Graduate Thesis Or Dissertation
 

Energy exchange of transplanted Douglas-fir seedlings on two cutover sites in southwestern Oregon

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  • An energy balance analysis was performed on each of four transplanted Douglas-fir [Pseudotsuga menziesii (Mirb.) Francol seedlings growing on two cutover sites in southwestern Oregon. The two sites were a clearcut and a partial-cut (shelterwood) side by side, with a pair of seedlings used on each site. One seedling of each pair had a shadecard to the southwest of that seedling. This way, the shelterwood harvest system and the use of shadecards were compared in relation to their success in ameliorating the microclimate of the seedlings. A model of seedling radiation geometry was used along with measurements of site radiation and environmental temperatures to calculate the net radiation "loading" upon each seedling. This heat load was then partitioned into the two major heat dissipation modes, latent heat (transpiration) and sensible heat convection. The resulting values of incident solar radiation, Bowen ratios, and water use calculations show that the partial-cut was more successful than the shadecards in improving the microclimate of the transplanted seedlings and, therefore, increasing the chance of survival during periods of heat and moisture stress. For August 7, 1981, the partial-cut was found to have reduced the daily solar radiation incident to a seedling by 29%. This compares to a reduction of 22% by a shadecard alone, and 47% by a partial_cut/shadeCa1 combination. The partial-Cut was, therefore, slightly more effective, in a quantitative sense, than a shadecard in reducing the amount of solar radiation incident to a seedling. The Bowen ratio increased greatly throughout the summer for the two seedlings on the clearcut, but very little for the two seedlings on the partial-cut. By late August, the seedlings in the clearcut had a sensiblet0latent heat loss ratio of between 40- and 60-to-i while the seedlings in the partial-cut had ratios of only 10- and 15-to-i. There was a greater difference in the clearcut/Partial cut comparison than in either of the shadecard/nO card comparisons. This indicates that the residual canopy of the partial-cut had a large effect upon the Bowen ratio, while the shadecard had little effect upon the Bowen ratio on either site. The water use by the seedlings on the clearcut changed markedly over the summer with the greatest use in May and the least use in August. In contrast, the two seedlings in the partial-cut had the lowest use in May, with greater use in either July or early August. The early August period coincided with one of the worst "heat waves" on record and this may have helped magnify the differences between treatments. As with the Bowen ratio, there was a greater difference between water uses in the clearcut/Partial cut comparison than in either of the shadecard/no card comparisons. There was a significant difference in total water use between the two sites. The seedlings on the clearcut used about half the amount of water that the seedlings on the partial-cut used. There was also a great difference in distribution of water use. For example, the seedlings on the clearcut used between 47 and 54% of their total summer water use in the May to July period compared to only 31 to 39% for the seedlings in the partial-cut. In contrast, the clearcut seedlings used only 10 to 12% of total water use in late August when the partial-cut seedlings used 18 to 23%. This suggests that the seedlings in the clearcut were not as active in water use as those in the partial-cut in the late summer. This may be a result of the clearcut seedlings being water stressed from a lack of available water.
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file details Vanderwaal__James_A_1982_MS.pdf 2017-08-02 Pubblico Scaricare