Overstory Removal and 

 Residue Treatments Affect 

 Soil Surface, Air, and Soil 

 Temperature: Implications 

 for Seedling Survival 



Roger D. Hungerford 

 Ronald E. Babbitt 



INTRODUCTION 



Logging results in microclimate changes that influence 

 seedling regeneration and subsequent forest development. 

 The most significant and immediate microclimate changes 

 are temperature, light, and moisture. Of the many plant 

 processes that are influenced by temperature, light, and 

 moisture (Kramer and Kozlowski 1979; Gates 1980), seed- 

 ling establishment is one of the most crucial in the early 

 life of a forest stand. 



Temperatures from 120 to 140 °F (Hare 1961) may be 

 lethal for first-year conifer seedlings, depending on species 

 (Baker 1929; Silen 1960) and exposure time (Baker 1929; 

 Levitt 1980). Condition of the protoplasm, internal water 

 relations, and tissue mass also influence mortality (Levitt 

 1980). Baker (1929) stated that the limit of safety was 

 122 °F for 2 hours. Maximum surface temperatures on 

 south-facing and west-facing slopes in western Montana 

 frequently exceed 138 °F and often 149 °F (Shearer 1967), 

 with maximums reaching 174 °F (Shearer 1981). Lotan 

 (1964) found soil surface temperatures often reached 149 

 to 167 °F on gentle slopes at high-elevation sites in south- 

 western Montana and eastern Idaho. Surface temperatures 

 on slash-burned clearcuts in Oregon exceed lethal levels on 

 69 to 97 percent of the south slopes and nearly 50 percent 

 of the north slopes (Silen 1960; Hallin 1968). The extent 

 (spatially and temporally) of temperatures exceeding lethal 

 levels on sites in the Northern Rockies is not known other 

 than for point-in-time measurements. 



High temperatures on exposed soil surfaces cause signif- 

 icant mortality of conifer seedlings (Baker 1929; Silen 

 1960; Cochran 1969). Shearer (1967) reported that high 

 temperature was the major cause of mortality of first-year 

 western larch (Larix occidentalis Nutt.) seedlings on 

 south-facing and west-facing slopes in Montana. In some 

 cases, however, seedlings of lodgepole pine (Pinus contorta 

 Dougl.) have survived temperatures of 149 to 167 °F 

 (Lotan 1964). High heat loads created by opening the 

 forest also cause drying of the surface through increased 

 evaporation. As the surface dries, maximum temperatures 

 increase (van Wijk and DeVries 1966; Cochran 1969). 



Low temperatures also cause significant mortality of 

 conifer seedlings during the growing season (Cochran and 

 Bernsten 1973; Lotan and Perry 1983). Topography, mois- 



ture conditions, and surface thermal properties contribute 

 to the occurrence of frosts (Cochran 1969; Fowler 1974). 

 Lethal low temperatures vary by species from 30 to 14 °F 

 (Cochran and Bernsten 1973; Levitt 1980). 



Temperature variation at the surface and in the air layer 

 surrounding seedlings is a function of the heat flux density 

 at the surface and the thermal properties of the seedbed 

 material. Expected temperature variations are described 

 by the amount of heat energy incident upon the surface 

 and how the heat energy is distributed at the surface. The 

 components of heat amount and distribution are described 

 by the equation (Rose 1966; Cochran 1969): 



G = Rn - H - LE 



where 



G = the heat flux density at the surface 



Rn = the net radiation flux density (measure of energy 



available at the surface) 

 H = the sensible heat flux density into the at- 

 mosphere, including heat dissipated by air 

 currents 



LE = heat used in latent heat of vaporization and used 

 in evaporation and transpiration 



How G influences temperature variation at the surface 

 and in the soil depends on the volumetric heat capacity (C) 

 and thermal conduct ivity (K) of the surface and lower soil 

 horizons. A factor (\[KC) called the thermal contact coeffi- 

 cient (Cochran 1969) integrates these two factors. Temper- 

 ature variations are inversely proportional to \[KC. The 

 nature of the material, moisture content, and amount of 

 air space (thus, compaction, texture, and soil type) in- 

 fluence these thermal properties. Temperatures are more 

 extreme on organic surfaces than on mineral soil (Cochran 

 1969; Fowler 1974). The thermal properties of the seedbed 

 are critically important (Cochran 1969; Fowler 1974). The 

 equation predicts that those practices providing shade 

 (decreasing Rn), allowing for increasing air circulation (in- 

 creasing H), or decreasing surface evaporation will lower 

 the temperature variation by reducing the amount of heat 

 at the surface or by dissipating the heat before it raises 

 the temperature to lethal levels. 



Some degree of shading is helpful in reducing seedling 

 mortality due to high and low temperatures (Shearer 1967; 

 Ryker and Potter 1970; Strothman 1972; Cochran 1975). 



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