DIVISION S-7-FOREST & RANGE SOILS

Canopy and Soil Modification of Precipitation Chemistry in a Temperate Rain ForestRobert L. Edmonds,* Ted B. Thomas, and Jon J. Rhodes

ABSTRACTLittle is known about the chemistry of solutions moving through

old-growth coastal temperate coniferous rain forests. The major ob-jectives of this study were to examine precipitation, throughfall,stemflow, and soil solution chemistry in an old-growth temperaterain forest in the Hoh River valley on the northwest Washingtoncoast 32 km from the Pacific Ocean, and to determine mechanismsinvolved with changes in solution chemistry. Dominant species wereDouglas-fir (Pseudotsuga menziesii [Mirb.] Franco), western hem-lock (Tsuga heterophylla [Raf.] Sarg.), western redcedar (Thujapli-cata Donn), and Pacific silver fir (Abies amabilis [Dougl.] Forbes).Stemflow was more acidic (avg. pH 4.3) than throughfall (avg. pHS.O) and precipitation (avg. pH 5.3). This precipitation pH is typicalfor a remote site. Organic acids were important contributors to ac-idity in throughfall and stemflow. Soil solution pHs were much high-er as a result of acid neutralization processes, averaging 5.7 in theforest floor and 6.2 at 40-cm depth. Sodium and Cl dominated pre-cipitation, followed by Ca and SO4, indicating a strong oceanic in-fluence. Throughfall and stemflow were generally enriched incations, especially K, but concentrations in soil solutions were lessthan those in stemflow. Organic anions contributed greatly to cationleaching in the canopy, with a much smaller contribution from SO4.Like precipitation, Na and Cl dominated throughfall, stemflow, andsoil solutions. The highest concentrations of NH4 were found instemflow, suggesting N fixation in the canopy. Throughfall and stem-flow under Pacific silver fir had the highest concentrations of bothcations and anions. Phosphate, NH4, and NO3 concentrations werelow in the soil solution, indicating strong retention of N and P inthis ecosystem.

R.L. Edmonds, College of Forest Resources, Univ. of Washington,Seattle, WA 98195; T.B. Thomas, U.S. Forest Service, Forestry Sci-ences Lab., Olympia, WA 98502; and J.J. Rhodes, Columbia RiverIntertribal Fishery Commission, 975 SE Sandy Blvd., Suite 202, Port-land, OR 97214. Received 16 Feb. 1990. Corresponding author.Published in Soil Sci. Soc. Am. J. 55:1685-1693 (1991).

"C'OREST CANOPIES typically modify precipitationr chemistry as a result of interaction with plant sur-faces (Tarrant et al., 1968; Eaton et al, 1973; Feller,1977; Cronan and Reiners, 1983; Parker, 1983; Lind-berg et al., 1986; Miller et al., 1987; Ugolini et al.,1988). Forest type and species strongly influence can-opy processes (Henderson et al., 1977; Larsen, 1979;Cronan and Reiners, 1983; Parker, 1983; Lindberg etal., 1986). For example, deciduous hardwoods producea throughfall solution that is less acid and higher inbasic cations than that from conifers (Cronan and Re-iners, 1983). The chemistry of water flowing throughthe forest floor and soil is further modified so, by thetime it reaches a stream, its ionic composition may bevastly different from that in precipitation (Feller,1977).

A number of studies examining throughfall, stem-flow, and soil solution chemistry have been conductedin forests in Oregon, Washington, and British Colum-bia (Cole et al., 1967; Tarrant et al., 1968; Turner andSinger, 1976; Cole and Johnson, 1977; Feller, 1977;Larsen, 1979;Sollinsetal., 1980; Van Miegroet, 1986).These studies suggest that coniferous canopies andsoils strongly influence the chemistry of solutions flow-ing through ecosystems. Only Larson's (1979) study,however, was conducted in an old-growth forest witha strong coastal influence. Little undisturbed oldgrowth remains, especially close to the Pacific Ocean,and knowledge of how these ecosystems function isstill incomplete. A better understanding of canopy andsoil processes in coastal forests in the Pacific North-

Abbreviations: dbh, diameter at breast height; DOC, dissolved or-ganic carbon; ANOVA, analysis of variance.

1686 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

west will be helpful in developing ecologically soundforest management practices.

Thus, the objectives of this study were to: (i) deter-mine concentrations of major cations and anions anddissolved organic C in bulk precipitation, throughfall,stemflow, and the soil solution in an old-growth tem-perate rain-forest ecosystem in the Olympic NationalPark, Washington, (iij determine effects of differentconifer species in altering precipitation chemistry, (iii)examine oceanic influences on throughfall, stemflow,and soil solution chemistry, and (iv) examine mech-anisms by which the canopy and soil alter solutionchemistry.

MATERIALS AND METHODSStudy Site

The study site is the 58-ha West Twin Creek Watershedlocated in the Hob River valley on the western side of Olym-pic National Park, Washington, 32 km from the PacificOcean (Fig. 1). The watershed is a steep (slopes range from40 to 110%), dissected, first-order valley wall watershed. Itgenerally faces southeast but many aspects are represented.Elevations range from 180 to 850 m. Mean January and Julyair temperatures at the Hoh Ranger Station (located 10 kmeast of the site at an elevation of 183 m; Fig. 1) are 4 and16 °C, respectively; annual rainfall averages about 3500 mm,with most falling from October to May. A winter snow packmay develop above 700 m elevation from December toMarch. Snow rarely falls at the base of the watershed andgenerally lasts for only a few days. Bedrock and soils areformed from marine deposits with minor conglomerates andbreccias and contain some green and black sandstones (Ta-bor and Cady, 1978). Soils are classified as coarse-loamy,mixed, mesic Typic Dystrochrepts (David Marrett, 1990,personal communication). There is, however, a moderatedegree of volcanic ash or other (e.g., spodic) amorphous ma-terial present, suggesting that soils might alternatively beclassified as medial, mesic Andic Dystrochepts (David Mar-rett, 1990, personal communication). A typical soil profilein the middle of the watershed is described in Table 1. Forest-floor depth is slightly greater at the top of the watershed (5-10 cm), but the mineral soil varies little throughout the wa-tershed because of frequent windthrow and resultant soilmixing.

