Root morphology and strain distribution during tree failure on mountain slopes.

To determinewhich are the most important characters governing mechanical resistance to rockfall and wind loading, static winching tests were carried out on three tree species: Silver fir (Abies alba Mill.),European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L.) in a mixed forest stand. Trees were winched to an angle of 0.25° at the stem base, both up- and downhill in order to compare how the same individual reacts when tested in two different directions. Trees were then winched to failure. Strain gauges were attached to the stem and one up- and downhill lateral root in order to determine the distribution of strain within the tree during overturning. Root morphology was then measured for all trees which uprooted during failure. No significant differences were found in the force necessary to winch trees up- and downhill in any species, either to an angle of 0.25° or to failure. Strain was significantly higher in lateral roots of Silver fir than in roots of Norway spruce and European beech when winched downhill. Downhill roots of Norway spruce were largely held in tension when trees were pulled downhill, whereas in Silver fir and European beech, they were held in compression. When trees were pulled uphill, no significant differences were found between species, and strain decreased along the lateral root of downhill roots only. European beech possessed a significantly greater number of roots than either Norway spruce or Silver fir. Norway spruce possessed a higher proportion of total root length near the soil surface, whereas European beech had the greatest proportion in the intermediate depth class and Silver fir had the highest maximal root depth. Norway spruce had a significantly lower proportion of oblique roots than the other two species, resulting in a plate-like root system which was less resistant to overturning than Silver fir or European beech. [ABSTRACT FROM AUTHOR]