effect of subalpine canopy removal on snowpack, soil solution, and nutrient export, fraser...

E�ect of subalpine canopy removal on snowpack,soil solution, and nutrient export, Fraser

Experimental Forest, CO

Robert Stottlemyer1* and Charles A. Troendle21U.S. Geological Survey, 240 W. Prospect Rd., Ft. Collins, CO 80526 USA

2U.S. Forest Service, Rocky Mountain Research Station, 240 W. Prospect Rd., Ft. Collins, CO 80526 USA

Abstract:Research on the e�ects of vegetation manipulation on snowpack, soil water, and streamwater chemistry and¯ux has been underway at the Fraser Experimental Forest (FEF), CO, since 1982. Greater than 95% of FEFsnowmelt passes through watersheds as subsurface ¯ow where soil processes signi®cantly alter meltwaterchemistry. To better understand the mechanisms accounting for annual variation in watershed streamwater ion

concentration and ¯ux with snowmelt, we studied subsurface water ¯ow, its ion concentration, and ¯ux inconterminous forested and clear cut plots. Repetitive patterns in subsurface ¯ow and chemistry were apparent.Control plot subsurface ¯ow chemistry had the highest ion concentrations in late winter and fall. When shallow

subsurface ¯ow occurred, its Ca2�, SO42ÿ, and HCO3

ÿ concentrations were lower and K� higher than deep¯ow. The percentage of Ca2�, NO3

ÿ, SO42ÿ, and HCO3

ÿ ¯ux in shallow depths was less and K� slightly greaterthan the percentage of total ¯ow. Canopy removal increased precipitation reaching the forest ¯oor by about

40%, increased peak snowpack water equivalent (SWE)4 35%, increased the average snowpack Ca2�, NO3ÿ,

and NH4� content, reduced the snowpack K� content, and increased the runo� four-fold. Clear cutting

doubled the percentage of subsurface ¯ow at shallow depths, and increased K� concentration in shallowsubsurface ¯ow and NO3

ÿ concentrations in both shallow and deep ¯ow. The percentage change in total Ca2�,SO4

2ÿ, and HCO3ÿ ¯ux in shallow depths was less than the change in water ¯ux, while that of K� and NO3

ÿ

¯ux was greater. Relative to the control, in the clear cut the percentage of total Ca2� ¯ux at shallow depthsincreased from 5 to 12%, SO4

2ÿ 5.4 to 12%, HCO3ÿ from 5.6 to 8.7%, K� from 6 to 35%, and NO3

ÿ from

2.7 to 17%. The increases in Ca2� and SO42ÿ ¯ux were proportional to the increase in water ¯ux, the ¯ux

of HCO3ÿ increased proportionally less than water ¯ux, and NO3

ÿ and K� were proportionally greaterthan water ¯ux. Increased subsurface ¯ow accounted for most of the increase in non-limiting nutrient loss. For

limiting nutrients, loss of plant uptake and increased shallow subsurface ¯ow accounted for the greater loss.Seasonal ion concentration patterns in streamwater and subsurface ¯ow were similar . Copyright # 1999 JohnWiley & Sons, Ltd.

KEY WORDS subsurface ¯ow; subalpine; clear-cut; snowpack; Colorado

INTRODUCTION

The major objectives associated with longer-term watershed studies include detection of ecosystem responsesto changes in atmospheric ion inputs, climate, and land use. In Rocky Mountain ecosystems, there isparticular interest in the fate of atmospheric inorganic nitrogen (N) inputs (Williams et al., 1996), and theinteraction between canopy removal and ecosystem N loss (Parsons et al., 1994; Reuss et al., 1997). Annualchange in the hydrologic cycle is the primary source of variation in longer-term trends in Rocky Mountain

CCC 0885±6087/99/142287±13$17�50 Received 27 April 1998Copyright # 1999 John Wiley & Sons, Ltd. Revised 30 November 1998

Accepted 18 March 1999

HYDROLOGICAL PROCESSESHydrol. Process. 13, 2287±2299 (1999)

