Wound periderm is formed by Juniperus occidentalis and Calocedrus decurrens in resistance to infection by the dwarf mistletoe Arceuthobium campylopodum. The pattern of wound periderm formation suggests that chemical as well as mechanical factors are involved. The concentration of hydrolases in the radicular apex of the parasite suggests that they may facilitate the penetration process by which dwarf mistletoes infect their hosts. Emzymatic assistance may also provide a partial explanation for the specificity with which dwarf mistletoes are adapted to given hosts. These resistance responses by the the non-host species were compared with the normal infection process Pinus ponderosa.

The viscin coating of seeds from Arceuthobium tsugense was analyzed histochemically. The results indicated that the viscin is composed of two components. The first is a muculagenous pectic compound which surrounds the entire seed and accounts for a number of observed phenomenon such as stickiness, water holding capacity and the ability to undergo repeated drying and rehydration. The second component is made up of a large number of cellulose strands which are attached to the endocarp and run throughout the pectic coating. Scanning electron micrographs show that they are thought to be important in maintaining the integrity of the pectic coating. The significance of these findings is discussed with respect to establishment of the parasite.

This research examines the influence of western dwarf mistletoe (Arceuthobium campylopodum) infection on the radial growth response of mature ponderosa pines (Pinus ponderosa) and its effects on dendroclimatic reconstructions. I hypothesize that trees with mistletoe have lower annual growth rates than uninfected trees, but exhibit higher mean sensitivities and stronger relationships between growth and climate variation. I tested these hypotheses using correlation and regression analyses to compare 100-year crossdated and standardized tree-ring chronologies from 26 infected and 29 uninfected trees. I compared both chronologies to climate variation as measured by changes in total precipitation, minimum, mean, and maximum temperature, and the Palmer Drought Severity Index (PDSI). Results show that trees infected with dwarf mistletoe have higher radial growth rates, exhibit greater sensitivity, and respond more strongly to climate variation. Both infected and uninfected chronologies are significantly correlated with the respective climate variables, but exhibit different patterns. The strongest correlations are between infected trees and PDSI for all months tested; significant correlations between uninfected trees and PDSI are limited to May through December lagged from the previous year. These results suggest mistletoe-infected trees are more sensitive to climatic factors than uninfected trees and may be useful for dendroclimatic analyses.

Dark respiration rates of the aerial shoots of Arceuthobium tsugense, obtained by manometric and IRGA techniques, show production of C02to range between 155-300 µl CO2 g-1h-1 with evidence of seasonal variation. Experiments with 14C02 indicate that the aerial shoots are capable of some photosynthetic CO2 fixation, with 10-15% of the available 14C incorporated by the plant tissue in one hour.The portions of the o aerial shoots which are most active in C02 fixation are the young terminal regions. Biochemical characterization of the products of photosynthesis reveals that 80-90% of the incorporated 14C is ethanol soluble. Ten percent of the ethanol fraction is lipoidal in nature, the rest is H20 soluble. Ion exchange separation of the H20 soluble portion shows that 16-25% of the 14C activity is cationic, about 25% anionic, with the balance neutral. Aspartic acid, glutamic acid, and valine are present in the cationic fraction, with additional free amino acids indicated. IRGA experiments indicate an apparent photosynthetic CO2 fixation capacity of 80-90 percent of the ethanol fraction is lipoidal in nature, the rest is H20 soluble. Ion exchange separation of the H20 soluble portion shows that 16-25% of the 14C activity is cationic, about 25% anionic, with the balance neutral. Aspartic acid, glutamic acid, and valine are present in the cationic fraction, with additional free amino acids indicated. IRGA experiments indicate an apparent photosynthetic CO2 fixation capacity of 80-90 µl CO2 g-1 h-1, or 25-30% of the amount of C02 produced by respiration. The significance of these findings is discussed with respect to nutrition of the parasite.

The goal of this investigation was to determine the potential of birds as vectors for the movement of dwarf mistletoe seeds and the means by which such movement could occur. Birds would greatly speed the rate at which these parasites move horizontally or vertically through the forest. Five areas in Oregon, heavily infected by six species of Arceuthobium were studied. Observations of avian behavior point to 13 species whose general behavior make them potential vectors in the distribution of these parasites. More specifically, the roosting and subsequent bathing behavior of most forest birds proves a very important means of potential seed transport. Additionally, an examination of nesting materials revealed that birds bring viable dwarf mistletoe seeds to infectible portions of host trees during the construction of their nests. Nesting, however, seems the least important of the vector mechanisms discussed. I observed no evidence of birds feeding on the seeds of dwarf mistletoe or of seeds being present in their droppings. The more probably means of seed transport is one of epizoochory.