Stem position and root infection influence heartwood formation in Douglas-fir plantations

Abstract

Heartwood formation influences timber quality and utilization, yet the factors driving its variation remain poorly understood. Understanding these variations can guide management strategies to influence heartwood development and enhance timber value through adapted silvicultural practices. This study examined the relationships between heartwood area and various tree characteristics in Douglas-fir plantations aged 25–34 years, across four sites within their native range in the southern interior of British Columbia, Canada. Stem analysis was used to quantify heartwood area and its relationships with tree-level variables, including sapwood area, tree height, root infection, and competition. To account for vertical variation along the stem, discs were categorized into three height classes based on relative stem height: lower (0–30%), middle (31%–60%), and upper (61%–100%). Heartwood area was primarily influenced by sapwood area, with the strongest effect at the middle stem, followed by the upper and lower positions. Tree height showed a position-dependent effect: positive in the lower stem and negative in the middle and upper sections. Root infection significantly increased heartwood area in the lower stem, with its effect weakening at higher positions. Competition had statistically significant but minimal effects, ranging from slightly negative in the lower stem to slightly positive in the upper stem. Our findings suggest that promoting sapwood development, particularly in the mid-stem, can enhance heartwood formation. While practices like thinning may contribute to this, the positive association between competition and heartwood in the middle stem indicates that maintaining moderate stand density may be more beneficial than aggressive spacing. Although root infection may locally stimulate heartwood near the base, its longer-term effects can disrupt the sapwood-heartwood balance and reduce tree vigor. Therefore, silvicultural strategies should aim to promote heartwood primarily through stand density management while limiting reliance on root stressors, integrating disease control measures to sustain overall tree health.