*Correspondence to: Dr R. Stottlemyer, U.S. Geological Survey, 240 W. Prospect Rd, Ft. Collins, CO 80526, USA

streamwater chemistry and ¯ux (Stottlemyer and Troendle, 1992), and processes accounting for hydrologicvariation are numerous. Sublimation of snowfall intercepted by the forest canopy is signi®cant (Schmidt andGluns, 1991) as is change in evapotranspiration with canopy loss (Troendle and Reuss, 1997). In alpine andsubalpine ecosystems of the Central RockyMountains, 70% of annual precipitation and495% streamwaterruno� originates as snowmelt even though there is considerable year-to-year variation in snowpack waterequivalent (SWE). Stream ¯ow response rarely re¯ects summer rain because of high soil moisture de®cits.

Annual nutrient ¯ux is in¯uenced by the pathway meltwater takes to reach the stream, and the relativeproportions which occur as near-surface or deep subsurface ¯ow. Canopy removal increases chemical ¯uxfrom the ecosystem (Reuss et al., 1997). But the increase in ¯ux varies greatly among chemical species, andthe mechanisms for this are not well understood particularly in subalpine ecosystems. In watersheds wherethe annual hydrograph is dominated by snowmelt, the rise of soil water level into near-surface soils duringsnowmelt can signi®cantly alter soil water ion concentration and ¯ux (Stottlemyer and Toczydlowski, 1996).Canopy removal in the subalpine forest of the Fraser Experimental Forest (FEF) increases water ¯ow andthe percentage of total ¯ow occurring in shallow soils (Troendle and Reuss, 1997). This e�ect couldaccentuate ion ¯ux during snowmelt especially from clear cut plots where plant uptake is reduced andmineralization of logging debris and the forest ¯oor may be increased.

The long-term e�ect of canopy removal on hydrologic budgets in western coniferous watersheds, andespecially the FEF in Colorado, is well de®ned (Troendle and King, 1985). The net e�ect of canopy removalon increased ion ¯ux in other ecosystems has been documented, and often signs of recovery occur withinthree to ®ve years. But in Central RockyMountain subalpine ecosystems, revegetation following disturbanceis slow. In such ecosystems, longer-term study is necessary to quantify the magnitude of e�ect followingcanopy removal (Reuss et al., 1997; Troendle and Reuss, 1997).

In this paper, we compare seasonal change in snowpack and soil water chemistry from clear-cut andforested hillslope subsurface ¯ow plots with nearby Lexen Creek watershed to better de®ne watershedprocesses regulating streamwater chemistry and ¯ux. The objectives are 1) to summarize longer-term (1982±1993) FEF research on the e�ect of canopy removal on physical and chemical alteration of winter precipita-tion; 2) to examine how vegetation removal alters seasonal subsurface ¯ow amount, ¯ow path, and chemical¯ux; and 3) to compare change in seasonal streamwater chemistry during snowmelt with change in soilsubsurface ¯ow, ion concentration, and ¯ux.

METHODS AND MATERIALS

Site description

The FEF is 137 km west of Denver, CO, west of the Continental Divide (Figure 1). The mean annualtemperature at FEF headquarters (elevation 2725 m) is 0.5 8C, and ranges from ÿ10.3 8C in December to11.8 8C in July. There is no signi®cant annual temperature trend for the period of record. Annual precipita-tion averages 600 mm, with no signi®cant long-term trend. Monthly mean precipitation ranges from 41 mmin January to 74 mm in May. Annual precipitation amount was above average during 1982±86 (mean of700 mm), the ®rst half of this study. Precipitation amount increases about 20% per 300 m elevation aboveFEF headquarters.

