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A study of soil temperature and seedling survival in a forest clearcut Stathers, Robert John


This thesis reports the results of a study of soil temperature and seedling survival on a steep south facing, high elevation forest clearcut on Vancouver Island, B.C., having a history of regeneration failures. During the first year of the study (1981) there were harsh weather conditions during the summer, while during the second year (1982) summer weather conditions were much milder. Lethally high surface soil temperatures were found to be the major cause of seedling mortality on this site. Of 2160 Douglas-fir, western hemlock and Pacific silver fir seedlings planted in April 1981, 87, 33 and 17% respectively had survived by the end of the second growing season. Shade cards significantly increased survival for each of these species. Of an additional 960 Douglas-fir, western hemlock, and Pacific silver fir seedlings planted in April 1982, 92, 74, and 88% respectively had survived by October 1982. Shade cards, trickle irrigation, and shade cards and trickle irrigation combined only significantly increased survival in western hemlock in the seedlings planted in 1982 because of the high overall survival rate resulting from the mild summer weather conditions. A physically based soil temperature model was developed to (i) quantify the effects of the factors which influence the soil thermal regime and (ii) to model the time course of soil profile temperatures from easily measured or estimated meteorological data and site specific characteristics. The model, based upon a finite difference solution of the heat flow equation, energy and radiation balance, and aerodynamic theory, calculates surface and subsurface temperatures in soils having thermal properties that vary with depth. A slightly modified version of the atmospheric stability correction method of Paulson (1970) was required to accurately predict surface temperatures under unstable atmospheric conditions. The model was tested with data presented in two other energy balance studies reported in the literature and found to estimate soil temperatures very accurately. In addition, good agreement was found between modelled and measured soil profile temperatures for 6 and 16 day periods in the forest clearcut mentioned above. Energy balance theory was used to show that maximum surface soil temperatures in a forest clearcut are not likely to exceed 85°C on a clear hot day. Wind speed, aerodynamic roughness of the surface, and soil thermal properties were also shown to strongly influence soil temperatures.

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