RESPONSES OF YOUNG TREES TO WIND AND SHADING - EFFECTS ON ROOT ARCHITECTURE

Two wind tunnels were designed to detect influences of wind on the development of the root systems of young trees: Picea sitchensis and Larix decidua were grown in the first and L. decidua only in the second. In the second experiment, the tap root of each L. decidua seedling was removed in order to mimic the formation of a shallow root-plate. Responses of shoot growth to wind stress were small, although uneven irradiance levels resulted in asymmetric growth of both shoots and roots; the distribution of root biomass around the tree was related spatially to that of shoot growth. In both experiments there was an increase in the number of large windward and leeward roots in both species, in the first experiment, the sum of the cross-sectional area (Sigma CSA) of lateral root bases was greater on the windward side of the tree in both species, whereas in the second experiment, it was greatest on the leeward side. Windward woody lateral roots of P. sitchensis had a higher incidence of branching than leeward roots. Woody tips of windward roots were significantly longer with a larger diameter than on leeward roots. It appears that wind action stimulates diameter growth and alters morphogenesis of those roots most important for anchorage. As a tree sways in the wind, leeward and especially windward lateral roots are placed under the most stress. Larger roots, or a greater branching density in these areas will help counteract wind stresses on the tree.