soil-landscape relationships of a climax forest in the allegheny high plateau, pennsylvania1

Soil-Landscape Relationships of a Climax Forest in the Allegheny High Plateau,Pennsylvania1

RICHARD AGUILAR AND R. W. ARNOLD2

ABSTRACTThe Tionesta Research Natural Area, located in the Allegheny

National Forest of northwestern Pennsylvania, is a unique remnantof the old growth American beech (Fagus grandifoliq L.)-Easternhemlock (Tsuga canadensis L.) forest that once covered over 2400ha of the Allegheny Plateau. During a detailed soil survey, the soilsoccurring on the various landscapes in the area were characterizedand the relationships between the vegetation and the soils were eval-uated. The development of the landscapes in the Tionesta area hasbeen strongly influenced by the contrasting credibility of alternatinglayers of sandstone, siltstone, and shale bedrock. The gentle, re-gional dip of the bedrock in a southwesterly direction has favoredthe preferential erosion of the south-facing slopes. The area expe-rienced a periglacial climate during the Late Wisconsin Glaciation.Evidence of this former cold climate is expressed by colluvial (so-lifluction) deposits and boulder fields on the ground surfaces of thefootslope landscapes. Seven distinct soils occur on the various land-scapes. These soils are extremely acid to very strongly acid and highin exchangeable aluminum. Low cation exchange capacity and basesaturation reflect the abundance of quartz and other primary min-erals which are highly resistant to weathering. Podzolization pro-cesses are evident and most pronounced in the coarse-textured, free-leaching soils under hemlock stands. Drainage appears to be anoverriding soil factor influencing the vegetation patterns on the dif-ferent landscapes with hemlock dominating the stands in wetter areas.Shade-intolerant species, such as black cherry (Prunus serotina L.),commonly occur on shallow soils on steep slopes where frequentcanopy openings associated with blow-downs have allowed their es-tablishment.

Additional Index Words: podzolization process, colluviation, CEC,exchangeable Al, acid soils, fragipan, soil parent material, residuum,soil classification.

1 Contribution from the Dep. of Agronomy, New York State Col-lege of Agriculture & Life Sciences, Cornell Univ., Ithaca, NY 14853.Agronomy Paper no. 1463. Received 17 July 1984. Approved 22Jan. 1985.2 Former Graduate Research Assistant and Professor, respec-tively. Principal author is now a Postdoctoral Research Associateat Colorado State Univ., Dep. of Agronomy, Fort Collins, CO. 80523,and the second author is now the Director, Div. of Soil Survey,USDA-SCS, P. O. Box 2890, Washington, DC 20013.

Aguilar, R., and R. W. Arnold. 1985. Soil-landscape relationshipsof a climax forest in the Allegheny High Plateau, Pennsylvania. SoilSci. Soc. Am. J. 49:695-701.

THE TIONESTA Scenic and Research Natural Areas,owned and managed by the USDA Forest Ser-

vice, are located in the Allegheny National Forest ofnorthwestern Pennsylvania. They contain a uniqueremnant of the climax forest that once covered over2400 ha of the Allegheny Plateau in Pennsylvania andNew York. Having slowly evolved over the course ofcenturies, the Tionesta Forest consists of a commu-nity of plants that represents the culminating stage ofnatural forest succession.

A detailed soil map (scale 1:12 000) of the 856 haTionesta Research Natural Area was made. The areais heavily forested and consists dominantly of easternhemlock (Tsuga canadensis L.) and American beech(Fagus grandifolia L.) stands. The soil survey wasmade to determine the soils occurring on the variouslandscape components and to establish the interrela-tionships between the vegetation and the soils.

DESCRIPTION OF THE STUDY AREAThe Tionesta areas are located within the Allegheny Na-

tional Forest of McKean County, PA (Fig. 1).The climate of the Allegheny High Plateau is cool and

humid. The average annual air temperature of the Tionestaareas is 8°C, and the average annual precipitation is 1067mm, with nearly half of this coming between the months ofJune and October. Annual snowfall is generally > 1500 mm.The average frost-free period is about 120 d.

