General Technical Report
Effects of Forest Harvest on Stream-Water Quality
and Nitrogen Cycling in the Caspar Creek
Randy A. Dahlgren2
1 An abbreviated version of this paper was presented at the Conference on
Coastal Watersheds: The Caspar Creek Story, May 6, 1998, Ukiah, California.
2 Professor of Soil Science and Biogeochemistry, University of California,
Department of Land, Air and Water Resources, One Shields Avenue, Davis, CA
Abstract: The effects of forest harvest on stream-water quality and nitrogen
cycling were examined for a redwood/Douglas-fir ecosystem in the North Fork,
Caspar Creek experimental watershed in northern California. Stream-water samples
were collected from treated (e.g., clearcut) and reference (e.g., noncut) watersheds,
and from various locations downstream from the treated watersheds to determine
how far the impacts of these practices extended. Additionally, a detailed nutrient
cycling study was performed in a clearcut and reference watershed to gain insights
into changes in nitrogen cycling after harvesting activities.
Stream-water nitrate concentrations were higher in clearcut
watersheds, especially during high stream discharge associated with storm events.
Elevated concentrations of nitrate were due to increased leaching from the soil
as mineralization (i.e., release of nutrients from organic matter) was enhanced
and nutrient uptake by vegetation was greatly reduced after harvest. The
elevated nitrate concentration in stream water from clearcut watersheds decreased in
the higher-order downstream segments. This decrease is believed to be primarily
due to dilution, although in-stream immobilization may also be important.
Although elevated nitrate concentrations in stream water from the clearcut
watershed might suggest a large nitrogen loss after clearcutting, conversion to a flux
indicates a maximum loss of only 1.8 kg N
ha-1 yr-1; fluxes decreased to <0.4 kg N
ha-1 yr-1 3 years after the harvest. Nitrogen fluxes from the reference watershed over
the same period were <0.1 kg N ha-1
yr-1. The increased nitrogen flux was due to
both higher nitrate concentrations and an increased water flux from the
In contrast to many forest ecosystems that show large nutrient losses
in stream water after harvest, this redwood/Douglas-fir ecosystem shows
relatively small losses. The rapid regrowth of redwood stump sprouts, which use the
vast rooting system from the previous tree, is capable of immobilizing nutrients in
its biomass, thereby attenuating nutrient losses by leaching. Rapid
regeneration also provides soil cover that appreciably reduces the erosion potential
after harvest. Removal of nitrogen, primarily in the harvested biomass, results in
an appreciable loss of nitrogen from the ecosystem. These data suggest that
nitrogen fixation by Ceanothus may be an important nitrogen input that is necessary
to maintain the long-term productivity and sustainability of these ecosystems.
he effects of forest harvest and postharvest practices on nutrient
cycling were examined for a redwood/Douglas-fir ecosystem
in the North Fork, Caspar Creek experimental watershed in
northern California. This ecotype is intensively used for commercial
timber production, and streams draining these ecosystems are
an important salmon-spawning habitat. Although the effects of
forest harvest practices on stream flow and sediment generation
have been intensively studied (e.g., Keppeler and Ziemer 1990, Rice
and others 1979, Thomas 1990, Wright and others 1990, Ziemer
1981, and papers contained in these proceedings), the impacts of
harvest practices on nutrient cycling processes have not been
rigorously examined for the coastal region of northern California.
Water quality and long-term nutrient sustainability are
major components addressed within the ecosystem approach to
forest management (Swanson and Franklin 1992). Forest harvest
practices are often considered to have adverse impacts on water quality
and sensitive aquatic communities owing to enhanced sediment
and nutrient losses due to erosion and leaching (Hornbeck and
others 1987, Likens and others, 1970, Martin and others 1986). The loss
of plant nutrients in drainage waters, suspended sediments,
and biomass removed by harvesting may further affect
nutrient sustainability of forest ecosystems (Hornbeck and others
1987, Johnson and others 1982, 1988). Sustainable forestry is based
on the premise of removing essential nutrients at a rate less than
or equal to that which can be replenished by natural processes.
As forest ecosystems become more intensively managed, it
is imperative that best-management practices be developed and
used to minimize environmental impacts and assure
long-term ecosystem sustainability.
This paper specifically examines the effects of forest
harvest and postharvest management practices on the nitrogen
cycle because nitrogen is the mineral nutrient required in the
largest amount by the vegetation and it is believed to be the most
limiting nutrient in this ecosystem. In addition, results for 10
additional nutrients are available in the final project report (Dahlgren
1998). This research uses a biogeochemistry approach to nutrient
cycling that examines processes and interactions occurring within
and between the atmosphere, hydrosphere, biosphere, and
geosphere (fig. 1). The process-level information obtained can be used
to decipher the complex interactions that occur in nutrient
cycling processes at the ecosystem scale. Results from this research can
be applied to the development of best-management practices
to maintain long-term forest productivity while minimizing
adverse environmental impacts from forest management.
