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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