Mixed Conifer Forests - Treatment and Results

Studies in mixcd conifer watersheds at Workman Creek demonstrated that large and significant increases in streamflow could obtained by replacing the forest with a grass cover on large or strategically located parts of a watershed or by greatly reducing the forest overstory density (Gottfried et al. 1999).

Workman Creek watershed in mixed conifer vegetation

The objectives of research in the White Mountains was to determined if results from Workman Creek experiments could be confirmed, and if they were transferable to other mixed conifer areas in Arizona. Information from the Workman Creek studies provided the basis for designing forest overstory treatments that were beneficial for timber production and wildlife habitat values and that would produce significant increases in streamflow. Two kinds of timber overstory removal were considered: thinning and patch clear cutting. These studies were designed to test multiple-use forest watershed treatments.

 
Mixed conifer watershed in White Mountains of Arizona

Results—Results from the experiments and studies conducted in the mixed conifer on the White Mountain watersheds have been reported in numerous publications including USDA Forest Service releases, journal ariticles, and special publication pulication on specific topics. A status-of-knowledge publication presented the results of water yield improvement experiments and other research conducted on the watersheds through the early 1970s (Rich and Thompson 1974).

Weir stream gage

• Annual water yield increases of 30 to 50 % (0.5 to 3.8 inches) were realized due to reduction in evapotranspiration and increase in snow accumulation in openings. These increases have remained stable for 21 years, probably because new tree roots had not fully occupied the soil mantle (Gottfried et al. 1999).
• Overland flow and sediment delivery from severely disturbed and undisturbed sites were low and inconsequential.
• Incresesed streamflow after treatment caused the natural channel adjustment process to accelerate.
• No increase in average annual water yields after a prescribed fire, which was understandable because the burn did not affect the forest overstory conditions or consume much of the forest floor.
• Concentrations of NH4-N and NO3-N, PO4, and K increased during the first 2 post-fire snowmelt periods. The changes in nutrient concentrations, while statistically signinficant, were small and of little consequence in terms of site productivity and downstream water quality.
• Mule deer, elk, and livestock benefitted from the harvested openings because of increased production of herbaceous species.

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