• Phillips, Leah M.
  University of Maine Graduate School
  

Growth characteristics of selected Picea rubens Sarg. (red spruce) and Abies balsamea (L.) Mill. (balsam fir) crop trees were studied in a northern conifer forest to determine the effects of pre commercial thinning (PCT) 25 years after initial treatment. Two measures of growth efficiency (GE, growth per unit of growing space) were examined: stemwood increment (dm3) per unit of projected leaf area (PLA) (m2) and stemwood increment (dm3) per unit of crown projection area (CPA) (m2).

Stem form differences were evaluated by comparing stem taper between species and treatments. Branch diameters were measured between 1.0 - 2.0 meters above breast height (BH, 1.37 m (4.5 ft)) for each crop tree, and the number and size of branches and the ratio of knots were determined. Volumes of all crop trees were calculated using Smalian's formula (Avery and Burkhart 1994) applied to different geometric forms of the tree to estimate total cubic foot volume from diameter measurements up the tree bole.  The efficacy of Honer's (1967) volume equation for estimating total cubic foot volume from diameter at BH (DBH) and total height (THT) was tested by comparing measured values to the estimated values. Differences in tree stability were determined by comparing height to diameter ratios (H/D) of all the crop trees by species and treatment.

GE did not differ between treatments using either definition, although average PLA and CPA per ffee were higher in the spaced plots. As expected, balsam fir was more growth- efficient than red spruce using both GE definitions. There were no significant differences in average PLA between the two species, but red spruce had a larger average CPA than balsam fir. Crop trees in the spaced plots had more stem taper than the un spaced plots and a lower (H/D) ratio. Stem taper differed between species; red spruce crop trees had more stem taper than balsam fir. The crop trees in the spaced plots had significantly more volume than those in the un spaced; total stand volume including non-crop trees was not measured. Balsam fir trees contained significantly more volume than red spruce in both treatments. Crop trees in the spaced plots had more and larger branches and also a higher percentage of knot volume than in the un spaced plots. There were no differences in the number and size of branches between balsam fir and red spruce, although red spruce crop trees had a greater knot volume than balsam fir trees. Results of this study are important for managers wanting to use PCT as a silvicultural tool to increase volume growth of selected crop trees without losing value or productivity.