When forest stands are thinned, the retained trees are exposed to increased light and greater mechanical strain from the wind. The consequent greater availability of photosynthate and localised mechanical strain in stems and roots are both known to increase cambial growth in conifers, but their relative importance has not previously been quantified. Light availability and wind movement were manipulated in a 10-year-old stand of Sitka spruce trees on an exposed upland site. Treatments were “Control”—no change in spacing or wind loading; “Thinned”—light availability and wind loading increased by removing neighbouring trees; “Thinned and guyed”—light increased and wind loading reduced by removing neighbouring trees and guying stems with wires. Twelve trees per treatment were maintained and monitored for four years before harvesting and removal of cross-sectional stem samples from four heights for measurement of radial growth response. Root systems were excavated from each treatment for observations of associated root growth responses. The “Thinned” treatment and “Thinned and guyed” treatment showed no significant stem growth response in the first year after treatment, but very large increases in the second and subsequent years. There were much larger growth responses in the “Thinned” treatment than in the “Thinned and guyed” treatment, especially in the lower stem. Similar growth responses were observed in the structural roots, close to the stem base. These increases in stem and root growth in response to wind movement corresponded with a reduction of branch growth. Such changes in allocation have implications for the hydraulic and biomechanical requirements of trees, and should be incorporated into tree growth and stability models.
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