Vegetation encompasses two vegetation zones identified

WESTERN OLYMPIC NATIONAL PARK

,Hoh Ranger Station

20 Km

by Franklin and Dryness (1973), the western hemlock zonein the lower watershed and the Pacific silver fir zone in theupper watershed. Western hemlock, Douglas-fir, and westernredcedar dominate the overstory vegetation in the lower wa-tershed. A few Sitka spruce (Picea sitchensis [Bong.] Carriere)are present. The understory vegetation is dominated byswordfern (Polystichum muniium [Kaulf] K. Presl) and Or-egon oxalis (Oxalis oregana Nutt. ex. T. & G.). Westernhemlock and Pacific silver fir are the dominant overstoryspecies in the upper watershed, while the main understoryspecies are ovalleaf huckleberry (Vaccinium ovalifoliumSmith), red huckleberry (Vaccinium parvifolium Smith), andOregon oxalis. Salal (Gaultheria shallon Pursh) occurs onsouth-facing slopes at mid to upper elevations. Western hem-lock, Pacific silver fir, western redcedar, Douglas-fir, and Sit-ka spruce represent 82, 10, 6, I, and 1%, respectively, of thetree population. Only 26% o:f the basal area, however, isrepresented by western hemlock. Pacific silver fir, westernredcedar, Douglas-fir, and Sitka spruce represent 26, 25, 15,and 6% of the basal area, respectively.

Forty-one trees were cored with an increment borer andaged at breast height. The fore st is unevenly aged. Douglas-fir, western redcedar, western hemlock, and Pacific silver firtrees range in age from 237 to 635, 123 to 600, 71 to 262,and 98 to 248 yr, respectively. Maximum tree height is >90m, but the canopy is broken up and very uneven. Ranges ofdbh are 10 to 310, 27 to 302, 5 to 260, and 9 to 190 cm forDouglas-fir, western redcedar, western hemlock, and Pacificsilver fir, respectively.

Field Sampling ProceduresPrecipitation

Precipitation samples were collected biweekly from Oc-tober 1986 through September 1988 in bulk precipitationcollectors. Each collector was a 20-cm-diam. polyethylenefunnel with a netted top to exclude debris. Precipitation wascollected in an autoclavable polyethylene bag. In 1986/1987four collectors were located ai: elevations from 229 to 640m in clearcuts in a 16-km2 area on Washington state De-partment of Natural Resources land adjacent to the park and2 km west of the watershed boundary (Fig. 1). Only twocollectors (at mid range) were used in 1987/1988, since therewas little variability with elevation among samplers. Sam-ples were collected in triple-rinsed 1-L polyethylene bottles.

Table 1. Typical soil profile in the West Twin Creek Watershed.Horizon Depth Description

Fig. 1. Map of the western Olympic National Park showing thelocation of the West Twin Creek Watershed, the Hoh RangerStation, precipitation collectors, and proximity to the PacificOcean.

O 2-0 Mixed coniferous litter and humified soil,unstratified ranging from fibric (O{) to sapric(OJ in properties.

BA 0-4 Dark brown (10 YR 3/3 moist), gravelly siltloam; moderate medium angular blocky; lowbulk density and somewhat smeary; many fineto coarse roots; 20% subangular sandstonegravel by volume; clear wavy boundary.

Bw 4-95 Dark brown (7.5 YR 3/2 and 7.5 YR 4/4 moist)grading to dark yellowish brown (10 YR 4/4moist) with depth, gravelly silt loam; weakcoarse to fine subangular blocky, low bulkdensity, and somewhat smeary; many fine tocoarse roots from 4 to 30 cm depth; few fineto coarse roots 30 to 95 cm; 30% subangularsandstone gravel by volume; gradual wavyboundary.

BC 95-105+ Grayish brown to yellow brown (10 YR 5/2 to10 YR 5/4 moist) very gravelly silt loam,weak fine angular blocky; no roots; 40%coarse fragments by volume, mostly angularsandstone gravel and some cobbles.

EDMONDS ET AL.: MODIFICATION OF PRECIPITATION CHEMISTRY 1687

Precipitation quantities were determined from daily rain-gauge data collected at the Hoh Ranger Station using a re-cording Belfort rain gauge (Belfort Instrument Company,Baltimore, MD). If precipitation fell as snow, it was collectedin 10-L snow buckets lined with acid-rinsed linear polyeth-ylene bags. Samples were transported in a cooler to the an-alytical laboratory at the College of Forest Resources,University of Washington, within 48 h of collection andstored at 4 °C.

Throughfall and Stem/lowThroughfall and stemflow were determined for four tree

species. Trees represented a range of diameters. In the lowerwatershed (at 180-m elevation) two western redcedar (40-and 112-cm dbh), two Douglas-fir (145- and 247-cm dbh),and two western hemlock (49- and 77-cm dbh) trees weresampled. In the upper watershed (at 800-m elevation), twowestern hemlock (73- and 81-cm dbh) and two Pacific silverfir (59- and 141-cm dbh) trees were sampled. Trees werelocated in a 2.2-ha area in the upper watershed and a 3.3-ha area in the lower watershed.

Throughfall collectors were constructed of 10-cm polyvi-nyl chloride rain gutters extending from the bole of a sampletree to the edge of the crown. One collector per tree was usedand they were placed so that there was contribution fromonly that tree. Collectors were lined with acid-washed andtriple-rinsed linear polyethylene and covered with 2-mm ny-lon chiffon mesh to keep out litterfall. Stemflow collars weremade from 2.5-cm-diam. Tygon tubing cut in half lengthwiseand fastened to the lower bole of trees encircling the stem.Throughfall and stemflow flowed into 120-L polyethylenecontainers lined with polyethylene bags. Samples were col-lected bi-weekly. Volumes were determined in the field anda 1-L sample from each sampler was returned to the labo-ratory for chemical analysis in the same manner as precip-itation samples.