Figure 1 Nutrient cycles consist of a series of interrelated processes occurring within and between
the atmosphere, hydrosphere, biosphere, and geosphere. Each nutrient is linked through a set of specific
interconnected steps that ultimately lead to a series of cyclic pathways.
Study Site Characteristics
Headwater catchments in the North Fork of Caspar Creek
were selected for this study (fig. 2). The watersheds are located in
the Jackson Demonstration State Forest, 11 km southeast of Fort
Bragg, California, and approximately 7 km from the Pacific Ocean.
The North Fork of Caspar Creek has a drainage area of 473 ha
and ranges in elevation from 37 to 320 m. The topography of the
North Fork watersheds ranges from broad, rounded ridgetops to
steep inner gorges. Slopes within the watershed are <30 percent
(35 percent of the area), 30-70 percent (58 percent of area), and
>70 percent (7 percent of the area) (Wright and others 1990).
The climate is Mediterranean, having dry summers
with coastal fog and mild temperatures ranging from 10 to 25
°C. Winters are mild and wet, with temperatures ranging between
5 and 14 °C. The average annual rainfall is about 1,200 mm with
no appreciable snowfall (Ziemer 1981). Soils are dominated by
well-drained Ultic Hapludalfs and Typic Haplohumults formed
in residuum derived predominately from sandstone and
weathered coarse-grained shale of Cretaceous Age.
The North Fork of Caspar Creek was originally clearcut
logged and burned in approximately 1910 (Tilley and Rice 1977).
Current vegetation is dominated by second-growth redwood
(Sequoia sempervirens (D. Don) Endl.) and Douglas-fir
(Pseudotsuga menziesii (Mirb.) Franco) with minor associated western
hemlock (Tsuga heterophylla (Raf.) Sarg.) and grand fir
(Abies grandis (Dougl.) Lindl.). The mean stand density based on cruise data
from three watersheds was 207 redwood stems
ha-1 (mean DBH = 56 cm) and 86 Douglas-fir stems
ha-1 (mean DBH = 66 cm). Timber volume at the onset of this study before logging was estimated at about
700 m3 ha-1 (Krammes and Burns 1973).
Solid-Phase Soil Analyses
Sites for six soil pits were randomly selected within a clearcut
(KJE) and reference (MUN) watershed (fig.
2). Soil pits (1.5 by 0.5 by 1-1.2 m; L ¥ W
¥ D) were excavated to a depth corresponding to the limit
of the major rooting zone (BC horizon; 100-120 cm). Each pedon
was described, and soil samples for chemical analyses and clods for
bulk density measurements were collected from across the entire pit
face for each morphological horizon. All soil samples were
collected during September 1992, the month during which the soil is driest.
Figure 2 Site map indicating the location of the individual subwatersheds within the North Fork of
Caspar Creek watershed. Letters indicate location of stream-water sampling sites; triangles, the location of piping
water samples; and years, the year in which felling activity was completed in clearcut watersheds.
Soil samples were air-dried, gently crushed, and
passed through a 2-mm sieve; roots passing through the sieve were
removed with a forceps. Bulk density was determined by the
paraffin-coated clod method using three replicate clods per horizon (Soil
Survey Staff 1984). Total carbon and nitrogen were determined on
ground samples (<250 µm) by dry combustion with a C/N analyzer.
Soil carbon and nitrogen pools (kg ha-1) were calculated for each
soil profile (n = 6) by summing the nutrient content of all
horizons within the major rooting zone. Nutrient pools for each
horizon were determined from the nutrient concentration in the <2
mm fraction, mean horizon thickness, and bulk density of each
horizon with a correction for the coarse fragment (>2 mm) volume.
Collection and Analysis of Ecosystem Waterflows
The chemistry of ecosystem waterflows along the hydrologic
cycle (e.g., precipitation, canopy throughfall, soil solution,
and streamflow) was used to compare nutrient cycling processes in
the clearcut (KJE) and reference (MUN) watersheds for 3 water
years commencing 1 October 1993 and ending 30 September
1996. Samples were collected on a monthly basis during the rainy
season (November-May) and on an event basis as necessary outside of
the rainy season. Bulk precipitation was collected from duplicate
sites within the clearcut watershed (KJE). Bulk precipitation
collectors are effective in capturing the wetfall component but have
been shown to only partially capture the dryfall (particulate and
gases) relative to the collection efficiency of a forest canopy (Lindberg
and others 1986). Thus, bulk precipitation fluxes
probably underestimate the total atmospheric deposition to a
forest ecosystem. Canopy throughfall was collected in triplicate
from beneath the canopy of redwood and Douglas-fir in the area
adjacent to the soil solution collection sites in the reference
watershed (MUN). Precipitation and throughfall collectors consisted of a
4-L polyethylene bottle containing a 15-cm diameter funnel with
Teflon wool inserted in the neck to act as a coarse filter.
