The process of sapwood/heartwood transition in trees is not fully understood. We tested whether the ontogenetically-stable apex-to-base conduit widening generates path length effects limiting the conductance of inner sapwood rings. The axial scaling ( b) of conduit hydraulic diameter ( Dh) was estimated at annual resolution in a spruce and beech tree. We compiled a global dataset of sapwood ring number ( NSWr), their average width ( SWrw), tree height ( H) and stem elongation rate ( ΔH) in conifer and angiosperm trees. A numerical model simulated the effects of H and ΔH on the conductance of each xylem ring ( KRING). b resulted ontogenetically stable. Simulations well predicted the observed patterns of increasing NSWr with H and decreasing NSWr with ΔH, assuming that heartwood forms when the marginal conductance gain of maintaining the functionality of an inner ring becomes negligible. Sapwood/heartwood transition minimizes the C costs associated to allocation to secondary growth and maintenance of living sapwood required to attain a given sapwood conductance. The number of sapwood rings depends on the effects of H and ΔH on the conductance of inner sapwood rings. The width of sapwood rings contributes to compensate for the lower conductance of inner sapwood rings at high ΔH.

Tree regeneration is a key demographic process influencing long-term forest dynamics. It is driven by many biotic and abiotic factors. Thus, predictions of tree regeneration are challenging because of complex feedbacks along climatic gradients. The stress gradient hypothesis (SGH) and life-history strategies (LHS) provide a framework for assessing such feedbacks across different species ranges. To address these topics, we analyzed regeneration for 24 tree species in 6,540 plots from 299 unmanaged European forests. Negative interactions predominated, with their intensity decreasing under stressful conditions for most species, as predicted by the SGH. However, positive interactions were only evident for a few species. Our study indicates that SGH and LHS can be combined to partially explain within- and between-species differences in tree recruitment. Moreover, our findings imply that projections of forest dynamics along wide climatic gradients must accommodate both negative and positive biotic interactions, as they strongly affect rates of community turnover.