Branch, crown, tree, and stand water flux in an 18-year-old loblolly pine plantation after thinning, fertilization, and throughfall exclusion treatments

Branch and tree sap flux density (SFD) was measured to evaluate the effects of thinning, fertilization, and throughfall exclusion treatments on water use by loblolly pine (Pinus taeda L.) trees. Variations in SFD at different radial xylem depths were also examined to improve estimates of tree water use from sap flux density measured in the outer sapwood.; This study was conducted in an 18-year-old loblolly pine plantation in central Louisiana in 1999. The results revealed that SFD was higher in the outer xylem than in the inner xylem and large trees had higher SFD than small trees.; Thinning increased daily tree SFD and whole tree water use four years after retreatment, but decreased stand-level tree water use. Fertilization increased tree SFD, whole-tree water use, and stand-level tree water use within the thinned plots.; Throughfall exclusion decreased mean daily water use by branches and trees. SFD and water use at different levels of scale (branch, tree, and stand) changed with available soil water and microclimatic factors. Generally, SFD was positively related to photosynthetic photon flux density and vapor pressure deficit.; Branch SFD was higher in the upper crown than in the lower crown. Fertilization significantly increased daily branch water use within the thinned plots, but did not significantly affect branch transpiration per unit leaf area.; This study shows that tree and stand water use is affected by spatial variation in the canopy environment as well as climatic variables and forest cultural practices. Scaling up water use, carbon dynamics and growth from needle level to branch-, tree-, and stand levels under various global scenarios should account for cultural practice differences, within canopy variation, and changes in micro-environmental factors. Cultural practices, such as thinning and fertilization, significantly increased SFD and whole tree water use by accelerating leaf area development. Correlated changes in other physiological processes eventually alter tree growth. The ability to model tree and stand growth response through monitoring of whole tree and branch water use has the potential to substantially improve our understanding of forest responses to management and climate change and should be useful in policy decision making.