The Regions Climate

Most of the southern forest is within the zone of Humid Subtropical climate, characterized by high temperature and abundant precipitation. The mountains are exceptions to this rule: At the colder upper reaches of the Southern Appalachians in North Carolina, the Fraser fir forest is similar to the balsam fir forests of coastal Maine.

Growing seasons are 180 days or longer in all but a few mountain sections, increasing to 320 days in southern Florida. High temperatures over these long growing seasons provide abundant energy for tree growth. For instance, loblolly pine in South Georgia may have as many as six flushes of height growth in a season, in contrast to three in the cooler part of the species' range.

The South lies mostly sunward of the isotherm that marks a 50°F mean annual temperature. The region's boundary approximates the 77°F summer isotherm—from northeastern North Carolina to northwestern Tennessee and, skirting the Ozark Mountains, to central Oklahoma. (This isotherm generally delimits the "land of cotton" of an earlier generation.)

For an area as vast as the South, and considering the influence of the large continental air mass that is confronted by ocean currents on two sides, precipitation is relatively constant throughout

Figure 1.2 Average length of the growing season, in days. (USDA, 1941)2

Figure 1.2 Average length of the growing season, in days. (USDA, 1941)2

the region. It averages 40 to 60 inches annually except in the higher, more southerly mountains where an excess of 80 inches may fall. At the upper reaches of the Appalachians along the North Carolina-Georgia line, precipitation often exceeds 120 inches in a 12-month period. That volume of rain and snow approaches the amount of precipitation received by the coniferous rain forest of the Olympic Peninsula of the Pacific Northwest and the tropical rain forests of Central America.

Not only is precipitation evenly spread throughout the region, it is also rather evenly distributed throughout the year. Some exceptions occur. More rain usually falls in winter than in summer in the western edge of the southern woodlands, as in the longleaf pine and loblolly pine forests of the Big Thicket in southeastern Texas. Even there, the difference between growing-season and dormant-season totals is not appreciable. Because southern Florida receives 60% of its rain in the summer, planting pines during the warmer months may be appropriate.

Annual and seasonal rainfall and snowfall tallies, together accounting for precipitation, suggest well-watered woods. However, the South is subject to the not infrequent "dry spell" which, during the growing season, can be disastrous to the forest. Seedlings die, fires rage, and beetles attack during these droughts that can last for more than a month. The 1995-1996 winter and the 1998 summer were notable examples.

The intensity of rain affects its availability for tree growth. Thunderstorms of high intensity and short duration may be experienced 50 or more days a year in all but the eastern and extreme southwestern parts of the region. Since these storms occur while the forest is in full foliage, the canopy intercepts some of the rainfall, wetting the leaves, stems, and trunks. Considerable moisture is so utilized before water begins to reach the forest floor by throughfall, by dripping from foliage, or by flowing down the trunks of the tree. Water held in the canopy is subject to rapid evaporation by free-flowing air. This interception may amount to 10 to 25% of the rainfall measured in adjacent open areas. It intensifies the effects of evaporation loss and the seasonal distribution of rainfall to make the already xeric southwestern part of the region drier than other climatic indicators might suggest.

Figure 1.3 Average temperature effectiveness provides a measure for quantifying climate that suggests the rate of plant growth. It is computed by adding together, for each day of the growing season, the difference between its mean temperature and 40°F. The line of 800 units approximately bisects the southern forest. In contrast, New England measures 300 units and the Rocky Mountains, 600. The numbers represent hundreds of units. A day with an average temperature of 41°F. produces 1 unit a day with an average temperature of 42°F. produces 2 units, and so on. (after Livingston and Shreve, 1921)3

Figure 1.3 Average temperature effectiveness provides a measure for quantifying climate that suggests the rate of plant growth. It is computed by adding together, for each day of the growing season, the difference between its mean temperature and 40°F. The line of 800 units approximately bisects the southern forest. In contrast, New England measures 300 units and the Rocky Mountains, 600. The numbers represent hundreds of units. A day with an average temperature of 41°F. produces 1 unit a day with an average temperature of 42°F. produces 2 units, and so on. (after Livingston and Shreve, 1921)3

0 0

Post a comment