Sapwood moisture in Douglas-fir boles and seasonal changes in soil water

Abstract

Large conifers, such as Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii), purportedly draw on water stored in their boles during periods of summer drought. The relation of seasonal changes in soil moisture to sapwood water content was evaluated in four forest stands dominated by mature Douglas-fir along a transect from the Pacific Coast to 1200 m in the western Cascade Mountains of Oregon, USA. The sites varied in stand age, elevation, topography, and soil characteristics, including available soil water capacity. At two sites, gravimetric measures of sapwood relative water content (SRWC) were taken approximately every 4 weeks from May 2002 through July 2004; two additional sites were similarly measured from February 2003 through July 2004. Automated meteorological stations located on the sites and in adjacent open areas continuously monitored weather and soil moisture. Plant-available soil water (ASW) in the upper 0.6 m of soil reached minimum values during the summer drought and rewetted during fall and winter. Large seasonal changes in ASW did not result in corresponding changes in SRWC. Minimum SRWC was lower at sites with higher ASW. At all sites, Douglas-fir trees apparently regulate water loss to maintain consistent (±10%) bole water content throughout the year despite large changes in soil moisture.