Are storage and tree growth related? Seasonal nutrient and carbohydrate dynamics in evergreen and deciduous Mediterranean oaks

Seasonal dynamics of branch carbohydrates differed sharply between coexisting evergreen and deciduous Mediterranean oaks. Branch carbon storage was crucial in the evergreen, while it played a minor role in the deciduous oaks. The aim of this study was to describe the seasonal dynamics of nutrients and NSC in relation to the aboveground phenology of coexisting winter-deciduous (Quercus faginea) and evergreen (Quercus ilex subsp. ballota) oak species, and to analyse the relationship between the resource budget of branches and shoot growth. Monthly concentrations of nitrogen (N), phosphorus (P), potassium (K) and non-structural carbohydrates (NSC) plus the aboveground phenology of branches were measured over 2 years. We also analysed the correlation between the resource budget of branches prior to the growing season and the subsequent shoot and stem growth. Seasonal branch nutrient dynamics could be explained by shoot growth phenology, showing similar patterns across species. However, NSC dynamics varied between the two species, owing to the differences in leaf phenology and the contrasting role of branches as storage sites. NSC and N branch storage were crucial for the early stages of shoot growth in the evergreen trees. Accordingly, branch N and NSC storage pools in late winter correlated positively with spring growth, and NSC concentrations dropped during bud burst in all branch organs of Q. ilex. Contrastingly, branch NSC concentrations of the deciduous Q. faginea were only marginally affected by spring growth and no relationship was observed between branch N and NSC stores prior to bud burst. These results challenge previous assumptions on the lower relevance of branch NSC storage for the spring growth of evergreen trees and call for further studies where closely related pairs of coexisting evergreen and deciduous tree species are compared.