Structure and hydraulic function of xylem in two tree species with contrasting amounts of sapwood, Pseudotsuga menziesii and Pinus ponderosa

Every wood anatomist knows that the wood near the center of a tree (juvenile wood) differs from the wood laid down at some distance from the pith (mature wood), and that the wood produced during the spring (earlywood) differs from the wood produced during the summer (latewood). There is a progressive increase in the dimensions of the cells from inner to outer growth rings. These differences affect the structure and function of the wood for water transport and mechanics. However, why do trees produce different wood quality as a function of cambial age? Is it as an adaptation to hydraulics or mechanical demands? No research has been undertaken in this area, because of the historic lack of communication between wood scientist and ecophysiologists, and because the hydraulic architecture field had few insights to offer until its recent wave of increasing sophistication. The chapters presented here are founded on these questions and provide new concepts and understandings of the trade-off between hydraulic properties and mechanical support in trees. We measured specific conductivity (k_s) and vulnerability to embolism (loss of k_s) to map tree hydraulic properties at different vertical and radial locations in the trunk and branches. In addition, field measurements, mechanical characteristics and anatomical features were determined. To investigate a wide range of wood properties, this research was conducted in two conifer species with contrasting amount of sapwood Douglas-fir (Pseudotsuga menziesii Mirb.) and ponderosa pine (Pinus ponderosa Dougl) and within three different age-classes. In Douglas-fir, the differences in wood properties from the pith to the bark, from the bottom to the top of a tree, and within earlywood and latewood had more effect on hydraulic than on mechanical properties. In ponderosa pine, change in wood properties did not affect the hydraulic characteristics as much as in Douglas-fir. We showed that ponderosa pine sapwood has so much potential storage of water that it can compensate and level off the differences in water transport. Any change in wood density from pith to bark is more reflective of the need to alter hydraulic efficiency than the need to increase mechanical strength as trees grow.