In situ soil solutions were collected from three sites in
the clearcut (KJE) and reference (MUN) watersheds using
zero-tension lysimeters. Lysimeters consisted of open-topped
polyethylene containers (15 by 10 by 4 cm; L
¥ W ¥ D) filled with acid-washed quartz sand (Driscoll and others 1988). Lysimeters were placed
in duplicate at the 20- and 40-cm depths along with a single
collector at the 60-cm depth. Lysimeters were installed by tunneling
from below and from the side of the excavated soil pit to the
desired depth. This installation technique minimizes disturbance to
the soil fabric and rooting system overlying the lysimeter.
Lysimeters were installed one year before soil solutions were collected
for chemical analysis. This equilibration period minimizes the
potential for artifacts due to disturbance from lysimeter installation
(Johnson and others 1995).
Concentrations of stream-water nutrients from
watersheds receiving various combinations of forest harvest practices
(e.g., clearcutting and burning) were compared to reference
watersheds having no disturbance (fig. 2). Additionally, samples were
collected along the main channel of the North Fork to determine
the magnitude and persistence of the cumulative effects of
timber harvest practices within the larger watershed. Grab samples
were collected biweekly (rainy season) to monthly (nonrainy season)
at 13 sites commencing in March 1991 and continuing through
June 1996. In addition to the regular grab-sampling protocol,
automatic pumping samplers were used to intensively collect water
samples during storm events from the clearcut (KJE) and reference
(MUN) watersheds. The autosamplers were programmed to collect
storm samples using a variable probability (SALT) random
sampling procedure (Thomas 1985, 1989). All stream-water samples
were collected by the USDA Forest Service Caspar Creek Research
Team. Before chemical analyses, all solutions were filtered through a
0.2-µm membrane filter. Ammonium and nitrate were determined
by ion chromatography. Data for 10 additional solutes are given in
the final report (Dahlgren 1998). During the 1994-1995 water
year, water samples from three storm events were bulked to
provide approximately a 20-L sample for isolation of the
suspended sediment fraction (>0.4 µm). Carbon and nitrogen
concentrations were determined by dry combustion using a C/N analyzer.
Nutrient Analysis in Biomass
Nutrient pools in biomass were determined for the
regenerating redwood spouts in the clearcut watershed (KJE) and for the
second-growth redwood/Douglas-fir stand in the reference
watershed (MUN). Ten randomly selected 10-m by 10-m plots were
sampled within the clearcut watershed in November 1995 to quantify
total aboveground biomass production 6 years after completion
of harvest. All shoots and their diameters were recorded within
each plot for the stump-sprouting redwoods. To develop
allometric relationships for the redwood sprouts, 10 individual stems
spanning the range of diameter classes (0.767.6 cm) were
destructively sampled. Biomass from each sprout was divided into foliage,
twigs (<2 mm), and branches/stems (>2 mm). The mass of each
category was recorded after drying at 70 °C.
Nutrient pools in the second-growth stand of the
reference watershed (MUN) were estimated by sampling various
biomass components from four replicates each of redwood and
Douglas-fir. Foliage and branch samples were obtained from the mid-point of
the upper, middle and lower one-third portions of each tree canopy
by climbing the tree. For each tree, all branch and foliage samples at
each canopy position were separately processed and chemically
analyzed. Individual root samples from each tree were obtained by
excavating at the base of each tree; samples were collected one meter away
from the base of the tree in the Oi/Oe and A horizons. Stemwood and
bark were collected by coring individual trees at breast height (1.4 m).
Carbon and nitrogen concentrations were determined
on ground samples (<250 µm) by dry combustion using a
C/N analyzer. Nitrogen recovery was 95.2 ± 2.3 percent based on
analysis of National Bureau of Standard's reference materials. Nutrient
pools in biomass of the redwood/Douglas-fir forest were determined
from stand density, allometric relationships obtained from Gholz
and others (1979), and nutrient concentrations determined from
the preceding analyses. Because no allometric relationships
were available for aboveground biomass in redwood, we estimated
the redwood biomass amounts using the Douglas-fir
allometric relationships. This extrapolation introduces a potential error
into the estimate for the redwood aboveground nutrients; however,
this error will most likely be on the order of <20 percent. Root
biomass for the second-growth forest and stump sprouts was estimated
from the data of Ziemer and Lewis3 obtained from comparable
forest stands in northern California.
