Nutrient concentration dynamics in an inland Paci c Northwest … · 2013-02-20 · Nutrient concentration dynamics in an inland Paci c Northwest watershed before and after timber

Nutrient concentration dynamics in an inland Pacific Northwestwatershed before and after timber harvest

J.A. Gravelle a,*, G. Ice b, T.E. Link c, D.L. Cook b

a University of Idaho, Environmental Science Program, 975 West 6th Street, Moscow, ID 83844-1133, United Statesb National Council for Air and Stream Improvement, Inc., 720 SW 4th St., Corvallis, OR 97333, United Statesc University of Idaho, College of Natural Resources, 975 West 6th Street, Moscow, ID 83844-1133, United States

1. Introduction

Soil and water chemistry within a watershed help determineforest productivity and provide essential nutrients for aquatic andriparian biota. Two key nutrients are nitrogen (N) and phosphorus(P). Excessive concentrations of these nutrients can lead toproblems such as eutrophication. National concerns aboutacceptable levels of nutrients in streams are highlighted byongoing efforts to update water quality standards (US EPA, 2000).

Concerns about forest management impacts on water qualityand aquatic ecosystem health have led to water quality manage-ment guidelines and regulations to help reduce nonpoint sourcepollution (Brown et al., 1993). Timber harvest and associated roadconstruction practices have changed dramatically in the last threedecades since state nonpoint source control programs wereimplemented under the federal Clean Water Act. These changesinclude increased stream protection requirements and implemen-tation of other best management practices (BMPs) such asoutsloped road design and installation of relief culverts near

stream crossings to minimize sediment routing to stream channels(Ice, 2004). With application of new BMPs as part of commercialforestry operations, data are needed to determine the effectivenessof contemporary forest management practices. Timber harvestingchanges several aspects of a watershed that can potentially alterstream water nutrient concentrations and cycling. These effectsmay include changes in water temperature (Brown, 1970; Mooreet al., 2005; Gravelle and Link, 2007), hydrologic regimes (Mooreand Wondzell, 2005; Hubbart et al., 2007) and flow pathways,primary production (Chapman, 1962), and organic matterdynamics (Bilby and Bisson, 1992) including rate of mineralization,and nutrient uptake by plants (DeLuca and Zouhar, 2000).

Changes in water yield and flow regime driven by land coveralterations (Stednick, 1996) can alter the timing and amount ofstream chemical concentrations (Caissie et al., 1996). Changes inflow and concentration can lead to differences in total nutrientexport from altered relative to unimpacted catchments (Campbellet al., 1995). Vegetation community changes over time can alsolead to variations in stream chemistry (Goodale et al., 2000).Timber harvest and road construction activities in mountainouswatersheds can increase the amount of sediment entering streams(Haggerty et al., 2004; Karwan et al., 2007), which has particularimportance for total phosphorus (TP) loads.

Forest Ecology and Management 257 (2009) 1663–1675

A R T I C L E I N F O

Article history:

Received 13 September 2008

Received in revised form 12 January 2009

Accepted 12 January 2009

Keywords:

Ammonia

Idaho

Mica Creek

Nitrate

Nitrogen

Nutrients

Orthophosphate

Pacific Northwest

Phosphorus

Timber harvest

Water quality

A B S T R A C T

The nutrient loads of water draining forested watersheds are generally lower than the loads in water

draining basins with other dominant land uses. Commercial forest management activities including

timber harvesting, site preparation, road construction, and maintenance can alter the chemical

properties of headwater forest streams, and there are concerns this can result in cumulative effects at

downstream locations. Monthly water samples were collected from 1992 to 2006 in the Mica Creek

Experimental Watershed (MCEW) in northern Idaho. This period of record included a pre-treatment time

interval from 1992 to 1997; post-road construction period from 1997 to 2001; and post-harvest period

from 2001 to 2006. Samples were analyzed for total Kjeldahl nitrogen (TKN), total ammonia nitrogen

(TAN), nitrate + nitrite (NO3 + NO2), total phosphorus (TP), and orthophosphate (OP). Statistically

significant increases (p < 0.001) were observed in NO3 + NO2 concentrations following both clearcut and

partial cut harvest practices. Downstream of the clearcut harvest activity, mean monthly increases of

0.29 mg-N L�1 were observed. Statistically significant increases were also observed at sites further

downstream, but changes were smaller than those immediately below the harvest sites and reflected

dilution and possibly instream processing and/or uptake. Continued monitoring at these sites will help

evaluate nutrient concentration trends during stand regrowth and hydrologic recovery.

� 2009 Elsevier B.V. All rights reserved.

* Corresponding author.

E-mail address: [email protected] (J.A. Gravelle).

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0378-1127/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2009.01.017

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Natural variations in nutrient concentrations in stream watercan be due to differences in geology and weathering, precipitation,stream discharge, and biological processes. In the western UnitedStates, differences in stream chemical concentrations, especiallyP, tends to be dominated by geology (Dethier, 1979). Previousstudies in this region have also found large seasonal variations inN and P concentrations. Seasonal variability can be due togeochemical weathering, as observed in western Montana(Nagorski et al., 2003), or snowmelt runoff (Sickman et al.,2003; Stottlemyer and Troendle, 1992). Primary producersremove N and P (Minshall et al., 2001; Mulholland et al., 2000),and there can be a high demand for N species, especially during thesummer (Peterson et al., 2001). However, compared with otherland uses, forestland streams generally have relatively low N and Pconcentrations (Omernik, 1977; Clark et al., 2001; Ice and Binkley,2003).

Because of the natural variability in geology, climate, atmo-spheric inputs, and vegetation between watersheds, as well as thewide range of forest management practices that can be applied, themeasured effects of timber harvesting in previous studies werehighly variable (Lovett et al., 2000) and hence produced mixedresults (Feller, 2005). In some cases little change was observed inthe overall nutrient budget following timber harvest (Martin andHarr, 1989), but generally an increase in nitrate concentrations wasobserved in the United States and southern Canada (Binkley andBrown, 1993), as shown by studies in Idaho (Snyder et al., 1975),British Columbia (Feller and Kimmins, 1984), and Oregon (Harr andFredriksen, 1988). Phosphorus concentrations can also showmuted responses to logging activities, but in general they werenot observed to increase after harvest, possibly due to fixation insoils (Tiedemann et al., 1988; Salminen and Beschta, 1991).

