Leaf phenology and water-use patterns of canopy trees in Northern Argentinean subtropical forests

Tree physiological processes are affected not only by environmental conditions, but also by phenological leaf stages. During foliar expansion, rapid changes occur, such as the activation of metabolic processes that encompass a hydraulic link between xylem and phloem pathways at a whole-tree level. Daily and seasonal changes in stem diameter may reveal different temporal dynamics of water use and recharge in tree reservoirs. Foliar phenological patterns were studied in relation to stem dimensional changes in 10 canopy tree species with different phenological patterns (three deciduous, three brevideciduous and four evergreen species). Additionally, we assessed (i) daily sap flow fluctuations in branch and main stem, (ii) diurnal changes in sapwood volumetric water content and (iii) stem radius variations during leafless, expanding and mature leaves periods in three of the 10 tree species (two deciduous and one brevideciduous). During the leaf expansion phase, the diameter of trees decreased in all 10 species, with a larger impact on deciduous and brevideciduous species. For the subset of deciduous and brevideciduous species, the movement of long-distance water transport occurred first near the branches and then in the main stem during the leafless stage. Changes in stored water use and a decrease in the volumetric water content and the radius of the main stem during this period suggest that there is a contribution of water from internal stem reservoirs toward the construction of new leaves.