Soil SolutionSoil solutions were collected using tension lysimetry (Cole,

1968). Modified 60-mm-diam. fritted-glass Buchner funnelswere placed beneath the forest floor and 5 cm and 40 cmbelow the top of the mineral soil. Three replicates for eachhorizon were located in the same areas as the throughfalland stemflow collectors in the lower watershed. Funnels werecut off 1 cm above the glass surface and the stem was bentat right angles. Tension (0.01 MPa) was applied using a 1-m hanging-water column. Solutions were collected in 10-Lpolyethylene bottles. Volumes were recorded in the field. Ifvolumes were insufficient for analysis, samples were com-posited by horizon. Soil solutions were sampled monthly,except during the drier months, and treated similarly to othersolutions.

Soil ChemistrySoil samples for chemical analyses were taken from the O

horizon, and from the 0- to 4-cm (BA horizon) and 5- to 40-cm (Bw horizon) depths of the mineral soil at three locationsin the same area as the stemflow and throughfall samplersin the upper and lower watershed. Samples were placed ina cooler and transported to the analytical laboratory at theCollege of Forest Resources, University of Washington,where they were air dried after the extraction of NH4 andNO3.

Laboratory Analytical ProceduresSolution Chemical Analysis

Specific conductance, pH, and alkalinity were determinedon unfiltered samples within a few days after arrival at the

laboratory. Specific conductance was determined with a YSIModel 31 conductivity bridge (Yellow Springs InstrumentCo., Yellow Springs, OH) and pH with a Radiometer PHM85 pH meter (Radiometer, Copenhagen, Denmark). Alka-linity was determined by titration (Rhoades, 1982) to an endpoint of pH 5.

Remaining samples were filtered through a Whatman GF/A filter, stored at 4 °C and analyzed within a month afterarrival at the laboratory. Solutions were analyzed for Ca, Mg,K, and Na using an IL 951 atomic absorption analyzer (In-strumentation Laboratory, Wilmington, MA). Ammoniumand NO3 were determined with a Technicon Autoanalyzer II(Technicon, Tarrytown, NY). Sulfate, Cl, and PO4 were de-termined with a Dionex 2100 ion chromatograph (Dionex,Sunnyvale, CA). Dissolved organic C was determined usingpersulfate oxidation at 100 °C and an OI Model 700 organicC analyzer (OI Corp., College Station, TX). Precipitation,stemflow, throughfall, and soil solution quantities were usedto determine volume-weighted average concentrations.

Soil ChemistryAvailable N (NH4 and NO3) was determined on field-

moist samples immediately after arrival at the laboratoryusing 1 M KC1 extraction (Keeney and Nelson, 1982) and aTechnicon Autoanalyzer II. Samples were then air dried forother analyses; moisture correction was applied by drying asubsample at 105 °C. Total C was determined using a LecoC analyzer (Leco Corp., St. Joseph, MI). Total N was de-termined using an H2SO4/LiSO4/H2O2/Se digestion (Parkin-son and Alien, 1975) and a Technicon Atomic AbsorptionAnalyzer II. Phosphorus (Bray no. 1) was determined usingthe method of Jackson (1958) and a Technicon AutoanalyzerII. Sodium, K, Ca, Mg, and Al were determined with 1 MNH4C1 extraction and an IL 951 atomic absorption analyzer.Cation-exchange capacity was determined by extraction with1 M NH4C1 and a saturation wash with 1 M KC1 (D.W.Johnson, 1985, personal communication). A 1:1 soil/waterpaste was used for determination of pH with a RadiometerPHM 85 pH meter.

Statistical AnalysisOne-way ANOVA was conducted on volume-weighted

precipitation, stemflow, throughfall, and soil solution chem-istry data (Sokal and Rohlf, 1981) to test differences amongmeans. Differences among means were determined at P =0.05.

RESULTS AND DISCUSSIONPrecipitation Chemistry

Precipitation pH from October 1986 through Sep-tember 1988 averaged 5.3 (Table 2), which falls withinthe range (4.9-5.5) observed in coastal Oregon, Wash-ington, and Alaska, and the Oregon Cascade Moun-tains (Sollins et al., 1980; Vong and Larson, 1983;Bormann et al., 1989). Lower precipitation pH values(4.5-4.8) were recorded at sites near Vancouver, BC(Feller, 1977), and near Seattle, WA (Van Miegroet,1986; Wolfe, 1988), probably because of anthropogen-ic inputs. Precipitation pH varied from 4.5 to 6.2 dur-ing the study period, but there was no distinct seasonalpattern. Acid precipitation was thus not prevalent overthe Hoh River watershed.

Concentrations of total cations and total inorganicanions were almost equal in precipitation with only asmall anion deficit (8.4 jtmolc L'1, Table 3), indicatingthat organic anions were low. This is supported by the

1688 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

Table 2. Average pH, electrical conductivity, and dissolved organiccarbon (DOC) in precipitation, Douglas-fir, western hemlock,western redcedar, and Pacific silver fir stemflow and throughfall,and soil solutions in West Twin Creek Watershed from October1986 through September 1988. Numbers in parentheses are stan-dard deviations.

ElectricalpH conductivity!

Precipitation

Throughfall .

