Interactions between xylem structure and water relations of Southern pines

This dissertation focused on the study of xylem structure and water relations of three major species of Southern pines: loblolly (LO), longleaf (LL) and slash pine (SL). The study was divided in two principal areas: (1) assessment of water availability and genetic family effects on water relation traits, growth and wood properties of mid-rotation LO; and (2) characterize water relation traits and secondary xylem structure of mature LL and SL. For the first study, water availability was controlled by irrigation into two fast growing families, one from Atlantic Coastal Plain (SC) and the other composed by a mix of Florida families (FL). The second study was carried out in a naturally-regenerated mixed stand of mature LL and SL on a flatwood site in north-central Florida.;For LO, increasing water availability via irrigation increased transpiration and stomatal conductance scaled at canopy level; whole-tree water conduction efficiency was maintained at high levels due to avoidance of xylem embolism. LO tends to maintain constant maximum water potential gradient from roots to shoots at a cost of loss of conductivity under water-limited conditions. The two genetic families evaluated showed differences in canopy conductance response to water-limited conditions: SC adjusted their overall canopy conductance in response to drought, while FL did not. At age 11, irrigation increased specific gravity and latewood percentage and the mechanism of this response was an extension of the basal area growing season by 24.6 days in irrigated trees. The main effect of irrigation was an increase in latewood growth. Before canopy closure irrigation caused null or negative effect on specific gravity and latewood percentage due to large effect on earlywood growth associated with fast leaf area index development. After year 7, earlywood growth was similar between control and irrigated trees but latewood growth was larger on irrigated plots, increasing the overall year-ring specific gravity and latewood percentage. Trees from SC family had more desirable wood properties than trees from FL family, independent of irrigation; this effect was associated with greater yearly latewood growth in SC.;In mature LL and SL, mean daily transpiration rate was higher for SL than LL trees and there were no significant differences between species in daily transpiration rate per unit leaf area. Species differences in transpiration rate were principally determined by differences in leaf area per tree; SL had 60% more leaf area per unit basal sapwood area than LL (p=0.086). LL had larger crown conductance than SL on days with high soil moisture and reduced to similar values than SL on days with low soil moisture. In terms of hydraulic architecture and tracheid anatomy, root sapwood-specific hydraulic conductivity of LL was larger than SL, but there were no species differences for any other organ tested. There were no differences in vulnerability to cavitation between species in any of organ evaluated and there was a weak trade-off between water conduction efficiency and safety. Tracheid hydraulic diameter was strongly correlated with sapwood-specific hydraulic conductivity across all organs. Tracheid allometry changed markedly between sapwood of roots, trunk and branches, reflecting different mechanical reinforcement needs. Higher sapwood to leaf area ratio and higher ks in roots of LL are anatomical traits that may allow LL to survive and dominate in drier soil microsites.