Precipitation chemistry at FEF headquarters is summarized for the study period in Reuss et al. (1997).Except for Ca2�, FEF precipitation ion concentrations are generally lower than much of Colorado (NADP,1982±1996). However, with the relatively large precipitation amount, ion deposition is similar to otherhigher elevation sites away from the Front Range. Precipitation chemistry does not signi®cantly change withincreased elevation, but ion input and snowpack content do (Stottlemyer et al., 1997). Snowpack NH4

� andNO3

ÿ content is higher in the alpine and on NE aspects.Lexen Creek, a 124 ha undisturbed watershed draining into the West St. Louis watershed, has an easterly

aspect, and ranges in elevation from 2984 m at the stream gauging station to 3515 m at the summit of BottlePeak. The bedrock includes remnants of a sedimentary sandstone cap at upper elevations while gneiss and

Copyright # 1999 John Wiley & Sons, Ltd. HYDROLOGICAL PROCESSES, VOL. 13, 2287±2299 (1999)

2288 R. STOTTLEMYER AND C. A. TROENDLE

schist parent material underlie the entire watershed. Soils are dominated by gravelly sandy loams withalluvial soils near the streams. The soils are mostly Inceptisols (Retzer, 1962).

About 18% of the watershed is alpine meadow which generally occurs above 3350 m elevation. The lowerand mid-elevation subalpine forested slopes with southerly and easterly aspects are dominated by lodgepolepine (Pinus contorta Dougl.) while upper elevations and northerly aspects are vegetated by Engelmannspruce (Picea engelmannii Parry)-subalpine ®r [Abies lasiocarpa (Hook.) Nutt.].

Since 1965, the peak snowpack water equivalent (SWE) in the Lexen Creek watershed averages390 (standard deviation � 100) mm, and shows no signi®cant time trend. Canopy snowfall interceptionaverages about 35% (Troendle and Leaf, 1980; Stottlemyer and Troendle, 1994; Troendle and Reuss, 1997).

Field methods

Daily precipitation is estimated using Belfort raingauges at FEF headquarters and adjacent to the top ofthe Lexen Creek watershed (3350 m elev., Figure 1). Precipitation chemistry is monitored weekly usingAerochem Metrics precipitation collectors (`event collector') alongside the Belfort raingauges. Details of theprocedures used are in Stottlemyer and Troendle (1987, 1992).

The peak SWE is measured in early April. In Lexen Creek, 2.5 km from the subsurface ¯ow plots,72 permanent stations are monitored (Figure 1). Ten stations are sampled in each subsurface ¯ow plot.Snowpack chemistry is determined at the FEF headquarters precipitation station, 1 km from the subsurface¯ow plots, and at several other locations. Details on snowpack sampling methodology including chemistryare in Stottlemyer et al. (1997).

Streamwater discharge (L sÿ1) at the mouth of Lexen Creek watershed is measured with a 1208 V-notchweir. Streamwater is sampled weekly from late April until late October at points just above the gaugingstation (station 49, elev. 2985 m), at the stream's origin in the alpine zone (station 49.1, elev. 3415 m), and atthree intermediate locations. During 1983±85, Lexen Creek was sampled throughout winter when streamdischarge was at base ¯ow and unchanging (mean � 5 0.02 L sÿ1 haÿ1) to see if stream chemistry also was

Figure 1. West St. Louis drainage including locations of Lexen Creek watershed, subsurface ¯ow plots, and alpine and Headquartersmeteorological stations, Fraser Experimental Forest, CO


SNOW HYDROLOGY 38: SUBALPINE CANOPY REMOVAL 2289

unchanged (Stottlemyer and Troendle, 1987). Full details on surface water sampling protocols are inStottlemyer et al. (1997).