3 Unpublished data from Ziemer and Lewis on file at USDA Forest Service,
Redwood Sciences Laboratory, Arcata, CA 95521.
Results and Discussion
Soils provide ecosystem resiliency after perturbations owing,
in large part, to their capacity to provide stored nutrients
for regenerating vegetation. The effects of clearcut harvesting on
the quantity and distribution of organic carbon and nitrogen in
soils were examined by comparing soils in the reference
watershed (MUN) to those in the clearcut watershed (KJE) 3 years after
the harvest. An important difference between soils in the
two watersheds was the loss of the majority of the litter layer
(Oi/Oe horizon) from the clearcut watershed. An Oi/Oe layer with
a thickness of 1-3 cm was found on all soils within the
reference watershed. The loss of the litter layer after harvest was due
to microbial decomposition and mixing of the litter layer with
the mineral soil during logging operations. The litter layer plays
an important role in forest ecosystems by protecting the mineral
soil from erosion, storing nutrients and water, and acting as mulch
to reduce temperature fluctuations and evaporative water loss.
Nutrient pools contained within the primary rooting
zone (upper 100-120 cm) were determined as a function of soil
horizon for the reference and clearcut watersheds
(fig. 3). In spite of the loss of the litter layer from soils in the clearcut watershed, there were
no statistical differences (p < 0.05) in organic carbon or nitrogen
pools between the clearcut and reference watersheds. The soils store
a very large pool of organic carbon (~170 Mg
ha-1), primarily in the A and AB horizons (the upper 30 cm). The loss of the Oi/Oe
horizon from the clearcut watershed appears to be compensated for by
an increase in organic carbon in the A horizon. This may reflect
the mixing of the litter layer with the mineral soil during
harvesting operations. The soils similarly store large concentrations of
nitrogen (>9 Mg ha-1); however, this nitrogen pool is not readily available
to the vegetation until mineralization releases the nitrogen from
the soil organic matter (fig. 1). Because the soil N pool is so large
(10,000 kg ha-1), even an appreciable decrease, such as 100 kg
ha-1, after harvest cannot be accurately determined by standard
solid-phase analysis, especially given the high spatial variability.
Therefore, analysis of solid-phase soil properties is not a sensitive method
for determining changes in nutrient cycling processes due
Figure 3 Carbon and nitrogen pools in reference watershed (MUN)
and clearcut watershed (KJE) 3 years after the harvest. The individual
segments of each bar indicate the nutrient amount contained within that
individual soil horizon (no Oi/Oe in clearcut). Error bar indicates the standard
error of the mean.
Analysis of ecosystem waterflows is a far more powerful
approach than solid-phase soil analysis for detecting treatment effects
after ecosystem disturbance because aqueous transport is a
primary mechanism for the redistribution of nutrients within an
ecosystem. Because ecosystem waterflows provide information about
current-day nutrient transport processes, their composition is very
sensitive to changes due to disturbance. The nutrient concentrations
were examined along the hydrologic cycle within the
second-growth redwood/Douglas-fir and clearcut ecosystems to examine
changes in nutrient cycling (fig. 4). The precipitation had trace inputs of
N (NH4 = 0.8 µM and NO3 = 1.0 µM) that were not appreciably
altered by canopy processes. Concentrations of NH4 in soil solutions were
very low (<3 µM) and showed no difference between the
reference and clearcut soils. Nitrate concentrations were also low;
however, nitrate concentrations were significantly greater throughout
the entire soil profile of the clearcut watershed. Enhanced
nitrogen mineralization rates coupled with reduced uptake were
responsible for the increased nitrate mobility after the harvest
(fig. 1). Soil solution nitrate concentrations in this study were very
low compared to values commonly exceeding 200 µM after
clearcutting of northern hardwood forests in the Hubbard Brook
Ecological Forest in New Hampshire (Dahlgren and Driscoll 1994). Within
the soil profile from the clearcut, the concentration of nitrate
decreases appreciably with depth because of nitrogen uptake by the
stump-sprouting redwoods. In the absence of this nitrogen
uptake, nitrogen losses to stream water would probably be much
higher. Concentrations of nitrate were higher in the stream water than
at the 60-cm depth in the soil profile in both watersheds. The
higher nitrate concentrations in the stream water most likely result
from changes in the hydrologic flowpath during storm events. Our
data suggest that water from the upper soil horizons (0- to 30-cm
depth) is preferentially routed to the streams during storm events owing
to the low saturated hydraulic conductivities associated with the
clay-rich B horizons that begin at a depth of about 30 cm.
Mean concentrations of nitrate in stream water were very similar to
those occurring in soil solutions at the 20- and 40-cm
depth (fig. 4).
