Effects of thinning on water stress and growth in Douglas-fir

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Abstract
The effects of a systematic thinning (alternate row) on water stress and growth in a 19-year-old Douglas-fir plantation (Pseudotsuga menziesii var. menziesii Franco) was studied durign 5 years in northeastern France. Thinning has an important effect on water balance. The increase of the soil water reserve led to a lower duration and level of water stress in the thinned plot, especially during the frist 3 years after thinning. This study revealed a rapid evolution of the water balance, since the evapotranspiration ratio of the two plots (control and thinned) had increased from 0.83 during the 1st year to 1.03 during the 5th year after thinning. Together with microclimate influences, thinning increased tree growth substantially (more than 100%); this growth increase resulted from a higher growth rate and longer growth periods (from 26 to 49 days).
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