The two conterminous subsurface ¯ow plots, also located in the West St. Louis watershed, wereestablished in 1978 and 1979 (Figure 1), monitored in an undisturbed state until 1984 when one was clear cut(Troendle, 1985). Harvesting removed all boles and merchantable material, and the branches were scatteredand left on site. Surface disturbance was minimal. No access roads were constructed, and boles were removedby winching. The plots have a west aspect, uniform 30% slope, and were fully forested with Engelmannspruce, subalpine ®r, and lodgepole pine. The soil is a Dystric Cryochrept underlain by a relatively imper-meable clay. The plots were monitored each year from 1980±1990, and again in 1993. Chemistry monitoringbegan in 1982. The surface lateral ¯ow and soil water collectors were located at the toe of the slope, and were15 m long for the plot to be treated and 35 m long on the control. Lateral ¯ow was collected from on orabove the mineral soil, the shallow subsurface which includes the developed horizons (0±1 m deep), and thedeep subsurface (1±4 m). Hydraulic conductivity below 4 m is negligible (Troendle, 1985). The surface lateral¯ow collector consisted of a trough located immediately upslope of the plastic-lined retaining wall forcollecting subsurface ¯ow from the 0±1 m soil depth. To collect subsurface ¯ow from the 0±1 m depth, a stepand trough draining to the center was cut with a back hoe and the step lined with plastic sheeting. The trenchwas back®lled with gravel placed between the plastic lined retaining wall and the upslope face. A similardesign was used for collecting soil water from the 1±4 m depth with the retaining wall located immediatelydownslope from the retaining wall and collector for developed soil horizons (0±1 m deep). Saturated ¯owfrom the 1±4 m depth would enter the gravel and drop to the trough. Out¯ow from both plots and the threedepths was piped to individual H or HS ¯umes ®tted with stage height recorders. Samples for chemicalanalyses were collected immediately above each ¯ume. Sampling was more frequent when the hydrographwas rapidly changing. Details on the plot layouts are described in Troendle (1985, 1987). The unboundedplot drainage areas have been estimated as 0.11 ha for the clear cut, and 1.64 ha for the control (Troendle andReuss, 1997).

Laboratory methods

Samples were taken to the FEF ®eld laboratory. Precipitation and snowpack samples were brought up toroom temperature for pH, speci®c conductance, and alkalinity determinations (Clesceri et al., 1989). AFisher AC meter was used to measure pH and alkalinity. Alkalinity was not determined on samples before1985. On the low concentration precipitation and snowpack samples, alkalinity was determined by doubleendpoint potentiometric titration (pH 4.5 and 4.2, Clesceri et al., 1989). The higher concentration, streamand lysimeter samples were processed in the same manner as precipitation samples except alkalinity wasdetermined by titration with 0.02N H2SO4 to a pH 4.5 endpoint (Clesceri et al., 1989). Analyses were usuallycompleted within eight hours following sample collection.

Filtered (0.45 mm) subsamples were then taken to the U.S. Forest Service Water Quality laboratory in Ft.Collins and analyzed for macro ions (Ca2�, Mg2�, Na�, K�, NH4

�, PO4ÿ, Clÿ, NO3

ÿ, SO42ÿ) on an

automated Dionex Model 2020 ion chromatograph (Dionex Corp., Sunnyvale, CA). Additional details onsample handling, analytical methods, and quality assurance procedures are in Stottlemyer and Troendle(1987, 1992) and Stottlemyer et al. (1997). Statistical analyses were conducted using Systat STATS andMGLH subroutines (Wilkinson, 1990).

RESULTS

Snowpack

During the 1980±84 pretreatment period, the peak SWE averaged 250 mm on both subsurface ¯ow plots(Troendle and Reuss, 1997). Following treatment in 1984, mean peak SWE on the clear cut was 35% greater(p5 0.05) than the control (Figure 2). The snowpack Ca2� content increased 32% (p5 0.07) to 79 eq haÿ1,H� 8% (p5 0.05) to 23 eq haÿ1, NH4

� 82% (p5 0.01) to 11.3 eq haÿ1, NO3ÿ 54% (p5 0.01) to



26 eq haÿ1, and SO42ÿ 5% (not signi®cant) to 28 eq haÿ1. The relative increase in snowpack NH4

� andNO3

ÿ content in the clear-cut was greater and K�, H�, and SO42ÿ less than the increase in peak SWE.

Subsurface ¯ow and chemistry

After 1984, control plot surface lateral ¯ow occurred each year from 1985 to 1987, but lasted an average of<10 d yrÿ1. Only in 1985 did surface ¯ow last two weeks. Shallow subsurface ¯ow (0±1 m soil depth)occurred an average of 32 days each year, and deep ¯ow 55 days. On the clear cut plot, surface lateral¯ow was not recorded at any time, shallow subsurface ¯ow occurred an average of 21 d yrÿ1, and deep ¯ow55 days. Relative to the control, annual evapotranspiration from the clear cut was reduced by 190 mm and

Figure 2. Peak snowpack water equivalent (SWE) and selected ion content for control and clear cut subsurface ¯ow plots, 1983±1993.Clear cut was done in 1984 (vertical line). Samples were not collected in 1991 and 1992



canopy interception by 80 mm (Troendle and Reuss, 1997), annual runo� increased from 85 to 356 mm(p5 0.001), and the percentage of total subsurface ¯ow at the shallow depth more than doubled ( from 5.8to 12.8%).

