Direct impacts of sheep upon Douglas-fir trees in two agrosilvopastoral systems |
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| Authors: | S H Sharrow D H Carlson W H Emmingham D P Lavender |
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| Institution: | (1) Depts. of Rangeland Resources and Forest Science, Oregon State University, Corvallis, OR, USA |
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| Abstract: | Livestock may provide important service and production functions in agroforestry systems. However, use of livestock in conifer/improved pasture agrosilvopastoral systems is currently limited by concerns about potential damage to trees by livestock. Effects of sheep grazing on Douglas-fir (Pseudotsuga menziesii) trees in two patterns of sheep/pasture/conifer agroforest (cluster and grid plantations) were studied from 4 years after planting (1983) until the first precommercial thinning at age 10 years (1988). Trees averaged over 1 m in height when grazing began in summer 1983. Some browsing of tree lateral branches by sheep occurred regardless of grazing season in 1983–1985. However, the 2 to 10% of current year's lateral branch growth removed by grazing sheep was too low to impact tree growth. Sheep removed the terminal leaders from only 3 to 9% of trees each year during 1983–1985. Most browsing of terminals occurred in the summer when other forages had become mature and were relatively unpalatable to sheep. Less than 13% of agroforest trees were debarked by sheep each year during 1983–1987. By the end of grazing in 1987, less than 8% of agroforest trees had sustained a level of debarking likely to impact future growth (>50% of tree circumference debarked). Grazing had no discernible effect upon tree diameter or height in any year (P > 0.05). Total tree mortality attributable to sheep grazing during 1983–1987 was only 0.9%, including three trees girdled by sheep and two debarked trees which were subsequently attacked by insects. Overall, grazing had no detrimental impact on timber stand growth or mortality.Submitted as Oregon Agricultural Experiment Station Technical Paper No. 9628. |
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| Keywords: | agroforestry browsing debarking sheep deer Pseudotsuga menziesii |
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