Aspen shoots are carbon autonomous during bud break

Current thinking holds that carbon autonomy of branches in trees is unlikely, particularly during bud break, when the new developing shoots require significant influx of carbon resources from more distant sources. Results from recent studies indicate that the impact of bud break on overall tree reserves might be small. In two studies the independence of flushing shoots from stored carbon reserves and the photosynthesis in developing new leaves and shoots of Populus tremuloides were explored. New developing shoots quickly became a positive carbon source and only a few days into flush, the photosynthetic system of the newly developing shoots was efficient enough to achieve positive carbon gain even at low light levels. Only 14% of the stored shoot reserves, without any mobilization from more distant reserves, were used during bud break and early shoot expansion. Without any underlying stress, shoots of deciduous trees appear to be carbon autonomous during bud break when demand on stored carbon should be the highest. The development of an efficient photosynthetic system in new shoots is critical in the recovery of carbon reserves in aspen. It minimizes the cost of bud break to the overall stored carbon reserves by optimizing the assimilation of carbon in the newly developed leaves, while eliminating the cost for mobilizing carbon reserves from more distant sources. This carbon autonomy of shoots has important implications for the whole tree carbon balance particularly to the non-photosynthetic tissues which functions solely depending on carbon export from the newly developing leaves and shoots.