Stability of Sitka spruce on mole-drained and ploughed surface water gley soil

摘要

A study compared the effects of two site preparation methods, double mouldboard ploughing and mole-drainage on the stability of Sitka spruce trees subjected to repeated and monotonic forced loads. The trees were planted on a low-permeability surface water gley soil in two experimental plots in the north-west of Ireland. The soil is highly productive with average Sitka spruce productivity in excess of 18 m~3 ha~(-1) yr~(-1). However, the major constraint on production is windthrow, which is dependenton a number of factors, including soil preparation. Repeated loading tests were carried out on 6-metre tall truncated tree stems in both cultivation treatments using a specifically constructed tree rocker. During a repeated load test in the double mouldboard plot, pore water pressure increased under the root plate and fractured the soil causing a washout of fines. Once this fracturing occurred, the sway of the tree stem and, as a result, the overturning moment increased substantially, making the tree very unstable. When a repeated load test was carried out in the mole-drained plot, there was little build-up of pore water pressure even though the loading was greater than that applied to the tree stem in the mouldboard plot. Monotonic tests, which consisted of pulling six trees over in each plot, were used to calculate the trees' maximum overturning moments. In the double mouldboard ploughed plot, the repeated loading test clearly showed that soil failure can be initiated at much lower overturning moments than the maximum moment applied during monotonic pulling tests. Both the repeated and monotonic loading tests indicated that the trees planted in the mole-drained plot were substantially more stable than those planted in the double mouldboard plot.