Repetitive patterns in subsurface ¯owpath and ¯ow chemistry were apparent (Figures 3 and 4). Seasonalchange in subsurface ¯ow ion concentrations from the control plot was similar to streamwater at the mouthof Lexen Creek (Figure 5). In most years, subsurface ¯ow chemistry from the control plot showed the highestconcentrations of base cations (CB) (p5 0.001), SO4

2ÿ (p5 0.01), and HCO3ÿ (p5 0.05) in late winter and

summer. NO3ÿ concentrations were highest (p5 0.001) in winter and early spring, but quickly fell to

detection limits during and following snowmelt. When shallow subsurface ¯ow occurred, its Ca2�, SO42ÿ,

and HCO3ÿ concentrations were generally lower and K� higher than deep ¯ow (Figure 3), but the di�erences

were signi®cant (p5 0.01) only for SO42ÿ. In some years, ions as Ca2�, K�, and NO3

ÿ showed an increase inconcentration, or `pulse', with onset of peak snowmelt.

Clear cutting accentuated seasonal di�erences in subsurface ¯ow Ca2� and NO3ÿ concentrations, and

increased the mean annual K� concentration (p5 0.001) in shallow subsurface ¯ow and NO3ÿ con-

centrations in both shallow and deep ¯ow (p5 0.001) (Figure 3). Deep subsurface ¯ow NO3ÿ concentration

increased from 4 meq Lÿ1 on the control plot to 131 meq Lÿ1 on the clear cut.In the control plot, the mean percentage of Ca2�, NO3


ÿ ¯ux in shallow depthswas less and K� slightly greater than the percentage of total runo� (Figure 4). Canopy removal increasedthe mean percentage of total subsurface ¯ow occurring at shallow depths from 5.8% (5 mm) to 12.8%(46 mm). As in the control plot, the percentage of total Ca2�, SO4

2ÿ, and HCO3ÿ ¯ux in shallow depths

was less (1±4%) than the percentage of shallow subsurface ¯ow, but NO3ÿ ¯ux was 4% and K� 22%

greater.Following treatment, total annual subsurface ¯ow of all ions increased (p5 0.05) in the clear cut com-

pared to the control. Relative to the control, in the clear cut the percentage of total annual subsurface Ca2�

¯ux occurring at shallow depths increased from 5 to 12%, SO42ÿ 5.4 to 12%, HCO3

ÿ from 5.6 to 8.7%, K�

from 6 to 35%, and NO3ÿ from 2.7 to 17%. The increases in Ca2� and SO4

2ÿ ¯ux were proportional to theincrease in water ¯ux, HCO3

ÿ ¯ux was less, and NO3ÿ and K� greater.

Streamwater

At the mouth of the undisturbed Lexen Creek watershed, streamwater ion concentrations showedpronounced seasonal patterns (Figure 5). In 1982±1984 and 1996, the rising limb of the hydrograph was sorapid that few samples were collected. Streamwater CB, SO4

2ÿ, and HCO3ÿ concentrations were greater

(p5 0.001) during winter and late fall. NO3ÿ concentrations also were highest (p5 0.001) in winter and fall,

but quickly declined to detection limits during and following snowmelt. During 1985±1987, the stream wassampled throughout the year to better quantify high winter ion concentrations. The relative change instreamwater CB and SO4

2ÿ concentrations from winter to summer was more subdued than in the subsurface¯ow plots, but the change in HCO3

ÿ concentrations were similar.In 1988, we began sampling Lexen Creek streamwater in the alpine just below where the stream forms. The

objective was to relate change in streamwater chemistry to snowmelt, the dominant hydrologic driver. In thealpine there is a gradual seasonal trend in streamwater discharge during the year, but little diurnal change indischarge or ion concentration (Stottlemyer et al., 1997). Even in years of record large late snowpack, as1995, surface runo� from snowmelt is rarely observed (Stottlemyer, unpub. data).

