Temporal and Spatial Variations in the Water Status of Forest Trees

This review focuses on the development of naturally occurring spatial and temporal variations in tree water relations and illustrates how the various components of a tree's hydraulic system interrelate. In developing this review, emphasis is given to mature forest-grown trees. Specifically, three points are developed: (1) the spatial variation of total water potential within a tree, (2) the state of the soil-plant-atmosphere continuum which causes these potentials, and (3) the tree properties which lead to the internal adjustment of total water potential. While examining tree water status as it is related to the soil-plant-atmosphere continuum, several deficiencies in current knowledge are revealed. For example, tissue capacitance has generally been ignored in describing waterflow even though it is known that internally stored water can supply as much as 15 percent of the total amount of water transpired during the summer. Over a 3-month, summer period a mature conifer may transpire 4,000 liters of water. Furthermore, there are various unanswered questions concerning physiological activities at the root-soil and leaf-air interfaces. This review also provides an appreciation of the similarities and differences in internal waterflow, water deficit formation, and stomatal activity between seedlings, saplings, and large trees and between deciduous and coniferous species. This review consolidates and synthesizes information valuable to those researchers currently involved in developing directions and methodologies for new physiological studies of tree water relations and should be useful to those presently evaluating experimental results as well as those interested in model simulations.