Variation of wood strength in tree roots

In order to determine if the distribution of lateral root strength is related to the shape of the system and the forces withstood, wood samples were taken from roots of various mature tree species and the strength tested. Root strength decreased along the root at different rates, depending on the type of root system present, Slightly tapered lateral roots in plate root systems were relatively stronger further away from the stem than the highly tapered laterals in heart and tap root systems. Wood strength in Norway spruce (plate system) was found to increase along the lateral roots before decreasing again. The increase in strength may coincide with the point of maximum bending of the root as the tree sways backwards and forwards in the wind. Strength was also found to increase on the underside of lateral roots in the plate systems of poplar, The undersides of these roots will experience high compressive stresses due to the weight of the tree pushing the root on to the hard bearing surface of the soil. External loading forces in plate root systems will be transmitted into the soil further away from the stem due to the lack of branches, therefore a high strength along the root will help resist mechanical stress, The high rate of branching near the stem, or large, rigid, main tap root, found in heart and tap root systems, respectively, allows a faster dissipation of forces nearer the stem, therefore a high investment in strength further along the root is not necessary.