Stem Dimensional Fluctuations in Douglas-fir of Different Crown Classes

Abstract

The influence of selected environmental factors and twig xylem pressure potentials on variations in stem basal area was examined under field conditions in Pseudotsuga menziesii of four crown classes. Progressive seasonal contractions in basal area indicated a gradual dehydration of living tissues near the cambium because daily water losses exceeded uptake. Thus, seasonal contractions were closely correlated with base (i.e., presunrise) twig xylem pressure potentials and soil moisture content. In contrast, stem shrinkage occurring during a particular day was more closely related to factors indicating the amount of daily water loss. Such diurnal contractions were greatest about 3 weeks prior to the end of the summer drought, when there was a coincidence of relatively well-hydrated internal tissues, low soil moisture contents, and high daily transpiration rates partially due to the lack of major stomatal control of afternoon water losses. Greater seasonal and diurnal contractions were generally associated with larger stem sections due to their relatively larger water storage areas. However, stem proximity to the transpiring crown and cambial growth rate often complicated this relationship. Forest Sci. 25:132-144.