Extreme weather conditions periodically occur in the studyarea. Bjorkbom and Larson (1977) discuss some of theseextremes and their effects on the forest stands in the Tio-nesta areas. Strong winds can occasionally cause local dam-age.

The boundary between Mississippian and PennsylvanianAge rock formations occurs in the study area (Fig. 2). The

696 SOIL SCI. SOC. AM. J., VOL. 49, 1985

McKEAN COUNTY AND THE STUDY AREA

Coastal Plain ProvincePiedmont Uplands

Conestoga ValleyTriassic Lowland

Fig. 1. Location of the Tionesta areas, McKean County, PA.

Knapp is the upper-most formation of the Mississippian Ageand the Pottsville Group is the lowermost formation ofPennsylvanian Age in the western portion of McKeanCounty. The Knapp formation consists largely of marineconglomerate, sandstone, and shale. The Pottsville Groupconsists of interbedded sandstone and shale, intermittentcoal seams, and a conglomerate unit at its base. Sandstoneis by far the dominant rock type in these formations. Thebedrock at the Tionesta areas appears to be flat-lying, butthere is a gentle regional dip of about 9 to 15 m/km to thesouth-southwest (Fettke, 1938).

Most seepage contributing to the waters of the two streamswhich dissect the area, East Fork Run and West Fork Run,originates on southfacing slopes. Precipitation infiltrating theupper portions of the bedrock readily moves through thepermeable sandstones. Upon encountering a less permeableshale layer, groundwater flows down-gradient along the dipof the shale bedrock.

On topographic profiles (Fig. 3) there are numerous nick-points, particularly at the higher elevations. In general, south-facing slopes appear to be more gentle than the north-facingslopes, which impart asymmetry to the valley profiles.Greater erosion on the south-facing slopes due to largeramounts of seepage waters (particularly during wetter pe-riods) may be contributing to valley asymmetry. The head-lands of both East Fork Run and West Fork Run are asso-ciated with these bedrock seepages. Field observationsrevealed that the nickpoints are expressions of resistantsandstone bedrock ledges. The more easily weathered shalelayers are preferentially eroded beneath the sandstone layerswhich remain to form structural benches on the upper reachesof the valley walls.

The Allegheny High Plateau was not glaciated (Shepps etal., 1959). By the time the ice advanced to its southernmostextent near the Allegheny High Plateau, it was weakenedsufficiently so that it circumvented the higher portions ofthe plateau and continued through the lower basins to theeast and west (Fig. 1).

Horizontal scaleFig. 3. Topographical cross-sections of the study area.

It is probable that during the Late Wisconsin Glaciationperiod, the Allegheny High Plateau experienced a vigorousperiglacial climate. Relict gelifluction sediments in the Ap-palachian Region are common within a zone > 100-km widebeyond the outer limit of glaciation (Denny, 1951). Land-form features characteristic of periglacial environments inthe Tionesta areas include solifluction (colluvium) depositson the lower portions of the hillslopes, tor-like rock ledgeson the plateau tops, and boulder fields in the lower footslopelandscape positions. The colluvial deposits now appear tobe stable except on slopes > 60%. In northern hemisphereperiglacial environments colluvium from south-facing slopesforces streams and rivers to preferentially erode and steepentheir north-facing valley walls (Currey, 1964). Under a per-iglacial environment, gelifluction processes would have beenmore intense on the valley walls facing the sun for the long-est time each day. Thus, north-facing slopes would havebeen cooler and less susceptible to gelifluction processes,thereby retaining steeper slope profiles. Gelifluction pro-cesses associated with a former colder climate may be re-sponsible for the valley profile asymmetry and the extensivecolluvial deposits observed in the lower elevations of thesouth-facing slopes.

