Starch concentrations in branches are not consistently changing with drought stress in mature temperate tree species

Starch is the ubiquitous carbon reserve in trees that is stored decentralized in sapwood parenchyma of branches, stems and roots. As a transitory carbon pool between photosynthesis and carbon sinks (growth, respiration,...), tissue concentrations of starch are assumed to mirror carbon source-sink-relations, with concentrations positively correlating with the net balance between gross primary productivity and the sum of all carbon sink activities of a tree. In this study, we investigated if starch concentrations in branch sapwood of mature trees are suitable indicators for drought induced changes of the trees’ carbon source-sink activities.

Taking advantage of the Swiss Canopy Crane II facility, we studied mature trees of 6 common European broadleaved species over five consecutive growing seasons that varied significantly in terms of temperature and precipitations. Despite the very different climatic conditions, we found surprisingly small variations of end-of-season starch concentrations in terminal branches for most years and species. This is in stark contrast to leaf gas-exchange and growth that both declined significantly in all species in years with extended drought periods. Further, among all investigated species, deviations from the species-specific average starch concentrations in some years were not consistently correlated with climatic anomalies (e.g., exceptionally dry seasons were not uniformly associated with decreased branch starch concentrations). Overall, these findings suggest that starch formation in branch sapwood possesses a high priority, and the fast refilling of starch reserves in wood parenchyma of younger branches after spring bud break occurs largely independent of the total tree annual carbon balance.