Internal respiration of Amazon tree stems greatly exceeds external CO<sub>2</sub> efflux

Abstract

Abstract. Respiration in tree stems is an important component of forest carbon balance. The rate of CO 2 efflux from the stem has often been assumed to be a measure of stem respiration. However, recent work in temperate forests has demonstrated that stem CO 2 efflux can either overestimate or underestimate respiration rate because of emission or removal of CO 2 by transport in xylem water. Here, we studied gas exchange from stems of tropical forest trees using a new approach to better understand respiration in an ecosystem that plays a key role in the global carbon cycle. Our main questions were (1) is internal CO 2 transport important in tropical trees, and, if so, (2) does this transport result in net release of CO 2 respired in the roots at the stem, or does it cause the opposite effect of net removal of stem-respired CO 2 ? To answer these questions, we measured the ratio of stem CO 2 efflux to O 2 influx. This ratio, defined here as apparent respiratory quotient (ARQ), is expected to equal 1.0 if carbohydrates are the substrate for respiration, and the net transport of CO 2 in the xylem water is negligible. Using a stem chamber approach to quantifying ARQ, we found values of 0.66 ± 0.18. These low ARQ values indicate that a large portion of respired CO 2 (∼ 35 %) is not emitted locally, and is probably transported upward in the stem. ARQ values of 0.21 ± 0.10 were found for the steady-state gas concentration within the stem, sampled by in-stem equilibration probes. These lower values may result from the proximity to the xylem water stream. In contrast, we found ARQ values of 1.00 ± 0.13 for soil respiration. Our results indicate the existence of a considerable internal flux of CO 2 in the stems of tropical trees. If the transported CO 2 is used in the canopy as a substrate for photosynthesis, it could account for up to 10 % of the C fixed by the tree, and perhaps serve as a mechanism that buffers the response of the tree to changing CO 2 levels. Our results also indicate, in agreement with previous work, that the widely used CO 2 efflux approach for determining stem respiration is unreliable. We demonstrate here a field applicable approach for measuring the O 2 uptake rate, which we suggest to be a more appropriate method to estimate stem respiration rates.

A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

“…In our case, assuming that only R soil and diffusion of O 2 and CO 2 give rise to ARQ, ARQ will be equal to RQ and the oxidative ratio (OR) of organic matter undergoing degradation. In this study, the annual mean RQ (calculated as ARQ/0.76) across all depths was 0.38, which was lower than RQ values for some soils ranging from 0.82 to 1 22,26–29 , but similar to or exceeding those of other soils ranging from 0.21 to 0.40 22,30–33 . Incubation studies have found a decrease in RQ values with time, often attributed to a depletion of labile organic matter (organic acids and carbohydrates).…”

“…From stoichiometric considerations, mean RQ values were calculated as 0.95 for different types of wood and 0.89 for humic acid and humin (OR values in Severinghaus 28 ). In soils, RQ values have been reported to vary from 0.83–0.95 for different biomes inside Biosphere 2 28 , 0.82–1.04 for Boreal, Temperate Subtropical and Mediterranean ecosystems 29 , 0.90 in a cool temperate deciduous forest 27 , and a mean value of 1 in the Amazonian tropical forest 26 . Therefore, based on previous research, an ARQ value of ca .…”

Using O<sub>2</sub> to study the relationships between soil CO<sub>2</sub> efflux and soil respiration

“…Seibt et al (2004) reported values in a forest soil chamber which correspond to the respiratory quotient (RQ) of 1.5 (and to 1.06 after removing one data point which was considered to be an outlier): this is higher than the 0.90 value reported recently for a soil chamber in a forest in Japan (Ishidoya et al, 2013). An average value of 1.0 for soil profiles were found in the Amazonian tropical forest in Peru (Angert et al, 2012).…”

“…We will define the ratio between the soil CO 2 efflux to O 2 influx as the soil ARQ (apparent respiratory quotient), which is similar to the definition for tree stems (Angert et al, 2012;Angert and Sherer, 2011), so ARQ = −P C /P O . If only respiration drives the soil ARQ, then it will be equal to the RQ or to the inverse of the oxidative ratio (OR, which is 1/ RQ).…”

Discrimination in Tree Stems O2 Uptake and the Dole Effect

“…In addition to the studies in these sites, we reassessed with the numerical model the D stem estimated for eight tropical trees (3 Tachigali paniculata, 2 Hymenolobium sp., and 3 Simaroura amara), which have been measured in a previous study (Angert et al, 2012a(Angert et al, , 2012b.…”