Abstract:
For nearly two decades, foresters in the Oregon Coast Range have been witnessing a substantial decease in Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco.) vigor and growth, caused by Swiss needle cast (Phaeocryptopus gaeumannii). Currently, no solutions are in sight but aerially-applied sulfur may alleviate Swiss needle cast and its growth impacts. In this trial, three treatments were imposed on Douglas-fir trees infected with Swiss needle cast; 1) sulfur, 2) sulfur + nutrients, and 3) no treatment. Volume growth, taper, and other responses to treatments were tested by intensive sampling of 120 trees. Tree attributes such as crown ratio, crown width, and sapwood area at crown base showed no significant differences between treatments. Means of both foliage mass and years of needle retention on a five-year-old sample branch were also not different between sulfur and control treatments. However, both of these attributes were different between the sulfur + nutrient and control treatments (p = 0.0599, 0.0205). Using a variable exponent taper model, it was determined that trees within the sulfur treatment did not have a significantly different taper from the control, whereas the sulfur + nutrient treatment did show decreased taper when compared to the control (p = <0.0246). This change in taper in the sulfur + nutrient trees however, has not translated into a significant increase in cubic foot volume. Likewise, numerical
integration of the taper equation did not reveal any volume differences between the control and sulfur + nutrient treatments (p = 0.6087). The dollar value of trees removed in the first thinning also did not differ among treatments, implying that sulfur and sulfur + nutrient treatments are not able to increase volume enough in four years to produce additional profits in the first commercial thinning.