More intense and widespread harvest impacts generally creategreater changes in nutrient cycling and stream chemistry (Felleret al., 2000; Fowler et al., 1988; Stark, 1979). Often site preparation(e.g., broadcast prescribed burning, mechanical treatments,herbicides) or other management practices (e.g., competitionrelease sprays, fertilization, roads) confound our ability todetermine the specific practice(s) contributing to changes (Fre-driksen, 1973; McBroom et al., 2007). These changes may be short-lived or persistent and must be assessed in relation to managementstages across a watershed, as well as compared to naturaldisturbance events, to gain a better understanding of the processesthat control nutrient dynamics.

These studies reveal the variability of responses that may arisebetween watersheds, indicating the need to improve our under-standing of N and P concentration responses to timber harvest bothwithin and between a broad range of watershed conditions. Animportant identified knowledge gap is the lack of understandingregarding stream chemical budgets, especially longitudinal varia-tion in stream chemistry (Feller, 2005). Relatively long-term N andP records, especially in relatively undisturbed mountain streams,are also rare (Vanderbilt et al., 2003).

The objectives of this study were to: (1) assess the effects ofcontemporary timber harvest practices on nutrient concentrationsand cycling and (2) provide representative nutrient concentrationdynamics for both undisturbed and intensively managed forestedmountain watersheds in the inland Pacific Northwest.

2. Study site

2.1. Site characteristics

The Mica Creek Experimental Watershed (MCEW) is a pairedand nested watershed study area in Shoshone County in northernIdaho (Fig. 1). The watershed is privately held by PotlatchCorporation. It is the site of one of the first comprehensive

assessments of contemporary (i.e., turn of the 21st Century)timber harvest and riparian buffer management practices in aworking forestland in the United States (McGreer et al., 1995;Gravelle and Link, 2007; Hubbart et al., 2007; Karwan et al.,2007). The entire study area is approximately 2700 ha and ismanaged primarily for timber production. The experimental areais located at approximately 47.178N latitude, 116.288W long-itude, and ranges from 1000 to 1600 m above mean sea level(amsl). Mica Creek is a tributary of the St. Joe River, and theresearch area includes the headwaters of the west fork and mainstem of Mica Creek. Average annual air temperature is 5 8C andaverage annual precipitation is approximately 1450 mm, overhalf of which typically falls as snow. The geology is characterizedby silt loam soil and parent material is comprised mainly of gneissand quartzite metasediments. The area is mountainous, with V-shaped valleys and moderately sloped hillsides of 15–30%.Stream gradients range from 3% to 14%, and large and smallorganic debris provide step-pool configurations, with riffle-runhabitats in lower gradient reaches. Substrate composition variesbetween reaches, but the majority of substrate consists of largegravels and sands.

The study area contains naturally regenerated, secondgrowth forest that is approximately 70–80 years old. There isa mixture of tree species, with the majority comprised of Grandfir (Abies grandis), Douglas-fir (Pseudotsuga menziesii), Westernred cedar (Thuja plicata) and Western larch (Larix occidentalis).Within the MCEW there are several stands of old growthWestern red cedar that are found occasionally in the riparianzones. Other species commonly found in the riparian zoneinclude Grand fir (Abies grandis) and Engelmann spruce (Picea

engelmanni), with high densities of alder (Alnus spp.) and otherherbaceous vegetation.

Fig. 1. Mica Creek Experimental Watershed, with road construction/timber harvest

areas and Parshall flume monitoring sites.

J.A. Gravelle et al. / Forest Ecology and Management 257 (2009) 1663–16751664

2.2. Site history and experimental treatments

The experimental area was a second growth forest that was notharvested since the early 1930s. In the early 1900s, approximately95% of the overstory canopy was removed from the MCEW usingsteam donkey, horse logging, log transport flumes, and a narrowgauge railroad (Schultz, 2000). Between the early 1930s andinitiation of experimental treatments in the late 1990s, the site wasrelatively undisturbed and is typical of many second growthforests in the region.

In 1990–1991, a paired and nested catchment study wasinitiated by installing seven steel Parshall flumes as long-termstream gaging stations. These seven flume (F) stations (F1–F7)isolate subbasins with clearcut (F1), partial cut (F2), downstreamcumulative (F4 and F7), and control (F3, F5, F6) treatments forcomparative analyses (Fig. 1). After installation of the Parshallflumes, Campbell Scientific CR10 dataloggers, Riverside Technol-ogy pressure transducers, ISCO 3700 portable water samplers, andCampbell Scientific 107 water temperature sensors were added tocollect data beginning in 1991. Meteorological instrumentationconsisted of both all-season and tipping-bucket precipitationgauges and shielded air temperature sensors. A NRCS SNOTEL sitewas also located within the experimental area and providedinformation on snow water equivalent (SWE), snow depth, andprecipitation.

Three headwater reaches (F1, F2, and F3) on the west fork ofMica Creek were used in a paired design to assess the effects ofclearcut and partial cut harvest practices. Flumes 4 and 7 wereused to assess downstream cumulative impacts. Table 1 sum-marizes the F1–F7 drainage areas, harvest treatment information,and channel characteristics.

In order to isolate the effects of road construction from theeffects of timber harvesting, these activities were separated intime. Roads to allow access for timber harvesting were constructedin the fall of 1997. Harvesting occurred 4 years later with acombination of line and tractor skidding in the late summer andfall of 2001. Clearcut harvesting occurred over 50% of the areadraining to F1, and partial-cut harvesting with a 50% canopyreduction target occurred over 50% of the area draining to F2,equating to approximately 25% canopy removal in this watershed(Table 1, Fig. 2).

