Taper and Volume Responses of Douglas-Fir to Sulfur Treatments for Control of Swiss Needle Cast in the Coast Range of Oregon
Abstract:For nearly 20 years, foresters in the Oregon Coast Range have been witnessing a substantial decease in Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco.) vigor and volume, caused by Swiss needle cast (SNC) disease. Currently, there are no solutions and disease severity is expected to worsen in coming years, but there is hope that aerially applied treatments of sulfur may be able to alleviate the effects of SNC. In this trial, volume, taper, and other attributes were examined on 120 Douglas-fir trees heavily infected with SNC for response to treatments of (1) sulfur, (2) sulfur + nutrients, and (3) control, which received no treatment. Tree attributes such as crown ratio, crown width, and sapwood area at crown base showed no statistically significant differences between treatments. Means of both foliage mass and years of needle retention also were 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 modified Kozak's (1988) variable exponent model form, taper analysis indicated that the taper of trees within the sulfur treatment was not significantly different from the taper of the control, while the sulfur + nutrient treatment showed decreased taper compared with the control (P = <0.0246). This improvement of taper in the sulfur + nutrient stand, however, has not translated into a statistically significant increase in cubic foot volume removed in the first thinning after adjusting for tree size differences between treatments. Comparing treatments by monetary value of removed trees in the first thinning also showed no significant differences, thereby implying that sulfur and sulfur + nutrient treatments are not able to increase volume enough in 4 years to produce additional profits in the first commercial thinning. It should be noted, however, that all conclusions drawn from this study are essentially from a single replication, and the scope of inference applies only to this particular type of stand in the Oregon Coast Range.
Document Type: Research Article
Publication date: July 1, 2008
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