Physiological response of large Douglas-fir to natural and induced soil water deficits

Abstract

The effect of soil water deficits on net photosynthesis, transpiration, stomatal activity, twig xylem pressure potentials, and cambial growth were studied in large, field-grown Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco). Abnormally high soil and tree water deficits were periodically induced during the summer by withholding supplementary irrigation water from a 28-m Douglas-fir contained in a weighing lysimeter. This eventually promoted persistent stomatal closure which limited gas exchange rates. At such times, net photosynthesis was confined to early morning daylight hours and daily totals were reduced by about 70% compared with the well-watered condition; xylem pressure potentials decreased only about 0.6 MPa during the day from a predawn level of −1.6 MPa. After irrigation or major rainfall, predawn xylem pressure potentials increased to −0.4 MPa and the tree stem abruptly increased in size; both indicated the hydraulic recharge of internal tree tissues depleted during the induced drought. Stomata then responded primarily to light, even though midday xylem pressure potentials remained near −2.2 MPa. These data are compared with those obtained from a nearby tree which was responding to natural fluctuations in soil moisture content.