From tree-tip to trunk-base: interplay of resistance to embolism formation and wood anatomy in two conifers

Efficient and safe water transport in plants depends on specific wood anatomical structures. Comparing the hydraulic vulnerability among tree species and individuals can be biased by differences in the sampling procedure resulting in unjustified conclusions. In fact, water-transporting conduits of the xylem widen from the tree-tip toward the base potentially influencing xylem vulnerability to embolism formation.The resistance to cavitation and embolism formation (hydraulic vulnerability curves) was empirically tested at different positions from the tree-tip to the trunk base in a mature Norway spruce and a silver fir tree. Anatomical analyses (conduit diameter, pit traits) were performed on the same material used for hydraulic measurements.Substantial intra-plant variation in xylem anatomy and vulnerability to embolism was observed in both species scaling with the distance from the apex. The P50 values (i.e. the water potential resulting in 50% loss of hydraulic conductivity) were about 3 MPa more negative at the tip than at the trunk base. Tracheid diameters and pit dimensions increased from the apex downwards, confirming the tip-to-base conduit widening. The highest variability of both anatomical and physiological traits was observed within the first 3 m from the apex. We propose a method for the prediction of drought vulnerability based on hydraulically weighed conduit diameter distribution.A clear pattern of anatomical traits’ variation along the longitudinal axes of the tree was observed, likely playing a role in the increase of vulnerability to embolism. In comparative studies, we thus recommend to standardize the sampling of material according to the distance from the apex.