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pierre laszlo

 
Ponderosa Pine

The Pacific ponderosa pine (Pinus ponderosa) ranges in the Western United States from latitude 52° N. to latitude 33° N. It is found from sea level to 10,000 ft. From north to south the species grows at progressively higher altitudes. In California, ponderosa pines are present at elevations from 500 to 3,500 ft in the north, and from 5,300 to 7,300 ft in the south. This tree draws on vigorous taproots for survival. Mature ponderosa pines put down a taproot to depths of more than 6 ft in porous soils. A thick bark is an asset for protection from fire damage. These trees have a life expectancy measured in centuries, in the range of 300 to 600 years. Ponderosa pines deserve their name, "Ponderosa," the Latin word for "big, hefty" refers to the impressive size and stature of the trees. Heights greater than 200 ft are recorded. The most salient feature may indeed well be the height. It is a tall plant. Towering over forests is risky for a tree. For one thing, it attracts lightning. Thus, survival of this species depends upon an ample supply of seeds for reproduction. Another issue is how the sap manages to ascend to such heights, a controversial problem which has puzzled botanists for centuries and has not yet been definitively settled. Transpiration pull theory holds that continued transpiration in the leaf/needle creates a tension in the xylem cells in the upper parts of the tree. Water is pulled up by this suction force. About 10 atm of tension is required to lift the water up to 250 ft. The measured tension in xylem sap is in the range of 25-300 atm, i.e., more than adequate. Other mechanisms may also contribute to raising water against gravity.