All timber harvest and road construction activities wereconducted in compliance with Idaho Forest Practices Act andstream protection zone (SPZ) requirements. Contemporary BMPsto reduce erosion and sedimentation include outsloped roadconstruction, installation of relief culverts prior to reaching streamcrossings, and creation of filter windrows along road fillslopes. InIdaho, SPZ requirements are divided based on two streamclassifications (IDL, 2000):

� Class I streams: Class I streams are used for domestic watersupply or are important for the migration, rearing, and spawningof fish (fish bearing). A Class I SPZ must be at least 75 feet(22.9 m) wide on each side of the ordinary high water mark

(definable bank). Harvesting is permitted, but 75% of existingshade must be retained. There are also leave tree requirementsfor a target number of trees per 1000 linear feet (305 m),depending on stream width. In Mica Creek, this was roughly 200trees in the 3–12 in. (8–30 cm) diameter class per 305 m of SPZ.� Class II streams: Class II streams are non-fish bearing. The Class II

SPZ in Idaho is 30 feet (9.1 m) of equipment exclusion zone oneach side of the ordinary high water mark (definable bank);skidding logs in or through streams is prohibited. There are noshade requirements and no requirements to leave merchantabletrees.

Two-sided riparian buffers were left on all Class I streamsduring harvest operations. No trees, with the exception of severalcedars in F2, were removed from the riparian buffers of the Class Istreams. Timber was removed from both sides of the Class IIstreams. In the post-harvest and post-burn conditions, Class IIstreams in clearcut treatments had only a small amount of greentree retention within the riparian zone, while in partial cuttreatments equal amounts of canopy cover (approximately 50%)were removed from both sides of the stream.

In the fall/winter of 2002 and 2003, heavy concentrations ofpost-logging debris at the harvest units were burned. At theclearcut areas, this included stacked slash piles and landing areas.Burning the heavy fuels concentrated at the landing areas iscommon hazard abatement work for spring broadcast burns, as

Table 1Watershed treatments and physical characteristics at Mica Creek.

Site Drainage

area (ha)

Treatment

type (%)

Drainage area affected

by treatment (%)

Canopy removal by

treatment (%)

Wetted

width (m)

Gradient (%) Substrate size,

d50 (mm)

Fines �2 mm (%)

F1 140 Clearcut 50 50 1.5 5 50 7

F2 175 Partial cut 50 25 1.9 8 50 9

F3 210 Control 0 0 1.9 14 70 14

F4 600 Cumulative 26 13 2.8 3 40 19

F5 650 Control 0 0 2.6 3 30 22

F6 1450 Control 0 0 3.1 5 110 12

F7 1200 Cumulative 13 6.5 3.5 3 80 14

Fig. 2. MCEW 2002 aerial photo showing clearcut and partial cut harvest units.

J.A. Gravelle et al. / Forest Ecology and Management 257 (2009) 1663–1675 1665

these areas can potentially generate summer holdover fires. In thepartial cut areas, stacked logging slash piles were also burned. InMay 2003 the clearcut areas were broadcast burned and replantedwithin 2 weeks with seedlings from a mixture of tree species. Nofertilization or widespread herbicide spraying took place withinthe post-harvest units during this investigation. The wetterriparian areas of the Class II streams did not burn, and ocularsurveys in the following spring runoff period indicated no directhillslope erosion inputs to the stream channel resulting from theprescribed burn.

3. Methods

3.1. Sampling protocol

Grab samples were taken monthly above each of the flume sitesbeginning in June 1992. High density polyethylene (HDPE) 250 mlbottles were used for total Kjeldahl nitrogen (TKN), nitrate + nitrite(NO3 + NO2), TP, total ammonia nitrogen (TAN) consisting of un-ionized (NH3) and ionized (NH4

+) ammonia, and unfilteredorthophosphate (OP) samples, which were fixed with 0.30 ml ofH2SO4 prior to sampling. TAN laboratory analyses were onlyavailable after August 1999. Filtered OP samples were also takenwith 60 ml syringe filters, where 20 ml of water was forcedthrough a 0.45-mm nylon filter into a HDPE 30 ml bottle. Sampleswere kept cool in the field, frozen, and shipped to the laboratory foranalysis. Samples were taken throughout the year except for somewinter months where inclement weather conditions made flumestation visits impractical.

Water samples were analyzed at water quality laboratories atOregon State University in Corvallis, OR (1992–1997), the NationalCouncil for Air and Stream Improvement, Inc. (NCASI) in Anacortes,WA (1998–1999), and NCASI in Corvallis, OR (2000–2006). Equip-ment used for analyses included the Perstorp Analytical 550 Auto-analyzer, Alpkem Flow Solution 3000, and Enviroflow 3000 wateranalyzers. The QA/QC program from 1998 to 2006 included analysesof method blanks, independent calibration verifications (ICVs) andcontinuing calibration verifications (CCVs) as well as precision andaccuracy assessments with each sample set for each target analyte.Precision was assessed by analyzing duplicate samples andcalculating the relative standard deviation. Accuracy was assessedby calculating the recovery of the target nutrient standards (ICV andCCV) and fortifying one sample in each set (20 or less) with the targetnutrient to determine matrix accuracy by calculating the spikerecovery. Table 2 summarizes methods, calibration range, andmethod detection levels for the nutrients measured.

3.2. Data analysis

Laboratory results from each of the seven flume sites were usedin a before–after/control-impact paired-series (BACIPS) design

(Stewart-Oaten et al., 1986; Stewart-Oaten, 2003) to assesschanges in nutrient concentrations as a result of road constructionand timber harvest activities. This method has been used in similarinvestigations involving harvest effects on water quality (Dahlg-ren, 1998). There were approximately 5 years of calibration datafrom June 1992 to August 1997 (calibration), 4 years of post-roadconstruction data from September 1997 to June 2001 (post-roads),and nearly 5 years of post-harvest data from July 2001 to May 2006(post-harvest). September and October 1997 were included in thepost-road timeframe for the analysis in order to include roadconstruction effects. Likewise, vegetation removal due to timberharvest began in mid-2001, so beginning in July 2001 allsubsequent data were included in the post-harvest time period.Sample sizes for each time period were n = 54 (calibration), n = 30(post-road), and n = 50 (post-harvest). Data were analyzed usingStudent’s t-tests between the observed and predicted data valuesfor post-treatment time periods. Predicted data values at treat-ment locations for post-treatment time periods were calculatingusing linear regression relationships between treatment andcontrol site pairs during the calibration time period. Using thenested and paired design at the MCEW, the treatment watersheds(F1, F2, F4, and F7) were compared against the control watershed(F3, F5, or F7) that closely matched their respective drainage areas.This resulted in comparisons between F1 and F3 and F2 and F3 atthe smallest scale (140–210 ha), F4 and F5 at the intermediatescale (600–650 ha), and F7 and F6 at the largest scale (1200–1450 ha). The various nutrient concentrations were then tested fordifferences between paired treatment and control sites based onthe BACI sampling design. All statistical computations wereperformed using the statistical package R (Ripley, 2001; Venablesand Ripley, 2002). Changes in nutrient concentrations were testedfor statistical significance at the a � 0.05 level.