Alpine streamwater ion concentrations showed a pattern quite di�erent than the watershed mouth. AlpineCB , SO4

2ÿ, and HCO3ÿ concentrations averaged half the levels at the mouth, and NO3

ÿ concentrations wereabout 50% greater. During snowmelt, alpine streamwater CB, NO3


ÿ showed littlechange in concentration compared with changes at the mouth of the watershed. The standard deviationexpressed as a percentage of the mean alpine streamwater ion concentration was 19% for CB, NO3

ÿ 14%,SO4

2ÿ 9%, and HCO3ÿ 45%. At the mouth of the watershed during snowmelt, the values were 88% for CB

concentration, NO3ÿ 183%, SO4

2ÿ 45%, and HCO3ÿ 85%.



Figure 3. Mean annual ion concentration (unweighted) in shallow and deep subsurface ¯ow from the control and clear cut plots,Fraser Experimental Forest, CO. Alkalinity was not determined until 1985. Date of clear cut shown by vertical line



Figure 4. Annual ion ¯ux from control and clear cut subsurface ¯ow plots, 1983±1993, W. St. Louis watershed, Fraser ExperimentalForest, CO. Date of clear cut shown by vertical line



DISCUSSION

Snowpack

At the FEF, canopy removal signi®cantly a�ects snowpack ion concentration and content (Stottlemyerand Troendle, 1994). While canopy removal has little e�ect on snowpack Ca2� and Mg2� concentrations,

Figure 5. Mean monthly streamwater discharge and ion concentration from mouth of Lexen Creek watershed, and alpine streamwaterion concentration, Fraser Experimental Forest, CO



K�, H�, and SO42ÿ concentrations decrease and NH4

� and NO3ÿ concentrations increase. In the subsurface

¯ow plots, the snowpack in the clear cut contained 30% less K� than the control plot. The loss of K� leachedfrom the canopy by throughfall and from organic debris in the snowpack reduce snowpack K� content. Inforest vegetation, most K� is contained in foliage cell solution where, with cell senescence, K� is rapidly lostby leaching. K� in organic debris or understory vegetation incorporated in the snowpack is quickly leachedwith snowmelt increasing snowpack K� content (Hornbeck and Likens, 1974; Stottlemyer et al., 1997). Thehigh snowpack NO3

ÿ and NH4� content in the clear cut re¯ects the absence of canopy interaction with

precipitation N inputs. The di�erences in snowpack N content between the clear cut and control plotsindicate an annual canopy retention of 0.2 kg N haÿ1 from snowfall. Arboreal lichen uptake of inorganic N,especially NO3

ÿ, has been found in the canopy of ®r forests (Olson et al., 1981), and canopy NH4�

uptake can reduce its ¯ux in throughfall (Parker, 1983). In the clear cut, the snowpack content of H�

increased slightly above levels in the control, but not proportional to the increase in peak SWE. Someincrease in snowpack H� content would be expected from increased precipitation inputs. We attribute theproportionally less snowpack H� content in the clear cut to reduced snowpack organic acids followingcanopy removal.

Unfrozen subalpine soils and periodic thaws at the FEF result in signi®cant snowpack solute loss beforepeak melt and minimize the potential for snowmelt ion pulses (Stottlemyer et al., 1997). More than 95% ofsnowmelt passes through the watersheds as subsurface ¯ow (Troendle and King, 1985) where soil processessigni®cantly alter meltwater chemistry.

Subsurface ¯ow

Beneath FEF canopies, ion concentrations in subsurface ¯ow typically show high spatial and temporalvariation (Stottlemyer et al., 1997). The longer winter contact period of soil water with the soil matrix,unfrozen subalpine soils which permit overwinter mineralization to occur, and limited biological uptakeaccount for the high subsurface ¯ow ion concentrations in winter. The periodic increase in subsurface ¯owion concentration with initiation of snowmelt has been observed in other studies in New England and theUpper Great Lakes (Rascher et al., 1987; Likens and Bormann, 1995; Stottlemyer and Toczydlowski, 1996).

