- This study examined the relationships between the frequency of occurrence and severity of Douglas-fir dwarf mistletoe (Arceuthobium douglasii Engelmann), environmental and stand conditions, and plant communities in the Southern Oregon Cascade Mountain Province. Data for the study was collected from the same ecology plots that were previously used to define the plant associations in the province. A pilot study of the variability in the
frequency of occurrence and severity of Douglas-fir dwarf mistletoe (DFDM) among plant associations was used to determine the sample size. The plant associations selected for the final sample were grouped into three climax series; Douglas-fir (Pseudotsuga rnenziesii (Mlrb.) Franco), white fir (A b/es concolor (Gord. and Glend) Lindl. cx Hildebr.) and western hemlock (Tsuga heterophylla (Raf.) Sarg.). Environmental and stand conditions were sampled using the variables elevation, mean annual temperature, mean annual precipitation,
dry season precipitation, site index, slope, total basal area, Douglas-fir basal area, percent basal area in Douglas-fir, number of tree canopy layers, age of each layer, diameter of Douglas-fir at breast height, aspect, topographic position, topographic shape, and soil parent material. DFDM was present in plots at significantly higher elevations, with lower mean annual temperatures and lower mean annual precipitation. The disease was never found in plots below 1066 meters elevation or at mean annual temperatures above 8°C. It occurred significantly more often in the white fir series than in the others. Within this series it was found more often in the coldest and driest plant associations. The relative frequency of DFDM among the series appeared to be related to the differences in their elevation, mean annual temperature and mean annual precipitation. The geographic distribution of the plots where DFDM occurred suggested that past timber harvesting, fire history and fire behavior may have influenced the present distribution of the disease in the Southern Oregon Cascades. The severity of DFDM was significantly associated with two stand variables. Severity increased as total basal area decreased and as the age of the oldest layer increased. Multiple regression analysis indicated that the disease was most severe in old, open stands on high, dry
sites. This study suggested that plant associations and climax series were useful indicators of the relative frequency of occurrence of DFDM in the Southern Oregon Cascades, but not of its severity. However, if the current distribution of DFDM was influenced by past harvesting and fire regimes, changes in these factors may change the diseases' distribution in the future.
A return to widespread partial cutting would be of particular concern because partial cutting often creates the stand conditions that were associated with severe DFDM in this study.