Stem form responses to differing areas of weed control around planted Douglas-fir trees Robin Rose, Douglas A. Maguire, and Scott Ketchum Department of Forest Science Oregon State University

Introduction Discerning differences in stem form and volume among silvicultural treatments.

Introduction DBH and height respond to control of competing vegetation. Consequently stem volume also responds to vegetation control.

Introduction As trees develop under intensive management: Form differs from a cone (  /12)D 2 H, Trees large enough for existing volume or taper equations.

Introduction Application of volume and taper equations: Regional development; May not include intensively managed stands; Averages lose subtle differences;

Introduction Volume and taper equations: Most are functions of only DBH and HT. Consequences: Insufficient for detecting treatment differences.

One solution Measure upper stem diameters. Assess existing volume or taper equations. Develop new site-specific equations.

Past work Upper stem measurements were made at sites near Marcola and Summit: Fit Kozak’s variable-exponent taper equation: Tested parameters across treatments; No treatment effect was found.

Past work Compared empirical volumes to Bruce and DeMars (1974) volume estimates. Bias increased with progressively more weed control.

Past work Compared volumes to a cone Control: Cone underpredict volume, Trees more parabolic. Weed control: Cone overpredicts volume, Tree more neiloid.

Objective Compare upper stem measurements to those predicted by Jim Flewelling’s taper system.

Methods Sites Summit, OR: Central region of the Oregon Coast Range. Marcola, OR: Western Cascade Mountain foothills.

Summit Marcola

Methods Experimental design Completely randomized design, 8 treatments, 3 replicates, Plot area = ac, 49 seedlings planted at 9.8 ft square spacing.

Methods Treatments No herbicide, 4 ft 2 full control, 16 ft 2 full control, 36 ft 2 full control, 64 ft 2 full control, 100 ft 2 full control, 100 ft 2 woody vegetation control, 100 ft 2 herbaceous vegetation control.

1-ft 2-ft 3-ft 4-ft 5-ft 4 ft 2 64 ft 2 16 ft 2 36 ft ft 2

Methods Diameter outside bark was collected at tree base, breast height, 8 ft, and every 4 ft above 8 ft. Observed DOB’s were compared to those predicted by Flewelling system. Consistent bias – inferences unaffected Changing bias – inferences questionable

Results Average differences: Summit: DOB overpredicted Across most of stem profile Marcola: Low at upper and lower stem positions. High at middle stem positions.

Results Averages by treatment: General overprediction over stem profile Largest overpredictions: Upper stem positions Greater for more intensive control

Discussion If Flewelling predictions are regional averages: More intensive weed control Narrower upper stem profiles. Less intensive weed control Slightly narrower lower stem profiles.

Results Treatment differences by site: Marcola: Underpredicts near base (greater weed control); Underpredicts near top; Overpredict near middle of tree.

Results Treatment differences by site: Summit: Overpredicts over entire stem profile; Overpredictions increase with greater weed control.

Conclusions Biases in predicted DOB’s are small. Systematic bias among treatments. Bias the estimated effects of treatments on stem profiles. Formal statistical analyses on implications are continuing.