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Abstract:
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 (14C) measurements to determine the mean age of carbon in CO2 emitted from and within
stems of three tropical tree species in Peru. Carbon pools fixed >1 year previously contributed to stem CO2 efflux in all trees
investigated, in both dry and wet seasons. The average age, i.e., the time elapsed since original fixation of CO2 from the atmosphere
by the plant to its loss from the stem, ranged from 0 to 6 years. The average age of CO2 sampled 5-cm deep within
the stems ranged from 2 to 6 years for two of the three species, while CO2 in the stem of the third tree species was fixed from
14 to >20 years previously. Given the consistency of 14C values observed for individuals within each species, it is unlikely that
decomposition is the source of the older CO2. 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 14C values observed in stem CO2 efflux and stem-internal CO2 result from respiration of storage C pools
within the tree. The observed age differences between emitted and stem-internal CO2 indicate an age gradient for sources of
CO2 within the tree: CO2 produced in the outer region of the stem is younger, originating from more recent assimilates,
whereas the CO2 found deeper within the stem is older, fueled by several-year-old C pools. The CO2 emitted at the stem–
atmosphere interface represents a mixture of young and old CO2. 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.