Douglas-fir

Western hemlock

Western redcedar

Pacific silver fir

Western hemlock

Stemflow

Douglas-fir

Western hemlock

Western redcedar

Pacific silver fir

Western hemlock

5.3a(0.3)

Lower watershed5.1a(0.2)5.1a(0.2)5.1a(0.2)

Upper watershed4.9ab(0.2)S.la(0.2)

Lower watershed4. led(0.4)4.5bc(0.4)4.0d(0.2)

Upper watershed4.5bc(0.4)4.4cd(0.3)

mS m-'l.lSa(0.63)

2.24ab(0.13)1.82ab(0.06)2.1 lab(1.97)

2.76ab(1.28)1.67a(0.79)

7.02c(2.68)4.03bc(1.71)5.25bc(1.78)

9.80c(5.20)4.56bc(2.42)

DOCJmgL'1

1.5a(1.1)

lO.Sab(17.9)7.5ab(5.9)7.1ab(5.1)

15.6b(6.6)8.5b(4.7)

25.5c(9.3)

26.5c(20.4)26.0c(12.2)

33.8c(25.9)26. Ic

(9.9)Soil solution (lower watershed)

Forest floor

Soil— 15-cm depth

Soil-^40-cm depth

5.7ae. (0.1)

5.8e(0.1)6.2f(0.1)

1.78a(0.82)1.53a(0.67)2.60ab(0.65)

lO.Sb(1.1)9.0b(4.6)2.9a(1.1)

t Numbers followed by a different letter within a column are significantlydifferent (P = 0.05).

t DOC determined only in 1987/1988.

low concentration of DOC in precipitation (1.5 mgL-1; Table 2), which is similar to that in precipitationat forested sites in Japan (Ugolini et al., 1988) and atHubbard Brook Watershed, New Hampshire (Mc-Dowell and Likens, 1988).

Electrical conductivity of precipitation was also low(Table 2), generally reflecting the low concentration ofions in precipitation (Table 3). Sodium, with an av-erage concentration of 54.3 jumol L-1 (Table 3) was themajor cation in precipitation, followed by Ca, Mg, andK. The high Na concentration (2-5 times higher thanthose reported from more inland sites (Feller, 1977;Sollins et al., 1980; Wolfe, 1988), reflects the closeproximity of the site to the ocean. Other studies nearsalt water also report high Na concentrations in pre-cipitation (Attiwill, 1966; Carlisle et al., 1966; Larson,1979; Bormann et al., 1989). At sites located >50 kmfrom the ocean in the Appalachian and CascadeRanges, the concentration ranking differs, with Ca >Na > K > Mg (Henderson et al., 1978). Concentra-

tions of NH4 in precipitation in the Hoh River valleywere less than those in Pu§;et Sound (Wolfe, 1988) andBritish Columbia (Feller, 1977), reflecting anthropo-genic inputs at the latter sites.

Chloride was the dominant anion in precipitation(57.6 Mmol L-1) followed by SO4, HCO3, NO3, and PO4(Table 3). Chloride concentrations were 1.5 to 4 timeshigher than those reported from sites further inland(Feller, 1977; Sollins etal., 1980; Wolfe, 1988), but weresimilar to those at other coastal sites (Larson, 1979;Bormann et al., 1989). Sulfate concentration in precip-itation (5.6 Mmol L"1) was lower than that in precipi-tation in Puget Sound (Wolfe, 1988) and in BritishColumbia (Feller, 1977), where anthropogenic inputscontribute, but was similar to that observed in the Cas-cades of Oregon (Sollins et al., 1980) and at coastal sites(Larson, 1979; Vong and Lirson, 1983; Bormann et al.,1989). Non-sea-salt SO4 (NSS) accounted for 47% ofthe SO4 and was calculated as follows:

NSS = cone. SO4 in precipitation —(cone. SO4 in ocean water X cone. Cl in precipitation)

cone. Cl in ocean water.Non-sea-salt SO4 is derived from oxidation of di-methyl sulfide produced by marine algae (Charlson etal., 1987), dust, and wood smoke from slash burning.Highest concentrations of SO4 were observed inspring, late summer, and fall (Fig. 2), particularly in1987.

Nitrate concentration in precipitation at the Hoh site(1.4 /and! L-1) was higher than that of NH4 (0.4 jumolL-'; Table 3) and was similar to that in the OregonCascades (Sollins et al., 1980) and coastal sites (Bor-mann et al., 1989). Wolfe (1988) observed higher NO3concentration in the Puget Sound area (9.7 nmol L-')due to anthropogenic inputs. There were no obviousseasonal patterns in NO3 and NH4 concentrations.

ThroughfallMost coniferous canopies show a tendency toward

net acidification of bulk precipitation inputs (Cronanand Reiners, 1983). In conifers, acidification processessuch as NH4 uptake, nitrification, wash-off of dry dep-osition, and leaching of plant-derived acids dominateneutralization processes such as plant-surface ion ex-change and release of organic or HCO3 salts (Cronanand Reiners, 1983). At the Hoh site, throughfall pHranged from 4.9 to 5.1 (avg. 5.0) (Table 2), but wasnot significantly lower than precipitation pH (5.3). Fell-er (1977) and Larson (1979) noted a similar trend inBritish Columbia and coastal Washington, respectively.In contrast, Cole and Johnson (1977) and Van Miegroet(1986) showed an increase in pH in throughfall com-pared with precipitation at the Thompson ResearchCenter in the Puget Sound region of Washington. Thistrend was also noted at Cascade Head, Oregon (Tarrantet al., 1968). Parker (1983) suggested that an increasein throughfall pH may be due to appreciable quantitiesof HCO3 in throughfall. The concentration of HCO3 inthroughfall at the Thompson Research Center was con-siderably higher (>40 /onol L-1) (Cole and Johnson,1977) than that in throughfall at the Hoh site (5.

EDMONDS ET AL.: MODIFICATION OF PRECIPITATION CHEMISTRY 1689

Table 3. Average concentrations of chemical elements in precipitation, throughfall, stemflow, and soil solution in the West Twin CreekWatershed from October 1986 through September 1988. Standard deviations are shown in parentheses.

