Direct impacts of sheep upon Douglas-fir trees in two agrosilvopastoral systems

✍ Scribed by S. H. Sharrow; D. H. Carlson; W. H. Emmingham; D. P. Lavender

• Publisher: Springer
• Year: 1992
• Tongue: English
• Weight: 561 KB
• Volume: 19
• Category: Article
• ISSN: 0167-4366
• DOI: 10.1007/bf00118780

✦ Synopsis

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.