The effects of wind speed and direction on radial growth of structural roots

Abstract

We used dendrochronological techniques to explore the relationships between tree wind loading and structural root annual radial growth. Study trees were shallow rooted 46-year-old Sitka spruce that had grown in an exposed upland plantation experiencing strong prevailing wind from the south west. We constructed growth ring chronologies using measurements of annual growth ring widths on stem sections and from the largest structural roots, and related them to wind climate data for the life of the trees. Separate root chronologies were constructed from samples from each quadrant around the trees relative to the prevailing wind direction, at 0.5 m, 0.75 m, 1.0 m and 1.25 m from the tree centre, i.e. 16 separate root chronologies. In this preliminary analysis, we correlated yearly growth data from the chronologies with mean hourly and maximum hourly wind speed in each month of the corresponding year. There were few significant correlations between wind speed and stem chronologies, but many positive correlations with the root chronologies. The analysis indicated an overall increase in allocation of assimilates to the structural roots in response to wind. Allocation to structural roots on the lee side was most strongly correlated with maximum wind speeds and this allocation appeared to be at the expense of roots further from the tree, that would have had less of a structural role. Development of windward roots was correlated well with maximum gusts in the corresponding years. There were, however, few significant correlations between wind maximum gusts and thickening of roots in the two sectors perpendicular to the wind direction. These results indicate that these adaptive changes in allocation within root systems resulted from flexing of structural roots in the vertical plane, rather than from twisting. We propose that the amount of secondary thickening in any part of the root system in any one year is proportional to the amount of bending stress experienced at that point. The time of year that the roots experience stress appears to be important: significant correlations between wind speed and root growth ring chronologies were predominantly from wind movement during January and February, and July to October. The January and February correlations could result from storage of assimilates in rays adjacent to the dormant cambium, in regions that experience the most stress. These stored assimilates would enhance radial growth when the cambium becomes active in the spring. There was little influence of wind on root development during spring and early summer when most assimilates are used in shoot growth and stem thickening.