One of the difficulties of analyzing the nutrient data from thisstudy is the large number of samples that had concentrationsbelow the detection limits (ND). For example, about 50% of TANsamples were ND. This is a common problem for forest watersheddata, where concentrations are typically low. For the purposes ofthis study it is assumed that ND data have values of half of thedetection limit. This is a common method used to address thisproblem (IDEM, 2008).

Since flow data were available at the sample sites, it waspossible to develop an estimate of total nutrient export from eachwatershed. An annual estimate for NO3 + NO2 was calculated ateach of the sample sites for each time period based on averagemonthly streamflow and the monthly concentration values.Average monthly flows were calculated from 30-min intervaldata collected at each Parshall flume. The average monthly flowvalue was then multiplied by the monthly sample concentrationto derive total estimated monthly export. Fig. 3 shows the year-round nutrient sampling dates relative to streamflows at F1. Forthe few months where concentration values were missing

Table 2Laboratory analytical methods summary.a.

Parameter Method number Description Calibration range Method detection

limit range

Total Kjeldahl nitrogen (TKN) USEPA 351.3 Colorimetric automated salicylate 0.05–5.00 mg-N/L 0.02–0.05 mg-N/Lc

Total ammonia nitrogen

(TAN) [NH3 + NH4+]

USEPA 350.1 Colorimetric automated phenate 0.01–2.00 mg-NH3/L 0.01–0.02 mg-NH3/L

Nitrate + nitrite (NO3 + NO2) USEPA 363.2 Colorimetric automated cadmium reduction 0.01–5.00 mg-N/L 0.01–0.02 mg-N/L

Total phosphorusb (TP) USEPA 365.4 and 365.1 Colorimetric automated ascorbic acid 0.01–2.00 mg-P/L 0.01–0.02 mg-P/L

Orthophosphate (OP) USEPA 365.1 Colorimetric automated ascorbic acid 0.01–2.00 mg-P/L 0.01– 0.02 mg-P/L

a Analyses conducted from 2000 to 2006 by NCASI, Corvallis, OR.b From 2000 to August 2006 total phosphorus was determined using a kjeldahl digestion followed by analysis using USEPA method 365.4. After August 2006 a persulfate

digestion was utilized followed by USEPA method 365.1.c This value should be viewed as a level of quantitation as the TKN results are effected by the background level of the digested blank which has varied over the time of this

study.


(generally winter months with low flows), a simple linearinterpolation was used to estimate a concentration for thatmonth. Annual estimates were calculated by taking the sum ofthese monthly export values and dividing by the number of years(number of months/12) for each time period. Results werecalculated as total mass (kg yr�1) and total mass per drainage area(kg ha�1 yr�1).

4. Results

Results focus on changes in nutrient concentrations betweenthe calibration period, road construction (post-road), and timberharvest (post-harvest). Student’s t-test p-values between treat-ment/control pairs, along with temporal trends of TKN, TAN,NO3 + NO2, TP, and OP, are presented.

4.1. Total Kjeldahl nitrogen

Based on the Student’s t-test results (Table 3), the onlystatistically significant change detected for TKN occurred as asmall decrease in concentrations at F2 (partial cut site) during thepost-harvest period (p = 0.016). Temporal trends from 1992 to2006, with treatment periods noted, are shown in Fig. 4A.

4.2. Total ammonia nitrogen

There was no calibration time period for TAN, as no data werecollected until August 1999. Despite the shorter record and lack ofcalibration data, Student’s t-test results were run between post-road and post-harvest data to determine if any changes occurredfollowing timber harvest. No statistically significant results werefound (see Table 3). Mean concentrations were low (�0.01 mg-NH3 L�1) and near the method detection limit, and the maximumobserved concentration (0.10 mg-NH3 L�1) occurred at both F4 andF5 (a control watershed) in October 2002 (Fig. 4B). Concentrationsappeared to remain relatively stable between seasons, years, andtreatment periods, as the largest difference between the post-road

Fig. 3. Nutrient sampling dates relative to streamflows at F1 for the different

treatment phases: (A) calibration: 1992–1997, (B) post-road: 1997–2001, and (C)

post-harvest: 2001–2006.

Table 3Observed nutrient concentrations, estimated concentration change, and Student’s

t-test p-value results.

TKN TANa NO3 + NO2 TP OP

Post-road

F1 (clearcut)

Observed (mg L�1) 0.17 – 0.06 0.05 0.01

Estimated change (mg L�1) +0.04 0.00 0.00 0.00

p-Value 0.219 0.794 0.698 0.143

F2 (partial cut)

Observed (mg L�1) 0.14 – 0.04 0.03 0.01

Estimated change (mg L�1) 0.00 +0.01 +0.03 0.00

p-Value 0.942 0.414 0.352 0.107

F4 (cumulative)

Observed (mg L�1) 0.13 – 0.04 0.04 0.01

Estimated change (mg L�1) �0.03 �0.01 0.00 0.00

p-Value 0.397 0.661 0.548 0.822

F7 (cumulative)

Observed (mg L�1) 0.04 – 0.04 0.05 0.02Estimated change (mg L�1) +0.04 +0.03 +0.01 +0.01p-Value 0.314 0.079 0.193 0.022

Post-harvest

F1 (clearcut)

Observed (mg L�1) 0.05 0.01 0.35 0.02 0.02

Estimated change (mg L�1) �0.01 0.00 +0.29 �0.01 +0.01

p-Value 0.301 0.630 <0.001 0.233 0.427

F2 (partial cut)

Observed (mg L�1) 0.04 0.01 0.05 0.02 0.01

Estimated change (mg L�1) �0.02 �0.01 +0.03 �0.01 0.00

p-Value 0.016 0.096 <0.001 0.131 0.562

F4 (cumulative)

