Ponderosa Pine Forests: Management Implications

Ponderosa pine forests cover more than 4 million acres in Arizona. Pine has the highest commercial value of any forest species in the Southwest, and basins within the pine type yield an average of 0.25 ac-ft of water per year. Pine forests supply forage and browse for game and livestock and provide a recreational haven from the hot and arid climate found in major portion of the state.

Treatments to increase water yield,
upper is strip cut and lower is a 100% clearing

Water Yield—The potential for increasing water yield in ponderosa pine is less than from other commercial forest types because these forests inherently occur on drier sites. Water yield from pine watersheds in the Southwest is derived mostly from snowmelt and is greatly influenced by the amount and distribution of precipitation, basin physiography, and soil type. Variation in actual first-year water response of 0.8 to 6.7 inches shows the importance of precipitation timing and amount of water yield and its apparent dominance over the percent of overstory removal in forests on shallow soils.

Two types of harvesting methods that have proved effective in increasing water yield from ponderosa pine forests in Arizona are overstory removal and strip-cutting. Water yield increases can be realized from forested watersheds with timber basal area in excess of 100 square feet per acre. However, under management practices that emphasize timber production, density reduction to 70 or 80 square feet per acre are generally not large enough to significantly reduce evapotranspiration demand for an extended period of time, particularly in pine forest on shallow, volcanic-derived soil types.

Original evapotranspiration levels can be regained by growth of shrubs and herbaceous plants or by root invasion from the remaining pine trees. Spacing and density of trees under managed stands generally allow tree roots to fully occupy the shallow soil mantle quickly. Although large variation in response can be expected, initial mean increases of 15 to 40% are realistic from pine forest on shallow, basalt-derived soils with basal area reductions of 30 to 100%.

On southern aspects, water yield response will generally be lost after 6 to 10 years following a uniform overstory reduction treatment and possibly as quickly as 3 years when using a strip-cut and thinning harvesting procedure. There are indication that water yield response on watersheds with northern exposure will persist for a longer time period.

Key Points

• The potential for increasing water yield in ponderosa pine is less than from other commercial forest types, presumably because the pine forests are drier. Short-term (3 to 10 yr) increases of 25 to 75 mm can be expected from clearcutting ponderosa pine with basal area in excess of 23 m²/ha.
• Under a multiple use management framework, where timber, range, wildlife, recreation, and water are all considered in the product mix, the long-term increases of 2 to 25 mm are a more realistic expectation (Brown et al. 1974). Low to intermediate stocking levels on approximately 2/3 of the ponderosa pine sites (Schubert 1974) can preclude water increases from these areas regardless of the management emphasis, except for clearcutting.
• No meaningful changes in total sediment production or water quality occurred as a result of the treatments applied in ponderosa pine forests. Average sediment production from untreated pine areas was 45 kg/ha and increased to 225 kg/ha after the clearing treatment (Brown et al. 1974). Relationships between the amount of sediment in suspension and streamflow discharge differed among the treated watersheds (Lopes et al. 1996). The highest sediment concentrations occurred after clearcutting, followed by stripcutting, thinning by group selection, and the combined shelterwood-seed tree silvicultural treatment. While changes in suspended sediment concentration are significantly different following treatment, these concentration are relatively low (generally less than 100 mg/l).
• Repeated inventories of the pine timber resource indicate that volume production has often been sustained, although at generally lower levels than those represented by pretreatment conditions (Baker 1999b). Exceptions to this finding were found on a watershed that was totally clearcut in 1966 and 1967, and on a watershed that had been converted from ponderosa pine forest to grass in 1958 and subsequently subjected to livestock grazing in the spring and fall starting in 1968. While these 2 watersheds, particularly the watershed cleared in 1966 and 1967, have Gambel oak and alligator juniper growing on them, the areas have been withdrawn from pine production.
• Reductions in the density of ponderosa pine forest overstories have generally resulted in increases in the production of herbaceous plants (Baker 1999b) and vice versa. These increases can approach 560 kg/ha after complete overstory removal including forage and non-forage plants. The untreated pine areas produced 225 kg/ha.
• Reducing densities of ponderosa pine forests have increased food for deer and elk, while retaining protective cover (Larson et al. 1986). Total clearcutting is detrimental to big game and Abert squirrel, although cottontail habitat can be enhanced when slash and Gambel oak thickets are retained.
• Fire can be prescribed to consume portions of the accumulation of dead organic material on mineral soil, impacting the hydrologic behavior of the burned site (Ffolliott and Guertin 1990b). Burning the L layer (unaltered organic material), the F layer (partly decomposed organic material), and into the H layer (well-decomposed organic material) affects postfire infiltration rates and erosion potentials. Other effects of fire can include thinning forest overstories from below, increasing seedling establishment, increasing production of herbaceous plants, and temporarily reducing fire hazard. Wildfire of moderate severity can have similar effects as observed with prescribed fire. However, wildfire of high severity often burns the forest floor to the mineral soil and induces a water-repellent layer in sandy soils (Campbell at el. 1977). The reduced infiltration rates can increase surface runoff from the burned site, causing soils to erode and removal of nutrients that have been mineralized. All small trees and many large trees can be killed, resulting in large increases in herbage.
• Public responses to vegetative treatments applied to the Beaver Creek watersheds were variable. Through applications of Scenic Beauty Estimation (SBE), which provides quantitative measures of esthetics preferences for alternative landscapes , the more natural-appearing watersheds were preferred by most publics (Baker 1999b). This conclusion adds weight to the often-heard, but seldom substantiated, claim that "naturalness" is a desirable forest landscape characteristic.
• Information obtained on resources in the ponderosa pine forests provided a framework for developing models to simulate the responses of natural resources to the treatments applied to the Beaver Creek watersheds, and production functions describing the trade-offs among the affected natural resources. This work resulted in a variety of publications related to hydrology, vegetation, and wildlife responses (Baker 1975, Bojorquez-Tapia et al. 1990, Brown and Daniel 1984, Ffolliott 1985a, Ffolliott and Guertin 1988, Larson 1975, Larson et al. 1979, Li et al. 1976a, Li et al. 1976b, Li et al. 1976c, Li et al. 1976d, Li et al. 1976e, O'Connell 1971, Rogers 1973, Rogers et al. 1982). A complete listing of publications on modeling and simulation techniques is found in Baker and Ffolliott (1998).
• Results from the Beaver Creek Watershed project were obtained on watersheds located on volcanic soils along the Mogollon Rim. The literature suggests that similar results might be obtained on volcanic soils elsewhere in the Southwest. However, extrapolation of the results from Beaver Creek to sites on sedimentary soils requires prior validation (Ffolliott and Baker 1977).

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