Response of annual growth ring components to soil moisture deficit in young, plantation-grown Douglas-fir in coastal British Columbia

Abstract

The response of 10 annual growth ring variables to drought in coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) was examined using 16- and 17-year-old trees growing in six progeny test sites in southwestern British Columbia. Width, density, and mass components of individual rings common to the same 11-year period were measured on 16 trees at each site using X-ray densitometry of increment cores. For each ring variable and site, the slope of the linear regression of the annual ring component (after adjusting for age trends across the core) on the log of the total growing season soil moisture deficit (SMD) for the same year was used to derive a drought response coefficient (DRC). DRCs quantified the sensitivity of ring components to changing annual moisture conditions across the 11 years on a particular site. SMD appeared to materially influence ring variables on only the driest of the six sites where mean SMD was two to three times greater than at any other site. On this site, DRCs of eight growth ring variables were significantly (p < 0.05) related to SMD. On the remaining sites only six of a total of 50 DRCs were significantly different from zero. These results suggest that the response of annual growth ring variables to drought may be useful for assessing drought hardiness of genotypes in Douglas-fir breeding programs, but only on sites where average SMD is high enough to elicit a drought response.