ALBERT

Description: Storage carbon (C) pools are often assumed to contribute to respiration and growth when assimilation is insufficient to meet the current C demand. However, little is known of the age of stored C and the degree to which it supports respiration in general. We used bomb radiocarbon ( 14 C) measurements to determine the mean age of carbon in CO 2 emitted from and within stems of three tropical tree species in Peru. Carbon pools fixed 〉1 year previously contributed to stem CO 2 efflux in all trees investigated, in both dry and wet seasons. The average age, i.e., the time elapsed since original fixation of CO 2 from the atmosphere by the plant to its loss from the stem, ranged from 0 to 6 years. The average age of CO 2 sampled 5-cm deep within the stems ranged from 2 to 6 years for two of the three species, while CO 2 in the stem of the third tree species was fixed from 14 to 〉20 years previously. Given the consistency of 14 C values observed for individuals within each species, it is unlikely that decomposition is the source of the older CO 2 . Our results are in accordance with other studies that have demonstrated the contribution of storage reserves to the construction of stem wood and root respiration in temperate and boreal forests. We postulate the high 14 C values observed in stem CO 2 efflux and stem-internal CO 2 result from respiration of storage C pools within the tree. The observed age differences between emitted and stem-internal CO 2 indicate an age gradient for sources of CO 2 within the tree: CO 2 produced in the outer region of the stem is younger, originating from more recent assimilates, whereas the CO 2 found deeper within the stem is older, fueled by several-year-old C pools. The CO 2 emitted at the stem–atmosphere interface represents a mixture of young and old CO 2 . These observations were independent of season, even during a time of severe regional drought. Therefore, we postulate that the use of storage C for respiration occurs on a regular basis challenging the assumption that storage pools serve as substrates for respiration only during times of limited assimilation.

Description: Understanding the variability and the current value of the Dole Effect, which has been used to infer past changes in biospheric productivity, requires accurate information on the discrimination associated with respiratory oxygen consumption in each of the biosphere components. Respiration in tree stems is an important component of the land carbon cycle. Here we measured, for the first time, the discrimination associated with tree stem oxygen uptake. The measurements included tropical forest trees, which are major contributors to the global fluxes of carbon and oxygen. We found discrimination in the range of 12.6–21.5 ‰, indicating both diffusion limitation, resulting in O2 discrimination values below 20 ‰, and alternative oxidase respiration, which resulted in discrimination values greater than 20 ‰. Discrimination varied seasonally, between and within tree species. Calculations based on these results show that variability in woody plants discrimination can result in significant variations in the global Dole Effect.

Description: Respiration in tree stems is an important component of forest carbon balance. The rate of CO2 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 CO2 efflux can either overestimate or underestimate respiration rate, because of emission or removal of CO2 by transport in xylem water. Here we used the ratio between CO2 efflux and O2 influx in stems of tropical forest trees to better understand respiration in an ecosystem that plays a key role in the global carbon cycle. This ratio, which we 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 CO2 in the xylem water is negligible. However, 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 CO2 (~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 tree, 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, for the first time, the existence of a~considerable internal flux of CO2in the stem of tropical trees. If the transported CO2 is used in the canopy as a substrate for photosynthesis, it could account for several percent of the C fixed by the tree, and perhaps serve as a mechanism that buffers the response of the tree to changing CO2 levels. Our results also indicate, in agreement with previous work, that the widely used CO2 efflux approach for determining stem respiration is unreliable. We demonstrate here a field applicable approach for measuring the O2 uptake rate, which we suggest to be a more appropriate method to estimate stem respiration rates.

Description: Understanding the variability and the current value of the Dole Effect, which has been used to infer past changes in biospheric productivity, requires accurate information on the isotopic discrimination associated with respiratory oxygen consumption in each of the biosphere components. Respiration in tree stems is an important component of the land carbon cycle. Here we measured, for the first time, the discrimination associated with tree stem oxygen uptake. The measurements included tropical forest trees, which are major contributors to the global fluxes of carbon and oxygen. We found discrimination in the range of 12.6–21.5‰, indicating both diffusion limitation, resulting in O2 discrimination values below 20‰, and alternative oxidase respiration, which resulted in discrimination values greater than 20‰. Discrimination varied seasonally, between and within tree species. Calculations based on these results show that variability in woody plants discrimination can result in significant variations in the global Dole Effect.

Description: Respiration in tree stems is an important component of forest carbon balance. The rate of CO2 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 CO2 efflux can either overestimate or underestimate respiration rate because of emission or removal of CO2 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 CO2 transport important in tropical trees, and, if so, (2) does this transport result in net release of CO2 respired in the roots at the stem, or does it cause the opposite effect of net removal of stem-respired CO2? To answer these questions, we measured the ratio of stem CO2 efflux to O2 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 CO2 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 CO2 (~ 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 CO2 in the stems of tropical trees. If the transported CO2 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 CO2 levels. Our results also indicate, in agreement with previous work, that the widely used CO2 efflux approach for determining stem respiration is unreliable. We demonstrate here a field applicable approach for measuring the O2 uptake rate, which we suggest to be a more appropriate method to estimate stem respiration rates.