Cationsj

Precipitation

Throughfall

Stemflow

Soil solutionForest floor

BA horizon

B horizon

"i

39§

401

264

9

9

8

H

5.5ab(6.6)8. lab(9.3)

54.4ab(43.8)

2.1a(0.7)l.Sa(0.5)0.6a(0.2)

NH4

0.4a(0.7)0.6a(1.1)2.7a(6.4)

3.1a(5.6)0.6a(0.9)l.Sa(0.9)

Ca

3.3a(2.9)5.5(8.8)

20.4ab(21.1)

32.8b(12.9)IS.Ob

(9.9)21.9b

(9.7)

K

2.8a(0.9)

25. lab(30.1)

119.6b(110.7)

33.7b(12.0)26.5b(13.0)21.6b

(7.4)

Mg

5.3a(1.5)9.8a(4.9)

23.1a(24.8)

22.5a(20.8)

9.8a(4.2)

12.2a(4.8)

Na— iimol L'1 -

54;3a(28.9)76.5a

(41.0)118.0a(68.9)

73.6a(22.1)7 l.Sa

(20.5)83.4a

(26.2)

HCO3

8.0a(5.1)5;4a(8.8)l.Sab

(15.6)

22.1c(2.8)

20.6a(4.6)

39.7c06.9)

AnionsfNO,

1.4a(0.9)O.lb(0.3)O.la(0.3)

O.lb(0.02)0.3ab(0.7)0.2ab(0.4)

Cl

57.6a(37.8)74.7a(65.2)

160.2a(121.4)

113.6a(99.6)98.1a(49.7)

108.9a(44.6)

SO4

5.6a(4.2)5.8a(3.9)7.3(6.7)

5.5a(6.6)7.9a(4.2)8.9a(3.7)

P04

O.la(0.2)2.0a(3.7)

10.4a(15.6)

l.Oa(1.2)O.la(0.2)0.2a(0.3)

Cations— anions

fimol,. L~'8.4

56.1

225.5

75.3

21.1

8.4

t Numbers followed by a different letter within a column are significantly different (P = 0.05).i Number of samples analyzed.§ Composite of all precipitation samples.

L"1), which was similar to that in precipitation (8.0jumol L~'; Table 3).

Although there was no significant difference betweenprecipitation and throughfall pH, there was a trendtoward acidification in throughfall. The dominantacidification mechanism in the canopy at the Hoh siteappears to be the leaching of plant-derived acids. Anincrease in DOC, indicating organic-acid leaching, oc-curred in throughfall (Table 2), although it was onlysignificantly different (P = 0.05) from precipitationDOC under Pacific silver fir and western hemlock inthe upper watershed. There were no significant differ-ences in throughfall pH or DOC (Table 2) among spe-cies at the Hoh site. Ugolini et al. (1988) andMcDowell and Likens (1988) also noted that DOCincreased in throughfall.

There was a slight seasonal pattern in throughfallpH, with the highest values observed in spring andsummer and lowest in fall and winter (Fig. 3A and3B) probably related to biological activity in the can-opy and lower rainfall in spring and summer. Pacificsilver fir throughfall had a consistently lower pH trendthan western hemlock (Fig. 3B), related to the highDOC in Pacific silver fir throughfall (Table 2). Pacificsilver fir tends to retain its needles longer than otherspecies (Kimmins, 1987) and these older needles,which may be starting to decompose, could be con-tributing organic acids to throughfall.

15

10 '

oO&cu

5 '

"' O N D J F M A M J J A S O N D J F M A M J J A S1986 1987 1988

Fig. 2. Average monthly SO4 concentrations in precipitation fromOctober 1986 through September 1988.

Electrical conductivity and cation concentrationswere not significantly higher in throughfall relative toprecipitation (Tables 2 and 3, respectively). There wasa trend, however, for both electrical conductivity andcation concentrations to be higher in throughfall thanin precipitation. Cronan and Reiners (1983) noted thatmost forest canopies show a marked enrichment inions as rainfall leaches the canopy. Like precipitation,the major cation in throughfall was Na (76.5 /urno! L"1;Table 3). The concentration ranking for cations wasNa > K > Ca > Mg. Throughfall was enriched rel-ative to precipitation in the order K > Na > Mg >Ca, indicating that K was most readily leached from

6.0

5.5-

£ 5.0-

4.5

4.0

western hemlockwestern redcedarDouglas-fir

O N D J F M A M J J A S O N D J F M A M J J1986 1987 1988

6.0

5.5-

oL 5.0-

4.5

4.0

B

western hemlockPacific silver fir

O N D J F M A M J J A S O N D J F M A M J J1986 1987 1988

Fig. 3. Average monthly pH in throughfall from October 1986through July 1988: (A) beneath western hemlock, western red-cedar, and Douglas-fir in lower West Twin Creek Watershed, and(B) beneath western hemlock and Pacific silver fir in upper WestTwin Creek Watershed.

1690 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

the canopy. Cronan and Reiners (1983) also noted thatK is typically highly enriched in throughfall.

Leaching, wash-off of deposited particles, and evap-oration from tree crowns all contribute to cation en-richment in throughfall. Leaching is probably thedominant mechanism at the Hoh site for both through-fall and stemflow, which is more enriched in cationsthan throughfall (Table 3). Wash-off of dry depositedsea-salt-containing aerosols seems important, based onthe increased concentrations of Na and Cl in throughfalland stemflow relative to precipitation (Table 3). Evap-oration is probably less important because SO4 con-centration changed little with passage through thecanopy (Table 3). Sulphate commonly shows little in-teraction with foliage (Lindberg et al., 1986).

Typically, NH4 concentrations in throughfall are de-creased after interaction with the canopy, oftenthrough foliar uptake (Feller, 1977; Parker, 1983), butno significant change occurred at the Hoh site (Table3). Nitrate concentrations in throughfall also were notsignificantly different from precipitation (Table 3). Ni-trogen dynamics in the canopy are discussed in moredetail below.