Figure 4 Mean±standard deviation for ammonium and nitrate in
precipitation (PPT); canopy throughfall (TF); soil solutions at 20-, 40-, and 60-cm
depths; and stream water for the period October 1993 to June 1996 in the
clearcut (KJE) and reference (MUN) watersheds.
Watershed-scale manipulations are a powerful approach
for studying the cumulative effects of forest management practices
on nutrient cycling processes. Watershed manipulation studies use
a paired watershed approach in which two watersheds with
similar characteristics are employed. In this study, stream-water
chemistry from the reference watershed (MUN) is compared to that
from clearcut (KJE) and clearcut/burned (EAG) watersheds to
examine the effects of harvesting practices on nutrient cycling
(fig. 2). The export of nutrients in stream water is often one of the
primary processes responsible for nutrient losses from forested
ecosystems. Monitoring of stream-water chemistry began in the KJE
watershed approximately 1.25 years (in March 1991) after completion
of felling operations and in the EAG watershed immediately
after harvest and burning activities.
Nitrate concentrations in stream water from the
reference watershed were generally less than our detection limits of 0.4
µM; however, concentrations exceeding 10 µM were measured
during two storm events in the 1995-1996 water year
(fig. 5). In contrast, nitrate concentrations in the harvested watersheds ranged
between 10 and 70 µM during storm events. Baseflow nitrate
concentrations in the clearcut watershed were also low, often below detection
limits (0.4 µM). It appears that nitrate concentrations showed
a progressive decrease in peak concentrations after the clearcut
(1991 to 1994), with the exception of increased concentrations
during storm events in the 1995-1996 water year. The increased
nitrate concentrations during the 1995-1996 water year were observed
in both the clearcut and reference watersheds, suggesting that
the increase was not solely due to the disturbance associated
with harvest practices.
Increased nitrate concentration in stream water
after clearcutting is a common observation; however, the magnitude
of nitrate leaching varies appreciably between ecosystems.
For example, maximum nitrate concentrations in stream water
after clearcutting of northern hardwood forests in the White
Mountains of New Hampshire were 500 µM (Dahlgren and Driscoll
1994), nearly an order of magnitude greater than those observed in
this study. We believe that the rapid immobilization of nitrogen by
the stump sprouting redwood biomass is an important factor
limiting the leaching losses of nitrate in this redwood/Douglas-fir ecosystem.
The maximum concentrations of nitrate often occur in
the second year after clearcutting (Martin and others 1986). This
results from enhanced immobilization of nitrogen during the first year
as microorganisms begin to decompose woody litter, with high
C/N ratios, that was added to the soil organic matter pool as slash
during the harvest. Because we missed the January 1990 to March
1991 monitoring period in the KJE watershed, we cannot
specifically determine the timing of peak nitrate concentrations after harvest
in this watershed. However, nitrate concentrations in stream
water peaked in the EAG watershed in the water year immediately
after harvest and burning. Removal of woody biomass by burning
may result in less microbial immobilization and a more rapid release
of mineral nitrogen in this watershed.
Figure 5 Stream-water nitrate concentrations and stream flow in the
clearcut (KJE), clearcut and burned (EAG; no stream flow shown), and reference
(MUN) watersheds of the North Fork, Caspar Creek.
Maximum nitrate concentrations occur during
high-discharge storm events and drop to low levels during baseflow
(fig. 5). We interpret these data to indicate that changing hydrologic
flowpaths during storm events result in the delivery of high-nitrate waters
to the stream during peak discharge. Data on soil solution
indicate that the highest nitrate concentrations occur within the upper
soil horizons (fig. 4). The soils within the watershed have a thick,
clay-enriched horizon beginning at a depth of approximately 30
cm. This horizon contains >40 percent clay, substantially reducing
the hydraulic conductivity that results in temporary saturation
above this layer. Given the steep slopes within the watershed,
thissaturated layer may move laterally downslope,
transporting nutrients from within the nutrient-rich rooting zone (upper 30
cm). This water enters the stream as subsurface lateral flow
(Keppeler and Brown, these proceedings) contributing to maximum
stream-water nitrate concentrations during peak discharge. The lateral
flow of water above the clay-rich horizon and through macropores
(e.g., root channels and soil pipes) was observed repeatedly on
roadcuts within the Caspar Creek drainage supporting this mechanism.
Nutrient Input/Output Budgets
Although nutrient concentrations in ecosystem waterflows
provide information on processes regulating nutrient concentrations
in stream water, the most important consideration from an
ecosystem sustainability perspective is the nutrient flux (kg
ha-1 yr-1) associated with atmospheric deposition and streamflow. Fluxes in
stream-water nutrients were calculated by combining stream discharge (L
s-1) with nutrient concentrations (mg
L-1). Because the water yields of clearcut and reference watersheds differ appreciably
(Keppeler, these proceedings; Ziemer, these proceedings), what appear to
be small differences in stream-water nutrient concentrations result
in much larger differences in nutrient fluxes.
