Management and Disturbance Effects on Water Yeild

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Table 4. The effect of the 2002 Hayman Fire on water yields from the burned area for the period 2020 - 2060. The year 1997 is the reference pre-fire year. Although included in the simulation total, bristlecone pine, cottonwood, limber pine, and lodgepole pine are not listed here as they contributed only about 1 percent to the total water yield from the burned area. Area Water yield (area inches) Specie (acres) 1997 2020 2040 2060 Aspen 4,662 4.0 7.0 6.2 5.3 Douglas fir 41,499 1.0 4.5 3.7 2.6 Ponderosa pine 54,453 0.6 3.8 3.1 2.5 Spruce-fir 6,073 3.6 8.2 7.5 6.8 Tota I 108,112 1.1 4.5 3.8 3.0 .j'?.???" Currently, a major infestation of insects, bark beetles and defoliators, is affecting forest stands throughout Colorado (Wulder et al. 2006), in nearby states, and in southwestern Canada. Although tree mortality resulting from bark beetle and defoliator attack are part of the ecosystem, the recent series of warm, dry years has caused the insect populations to explode. In addition, the dry years, warm temperatures, and greater plant density have caused greater stress to trees and trees are more susceptible to attack, particularly from the mountain pine bark beetle. Over time, tree mortality after large-scale infestations will affect watershed processes such as canopy interception, soil moisture storage, transpiration, and ground water recharge in a manner similar to timber harvest. Love (1955) reported on the impact of a beetle infestation in spruce stands on the White and Yampa Rivers in Colorado. About 80 percent of the trees over hundreds of square miles were killed. The forested area consisted primarily of Engelmann spruce and Subalpine fir. Water yield, measured at U.S. Geological Survey streamflow gauges, increased by as much as 22 percent (Love 1955) with one-half of the initial increase still detectable 25 years later (Bethlahmy 1974). In the past, spruce beetle outbreaks were known to occur at 117-year intervals in northwestern Colorado subalpine forests (Veblen et al. 1994). Although a variety of tree species are affected by the mountain pine beetle (ponderosa pine, lodgepole pine, and limber pine) or by the spruce beetle (Engelmann spruce), the lodgepole pine type seems to be impacted the most severely and extensively during this current epidemic. Of the 456,306 acres of lodgepole pine on NFS land in the South Platte River Basin (Table 1), almost 95 percent are classified as pole or sawtimber stands (Table 5) and are subject to mountain pine beetle attack. Stands classed as pole size have an average DBH between 4.9 and 9.0 inches, and the sawtimber size class represents stands with an average DBH greater than 9.0 inches. Historically, the mountain pine beetle has tended to attack the larger more decadent sawtimber trees. In the current outbreak, the warmer weather and drought conditions have allowed the insect population to thrive while trees 22 2/2/2007