BREAST-HEIGHT RELATIVE DENSITY AND RADIAL GROWTH IN MATURE JACK PINE (PINUS-BANKSIANA) FOR 38 YEARS AFTER THINNING

Sawlogs are in short supply in northern Ontario, and thinning has been suggested as one way to improve the situation. The only rotation-age jack pine (Pinus banksiana Lamb.) thinning trial in the region was exa mined to assess how commercial thinning influenced wood quality. This report covers an unreplicated trial of a 65-year chronology of pith to bark relative densities and growth rates based on X-ray densitometry of breast-height increment cores taken from trees on two thinned plots (average spacing 2.6 and 3.4 m) and an unthinned control (average spa cing 1.7 m). The trees on the treatment plots responded to thinning by producing wood with significantly lower relative density than those o n the control plot. This trend continued much longer than reported for other pines and could negatively affect pulp yield or mechanical prop erties of lumber. Enhanced earlywood growth caused a drop in the propo rtion of latewood that resulted in the decline in density. Thinning ma y have improved moisture availability during the early and middle seas on and encouraged earlywood growth. Density and growth rate difference s became apparent soon after treatment. Early, rapid, and inexpensive estimates of the product potential of younger thinning trials are poss ible using the techniques demonstrated here.