Nitrogen fluxes in precipitation are shown for the 5 water
years of the study in table 1. The precipitation fluxes are regulated to a
large degree by the precipitation amount for a given year.
Precipitation amounts from nearby Fort Bragg during the study period
ranged from a low of 78 cm during the 1993-1994 water year to a high of
148 cm during the 1994-1995 water year. Nitrogen fluxes in
bulk precipitation were very low, ranging between 0.1 and 0.4 kg N
ha-1 yr-1 during the study period. Actual nitrogen inputs to this
ecosystem may be somewhat higher because the forest canopy has a much
higher efficiency of capturing atmospheric gases, aerosols, and
particulate matter than the funnel used to collect the bulk precipitation.
In contrast, the capture efficiency of the clearcut watershed would
be greatly attenuated by the removal of the canopy.
Nitrogen fluxes in stream water were substantially higher
for the clearcut watershed than for the reference watershed
(table 1). The maximum nitrogen flux in the clearcut watershed was 1.85
kg N ha-1 yr-1 in the 1991-1992 water year. Nitrogen fluxes
decreased over time to 0.15 kg N ha-1
yr-1 in the 1995-1996 water year (the
7th water year following harvest). In contrast, nitrogen fluxes were
low (<0.10 kg N ha-1 yr-1) in the reference watershed. The
increasednitrogen flux in stream water after harvest results from
the combination of increased stream-water nitrate concentrations
and an increase in water yield due to reduced evapotranspiration
and interception of water by the canopy. Nitrogen fluxes return
to background levels when the immobilization capacity of
the regenerating vegetation approaches the rate of
nitrogen mineralization. The recovery period of 5-7 years is consistent
with the findings of other studies examining the effects of clearcutting
on stream-water nutrient fluxes (Dahlgren and Driscoll
1994, Hornbeck and others 1987, Martin and others 1986).
Table 1 Nitrogen fluxes contained in precipitation and stream water from the
reference (MUN) and clearcut (KJE) watersheds for the 5-year study period.
|Water year||Precipitation||Stream water|
| || ||Reference||Clearcut|
| ||kg ha-1|
Cumulative Effects in Stream-Water Nitrate Concentrations
The cumulative effects of timber harvest operations (i.e.,
the distance to which a harvesting effect is observed downstream
from the harvested watershed) is a very important attribute of
watershed biogeochemistry. Some impacts may be observed far
downstream of the actual disturbance, whereas other impacts may not
be detectable downstream of a disturbance. Figure
6 shows mean stream-water nitrate concentrations in nondisturbed
reference watersheds (HEN, IVE, and MUN), harvested watersheds
(BAN, CAR, EAG, GIB, and KJE), sampling points along the main
stem that combine water from reference and harvested watersheds
(DOL, FLY, LAN, and JOH), and the main stem just before it exits
the experimental watershed (ARF).
Figure 6 Cumulative effects of nitrate concentrations
(mean±standard deviation) in stream waters of the North Fork, Caspar Creek
experimental watershed. Data shown are for reference watersheds (HEN, IVE, and
MUN), harvested watersheds (BAN, CAR, EAG, GIB, and KJE), locations
downstream from harvested watersheds (Main stem; DOL, FLY, LAN, and JOH), and at
the exit point (Exit; ARF) of the stream from the experimental watershed.
The reference watersheds showed very low concentrations
of nitrate with a relatively low standard deviation.
Nitrate concentrations in stream water draining the clearcut
watersheds showed elevated nitrate concentrations with a mean of about 4
µM over the 5-year study period. The high standard deviation
associated with harvested watersheds reflects the temporal variability
that occurs in nitrate leaching after the harvest.
Maximum concentrations of about 70 µM were measured from
clearcut watersheds. Also contributing to the large standard deviations
are the large fluctuations that occur in nitrate concentrations
during storm events because of changing hydrologic flowpaths.
Nitrate concentrations decreased at sampling points downstream from
the harvested watersheds and reached a mean value of about 1 µM
as the stream water exited the experimental watershed. The
decrease in nitrate concentrations as the water enters higher-order
streams appears to be largely due to dilution with low-nitrate
waters entering from nondisturbed portions of the watershed. There
may be some attenuation of nitrate concentrations by
in-stream immobilization of nitrate by biota; however, the short
residence time of water in the watershed provides very little time for
biological processes to affect water quality.
