Roots are the key to stabilising soil through bioengineering. Poplar and willow poles take some time (up to five years) to grow sufficient roots to make a contribution to hill slope stabilisation. While large buttressing roots are clearly visible growing off the base of the tree trunk, smaller roots that extend away from the trunk into the surrounding area are contributing the soil binding function.

Roots with diameters <2 mm comprise ~90% of tree root length and it is these roots that are in intimate contact wilth soil particles. The bulk of the roots are found in the top 40 cm of soil. Other roots penetrate deep into the soil and will grow in to the underlying rock. Where this happens the tree is better able to resist and reduce movement of saturated soil downslope.

Poplar and willow trees planted on erodible slopes come into their own in the event of prolonged rain or a heavy rainstorm event when the soil becomes saturated. The trees can prevent slippage, reduce the degree of slippage and hold up slipped material preventing it from falling all the way downslope.

Hence our interest in where roots grow, how they respond to soil type, soil density, gravity and wind direction, and how they interact with roots of neighbouring trees to stabilise soil.

Root research is laborious since excavation is difficult, measuring root parameters takes a lot of time, and in most situations there is no quick way of obtaining reliable data.

Current research projects are investigating root development at the following stages

♦ in new clones at the cutting stage

♦ within the first three years after field planting

♦ following pollarding in the field

♦ fine root presence over the annual cycle.

For further information see the Reports section.