Observed (mg L�1) 0.04 0.02 0.1 0.02 0.01

Estimated change (mg L�1) �0.01 +0.01 +0.07 �0.01 0.00

p-Value 0.220 0.360 <0.001 0.733 0.179

F7 (cumulative)

Observed (mg L�1) 0.04 0.01 0.05 0.02 0.02Estimated change (mg L�1) �0.01 0.00 +0.04 0.00 +0.01p-Value 0.109 0.781 <0.001 0.937 0.007

All p-values are based on comparison of 1992–1997 (calibration) to 1997–2001

(post-road) and 2001–2006 (post-harvest) time periods except for the TAN

comparision where water analysis results were available after August 1999. The

four treatment sites were compared against control sites as follows: F1 (50%

clearcut) and F2 (50% partial cut) compared to F3, F4 (cumulative) to F5, and F7

(cumulative) to F6. Bolded values indicate significance at a � 0.05.a Since TAN (NH3 + NH4

+) data collection started in August 1999, comparison

between post-road and post-harvest time periods only.


and post-harvest treatment periods at any of the sites was<0.01 mg-NH3 L�1.

4.3. Nitrate + nitrite (NO3 + NO2)

No changes in monthly NO3 + NO2 concentrations weredetected following road construction, but every treatment siteshowed statistically significant increases (p < 0.001) followingtimber harvest (Table 3, Fig. 5). Increases at the clearcut treatmentsite (F1) were greatest, where mean monthly concentrationsincreased from 0.06 mg-N L�1 during the calibration and post-roadperiods to 0.35 mg-N L�1 during the post-harvest period. Due tothe paired and nested design at the MCEW, it is also possible toassess longitudinal effects of timber harvest on nutrient concen-trations. Fig. 6 shows F1 (clearcut), F4 (cumulative), and F7(downstream cumulative) monthly NO3 + NO2 concentrations. F1,the site impacted with the greatest proportion of upstream landcover change, shows the highest concentrations following timberharvest, with attenuation further downstream. This pattern canalso be observed in the mean monthly concentrations betweentime periods (Fig. 7). At F1, the post-harvest mean NO3 + NO2

concentration increased by 0.29 mg-N L�1, at F2 by 0.03 mg-N L�1,at F4 by 0.07 mg-N L�1, and at F7 by 0.04 mg-N L�1(Fig. 7). Allcontrol sites (F3, F5, F6) displayed differences of <0.01 mg-N L�1

between the calibration and post-harvest periods.The peak concentration of 0.89 mg-N L�1 occurred at F1 in April

2004, with mean monthly concentrations of 0.43 mg-N L�1 and0.59 mg-N L�1 in water years (October–September) 2004 and2005, respectively.

Annual estimates of total NO3 + NO2 export were alsocalculated at each site for all time periods (Table 4). The highestestimated total export (442 kg yr�1) and highest export relative todrainage area (3.16 kg ha�1 yr�1) occurred during the post-harvesttime period at F1.

4.4. Total phosphorus

There were no significant changes in TP detected following roadconstruction or timber harvest. Observations at all sites showedmean TP concentrations of approximately 0.03 mg-P L�1. Temporaltrends from 1992 to 2006 showed concentrations remaining lowthroughout the study period (Fig. 8A).

Fig. 4. Monthly TKN (A) and TAN (B) data by treatment period, 1992–2006. F1 (clearcut), F2 (partial cut), and F3 (paired control) data are shown.

Table 4Estimated annual NO3 + NO2 export for each watershed by treatment period.

Calibration Post-road Post-harvest

F1: clearcut 59.6 (0.43) 60.2 (0.43) 442.1 (3.16)F2: partial cut 11.2 (0.06) 56.5 (0.32) 59.0 (0.34)F4: cumulative 81.0 (0.14) 162.9 (0.27) 431.3 (0.72)F7: cumulative 73.0 (0.06) 178.4 (0.15) 323.2 (0.27)F3: control 16.4 (0.08) 35.3 (0.17) 23.1 (0.11)

F5: control 43.0 (0.07) 90.8 (0.14) 46.1 (0.07)

F6: control 184.5 (0.13) 276.6 (0.19) 85.0 (0.06)

Estimates are based on monthly samples and discharge data (recorded every

30 min) from each site. Total export is listed in kg yr�1, with kg ha�1 yr�1 in ( ).

Bolded values indicate where concentration differences were found to be significant

at a � 0.05.


4.5. Orthophosphates

The only site to show statistically significant changes inOP concentrations following treatments was F7, the down-stream cumulative site (see Table 3). Small increases ofapproximately 0.01 mg-P L�1 were detected following road

construction (p = 0.022) and timber harvest (p = 0.007).Mean concentrations were relatively constant around 0.01mg-P L�1 at all sites. Maximum OP concentrations reached0.07 mg-P L�1 at all seven sites during the fall of 2004, whichwas during a period of high rainfall and moderate stormflows(Fig. 8B).

Fig. 5. Monthly NO3 + NO2 data by treatment period, 1992–2006 for (A) F1 (clearcut), F2 (partial cut), and F3 (paired control) and (B) downstream sites F4 (cumulative), F7

(cumulative), F5 (control), and F6 (control).

Fig. 6. Monthly NO3 + NO2 data by treatment period, 1992–2006, showing longitudinal cumulative effects of post-harvest nitrate/nitrite concentrations. F1 (clearcut), F4

(cumulative), and F7 (downstream cumulative) sites are shown.


5. Discussion

The long-term monitoring at the MCEW provides valuableinsight to nutrient concentration responses to different aspects oftimber harvest activities, including road construction, treeremoval, and burning. An additional benefit to understandingnutrient dynamics at the MCEW is the collection of relatedenvironmental data that are otherwise unknown variables in otherwatersheds. Both abiotic (streamflow, water temperature, sedi-ment) and biotic (fish, amphibian, macroinvertebrate) responses to

timber harvest activities were also measured. The streamflow data,in conjunction with nutrient concentration results, permitestimation of stream chemistry budgets and estimates of bothnutrient load responses following harvest.

