Axial and radial profiles in conductivities, water storage and native embolism in trunks of young and old-growth ponderosa pine trees

Ponderosa pine has very wide sapwood, and yet the spatial and temporal use of that sapwood for water transport is poorly understood. Moreover, there have been few comparisons of function in tips of old-growth trees in comparison with young trees. In the present study, axial and radial specific conductivity (k(s)), leaf specific conductivity (LSC), leaf specific conductance (k(l)), native embolism and the compartmentalization of sapwood water storage were characterized in trunks of young and old-growth trees. Trunks of young trees had lower k(s), lower LSC and lower native embolism [corresponding to 5% loss of conductivity (PLC)] than trunks of old-growth trees. However, k(l) in young trees was 3.5 times higher than in old-growth trees, supporting the hypothesis that tall trees have a reduced ability to transport water to their leaves. Water storage (capacitance) of young trees was not significantly different than at the base of old-growth trees. Although the top of the old-growth trees had similar k(s), LSC and k(l) to the young trees for a given cambial age, they had higher native embolism and lower capacitance. There was no trade-off between k(s) and native embolism at any height. In the tree crown, outer sapwood had 35-50% higher k(s) than the inner sapwood and 17-25 PLC lower native embolism. At the base of the old trees, there was no significant difference in native embolism between the outer, middle and inner sapwood, showing that refilling of embolisms was complete despite the 130-year difference in wood age among these radial positions. Although during the dry season the inner sapwood tended to be more saturated than the outer sapwood, the outer part of the sapwood contributed up to 60% of the overall stored water. Safer xylem, higher capacitance and higher k(l) would appear adaptive in the young trees for regulating their water resource, which is likely to be less reliable than the water availability of older trees with their more developed root system.