Graduate Thesis Or Dissertation

 

Influence of cattle grazing and forage seeding on establishment of conifers in southwest Oregon Public Deposited

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  • The ability of controlled livestock grazing, in combination with seeding of palatable forages, to reduce understory competition and enhance conifer establishment, was evaluated during 1985 and 1986, on two adjacent sites in southwestern Oregon. In 1984, Site 1 was clearcut and broadcast burned to remove slash, and Site 2 was machine scarified, ripped to ameliorate compacted soil layers, piled and burned. Both sites were planted in the spring of 1985 with Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and ponderosa pine (Pinus ponderosa Dougl.) and the following treatments applied during both years: silviculture-control, native vegetation-grazed, seeded vegetation-ungrazed, and seeded-grazed. Fifteen permanently marked 3.6m circular plots were located within each treatment replication, giving a total of 1950 trees from which growth and survival of conifers, and browsing and trampling by livestock and wildlife were monitored. Douglas-fir seedlings were also assessed for predawn and midday xylem potentials at each sampling date. Gravimetric soil moisture content and understory vegetation cover were also assessed on Site 1 and Site 2. Interpretation of conifer survival and growth response was complicated by severe frost damage and heavy browsing by elk during the first year. Analysis of variance determined Douglas-fir survival and growth was not significantly different between treatments both years. However, a binomial response model describing predicted mortality of Douglas-fir indicated differences in survival were present. Mortality as predicted by the model was a result of the factors of frost, wildlife browsing, and livestock browsing and trampling, rather than treatment applications. Frost damage had the greatest impact on Douglas-fir mortality, both because it affected a large percentage of the trees on the site (43.6 percent overall), and because it dramatically increased seedling mortality. Elk, though impacting an equally large percentage of trees (23.1-57.7 percent), appeared to selectively browse the healthiest trees, or those not affected by frost damage. This resulted in much lower predicted mortalities. Although livestock browsing and trampling increased mortality of Douglas-fir seedlings, livestock activities were much less prevalent than wildlife browsing or frost. Each year, the controlled grazing program maintained livestock browsing at 2.6 percent, and trampling at 6.0 percent. In 1985, early season, intense grazing by cattle did not result in treatment differences for Douglas-fir xylem potential. In 1986, the general trend was for seedlings growing in the seeded-grazed and silviculture-control treatments to have similar and less negative xylem potentials than trees growing in the seeded-ungrazed and native-grazed treatments. In 1986, gravimetric soil moisture content differed between treatments only during the June sampling date, when soil moisture content was significantly higher in the silviculture-control treatment. Total herbaceous and total shrub cover did not differ between treatments either year. The generally low xylem potential levels for seedlings in the seeded-ungrazed treatment, suggests seeding with similar forages and with the exclusion of livestock grazing in this area may result in increased water stress for Douglas-fir seedlings. Grazing improved the water relations of seedlings in comparison to ungrazed plots.
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