Nutrient Pools in Biomass
An inventory of the number and size distribution of
stump-sprouting redwoods in watershed KJE was made 6 years
after clearcut harvesting. This inventory showed 5020 ± 1970 stems
ha-1 (mean ± std. dev.; range = 2600 - 8300) having a DBH of 2.59 ±
0.71 cm (mean ± std. dev.). The DBH of the stump sprouts ranged
from <1 to 10 cm. The diameter distribution indicates that the
majority (76 percent) of the stems have DBH values 3 cm with far
fewer stems in the larger diameter classes (Dahlgren 1998). Within
a cluster of stems surrounding a stump, there were generally 1 to
4 dominant stems with DBH values greater than about 6 cm.
Total carbon storage in the aboveground biomass of the
6-year-old redwood stand was 7.8 Mg
ha-1 (table 2). On the basis of
an average carbon content of about 50 percent for the biomass,
there was more than 15 Mg ha-1 of aboveground biomass within
the redwood sprouts 6 years after harvest. This rapid accumulation
of biomass after harvest is the result of the rapid regrowth
associated with regeneration from stump sprouting versus establishment
from seed. The large intact rooting system can acquire an abundance
of nutrients to support regrowth. Also important in this rapid
regrowth is the immobilization of potentially mobile nutrients, such as
nitrate, into the aboveground biomass. The accumulation of 70 kg N
ha-1 in the aboveground biomass and the retention of 16 kg N
ha-1 in the living rooting system greatly attenuate leaching of nitrogen
after harvest (table 1). Warmer and moister soil conditions combined
with higher organic matter concentrations from logging slash
after clearcutting result in higher decomposition, mineralization,
and leaching (fig. 1). Thus, rapid immobilization of nutrients by
the aggrading redwood forest will have a strong influence on
nutrient dynamics and leaching after harvest in these ecosystems.
Nutrient pools calculated for the second-growth
redwood/Douglas-fir stand showed 644 Mg C
ha-1 (~1288 Mg ha-1 of
biomass) stored in this forest ecosystem that is more than 80 years old
(table 2). The wood and bark components contain about 86 percent
(555.5 Mg ha-1) of the biomass carbon, and only 6.4 percent (41 Mg
ha-1) of the biomass carbon pool is found in the belowground root
Table 2Carbon and nitrogen pools contained in stump-sprouting redwood 6 years
after clearcutting (KJE) and in the second-growth redwood/Douglas-fir stand (MUN).
| || || ||kg ha-1|
|Branches/redwood stems||5,915||34,900||27.5|| 95|
|Wood|| ||488,000|| ||735|
|Bark|| ||67,500|| ||214|
The biomass contains nearly four times the amount of
organic carbon stored in the soil profile (~170 Mg
ha-1). There was a total of 1480 kg
ha-1 of nitrogen in the redwood and Douglas-fir biomass
in this ecosystem (table 2). Only 10 percent (166 kg
ha-1) of the nitrogen pool is found in the belowground root
biomass. Approximately 64 percent (949 kg
ha-1) of the total nitrogen in the biomass is contained within the wood and bark
components. Because conventional clearcutting removes only the wood and
bark components, it is this 949 kg ha-1 of nitrogen that will be
removed from the ecosystem by traditional clearcut harvesting.
Nutrients in Suspended Sediments
The transport of nutrients in suspended sediments can
be substantial if steps are not taken to minimize erosion after
harvest. We determined the nutrient concentrations of suspended
sediment collected from both the clearcut (KJE) and reference
(MUN) watersheds in order to provide an estimate of the amount
of nitrogen lost from these watersheds as suspended sediment.
The nitrogen content of suspended sediment in the reference
watershed (4.4 ± 1.9 g N kg-1) was 2.7 times higher than in the
clearcut watershed (1.6 ± 0.5 g N
kg-1). This difference possibly reflects
the origin of the suspended sediment. The soil surface of the
reference watershed is covered by a litter layer that would produce a
relatively organic-rich sediment. In contrast, the loss of the litter layer
from the clearcut watershed results in a soil surface with a lower
organic matter concentration. Thus, the difference in the nitrogen
content of suspended sediments between the two watersheds
probably reflects the contrasting nature of the soil surface in the
two watersheds, or different source areas. The source of much of
the sediment in KJE was likely from sediments stored in the
channel (Lewis, these proceedings).