Dominant anions in throughfall were Cl (74.7 jtimolL-1) and SO4 (5.8 jumol L-1; Table 3). The mean con-centrations of all anions were not significantly differentfrom those in precipitation. Sulfate concentration inthroughfall was lower than that in throughfall in Brit-ish Columbia (Feller, 1977) and Cedar River, Wash-ington (Johnson, 1975), but was similar to that in

throughfall in the Oregon Cascades (Sollins et al.,1980). Higher SO4 concentrations in the Puget Soundarea are due to anthropogenic inputs.

There were no significant differences in the concen-trations of any of the cations or anions in throughfallamong species (Table 4). The highest Ca concentrationin throughfall, however, occurred under western red-cedar, a known Ca accumulator (Bledsoe and Zasoski,1981). Pacific silver fir throughfall also consistentlyhad the highest mean concentrations of K, Mg, Na,Cl, SO4, and PO4 (Table 4), indicating that this speciestends to influence throughfall chemistry to a greaterextent than the other species.

In summary, it appears that the chemistry ofthroughfall at the Hoh site was very similar to that ofprecipitation and was dominated by oceanic influ-ences. There were trends toward increases in acidityand enrichment of DOC and cations, probably duemostly to leaching, but in no case were concentrationsof cations in throughfall significantly greater thanthose in precipitation.

StemflowStemflow solutions typically had higher ionic con-

centrations than precipitation and throughfall. This isusual for most forest ecosystems (Parker, 1983). Stem-flow pH was lower and DOC and electrical conduc-tivity were higher (P — 0.05) than in throughfall (Table2). Stemflow pH ranged from 4.0 to 4.5 (avg. 4.3) andwas significantly lower than precipitation and through-

Table 4. Average concentrations of chemical elements in throughfall and stemflow solutions from different tree species in West Twin CreekWatershed from October 1986 through September 1988. Numbers in parentheses are standard deviations.

Cationsf AnionstH NH4 Ca Mg Na HCO3 NO3 Cl SO4 PO4

Throughfall

Douglas-fir

Western hemlock

Western redcedar

Pacific silver fir

Western hemlock

Stemflow

Douglas-fir

Western hemlock

Western redcedar

Pacific silver fir

Western hemlock

t Numbers followed by

85

86

84

70

80

49

57

58

46

55

7.3a(3.4)7.5a(4.1)7.1ab

(18.0)

12.7a(5.6)7.9a(3.4)

78.1bc(44.5)29.4a(13.2)95.1c(28.4)

41.3ab(26.8)38.2ab(23.6)

0.4a(0.7)0.7a(1.0)0.8a(1.6)

0.5a(0.9)0.6a(1.0)

2.1a(2.0)1.8a(8.8)4.5a(6.8)

2.6a(2.8)2.1a(8.6)

Lower watershed11. 3a 22.9a(13.8) (50.5)

8.7a 21.5ab(6.0) (25.3)

15.5ab IS.Oa(13.4) (16.9)

Upper watershed13.4ab 57.8a(9.5) (62.0)

11.9a 19.0a(5.4) (12.7)

Lower watershed39.5b 127.9c(18.5) (40.0)22.6ab 114.7v(17.5) (80.7)25. la 23.0ab(13.4) (15.4)

Upper watershed100.8b 270.5b(67.5) (152.6)25.4ab 98.4b(21.1) (62.4)

a different letter within a column are significantly different (P =

lO.la(12.9)

9.7a(6.7)8.8a(6.7)

12.9a(10.7)

8.2a(4.7)

28.1a(14.8)14.2a(12.2)17.4a(34.4)

40.5a(28.4)16.2a(13.8)0.05).

83.9ab(48.0)72.2ab(34.0)63.4a(33.2)

99.1ab(62.3)70. lab(35.2)

154.5c(62.4)85.2ab(36.9)73.9ab(33.0)

184.3b(82.1)97.4ab(45.7)

5.0a(4.5)4.9a(4.0)

10.3a(16.1)

1.3a(2.9)4.1a(4.6)

T§

0.8a(7.6)O.la(0.2)

7.2b(36.8)

0.7a(2.5)

O.la(0.1)O.la

(0.3)O.la

(0.3)

O.la(0.6)O.la

(0.6)

O.la(0.1)O.la

(0.5)O.la

(0.01)

O.la(0.1)O.la

(0.5)

92.5a(67.8)88.1a(60.1)73.3a(51.1)

107.0a(79.9)76.1a(89.6)

208.2bc(88.1)

103.4a(58.9)98.4a(69.5)

270. la(184.2)129.5a

(84.4)

6.4a(4.7)5.7a(4.5)4.9a(3.1)

13.4a(10.5)11. 2a

(5.8)

9.8bc(6.9)3.6a(2.5)4.9ab(3.2)

12.6a(10.0)5.7a(4.1)

2.9ab(4.7)l.Oa(1.6)0.9a(1.6)

8.9ab(12.2)2.5a(3.0)

16.1ab(10.8)2.9a(5.1)1.9a(2.8)

28.1b(24.9)4.3ab(7.5)

t n = total number of samples analyzed.§ T = trace.

EDMONDS ET AL.: MODIFICATION OF PRECIPITATION CHEMISTRY 1691

fall pH. There was also a much larger anion deficit instemflow (225.5 pmol c L-1) than in throughfall (56.1jtmolc L"1; Table 3). This was largely due to the pres-ence of organic anions in stemflow, as indicated bythe high DOC concentration (Table 2). Plant-derivedacids strongly controlled stemflow acidity.