5.1. Total Kjeldahl nitrogen

When assessing temporal trends through time for TKN,equipment detection capabilities appeared to hinder the abilityto potentially detect small changes due to road construction andtimber harvest activities. Although a significant change in TKNconcentration at F2 (p = 0.016) was detected after timber harvest,there were also improved laboratory TKN detection capabilities forthis time period. During the time of this research, the NCASIlaboratory improved its ability to lower detection limits fornutrient-related projects, and TKN data at the MCEW attest to thissuccess. Although a statistically significant change was noted,post-treatment concentrations declined by only 0.02 mg-N L�1,likely due to the difference in values placed on non-detectablevalues, which constituted approximately 60% of all samples at F2.Reduced variability in post-harvest mean monthly concentrations(see Fig. 4) was observed at all sites as blank corrections usingcopper sulfate digestion were employed to enhance accuracy.

Despite varying detection limits during the calibration andpost-road time periods, the MCEW still exhibited low TKNconcentrations (<0.20 mg-N L�1) compared to other forestedwatersheds. Binkley et al. (2004) reported that dissolved organicN for forested watersheds nationwide averaged 0.31 mg-N L�1 andboth western and coniferous watersheds were found to generallyhave higher concentrations of organic N.

Fig. 7. Comparison at all sites of NO3 + NO2 concentrations by treatment periods

(calibration: 1992–1997; post-road: 1997–2001; post-harvest: 2001–2006). Values

represent the sample concentration average of the monthly samples for each time

period.

Fig. 8. Monthly TP (A) and OP (B) data by treatment period, 1992–2006. F1 (clearcut), F2 (partial cut), and F3 (paired control) data are shown.


5.2. Total ammonia nitrogen

Biological activity by saprophytic bacteria accounts forproduction of TAN (Baron, 1979). Despite the shorter data record,there appear to be no high concentrations of TAN in MCEWstreams, and mean concentrations near 0.01 mg-NH3 L�1 approachdetection limits. This is typical of what is expected for highgradient forest streams (Binkley et al., 2004). Given the lowconcentrations observed, pH values ranging from approximately7.7–8.2, and relatively low water temperatures (summer meandaily temperatures ranging from 7 to 12 8C) in MCEW streams, thefraction of the TAN that occurs in un-ionized (NH3) form is unlikelyto pose any toxicity threat to fish or other organisms (US EPA,1998).

5.3. Nitrate + nitrite (NO3 + NO2)

The most notable changes following timber harvest occurred inNO3 + NO2 concentrations. Although the accuracy of these resultsmay be somewhat affected by limitations associated with monthlysampling that miss potential daily or hourly concentrationfluctuations, they agree with other studies that found increasesin NO3 + NO2 concentrations following timber harvest (Likenset al., 1970; Snyder et al., 1975; Feller and Kimmins, 1984; McHaleet al., 2007).

It should be noted that F1 consistently had higher NO3 + NO2

concentrations than the other sites during all time periods (Fig. 7).Variation in NO3 + NO2 levels at the MCEW may be due to slightlydifferent geology and geologic weathering processes, which haspreviously been observed to play a particularly important role instream water NO3

� concentrations (Clow and Sueker, 2000;Williard et al., 2005). However, a vegetation survey at the MCEWin 1999 revealed the presence of N-fixing alder upstream from F1,which may have contributed to consistently higher levels of N andNO3 + NO2. The role of riparian vegetation on elevated nitratelevels was also observed in the coastal Pacific Northwest (Comptonet al., 2003; Homann et al., 1994; Wigington et al., 1998). The scaleof the NO3 + NO2 response observed here may in part reflect asubbasin that was predisposed to larger N losses. This is considereda factor in some watersheds in the northeastern U.S., whereatmospheric deposition and base ion depletion are believed tohave predisposed watersheds to NO3 + NO2 losses followingclearcut harvesting (McHale et al., 2007).

What is especially interesting is that the nitrate responseappeared to progressively increase during the post-harvest periodthrough late 2005, followed by an apparent decline. In the clearcutF1 watershed there is also an intra-annual temporal pattern of theNO3 + NO2 dynamics (Fig. 9). After timber harvesting in thesummer and fall of 2001, NO3 + NO2 concentrations noticeablyincreased in February 2003. This was not the first runoff periodafter the harvest, but it was the first runoff period after hazardabatement burning. There was an even larger response in thespring of 2004. Several factors may be responsible for this pattern.The hazard abatement (landing areas and slash piles) andbroadcast burning activities that occurred from fall of 2002 toMay 2003 further reduced vegetation and hence may have reducedplant uptake and accelerated mineralization rates (Fredriksen,1973). The spring 2003 peak concentrations could be a reflection ofthe hazard abatement burns. The broadcast burn did not occuruntil the end of May 2003 (at the end of the 2003 spring runoff),and the next runoff period was spring 2004. This coincides with thehighest concentration observed at F1, which was 0.89 mg-N L�1, inApril 2004.

Other than post-fire effects, there also may have been otherprocesses that contributed to the observed nutrient dynamics.Slash and other debris left on the site may have experienced delays

in decay and conversion from organic-N to more mobileNO3 + NO2, especially since this high elevation watershed issubject to long periods of snow and freezing temperatures. Inaddition to runoff and decay/conversion delays that occur in asnow-dominated environment, subsurface flow pathways forforest watersheds can also create long delays. Dunn et al. (2007)found mean residence times (MRT) for runoff from watershedsranging from months to several years. Another key factordetermining N flux rates could be the amount of carbon (C)available in soils and aboveground biomass. From the abovegroundbiomass, fungal fruiting bodies on decomposing logs potentiallyprovide a major pathway for N export (Harmon, 1992). Pinpointingthe precise causes and contribution of these factors are beyond thescope of this investigation, but it is valuable to recognize thatmultiple processes likely affected streamwater nutrient concen-tration levels following disturbance.