The suspended sediment load predicted for an
unlogged condition in water years 1990-1996 for the entire North Fork
Caspar Creek experimental watershed was about 385 kg
ha-1 yr-1 before harvest activities (Lewis, personal communication). If the
nitrogen content (4.4 g N kg-1) of the suspended sediments from the
reference watershed (MUN) is representative of that for the entire
watershed before harvest, 1.7 kg ha-1
yr-1 of nitrogen would be lost from the watershed as suspended sediment. Harvest activities within
the entire North Fork experimental watershed resulted in an increase
of 345 kg ha-1 yr-1 of suspended sediment (Lewis,
personal communication). However, the total nitrogen lost from the
clearcut watershed would actually be somewhat lower than that lost
from the reference watershed because of the lower nitrogen
content associated with suspended sediment from the clearcut
watershed (1.6 g N kg-1). Because of the limited data collected in this study
and the large temporal variability associated with suspended
sediment fluxes over the course of a harvest rotation, it is very difficult
to estimate the long-term nitrogen fluxes from these watersheds.
Ecosystem Nitrogen Sustainability
Sustainable forestry is based on the premise of removing
essential nutrients at a rate less than or equal to that which can be
replenished by natural processes. As shown in the preceding
discussion, nitrogen is lost from the ecosystem primarily by biomass
removal, suspended sediment, and leaching. Denitrification may also
result in nitrogen loss; however, we have no estimates of how
much nitrogen may be lost by this mechanism. The primary inputs
of nitrogen into the ecosystem are atmospheric deposition
and nitrogen fixation, primarily by
Ceanothus. A nitrogen mass balance was calculated on the basis of estimated nitrogen inputs and
outputs over the course of an 80-year harvest rotation
(table 3). Regardless of the amount of nitrogen lost in the suspended sediment
fraction, there is a net loss of nitrogen from this ecosystem. Nitrogen
losses are dominated by biomass removal (~950 kg
ha-1), which removes about 60 percent of the nitrogen contained in the
biomass. Although the nitrogen loss in the suspended sediment
fraction cannot be precisely estimated, it appears to be on the order of 1.0
- 2.0 kg N ha-1 yr-1. These losses greatly exceed the only
measured input of about 20 kg N ha-1 in the bulk precipitation. This
input/output balance suggests a nonsustainable forest
management practice over the long term; however, nitrogen fixers such
as Ceanothus can contribute appreciable nitrogen inputs into
these ecosystems. Ceanothus thyrsiflorus (blue-blossom ceanothus) is
an aggressive invader after clearcutting, and it has the potential to
fix large quantities of nitrogen to replenish the nitrogen
deficit imposed by harvesting. Nitrogen fixation rates for
Ceanothus velutinus in the Oregon Cascades range from 70 to 100 kg N
ha-1 yr-1 (Binkley and others 1982, Youngberg and Wollum 1976).
These data, as well as data reported in the literature (e.g., Swanson
and Franklin 1992), suggest that nitrogen fixation by
Ceanothus may be necessary to maintain the long-term productivity and
sustainability of these ecosystems. Additional research appears warranted
to determine the importance of Ceanothus in the postharvest
recovery of the nitrogen budget in this ecosystem.
Table 3 Nitrogen mass balance for clearcut harvest management based on an
80-year harvest rotation. The suspended sediment flux is estimated based on limited data from
|Nutrient component||Nitrogen pools and fluxes|
| ||kg ha-1|
|Soil pool||9,500|| |
|Biomass pool||1,480|| |
|Atmospheric deposition flux|| ||+20|
|Nitrogen fixation flux|| ||+?|
|Harvest removal flux|| || -950|
|Stream-water flux|| || -10|
|Suspended sediment flux|| || -80 to -160|
Clearcut harvesting of this redwood/Douglas-fir ecosystem did
not result in any short-term detectable decrease in soil carbon
and nitrogen pools. Stream-water nitrate concentrations were
increased after clearcutting, especially during storm events with high
stream-discharge volumes; however, fluxes in stream water were
relatively low compared to results from other forest
ecosystems. Immobilization of nutrients by the rapid regrowth of
redwood stump sprouts appears to make this ecosystem relatively resilient
to nutrient loss by leaching after harvest. The elevated
nitrate concentration in streams draining clearcut watersheds
was substantially decreased at downstream sampling points. By the
time the stream left the experimental watershed, nitrate
concentrations were near those of the nonperturbed reference watersheds.
Removal of nitrogen in the harvested biomass results in an appreciable
loss of nitrogen from the ecosystem. These data suggest that
nitrogen fixation by Ceanothus may be an important nitrogen input that
is necessary to maintain the long-term productivity and
sustainability of these ecosystems.
This research was supported by a grant from the
California Department of Forestry and Fire Protection. We acknowledge
the valuable guidance and logistical support from E. Keppeler, R.
Ziemer, J. Munn, and N. Henry. Field assistance by Z. Yu, J. Holloway, D.
Baston, R. Northup, and the entire field staff of the USDA Forest
Service/Jackson State Forest Caspar Creek Research Team is greatly appreciated.
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