There was a trend for concentrations of both cationsand anions, except HCO3 and NO3, to be higher instemflow than in throughfall and precipitation, butonly in the case of K was stemflow concentration sig-nificantly higher (P = 0.05; Table 3). Potassium (119.6jumol L"1) and Na (118.0 jumol L"1) were the dominantcations in stemflow, while the dominant anion was Cl(160.2 jumol L~'; Table 3). The ranking for cation con-centrations was K > Na > Ca > Mg. Stemflow wasenriched relative to precipitation in the following or-der: K > Ca > Mg > Na, indicating that K was easilyleached from the canopy. Like throughfall, leaching isprobably the dominant mechanism for cation enrich-ment, with organic anions playing an important role.

Commonly, NH4 and NO3 in precipitation are takenup by the canopy, resulting in negative net throughfalland stemflow (Parker, 1983). This was observed forNH4 in throughfall by Feller (1977) in British Colum-bia and for NO3 by Sollins et al. (1980) in Oregon andJohnson (1975) in Washington. At the Hoh site, therewere no significant differences in NH4 concentrationsin stemflow (2.7 jumol L~'), precipitation (0.4 jumolL-'), or throughfall (0.6 /mio! L-1; Table 3). The trendtoward a higher NH4 concentration in both throughfalland stemflow, however, suggests that N fixation wasoccurring in the old-growth canopy. Denison (1973)noted that the epiphytic lichen Lobaria oregana(Tuck.) Muell. Arg. fixes N in the canopy of old-growthtrees in the Oregon Cascades and this species occursat the Hoh site. Nadkarni (1984) also found higheramounts of N in throughfall beneath branches withepiphytes than beneath stripped branches in the Olym-pic rainforest.

There were some differences among species with re-spect to stemflow pH (Table 2). Western redcedar andDouglas-fir had the lowest stemflow pH (4.0 and 4.1,respectively), while Pacific silver fir and western hem-lock had the highest (4.5). Stemflow pH was similarin western hemlock in both the upper and lower wa-tershed (Table 2). The bark of western redcedar andDouglas-fir is rougher than that of the other species,providing a greater surface area for contact. Westernredcedar bark also contains high concentrations ofphenolics (Barton, 1963). Interestingly, Pacific silver

fir stemflow had the highest DOC concentration, buta higher pH (4.5; Table 2). The high cation concen-trations, especially Ca, in Pacific silver fir stemflow(Table 4) apparently had a neutralizing influence.

Tree species also influenced cation and anion con-centrations in stemflow. There was a trend for Pacificsilver fir stemflow to have the highest concentrationsof Ca, K, Mg, Na, HCO3, Cl, SO4, and PO4 with west-ern redcedar having the lowest concentrations of theseions (Table 4). There also appeared to be a trend forPO4 concentrations to be high in both Pacific silver firand Douglas-fir stemflow (Table 4), suggesting that Pis being leached from the canopies of these species.

Cronan and Reiners (1983) suggested that cationenrichment in solutions flowing through forest cano-pies is mostly from plant recycling, while most of theinorganic anions are from atmospheric sources. Thisalso seems to occur at the Hoh site, although wash-offof sea-salt-derived aerosols is probably responsible formuch of the Na enrichment. The anions Cl, SO4, andNO3 were dominantly from atmospheric sources, al-though PO4 enrichment from plant sources could beoccurring.

In summary, there was a trend for cations in stem-flow to be more concentrated than in throughfall andprecipitation. Stemflow also had a much lower pH,resulting from organic acids from plant sources. Al-though only 5 to 20% of the volume of precipitationreaches the forest floor in stemflow (Parker, 1983), itis a major source of plant nutrients to fine roots nearthe base of trees, and could be an important source ofnutrients to plant roots at the Hoh site. Organic anionsfrom plant sources dominated inorganic anions instemflow and were associated with cation leaching inthe canopy.

Soil Solution and Soil ChemistryThe pH of solutions moving through the old-growth

rain-forest canopy was 5.3 in precipitation, 5.0 inthroughfall, and 4.3 in stemflow. It increased in theforest floor to 5.7 and to 6.2 at the 40-cm depth in thesoil (Table 2). Soil pH also increased with depth from4.2 in the forest floor to 4.9 in the lower mineral soil(Table 5). The lower pH of the soil relative to the soilsolution is commonly observed (Feller, 1977). Ad-sorption of organic anions onto soil particles or weath-ering of silicate minerals is probably responsible forthe increasing solution pH with soil depth along withincreased HCO3. Concentrations of DOC decreased

Table 5. Average soil chemistry for West Twin Creek Watershed, Olympic National Park, Washington. Standard deviations are shown inparentheses (n = 3).f

Horizon

O

BA (0-4 cm)

Bw (5-40 cm)

pH

4.2a(0.3)4.6a(0.5)4.9a(0.5)

NO3

0.4a(0.01)0.2b

(0.1)0.2b

(0.1)

NH4

- mg kg"1 -10.2a(1.2)2.0b

(0.8)

1.2b(0.8)

P04

43.0a(27.2)

S.Ob(7.7)2.2b(0.7)

N

l.Oa(0.05)0.3b

(0.1)0.2b(0.01)

C

35.3a(12.9)

8.8b(0.9)3.1b(1.3)

Na

O.la(0.05)O.OSa(0.03)O.la(0.03)

K

0.8a(0.1)0.3b

(0.04)O.lc

(0.05)

Ca- cmol kg ' -

6.0a(1.6)1.2b(1.6)O.lb(0.1)

Mg

2.6a(0.7)0.7b

(0.5)0.2c

(0.03)

Al

3.5a(2.4)8.5a(5.8)8.1a(6.0)

Cation-exchangecapacity

cmol,, kg~'28.3a(4.0)24.8a(2.6)20.5a(8.4)

t Numbers followed by a different letter within column are significantly different (P = 0.05).

1692 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

with soil depth (Table 2) as did the anion deficit (Table3), indicating that adsorption of organic anions is animportant mechanism for neutralizing the natural ac-idity in stemflow and throughfall in this ecosystem.