The partially harvested F2 watershed did not experiencebroadcast burning, but did have slash piles that were burnedwhen the clearcut hazard abatement burns also occurred. Theapparent (although muted compared to F1) NO3 + NO2 response inMarch 2003 (Fig. 9) may reflect concentrated sources from thoseburns areas. Given that this pulse was observed at F2 following theburning of slash piles, it does suggest that the hazard abatementburning in the clearcut areas also provided concentrated sources ofNO3 + NO2 that were observed at F1 during the spring 2003 runoffperiod. Higher severity burning was shown to enhance post-fire Navailability for increasing streamwater NO3 + NO2 concentrationsin Ontario (Bayley et al., 1992). At MCEW, high severity burningoccurred on landing areas and at slash piles, which may similarlyhave affected streamwater NO3 + NO2 concentrations at F1 and F2.

Year to year patterns must also be interpreted within theseasonal variations that are observed (Fig. 9). Typically, the highestconcentrations occurred in spring or early summer. In Oregon’swest central Cascades, higher nitrate concentrations werepronounced in younger stands during the wet season of Decemberto May (Cairns and Lajtha, 2005). Hale (2007) also observed a flushof NO3 + NO2 with fall storms and then a decline over the year inrain-dominated coastal Oregon watersheds. In snow-dominatedareas like the MCEW, fall rainfall can be unpredictable, andprecipitation often occurs as snow. For this reason, watersheds likeMica Creek probably delay this flush of nutrients until later in thewater year (late winter and spring), when rain-on-snow andsnowmelt runoff events typically occur. In streams with sub-stantial snow accumulation, these peak concentrations duringspring runoff are expected (Coats and Goldman, 2001).

Even with the observed increases in NO3 + NO2 observed in thisstudy, the absolute concentrations are low compared to otherforested watersheds. Binkley et al. (2004) reported that NO3 + NO2

concentrations nationwide for forested watersheds average0.31 mg-N L�1, which is a higher concentration than the partialharvest basin (F2) experienced for a peak concentration measuredfollowing harvesting. The increase to a peak of 0.89 mg-N L�1 forthe clearcut (F1) watershed is within expected ranges for anunfertilized forested watershed, as most peak concentrations forNO3 + NO2 have been observed to be generally <1.0 mg-N L�1

(Binkley et al., 1999).

5.4. Longitudinal effects and nutrient export

It is also noteworthy that the F1 and F2 post-harvest increasesin NO3 + NO2 were observed at the downstream cumulative sites(F4 and F7). Concentrations decrease (Figs. 5 and 6) from F1downstream to F4, and F7, at the mouth of the west fork of MicaCreek. Since the proportional amount of mature forest in theMCEW increases as one moves downstream, this dilution effect onconcentrations is not unexpected. Some uptake or processing of


these increased levels of nitrate could also be occurring in the2.1 km of stream between F4 and F7. Instream processing can playa major role in N retention and transformation (Mulholland, 1992;Peterson et al., 2001). This can occur over relatively short distance,as uptake rates of approximately 20% of terrestrial inputs occurredover a 300 m reach in Tennessee (Mulholland, 2004).

The dilution effect observed in concentration levels also showsup in total load estimates. Based on annual export estimatescalculated from the monthly samples (Table 4), it appears thatsome uptake could be occurring between F4 and F7, as differencesin total mass (kg yr�1) and drainage weighted export values(kg ha�1 yr�1) are much lower at F7. In the post-harvest timeperiod, there was an estimated annual NO3 + NO2 export of442 kg yr�1 (3.16 kg ha�1 yr�1) at F1, 431 kg yr�1

(0.72 kg ha�1 yr�1) at F4, and 323 kg yr�1 (0.27 kg ha�1 yr�1) atF7. The estimated post-harvest export load at F1(3.16 kg ha�1 yr�1) is similar to other findings, where watershedsdominated by young succesional status had estimated nitrateexports of up to 3.04 kg ha�1 yr�1 in the Cascade Mountains ofOregon (Cairns and Lajtha, 2005). A substantial increase in annualexport did occur in the post-harvest treatment phase, as theestimated annual export at F1 was only �60 kg yr�1

(0.43 kg ha�1 yr�1) for both the calibration and post-road timeperiods. The total export increase in the post-harvest time period isdue to both post-harvest NO3 + NO2 concentration increases andpost-harvest streamflow increases (Hubbart et al., 2007). Overall,the total estimated load budgets calculated at the MCEW arereasonable when compared to other undeveloped watersheds. In

an examination of 85 sites across the United States ranging from6.1 to 2500 km2, Clark et al. (2001) showed a median annualNO3 + NO2 yield of 0.26 kg ha�1 yr�1. At F6 (14.5 km2) the annualestimated yield ranged from 0.06 to 0.19 kg ha�1 yr�1 across alltime periods, and at F7 (12.0 km2) the annual estimated yieldranged from 0.06 to 0.27 kg ha�1 yr�1. Even higher values found inthe smaller watersheds like F1 during the calibration and post-road periods (0.43 kg ha�1 yr�1) are consistent with other findings,where median concentrations in smaller basins (<6 km2) havebeen found to be 200% higher than larger (>6 km2) basins (NCASI,2001).

Monthly sampling may cause limitations for examining allaspects of concentration changes and accurately estimating annualexport budgets. However, this is the nature of conducting researchin remote and challenging terrain. The MCEW is a relatively remotestudy area that provides problems for frequent sampling,especially during the snowmelt period. Visits to the samplingsites require snowmobile trips up to 80 km in length, and otherhazards such as downed trees, steep slopes with drifted-inroadbeds, and patchy snowcover prevent optimal temporalsampling during the snowmelt season. Despite these challenges,the year-round sampling appears to have provided relatively eventemporal coverage in relation to flow variability as shown in Fig. 3.

5.5. Total phosphorus

No changes were detected due to road construction or timberharvest. Observations at all sites showed mean TP concentrations

Fig. 9. Monthly NO3 + NO2 data separated by water year (WY) for (A) F1 (clearcut) watershed and (B) F2 (partial cut). F1 shows largest changes in 2003–2005 and apparent

decline in 2006. Note F2 showing a response in March 2003 and lower scale of change compared to F1 (clearcut) watershed.


of approximately 0.03 mg-P L�1. There appear to be some seasonalfluctuations, with higher concentrations during spring runoff andin fall rainy periods, when elevated nutrient flushing would beexpected to occur.

