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: Non-structural carbohydrates (NSCs) are critical to maintain plant metabolism under stressful environmental conditions, but we do not fully understand how NSC allocation and utilization from storage varies with stress. While it has become established that storage allocation is unlikely to be a mere overflow process, very little empirical evidence has been produced to support this view, at least not for trees. Here we present the results of an intensively monitored experimental manipulation of whole-tree carbon (C) balance (young Picea abies (L.) H Karst.) using reduced atmospheric [CO 2 ] and drought to reduce C sources. We measured specific C storage pools (glucose, fructose, sucrose, starch) over 21 weeks and converted concentration measurement into fluxes into and out of the storage pool. Continuous labeling ( 13 C) allowed us to track C allocation to biomass and non-structural C pools. Net C fluxes into the storage pool occurred mainly when the C balance was positive. Storage pools increased during periods of positive C gain and were reduced under negative C gain. 13 C data showed that C was allocated to storage pools independent of the net flux and even under severe C limitation. Allocation to below-ground tissues was strongest in control trees followed by trees experiencing drought followed by those grown under low [CO 2 ]. Our data suggest that NSC storage has, under the conditions of our experimental manipulation (e.g., strong progressive drought, no above-ground growth), a high allocation priority and cannot be considered an overflow process. While these results also suggest active storage allocation, definitive proof of active plant control of storage in woody plants requires studies involving molecular tools.

Description: Trees contain non-structural carbon (NSC), but it is unclear for how long these reserves are stored and to what degree they are used to support plant activity. We used radiocarbon ( 14 C) to show that the carbon (C) in stemwood NSC can achieve ages of several decades in California oaks. We separated NSC into two fractions: soluble (~50% sugars) and insoluble (mostly starch) NSC. Soluble NSC contained more C than insoluble NSC, but we found no consistent trend in the amount of either pool with depth in the stem. There was no systematic difference in C age between the two fractions, although ages increased with stem depth. The C in both NSC fractions was consistently younger than the structural C from which they were extracted. Together, these results indicate considerable inward mixing of NSC within the stem and rapid exchange between soluble and insoluble pools, compared with the timescale of inward mixing. We observed similar patterns in sympatric evergreen and deciduous oaks and the largest differences among tree stems with different growth rates. The 14 C signature of carbon dioxide (CO 2 ) emitted from tree stems was higher than expected from very recent photoassimilates, indicating that the mean age of C in respiration substrates included a contribution from C fixed years previously. A simple model that tracks NSC produced each year, followed by loss (through conversion to CO 2 ) in subsequent years, matches our observations of inward mixing of NSC in the stem and higher 14 C signature of stem CO 2 efflux. Together, these data support the idea of continuous accumulation of NSC in stemwood and that ‘vigor’ (growth rate) and leaf habit (deciduous vs evergreen) control NSC pool size and allocation.