The chemical composition of a soil solution dependson a complex series of equilibrium, adsorption, im-mobilization, plant uptake, displacement, weathering,and decomposition reactions (Feller, 1977). The leach-ing of cations depends on the amount of anions presentin the solution. Organic anions and HCO3 are the dom-inant anions associated with leaching in undisturbedPacific Northwest forests, with organic anions domi-nating at low pH (McColl, 1972; Johnson et al., 1977;Ugolini et al., 1977). Organic anions dominated thecanopy leaching of cations at the Hoh site, but as thesolution moved through the forest floor organic anionsbecame less important and HCO3 increased in impor-tance until it dominated at the 40-cm depth (Table 3).

There were no significant differences in mean con-centrations of cations and anions among stemflow,throughfall, forest floor, and soil solutions (Table 3).There was a trend, however, for concentrations of cat-ions in forest-floor leachates to be lower than those instemflow and slightly higher than those in throughfall.There was also a trend for cation concentrations inthe mineral soil to be less than those in the forest floor(Table 3). Feller (1977) and Sollins et al. (1980) alsonoted a decrease in mineral-soil cation concentrationsrelative to forest-floor leachates. Electrical conductiv-ity was also high in the forest floor and declined withdepth in the mineral soil (Table 2).

The dominant cation in the forest floor and mineralsoil solutions was again Na (Table 3). Cation concen-trations generally decreased in the order Na > Ca >Mg > K in both the forest floor and mineral soil. Atsites away from coastal influence, Ca or Mg dominatesNa (Feller, 1977; Sollins et al., 1980). Anion concen-trations decreased in the order Cl > HCO3 > SO4 >PO4 > NO3. The dominant anion in the soil solutionaway from the coastal influence is HCO3, particularlyin low-elevation forests (Feller, 1977; Johnson et al.,1977; Sollins et al., 1980). Chloride, although the dom-inant anion in the soil solution, had little influence onthe leaching of nutrient cations because it moved withNa.

There were no significant differences among SO4concentrations in precipitation, throughfall, stemflow,and soil solutions (Table 3), but there was a slight trendtoward increasing concentrations with depth in themineral soil, perhaps as a result of weathering. Soilsin the area are derived from uplifted marine sedimentsand contain considerable pyrite. Feller (1977), in Brit-ish Columbia, noted a decrease in SO4 with depth, asdid Johnson and Cole (1977) in the Puget Sound areaof Washington. Thus SO4 dynamics in soil solutionsseem strongly related to soil type and parent material.Phosphate in the soil solution tended to be lower thanthat in stemflow and throughfall (Table 3). Soils at theHoh site were found to have considerable PO4 ad-sorption capacity (R. Harrison, 1990, personal com-munication).

Nitrate concentrations were low in the soil solutionand changed little with depth (Table 3), as did bulksoil NO3 (Table 5). Nitrate concentrations in the forest

floor and soil solutions (0.1 to 0.3 /tmol Lr1) were gen-erally lower than those of NH4 (0.6 to 3.1 /umol Lr1;Table 3). A similar trend occurred in the bulk soil(Table 5). The same trend was observed by Cole andJohnson (1977) in the Puget Sound area of Washing-ton. Slightly higher concentrations of NO3 (2.9 to 4.8^mol Lr1) and similar concentrations of NH4 (2.8 /tmplL"1) were observed in forest floor and soil solutions inBritish Columbia (Feller, 1977). Thus, it appears thatthere is strong retention of N in the ecosystem despitethe very high rainfall.

CONCLUSIONSThe chemistry of precipitation, throughfall, stem-

flow, and forest floor and soil solutions in this coastalold-growth temperate rain-forest ecosystem wasstrongly dominated by oceanic influences. Sodium wasthe dominant cation and Cl the dominant anion inprecipitation at the Hoh site. There was no evidenceof acidic precipitation from anthropogenic sources atthis remote site. Some SO4 from oceanic sources waspresent in precipitation, but SO4 concentrationschanged little with downward passage of solutions. Or-ganic acids contributed strongly to acidity in through-fall and stemflow solutions. This natural acidity,however, was neutralized in the forest floor and soilsolution.

There was a general enrichment of cations in stem-flow and throughfall relative to precipitation withhighest cation concentrations in stemflow, particularlyK. Wash-off of sea-salt-containing aerosols is an im-portant contributor to enrichment, especially for Na.Concentrations of cations in soil solutions were gen-erally less than those in stemflow. Organic anions wereassociated with most of the cation leaching in the can-opy, but were arrested in the soil, where the influenceof HCO3 increased. Chloride, although the dominantanion in soil solutions, had little influence on nutrient-cation leaching. Pacific silver fir generally had thegreatest influence in modifying solutions, with highestconcentrations of K, Na, Mg, Cl, SO4 and PO4 inthroughfall and stemflow. Western redcedar had thehighest Ca concentration in throughfall.

Concentrations of NH4 and NO3 were low in pre-cipitation. There was a trend for NH4 concentrationsto increase in stemflow, suggesting N fixation in thecanopy, and for NO3 concentrations to decrease, sug-gesting canopy uptake. Ammonium, NO3, and PO4concentrations were very low in both the forest floorand soil solutions, indicating strong retention of N andP in the ecosystem.

ACKNOWLEDGMENTSThis research was supported by grants from the National

Atmospheric Precipitation Assessment Program (NAPAP)through the National Park Slervice. Technical support forlaboratory and data analysis was provided by Brian High,Sarah Cooke, and James Marra. Field assistance was pro-vided by Teresa Hagan, Janet Kailen, and Hunter Sharpfrom Olympic National Park. We thank John Aho, OlympicNational Park, for logistical support and David Marrett, De-partment of Soil and Environmental Sciences, University ofCalifornia, Riverside, for classifying soils. The suggestionsof the three reviewers were ve:ry helpful in revising the man-uscript.

EDMONDS ET AL: MODIFICATION OF PRECIPITATION CHEMISTRY 1693