METHODS AND MATERIALSStream geometries, general vegetation patterns, and can-

opy openings were noted and located on aerial photographs.Transects across major landscape physiographic units wereselected, and observations were made at 30-m intervals. De-tailed soil profile descriptions were compiled and the soilswere classified in accordance to the guidelines and regula-tions of the National Cooperative Soil Survey (Soil SurveyStaff, 1975). The area was mapped at a scale of 1:12 000. Anenlarged version of the USGS topographic map (scale1:24 000) including the Tionesta areas was used as a basemap. The topographic map was enlarged to twice its original

ALLEGHENYRIVER

KENDALLCREEK

KINZUACREEK

STUDYAREA

75°elevation 550(m) 350

150

OSWAYOCATTARAUGUSCHEMUNG ———

Fig. 2. Structural cross-section of the bedrock in the Bradford District of north-central Pennsylvania.

AGUILAR & ARNOLD: SOIL-LANDSCAPE RELATIONSHIPS OF A CLIMAX FOREST IN THE ALLEGHENY PLATEAU 697

TYPICFRAGIAQUULT

(Nolo)

OCHREPTICHAPLUDULT

(Clymer variant)

TYPICDYSTROCHREPT

(Hazelton) TYPICFRAGIAQUALF(Brinkerton)

•JCookport taxadjufcO/'

V / AQUT&/

(Ernest^. ̂ ~ / "- --/

TTFmil-n WITHSFTAI F AMD SILISrONE

MIXEDALLUVIUM &COLLUVIUM

COLLUVIAL SEDIMENTSFig. 4. Soil-landscape diagram of the Tionesta Research Natural Area.

scale in order to obtain the desired scale (Fig. 6). Pedonsbest representing the overall properties of the various soilswere sampled by genetic horizon for laboratory analyses.

Bulk densities for each horizon were determined on soilclods as outlined by Olson (1979). Particle size analysis wasdetermined by the method described by Dower and Olson(1980). Coarse fragments (> 2 mm) were estimated in thefield.

Samples were analyzed for cation exchange capacity (CEC),exchangeable bases, pH, total organic matter, and exchange-able aluminum. The CEC was determined on sample solu-tions extracted with ammonium acetate at pH 7 (Peech, 1945)and at pH 8.2 by leaching the soil samples with 0.25 M BaCl2and 0.55 Mtriethanolamine. Extractable bases (Ca2+, Mg2+,K+, Na+) were determined by atomic absorption and flamephotometry using these extracts. The pH measurements weremade with a standard pH meter in 1:1 soil-water pastes.Exchangeable aluminum was extracted with 1 M KC1. Or-ganic matter was determined by the titrimetric method out-lined by Greweling and Peech (1965).

DISCUSSIONSoils in the Uplands

The four major soils encountered in the uplandsdiffer primarily by drainage class, coarse fragmentcontent, and particle size distribution. The Aquic Fra-giudults and Typic Fragiaquults on the plateau topshave formed from very similar parent material anddiffer primarily in drainage (Fig. 4). These soils occuron level to gently sloping terrain and, as indicated byuniform increase in coarse fragments with depth, haveweathered in place from sandstone interbedded withsome siltstone and shale (Table 1 and 2).

The Aquic Fragiudults are very similar to the Cook-port series (fine-loamy, mixed, mesic Aquic Fragiu-dults) except that these soils have a coarse-loamy fam-ily textural class and thus, were classified as ataxadjunct of the Cookport- series. The Typic Fragia-quults, with fine-loamy, mixed, mesic family charac-

Table 1. Physical and chemical data of the Cookport taxadjunct soil—coarse-loamy, mixed, mesic Aquic Fragiudults.

Particle size distribution( < 2 mm material)

Depth

cm2 + -0

0-88-13

13-2929-6060-104

104 +

Horizon

OaEBtlBt2BBxlBx2

Sand

_494342475253

Silt— % —

-383638403229

Clay

..132120131618

Bulkdensity

Mg/m'-

1.541.071.161.531.691.77

Coarsefragments

by vol

%<5

55

10152560

CEC

OM

%37.01.94.63.01.20.50.5

PH

3.33.53.64.34.54.54.4

Exch. Al Sum of(KC1 extract.) cations

2.62.15.93.62.02.72.1

- cmol( + )kg~'-8.3

25.417.18.16.15.1

Effective

5.52.46.33.72.02.82.2

Base saturationSum ofcations Effective

———— % ————52.3

3.7 13.81.6 6.40.7 3.40.9 3.21.0 3.22.6 5.9

Table 2. Physical and chemical data of the Nolo loam soil—fine-loamy, mixed, mesic Typic Fragiaquults.

