Graduate Thesis Or Dissertation
 

Forage production and utilization in relation to deer browsing of Douglas-fir seedlings in the Tillamook Burn, Oregon

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  • The main objective of this investigation was to evaluate relationships between the production and utilization of forage and deer browsing of hand-planted Douglas-fir seedlings. A secondary purpose was to study some effects of selected physical and biological site factors on the survival and growth of fir seedlings. Field work was undertaken between June, 1961, and February, 1964, in a 340-acre deer-tight enclosure used by known numbers of deer since 1959. The study area located on steep topography in the Tillamook Burn of northwest Oregon was subjected to three wildfires between 1933 and 1945 destroying all coniferous forest growth. Since 1945, a large portion of the enclosure area has been highly disturbed by erosion and salvage logging operations. Present vegetation consists mainly of seral species which have been classified into six plant communities. Vegetation studies showed that overstory plant cover averaged about l2 percent and understory cover 77 percent in the summer of 1963. Yields of summer forage averaged 2,600 pounds per acre, and forage available for winter use averaged 640 pounds per acre on September 1. Subsequent weathering losses and deer utilization greatly reduced the latter value by mid-winter. Growth of winter-active herbaceous forage was approximately 90 pounds per acre in March, and by May, normal forage supplies were again available. From winter browsing trials, utilization studies, and examinations of deer stomachs it was concluded that: Blackberry leaves were the most highly preferred forage during the winter season. Salal ranked high but was limited in distribution. Huckleberry and cascara were preferred woody plants whereas alder, hazel, and vine maple were utilized only when herbaceous forage was unavailable. Planted Douglas-fir occupied a prominent place on the food preference list, ranking higher than most common woody plants. Winter-active herbaceous forage was highly important in winter and early spring diets. Leaves of herbaceous and woody plants supplied nearly all the summer forage and were important in all seasons when available. Browsing of Douglas-fir seedlings began at lower elevations when the enclosure was first covered with snow. Later it occurred at all levels but was concentrated on lower areas. Chemical analyses of forage samples collected through the year indicated that blackberry was the most nutritious forage plant commonly eaten by deer. Leaf material from other plants was highly nutritious but twigs of the same species generally provided poor quality forage. Winter-active grasses and herbs were highly nutritious but an excessive moisture content reduced their unit-intake value. Douglas-fir was higher in nutrient content than most woody plants and rated as good feed when herbaceous forage was unavailable. Forage preference appeared to be positively related to nutrient content in a majority of the plants which were compared. Notable exceptions were alder in the winter season and huckleberry during most of the year. Artificial rumen analyses revealed that all forages were lower in digestibility than alfalfa pellets. In general, digestibility seemed to be related to forage preference. Green leafy feeds were more highly digested than woody twigs. The incidence of Douglas-fir browsing was found to be significantly related to several physical and biological site variables. It was concluded that Douglas-fir browsing will continue unless or until seedlings are completely protected from deer by physical or physiological (chemical) isolation because fir seedlings are an important source of forage during certain periods of the winter season.
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