Microclimate, especially temperature extremes, was studied on various plots of pine and spruce sites in Finland. Thermoelement measurements were made on clear, calm summer days. Comparisons showed that the climate of the exposed sites varied greatly from that of the sites with protective ground vegetation or canopy. Very large temperature differences between the sites existed at ground-level and moderately large differences at the 5 and 20ensp;cm. levels. At exposed sites a temperature range of 57 deg;C. in 24ensp;hr. was recorded in sandy surface soil, and still larger ranges seemed probable. Extreme diurnal fluctuation in temperature is evidently not closely correlated with general climate; it is favored by dryness of the soil and presence of ground cover, such as humus and fine litter, with low thermal conductivity. Maximum temperatures of 50deg; to 70 deg;C, are considered of normal occurrence in thin surface soil at exposed sites in large areas of northern coniferous forests. At the same time, maximum temperatures may remain from 9deg; to 13 deg;C. under the protection of canopy and surface cover, and night minima may be below 0 deg;C. in thin surface soil. A negative correlation was found between evaporation rate and drought injuries of plants on dry sites. Root competition was a more important factor to plants than evaporation conditions, at least as shown by the measurements from large vessels.
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