5.6. Orthophosphates

The only site to show significant change in OP followingtreatments was F7, the downstream cumulative site (Table 3).Although these increases were observed in both the post-road andpost-harvest time periods, statistical detection could be affected byconcentrations that regularly approach the limits of detection.Mean concentrations were relatively stable around 0.01 mg-P L�1

at all sites and during all treatment periods, and the significantchange detected at F7 was <0.01 mg-P L�1. Because of such lowconcentrations, this change could be an artifact of laboratoryprecision and accuracy. Regardless, F7 concentrations are stillessentially the same as F6, its paired control site, and the rest of thesites in the MCEW (Fig. 10).

5.7. Management disturbance compared with natural disturbance

It is also important to consider the immediate impacts of forestmanagement in the context of watershed outputs over an entirerotation and in comparison to natural disturbance events. Studiesof prescribed fires showed that severe burning can result inelevated nutrient loads. For example, in the H.J. AndrewsWatershed 1 in the Oregon Cascades NO3-N increased from lessthan 0.1 mg-N L�1 after clearcutting to greater than 0.4 mg-N L�1

following a prescribed burn (Fredriksen, 1973). In the Caspar CreekWatershed in northern California, the highest nitrate concentra-tions occurred in the water year immediately following harvestand burning (Dahlgren, 1998), as was similarly observed in thisstudy. A similar type of response could be expected for watershedsthat experienced severe wildfire, however these events cansometimes cover most or all of a watershed compared to 5–20%of a drainage area under recent alteration by typical commercialforest management. In severe wildfire situations, nitrate levels canremain elevated at least 9 years following disturbance (Bayleyet al., 1992).

5.8. Nutrient response to forest management compared to alternative

land uses

Although examining comparisons between land uses is beyondthe scope of this research, it can be helpful to compare this short-

term response in nutrient outputs to that from other land useactivities. For example, Omernik (1976) found that TP increasedwith an increase in the percent of area in agricultural or urban landuse. Poor and McDonnell (2006) compared nitrate concentrationsfrom subbasins in the same watershed in Oregon that had differentland uses and found fluxes of NO3 + NO2 that were approximatelythree to five times higher from agricultural and urban watershedsduring storm events than from a forested watershed. In easternMontana, agricultural watersheds have reported TKN values of6 mg-N L�1 during the summer season (MDEQ, 2001). Overall, ithas been observed that agricultural stream basins have N and Pconcentrations almost ten times greater than forested basins(Omernik, 1977), and generally forested streams will have muchlower nutrient loads than streams associated with agriculture orurban land use (NCASI, 2001). While this research helpsdemonstrate that forestland generates much lower nutrientoutputs to streams when compared to other land uses, it shouldbe stressed that these comparisons do not reduce the need foreffective streamside management zones and BMPs for commercialforest operations.

6. Conclusions

Understanding stream chemistry, nutrient cycling, and nutrientloading is important in comprehensive watershed management. Asa relatively undisturbed watershed before road construction andtimber harvest, the MCEW provided opportunities to gatherinformation on background nutrient concentrations in undis-turbed forested watersheds as well as evaluate changes to N and Pconcentrations following harvest activities. Because the studydesign also isolated the road construction component from treeremoval, the BACIPS design enabled the effect of each impact to beevaluated separately.

The objectives of this investigation were to assess generalchanges in N and P concentrations before and after timber harvestactivities. Statistically significant increases in NO3 + NO2 wereobserved below harvest units but concentrations were diluted andfurther attenuated as they moved downstream. Increases inNO3 + NO2 appeared to be delayed after harvesting and may bepartially a response to burning of slash. In 2006, 3 years after post-harvest burning, there appeared to be the beginning of a recoveryback toward pre-treatment NO3 + NO2 concentrations. Othernutrients showed little or no response to forest management.

Results from this study show an increase in NO3 + NO2 inresponse to forest management. Based on longitudinal concentra-tion and annual export estimates, it also appears that dilution andpossibly attenuation occurs as flow moves downstream. Down-stream dilution effects observed at MCEW, with no apparentdetrimental biological response, do not necessarily mean thatresults in other watersheds will be similar. Macroinvertebratecommunity responses in the MCEW were observed to be relativelyinsensitive to timber harvest, and there were no major changes tofunctional feeding groups (Gravelle et al., submitted for publica-tion). Differences in proportions and extent of land use, location ofharvest activities, vegetative cover, and physical characteristics ofthe stream could produce differing results than seen in thisresearch. Downstream reaches could also be more sensitive tochanges than at MCEW (e.g., upper reaches of streams with highturbulences are less sensitive to oxygen demand loads thandownstream low energy reaches). However, the observations atMCEW do point out that determining cumulative effects frommultiple forestry activities should take dilution, attenuation, andother processes into consideration.

A number of future investigations that are beyond the scope ofthis assessment would be useful to explore in order to more fullyunderstand the subtleties and effects of temporal variability in

Fig. 10. Comparison at all sites of OP concentrations by treatment periods

(calibration: 1992–1997; post-road: 1997–2001; post-harvest: 2001–2006). Values

represent the sample concentration average of the monthly samples for each time

period.


nutrient concentrations. Specifically, it would be beneficial tobetter understand nutrient concentration fluctuations betweenseasons and during storm runoff events. Frequent sampling duringnutrient flushing cycles would be useful to assess the specificmechanisms that produce higher nutrient concentrations (Kirch-ner et al., 2004). Continued monitoring at these sites will helpevaluate nutrient concentration trends following future MCEWtimber harvesting, stand regeneration, fertilization, and hydrologicrecovery.

Acknowledgments

The authors wish to recognize Dale J. McGreer for initiating theMica Creek Experimental Watershed study. Support of the basicdata collection was provided by Potlatch Corporation. Lab analyseswere provided by NCASI, with a special thanks to David Campbell,Bill Author, Nikki Frum, and Ron Messmer. This work was partiallysupported by the National Research Initiative of the USDACooperative State Research, Education, and Extension Service,grant number 2003-01264. Gratitude is also extended to TerryCundy at Potlatch Corporation for his help in this research as wellas two anonymous reviewers whose comments greatly improvedthis manuscript.

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Nutrient concentration dynamics in an inland Paci c Northwest … · 2013-02-20 · Nutrient concentration dynamics in an inland Paci c Northwest watershed before and after timber