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.