Some processes involved in the wind shaping of conifer trees

This thesis reports a four year study to investigate the cause of wind-shaping in trees. The main study site was a Sitka spruce plantation at Dunslair Heights, Scotland. The fates of individual shoots were followed over the study period. Shoot breakage occured mainly during the season of active extension growth in strong winds. Windward shoots were broken much more often than leeward shoots. Surviving shoots on the windward side displayed strong curvature, the extent of curvature for more sheltered shoots being less. A parallel experiment in the greenhouse showed that deflections applied for only a few hours each day were 'fixed' if the deflections were applied during the stage of growth when the stem is hardening. In the field the extension rates of terminal shoots were also measured. The largest length increments were recorded on leeward branches, arising each year from their larger initial bud size, in comparison with the buds on the windward branches. The energy balance of shoot apices was considered in relation to the possible temperatures of the buds and apices. Preliminary measurements of these temperatures were made on Arolla pine at the tree-line in the Austrian alps; and later, more extensive measurements were made on Sitka spruce at Dunslair Heights. A maximum difference of 5°C between sheltered and exposed shoots was recorded. Such maxima occured at high solar irradiance and low wind speed. A subset of the data was used to estimate boundary layer resistances of sheltered and exposed shoots. The significance of the temperature difference as a causative agent in asymmetry of the tree is discussed. In a parallel experiment in the greenhouse an attempt was made to apply a gradient of surface temperature across the canopies of young trees by using radiative heat transfer. Despite technical difficulties in simulating an ecologically realistic regime, the warmed shoots did achieve greater extension not only in the year of treatment but in the following year as well. Finally, the relative importance of the various processes is considered, and areas for further study are identified. CHAPTER