Forests and Water: A State of the Art Review for Colorado

• Year: 2003
• Title: Forests and Water: A State-of-the-Art Review for Colorado
• Author: Colorado State University
• Description: Colorado State University 2003
• Publications - Doc Type: Other

tion of a potential change, the organisms that might be
affected, and the beneficial uses of a given water body
(MacDonald et al., 1991).
3.4. Nutrients
Forest management activities such as forest thinning and
harvesting can alter the rates and processes of nutrient
cycling. Catchment -scale studies have produced a large
body of data on the effects of forest management on
nutrient concentrations and total loads. The observed
changes in nutrient concentrations vary substantially
between locations, even when the studies are within a
single physiographic region. Most recent studies in the
U.S. have found only small nutrient losses following for-
est harvest and no significant degradation of water qual-
ity. There usually is minimal opportunity for a buildup
of these nutrients in streams or lakes after timber harvest
because the period of increased nutrient flux to the
stream is normally very brief (Currier, 1980). Nutrient
losses by leaching are usually minor compared to the nu-
trient losses by biomass removal (Johnson, et al., 1988;
Mann et al., 1988; Clayton and Kennedy, 1985; Martin
and Harr, 1989).
Streams emanating from undisturbed forested catch-
ments generally have very low nitrate- nitrogen concen-
trations (0.002 - 1.0 mg L -1 NO3 -N) ( Binkley and Brown,
1993a;b). Concentrations are low because nitrogen is
rapidly taken up by plants and other organisms, and
nitrate formation (nitrification) rates are slow. The low
rates of nitrification in forested environments are due to
the slow rates of organic matter decomposition, acid soil
conditions, and bacterial allelopathy.
In general, nutrient mobility from disturbed forests fol-
lows the order of nitrogen > potassium > calcium and
magnesium > phosphorus. Thus forest harvest or other
disturbances such as fire will generally produce larger
differences in nitrogen concentrations than other con-
stituents.
Timber harvesting may temporarily increase nitrate -
nitrogen concentrations in soil and stream waters. If
vegetation is quickly reestablished, the increases in
nitrate - nitrogen are short-lived and usually do not repre-
sent a threat to water quality or site productivity. At the
Fraser Experimental Forest clearcutting increased the
amount of ammonium and nitrate in the snowpack but
decreased the amount of potassium. Nitrate concentra-
tions increased in both shallow and deeper subsurface
flows, while potassium concentrations increased in soil
water and shallow subsurface flow (Reuss et al., 1997).
27
The increase in nitrate - nitrogen concentrations was
from 0.006 to 0.06 mg L-1, but these values are still far
below the criterion for drinking water of 10 mg L-1. The
increased ion flux was still apparent at the plot scale a
decade after treatment (Reuss et al., 1997; Stottlemeyer
and Troendle, 1999), but the increased nutrient flux was
not detectable at the watershed scale. The estimated loss
of nitrogen due to leaching over the first eight years after
harvest was 48 kg ha', while the atmospheric input over
the same time period was approximately 59 kg ha' (Re-
uss et al., 1997).
The Hubbard Brook study in the northeastern U.S. is of-
ten cited as an example of the effects of timber harvest-
ing on water quality (Likens et al., 1970). In this study
the vegetation was cut and left on -site, and the hillslopes
were sprayed with a general herbicide for three years to
prevent any plant regeneration. Nutrient concentrations,
particularly nitrate- nitrogen, increased significantly.
This study helped identify nutrient cycling processes,
but it does not represent the typical effects of timber
harvesting on water quality.
In summary, timber harvest has only a minor effect on
nutrient concentrations in surface waters. In the absence
of extensive erosion and sediment delivery to the stream
network, total phosphorus concentrations should show a
significantly increase. Increased exports of nitrate -nitro-
gen are usually short-lived, and the concentrations are
low relative to drinking water standards. Losses often are
less than atmospheric inputs.
3.5. Fertilization
Fertilization to improve forest growth rates is uncommon
in Colorado. Studies in other areas have reported widely
varying effects of forest fertilization on water quality
(e.g., Fredriksen et al., 1975; Stephens, 1975; Bisson et
al., 1992; Binkley and Brown, 1993b; NCASI, 1999b).
Most studies indicate no significant increase in ammo-
nium- nitrogen and phosphorus concentrations in streams
after fertilization. Nitrate - nitrogen concentrations may
increase, but the increases are short-lived as nutrient
retention by forest soils is excellent. As long as aerial
applications do not put fertilizers directly into streams or
other water bodies, forest fertilization should have little
effect on nutrient concentrations in surface waters.
3.6. Prescribed Fire and Wildfire
There is an increasing body of literature on the effects
of both prescribed and wild fires in forests. A number of
studies have documented the changes in runoff, erosion,