Nearctica - Biomes - Eastern Deciduous Forest - Local Temperature Variations

Local Temperature Variations

Special Segments General Topics

Return to Eastern Deciduous Forest Main Page

Everyone knows that its warmer, on average, down south than up north. There are, however, smaller local variations in temperature in addition to the large geographical differences. A man-made local variation in temperature is caused by cities. Cities are always a little warmer than the surrounding countryside. Some naturally occurring local differences in temperature are discussed below. These local differences in temperature can affect the plant species in the area.
COLD POCKETS
	The weather forecast sometimes contains the statement "Colder in low lying areas". Cold air is heavier than warm air and sinks to the ground, accumulating in low lying pockets on the landscape. These pockets may be valleys or depressions among the hills in a region. The difference in average temperature is not necessarily great (a few degrees perhaps), but in marginal areas of a species range, may determine whether or not the species can survive. Example: American Beech (Fagus grandiflora) is found throughout the Eastern Dedicuous Forest, but is more common in the north. In the south it tends to concentrate in the mountains and other high areas. American Beach is favored by cooler temperatures. Near our house in southern Maryland is a "Cold Pocket" surrounded by low hills on all four sides. American Beach is common in this area even though it does not occur anywhere else within the surrounding region.
EXPOSURE TO SUNLIGHT
South Side of the Street North Side of the Street	Steep valleys oriented east and west can have distinctly different species of trees and other plants on their north and south sides. In the hottest period of the summer, the sun is located to the north and the south slope of the valley receives more sunlight than the shaded north side. Therefore the south slope is hotter than the north side. More sunlight translates into warmer temperatures and greater evaporation of soil moisture and respiration of water by the trees. Trees better adapted to hotter, drier conditions are favored on the south slopes, but not on the northern, shaded side of the valley. An example of exposure: In the winter time the sun rises and sets to the south. There is a street in suburban Maryland with houses on a small hill on the south side of the street and houses on a flat on the north side of the street. The houses on the south side of the street are shaded (the sun is behind them) by the trees, the houses, and other houses behind them. The houses on the flat, north side of the street are not shaded from the sun. The pictures on the left were taken in January during a thaw after a snow storm by standing in the middle of the street, taking a picture to the north, turning around, and taking a picture to the south. Snow persists on the south side of the street, but is gone on the north side. In fact snow sometimes stays as long as two weeks longer on the south side as on the north (depending on how high temperatures get during the thaws in January).