Annual cycles of soil and water temperatures at Hubbard Brook

Soil temperatures in the Hubbard Brook Experimental Forest in central New Hampshire decline very slowly from December to March and are restricted from falling below OºC by insulation of snow and organic matter. Soil in the hardwood forest on a moderate south slope warms rapidly in the spring leafless period after snowmelt and reaches a maximum temperature in early August that averages 17.5ºC near the surface and 12.5ºC at 91-cm depth. The soil cools to nearly isothermal conditions at 11ºC in October. The mean annual cycle of soil temperature near the surface corresponds closely to that of air temperature under the canopy, except in winter. Stream temperatures are about the same as soil temperature at a depth of 31 cm throughout the year. Windthrow mounds are cooler than uniform slopes in winter and warmer in summer. Simultaneous soil temperatures at different locations in the forest generally differ by no more than 2ºC at any given depth.