Graduate Thesis Or Dissertation
 

Effects of grazing management and tree planting pattern in a young Douglas-fir argoforest

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  • A silvopastoral system using a Douglas-fir (Pseudotsuga menziesii) tree crop and subclover (Trifolium subterraneum) as a nitrogen-fixing forage crop was investigated near Corvallis, Oregon, during 1983-1985. Treatments included all possible combinations of three planting patterns (no trees, trees planted 2.5 m apart in a grid, and clusters of five trees each with clusters 7.7 m apart) and two grazing/understory management systems (seeded to subclover and grazed by sheep or ungrazed and unseeded). Forage production and use by sheep was recorded each summer, fall/winter and spring grazing periods during 1983- 1985. Averaged over the three years, clover/grazed plots produced approximately twice as much forage as ungrazed/unseeded plots. The tree crop had no apparent effect on forage production. However, sheep consumed only half as much forage on grid plantations as on either cluster plantations or open pasture. Livestock management became more difficult on grid plantations over time. Grazed plots received an average of 34 kg N ha⁻¹ yr⁻¹ from livestock urine. This was associated with slightly higher N content of Douglas-fir foliage in grazed than in ungrazed plantations. Twig xylem pressure potential, measured with a pressure chamber during July-September of 1984 and 1985, indicated that trees experienced greatest summer moisture stress in August and September. During the summer, trees in grazed plantations had about 0.1 MPa and 0.05 MPa higher water potentials than trees in ungrazed plantations at pre-dawn and midday, respectively. Gravimetric soil moisture generally followed patterns in monthly precipitation and potential evaporation , but was unaffected by tree planting pattern. Averaged over the two years, the decrease in soil moisture from July to August was greater on ungrazed than on grazed plots. Sheep removed a small percentage (< 5%) of twigs during each grazing period. Breakage, debarking and removal of terminals were minor problems which occurred during spring and summer grazing periods. Terminal shoots of trees within reach of animals in 1983-84 were browsed mostly by deer in spring prior to stocking with sheep. By 1985, most trees were out of the reach of sheep and wildlife. Tree height and diameter were measured in the fall from 1982-1985. No effect of grazing management on tree height and diameter growth was apparent. Differences in height and diameter growth between cluster and grid planting patterns are likely a function of planting density and tree size at the commencement of our experiment.
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