Effects of precommercial thinning on annual radial growth and wood density in balsam fir (Abies balsamea)

This study examined effects of precommercial thinning (PCT) on annual radial growth (ring width) and wood density in balsam fir [Abies balsamea (L.) Mill.]. In addition, the responses to PCT were measured and compared at several stem heights (0.2 m, 0.7 m, 1.3 m, 3 m, 5 m, 7 m, 9 m). Fifty-four trees were collected from plots subjected to moderate thinning (nominal stand density of 2,150 stems/ha), and light thinning (nominal stand density of 4,200 stems/ha), and from control plots. Ring width and wood density of individual rings were measured by X-ray densitometry. Our results show that in balsam fir the annual radial growth rate showed a positive response to PCT, especially in the low part of the stem (up to 5 m high), and this response lasted for 7 years. To achieve a significant increase in annual diameter growth in this species, however, a moderate thinning intensity is needed. This study also revealed that the response of annual radial growth to PCT was limited primarily to the earlywood width, whereas the latewood width showed little response. As a result, the latewood percentage was affected by the moderate thinning. The light thinning and the control plots, however, had a comparable earlywood width and latewood percentage. Both earlywood density and latewood density showed little response to PCT. However, the wood density of growth rings tended to decrease following the moderate thinning, due to a decreased latewood percentage. In addition, the moderate thinning might somehow reduce the intra-ring variation in wood density and thus produce more uniform wood. This study also revealed that the responses to thinning in this species tended to weaken appreciably with increasing stem height. The remarkable responses were observed in the low part of the stem (up to 5 m high), whereas little response was found at the upper part of the stem. As a result, an increased stem taper may result from PCT in balsam fir.