In the clear cut, the gain in shallow subsurface ¯ow increased K� concentration. A more pronouncede�ect of canopy removal was the increase in subsurface ¯ow NO3

ÿ concentration. We attribute the two orthree year lag in increased NO3

ÿ concentration following treatment to slow reinvasion of nitri®ers inmature western conifer ecosystems, increased microbial immobilization rates in decaying organic debris, orcontinued short-term root NO3

ÿ uptake by the cut trees (Parsons et al., 1994). By 1993, NO3ÿ con-

centrations in subsurface ¯ow from the clear cut remained well above the control plot (mean of 49 versus2 meq Lÿ1). The increase in NO3

ÿ concentration from the clear cut at both subsurface ¯ow depths re¯ectsreduced root uptake and high NO3

ÿ mobility in soils. In contrast, subsurface ¯ow SO42ÿ concentrations on

the clear cut peaked in 1986 (Figure 3), then declined to below control plot levels likely due to dilution (Reusset al., 1997). FEF ecosystems appear to have limited capacity to adsorb SO4

2ÿ (Stottlemyer and Troendle,1987; Reuss et al., 1997).

In the control plot, subsurface ¯ow CB, SO42ÿ, and HCO3

ÿ concentrations generally declined at shallowerdepths suggesting a common process, mineralization, accounts for the increased concentrations with soildepth. Such change in CB , especially Ca2�, and HCO3

ÿ with depth would be expected because of morereadily mineralized substrates at depth (Rice and Bricker, 1995). The reduced ion concentrations at shallowerdepths could also re¯ect shorter residence times. The increased SO4

2ÿ concentrations with depth would showan opposite trend if FEF ecosystems were responding to past and present atmospheric SO4

2ÿ inputs. Ca2�

and HCO3ÿ concentrations in the control plot were only slightly below alpine streamwater levels indicating

substantial and rapid mineral weathering in FEF soils (Stottlemyer et al., 1997).The e�ect of treatment on ion ¯ux was a function of both increased runo� and percentage of runo� in

shallow soils. Total subsurface ¯ow increased 4.1 times on the clear cut relative to the control, but shallowsubsurface ¯ow increased nine times. The increase in total subsurface ¯ow amounts in the clear cut accounts



for 91% of the increase in Ca2� output. The increase in shallow subsurface ¯ow coupled with its higher Ca2�

concentration likely accounts for the remaining 9%. The Ca2� in shallow subsurface ¯ow is likely fromsurface mineral soil exchangeable pools which are large relative to Ca2� output (Reuss et al., 1997). K� ¯uxin shallow ¯ow from the clear cut was 20 times the control or twice the ¯ux accounted by the increase inshallow subsurface ¯ow amount. The increase, 5.9 to 118 eq haÿ1 or 4 kg K� haÿ1 for the period 1985±1993,came primarily from shallow soil exchange sites, decomposition of logging slash deliberately left in place,and reduced plant uptake. With the available data, we can not separate the relative importance of thesesources. The increase in NO3

ÿ ¯ux from the clear cut was the most pronounced response (Figure 4). Shallowsubsurface NO3

ÿ ¯ux increased 4650 times and deep subsurface NO3ÿ ¯ux 4100 times the ¯ux from the

control. The increase from the clear cut, about 4.5 kg N haÿ1 yrÿ1, is equal to the low end of the range ofannual N uptake and net N mineralization rates for this ecosystem (Reuss et al., 1997). Conversely, theincreased SO4

2ÿ and HCO3ÿ ¯ux in shallow subsurface ¯ow from the clear cut was proportionally only half

the increase in subsurface ¯ow amount. The reduced ¯ux is consistent with the concentration trend with soildepth for these ions. Greater contributions from soil weathering occur at increased depths. The increase inSO4

