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Reducing windthrow losses in farm forestry

The annual loss to the forest industry due to windthrow is estimated at approximately €1.3 million. Windthrow is caused by the failure of poorly drained soil beneath the root plate to anchor trees. This results in the stem and rootplate of a tree overturning.
In addition to the economic losses, windthrow can also result in increased soil erosion, run-off and adverse visual impact. Factors affecting the occurrence of windthrow include tree type, soil type, soil drainage, soil strength, elevation, aspect and location.

Implications

  • Coniferous crops have improved anchorage and stability on gley soils which are mole drained.
  • Mole drainage results in deeper and more radially symmetric tree rooting systems than in double mould board ploughed sites.
  • A spacing of 1.0 m between the mole drains is necessary for adequate drainage for forestry applications.

Background

  • This study was established to ascertain the mechanism by which windthrow occurs and the effects of different drainage methods on crop anchorage and stability.
  • The trials were conducted in an existing plantation of Sitka spruce, which was established on a surface water gley. The high water table is due to low soil permeability and this results in shallow rooting of trees and predisposition to windthrow.
  • Three treatments were assessed:
    • Mole drainage – This involved creating unlined tunnels (with a diameter of 0.075 m) at a depth of 0.45 m in the soil. These tunnels were spaced at 2 m intervals in the soil. The installation resulted in the cracking and breaking of the soil above the mole thereby increasing soil permeability and allowing excess water to drain away rapidly.
    • Double mouldboard ploughing (DMB) – A furrow, 0.3 m in depth, is formed by the plough and the excavated soil is upturned and laid as ribbons on either side of the furrow. These ribbons are spaced at 2.0 m apart.
    • Undrained control plot - The effect on the water table of mole drains at 1.025 m and 2.0 m apart were compared.

Findings

Destructive tree pulling and dynamic forced rocking on semi-mature trees

  • Tree pulling tests and dynamic forced rocking were used to quantify the degree and nature of root anchorage. The trees in the mole drained plots had superior anchorage to the other treatments when tested by pulling and by forced rocking to and fro.
  • The dynamic forced rocking test resulted in a significant increase in pore water pressure in the soil beneath the root plate of a test tree in the control plot. This resulted in liquefied soil rising to the surface around the root plate. Subsequently the stem movement of the tree became erratic. These events were considered to result in tree failure.
  • A bending moment of 4 kNm* started the rise in pore water pressure, which ultimately led to liquefaction of the soil beneath the root plate in the control. Failure was initiated in the DMB tree at a bending moment of 9.7 kNm. No such failure was initiated in the tree in the mole drained plot despite application of bending moments of up to 15 kNm. These low values in comparison with those of simple pulling tests where loads are applied slowly indicate that the application of rapid repeated loading by wind weakens the soil.

*10 kN = 1 tonne force

Shear testing of soils
The deformation of a layer of soil when the top boundary is displaced relative to the bottom layer under plain strain conditions is defined as simple shear. Such conditions occur beneath the stem of a tree subject to wind loading. The study showed that repeated loading results in the generation of substantial excess pore water pressures. This causes a reduction in the strength of the soil.

Computer aided modelling
Mole drainage systems at spacings of 1.025 m and 2 m were modelled based on a field experiment at Ballinamore, Co Leitrim. Three successively higher design rainfalls were applied and the depth to the water table was measured at a point mid-way between mole drains. The results are summarised in table 1 and show that the closer spacing of mole drains (1.025 m spacing) maintained the water table at a greater depth.

Table 1: Summary of results of steady state analyses

Steady state rainfall
(mm/day)
Depth to water table (m)
1.025 m spacing2 m spacing
5 0.400 0.225
12 0.338 0.125
30 0.225 0.000

Five days of rain as recorded in the field experiment were applied to the two models. The closer spacing of drains (1.025 m spacing) compared favourably to the wider 2 m spacing. This indicates that for forestry applications a 1 m spacing of mole drains is preferable to 2 m spacing on the site types tested.

Based on End of Project Report

Reducing windthrow losses in farm forestry.  ARMIS No. 4315.
Authors:  John Mulqueen, Teagasc, c/o Civil Engineering Dept. NUIG, J. McHale and M. Rodgers Civil Engineering Dept., NUIG 091 524411, 1999

Compiled by:
Nuala Ni Fhlatharta, Teagasc, Athenry, Co. Galway; Tel. 091 845200