The effects of spacing and genetic factors on growth and tree form quality traits of plantation-grown jack pine

Abstract

Overall tree form ofjack pine (Pinus banksiana Lamb.) growing in natural stands varies from straight slender trees with compact crowns to broad, limby and even multi-stemed trees. Jack pine grown in plantations at conventional spacings of 2 m develops form traits undesirable for forestry utilization. The goal of this study was to investigate the effects and interaction of spacing and genetic factors on plantation-grown jack pine to determine the potential benefit of a selection program based on form traits. Ten wind-pollinated families were collected from each of six natural fire-origin stands from east and west of Lake Nipigon in northwestern Ontario that had been subjectively rated as good, average and poor in form. These sixty families making up six form-quality groups, together with an additional twenty plus tree families making up two more form-quality groups, were established at two planting sites with contrasting soil texture and fertility at spacings of 1, 2 and 3 m. Eight tree form quality traits—branch length, branch diameter, branch angle, branch number, relative branch diameter, relative crown width, number of crooks, and taper were measured and analyzed together with two growth traits—height and diameter at breast height (DBH) at age eight. General linear models were used to evaluate the effects of site, spacing, form-quality group, family and the relevant interactions on all traits except number of crooks which was not normally distributed. All jack pine growth and form traits except branch number were greatly affected by spacing. Most of the change in form traits occurred when spacing increased from 1 to 2 m with lesser change from 2 to 3 m. This effect was more pronounced at the more fertile test site. Form-quality groups were significantly different for only two of the form traits—branch length and branch angle with this effect again being more evident at the fertile test site. However, the family effect was significant for all seven tested form traits as well as the two growth traits suggesting that any of these traits would readily respond to family selection. A definite provenance effect was demonstrated for growth traits and stem taper; the western sources grew faster with more favourable taper. None of the form quality group X spacing nor the family x spacing interactions were significant. Heritabilities and genetic correlations were determined for all traits separately for each spacing at each test site. Traits height, branch angle and taper had the highest heritabilities, with individual heritabilities of greater than 0.15, and the traits branch diameter, relative branch diameter and relative crown width had values less than 0.05. The two growth traits together with growth-related form traits tended to be strongly positively correlated with the exception of height and relative crown width. However, branch angle and branch number showed no strong correlations with any other tree form and growth traits. Both heritabilities and genetic correlations were more meaningful for the more fertile test site due to larger components of family variance.