) and TFE (grey) plots. * indicates significant difference with P < 0.05.

Figure 5

Ratio of TFE: control plot V _cmax (a) and R _d (b) values for various measurement campaigns made at the Caxiuanã TFE experiment from prior to the start of the experiment (2001) to 2014; see Methods for details. The symbols indicate the differences in the sample selection. ‘Uncut tower trees’ are data from leaves on attached branches accessed via a walk‐up through‐canopy tower. ‘Metcalfe sample’ are from leaves on the same trees sampled by Metcalfe et al. (2010a). This study used cut branches with fully sunlit leaves and the results from Fig. 2 are replotted here.