Varying densities of Pacific madrone in a young stand in Oregon alter soil water-potential, plant moisture stress, and growth of Douglas fir

Abstract

Soil water-potential and Douglas fir [ Pseudotsuga menziesii (Mirb.) Franco] moisture stress and growth were studied on a droughty, 2-ha site in southwest Oregon where Pacific madrone ( Arbutus menziesii Pursh) and associated shrub and herbaceous vegetation were thinned to represent the following range of conditions: high-density madrone (H), with associated shrubs and herbs controlled; medium-density madrone (M), shrubs and herbs controlled; low-density madrone (L), shrubs and herbs controlled; no madrone (N), shrubs and herbs controlled; and no madrone (U), shrubs and herbs predominate. Soil water-potential ( Ψ s) at a depth of 0–30 cm was consistently higher in treatment N than in all other treatments; in 1987 this difference was significant ( P<0.025). Average Ψ s in treatment U reached −1.5 MPa (permanent wilting point) between June and July in both years of the study, whereas in the other treatments that level was never reached. Soil water conditions were also relatively severe in treatment H. Predawn plant moisture stress ( pms) of Douglas fir was significantly ( P=0.0001) less in treatment N than in all other treatments. Seasonal moisture-stress relief ( smsr) of Douglas fir was significantly related to madrone leaf area index ( L) ( r 2=0.75, P=0.003), and was greatest in treatment N. Seasonal moisture-stress relief of madrone was also significantly correlated with L ( r 2=0.66, P=0.03). There were highly significant linear relationships between both predawn and midday pms and soil water-potential for Douglas fir ( r 2=0.76 and 0.75) and madrone ( r 2=0.69 and 0.79). Results clearly show that conditions for maximum Douglas-fir growth occurred in treatment N, in which all madrone and the associated vegetation were controlled. Average diameter growth of Douglas fir was greatest in treatment N, although not significantly different from that in treatment U, and least in treatment H. In 1987, Douglas-fir growth in diameter, stem basal-area, and stem volume was strongly related to smsr ( r 2=0.62–0.81) and madrone L ( r 2=0.66–0.87), and to a lesser extent, seasonal soil tension relief ( sstr) ( r 2=0.51–0.62).