2ÿ with depth again indicates most SO42ÿ output from FEF watersheds is not from present or past

precipitation inputs.Subsurface ¯ow rarely showed more than trace NH4

� concentrations, so essentially all N leaving the plotswas NO3

ÿ. The control plot retained 97% of annual precipitation N inputs, or 1.4±1.5 kg N haÿ1, consistentwith budgets for Lexen Creek and other FEF watersheds (Stottlemyer and Troendle, 1987, 1992; Stottlemyeret al., 1997). The clear cut lost an average of 4.6 kg N haÿ1 yrÿ1 (Figure 4) which, coupled with net retentionin the control, results in a net N loss from the clear cut of about 6 kg N haÿ1 yrÿ1 (Reuss et al., 1997).Estimates of annual N uptake for this ecosystem range from 6 to 12 kg N haÿ1 so it appears most of the Nloss was from lack of uptake rather than increased N mineralization in response to disturbance.

Streamwater

The repetitive discharge and ion concentration patterns observed in streamwater at the mouth of LexenCreek watershed and in both subsurface ¯ow plots show the importance of hydrology in controlling soil andstreamwater chemistry and ¯ux. The rapid decline in early summer stream discharge re¯ects the loss ofsnowmelt, the seasonal decline in precipitation, and increased evapotranspiration.

The similarity in seasonal streamwater Ca2�, SO42ÿ, and HCO3

ÿ concentrations at the mouth of LexenCreek watershed and in the subsurface ¯ow control plot suggests the processes de®ned in the plot study maybe important in determining seasonal change in streamwater chemistry. The late summer and early fallincrease in streamwater ion concentrations re¯ect weathering products from deeper soils. The stream-water SO4

2ÿ trend is further evidence sulfur is coming primarily from mineralization. During the decade-long study, NO3

ÿ concentrations in streamwater at the mouth of Lexen Creek watershed and subsurface ¯owfrom the control plot had identical means (4 meq Lÿ1) and standard deviations (9 meq Lÿ1), and similarseasonal concentration patterns. The periodic increase in streamwater NO3

ÿ concentrations (Figure 5) withinitial snowmelt, similar to what was observed periodically in subsurface ¯ow chemistry, is attributed tothe seasonal change in soil mineralization and nitri®cation rates (Johnson et al., 1969). With snowmeltinitiation, `older' subsurface water can be forced out resulting in a small increase in streamwater NH4

� andNO3

ÿ concentration.The lack of variation in alpine streamwater ion concentration was not observed in the subsurface ¯ow

plots which suggests another process must regulate seasonal change. FEF alpine soils are very porous andpoorly developed which permits rapid snowmelt penetration. This characteristic accounts for why little or nosurface lateral ¯ow occurs during snowmelt in the alpine. The lack of signi®cant seasonal change in alpinestreamwater chemistry suggests thorough mixing of snowmelt and soil water before it enters the stream(Stottlemyer et al., 1997). The rather constant streamwater Ca2�, SO4

2ÿ, and HCO3ÿ concentration in the

alpine is similar to trends observed in ground water (Rice and Bricker, 1995). The low concentration relativeto levels at the watershed mouth likely re¯ects reduced residence times and a shorter ¯owpath length to the



alpine stream. The relatively high alpine streamwater NO3ÿ concentration is evidence of rapid movement

of soil water beneath the shallow rooting zone, and the limited ability of the ecosystem to retain NO3ÿ even

in the growing season.

CONCLUSIONS

At the FEF, canopy removal signi®cantly a�ects snowpack ion concentration and content. Canopyremoval decreased snowpack K� content but increased NH4

� and NO3ÿ content. The major e�ects of

canopy removal on soil water chemistry were an increase in K� and NO3ÿ concentrations. Following

treatment, total annual subsurface ¯ux of all ions signi®cantly increased in the clear cut compared to thecontrol. The increase was most evident for K� and NO3

ÿ. The clear cut also increased the percentage of totalion ¯ux occurring in shallow soils. The e�ects of the clear cut on increased subsurface ¯ow and ion ¯ux arestill apparent a decade following treatment.

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