Dew as an Ecological Factor: II. The Effect of Artificial Dew on the Survival of Pinus Ponderosa and Associated Species

Abstract

Pinus ponderosa is widely distributed throughout the mountainous area of western North America. To the east it extends into the Dakotas and Nebraska; to the north it follows the Rocky Mountains on into southern British Columbia; and to the south it runs along the mountain tops into Baja California and Mexico proper. In California it is found throughout the mountains of the State. At middle and lower elevation in the Sierra and inner north coast ranges it has a continuous distribution, while in the middle and south coast ranges it is found only in a few scattered locations. Throughout its range the associated species change. For example, in the coastal mountains near Santa Cruz, California, where precipitation may exceed 100 inches per year, it grows in a few scattered stands in association with Sequoia sempervirens. On the other hand, in the northeastern part of the State, near Susanville, where precipitation rarely exceed 20 inches per year, it grows in association with Juniperus occidentalis, Artemisia tridentata, and other xeric species. However, over its entire range P. ponderosa is exposed to long periods of drought. Even in the coastal mountains where it grows in association with Sequoia sempervirens it is largely restricted to sandy soils which retain only a small percent of the heavy rainfall received during the winter. In the arid northeastern part of the state, stands of P. ponderosa often become locked or stagnated at an early age. Clumps of trees 6-10 feet high and 2-4 inches in diameter may average 50-75 years of age. This is in sharp contrast to areas immediately to the west where precipitation is 30-40 inches per year. With increased soil moisture individual trees assert their dominance at an early age and young stands do not stagnate; fifty year old trees are 50-75 feet high and 15-30 inches in diameter. Again in the arid area, mature stands of P. ponderosa are relatively open with as much as 60 percent of the full light intensity reaching the forest floor. Even so, the young trees in the scattered understory are badly suppressed. However, once the mature trees are removed these suppressed trees resume growth. Apparently, this