Axial conduit widening and growth rate determine the radial patterns of sapflow and the transition of sapwood into heartwood

Sap flows through a network of xylem conduits compartmentalized into growing rings. We implemented empirical observations of conduit dimensions into a numerical model to simulate the effects of the three dimensional xylem architecture on the pattern of sapflow profiles across the sapwood and the mechanism of sapwood transition into heartwood. We reconstructed the axial variation (from stem apex to base) of the xylem conduit diameter (Dh) within each growth ring in three mature individuals (a Picea abies, a Pinus cembra, and a Fagus sylvatica). For every year of growth, Dh widens from the stem apex towards the base according to an ontogenetically stable power-like trajectory. Accordingly, the axial hydraulic resistance of a ring (RL) increases with the distance from the apex (L) more rapidly close to the apex. Therefore, the total R of the apical shoot is dependent to its length. We modeled the sap flow of a mature tree generated by a difference of water potential between soil and leaves (ΔΨ), assuming the crossing-ring resistance (RR) being a function of L. The total axial resistance (i.e., from stem apex to base: RR_TOT) of the different rings at the stem base increased from the outmost ring towards the pith more rapidly with increasing stem elongation rate (ΔL). Accordingly, sapflow decreased from the outmost ring towards the younger sapwood rings more rapidly when ΔL is higher. If the hydraulic functionality of xylem conduits is set by a maximum RR_TOT, then our model predicts that sapwood transition into non-functional heartwood occurs sooner in fast growing trees, in agreement with empirical data showing that the number of sapwood rings scales negatively with stem elongation rate. We conclude that sapwood is not a hydraulically homogeneous tissue and heartwood formation is determined by hydraulic limitations to water flow in the inner sapwood rings.