Depth


Horizon Sand Silt Clay

CoarseBulk fragments

density by vol OM

CEC Base saturationExch. Al Sum of Sum of

pH (KC1 extract.) cations Effective cations Effective

3 + -00-99-21

21-3434-7070-108

108-120 +

OaEBtBgBxlBx2Bx3

393051484243

423431313434

193618212423

1.761.081.301.751.841.79

555

121530

62.02.53.91.30.60.30.5

3.43.54.04.44.64.95.4

10.77.19.64.13.92.20.7

20.239.514.39.58.39.5

13.37.3

10.24.44.43.54.2

1.01.32.23.8

15.836.8

19.62.85.27.1

11.037.084.0


Table 3. Physical and chemical data for the Clymer variant soil—coarse-loamy, mixed, mesic Ochreptic Hapludults.

Particle size distribution

Depth

cm1.5 + -0

0-77-11

11-2424-4040-5656-9090 +

Horizon

OaEBtl2Bt22BC12BC22C2R

( < z mm material;

Sand

-413437444249-

Silt

— % —-

464141404437-

Clay

-132522161414-

Bulkdensity

Mg/m1

-1.491.001.221.191.411.45

fragmentsby vol

——— %

158

10153575-

CEC

Exch. Al Sum ofOM

51.52.16.83.83.01.00.6-

pH

3.43.63.74.24.64.54.5-

(KC1 extract.) cations

3.44.28.06.32.62.53.1-

cmol ( + ) kg"1

..13.239.527.218.110.18.1-

Effective

6.44.38.56.52.72.53.2-

Base saturation

Sum ofcations Effective

———— % ————

47.01.3 3.91.2 5.80.6 2.30.4 2.90.7 2.61.7 4.2-

teristics, were classified at the series level as Nolo loam.The Cookport taxadjunct is the dominant soil occur-ring in the Tionesta area. All soil series described ormentioned in this paper have been established and arecurrently recognized by the National Cooperative SoilSurvey.

The Cookport taxadjunct and the Nolo loam soilshave well expressed argillic and fragipan horizons. Inmany Pennsylvania soils, it has been observed thatthe degree of fragipan expression increases and thedepth to the upper boundary of the pan decreases assoil drainage grades from moderately well drained tosomewhat poorly drained (Peterson et al., 1970, andCiolkosz et al., 1979). The soils in the Tionesta areadisplay similar characteristics.

The Nolo loam soil has a strongly developed fra-gipan that slows percolating water and retards trans-location (Table 2). Gleyed conditions were observedthroughout most of the soil profile. The Cookport tax-adjunct soils range from moderately well drained tosomewhat poorly drained. The degree of fragipanexpression varies significantly within this range, beingmost strongly developed in the somewhat poorlydrained soils. Hemlock, well adapted to cool, wet sites,is the dominant species occurring on the poorly drainedNolo soil. Beech is the most abundant species estab-lished on the Cookport taxadjunct soil; however, theproportion of hemlock rapidly increases as 'this soilgrades into the poorly drained Nolo soil. The pod-zolization process is more strongly expressed in soilsunder hemlock (Tsuga canadensis L.) than under beech(Fagus grandifolia L.) and other hardwood stands.

The soils occurring on the very steep slopes in theconvex creep slope and upper shoulder landscape po-sitions (Fig. 4) are coarse-loamy, mixed, mesicOchreptic Hapludults. The uniform increase in coarsefragments with depth suggests that the subsoil isweathered in place from sandstone and interbedded

shale (Table 3). The chemical and physical propertiesof these soils do not fall within the range of charac-teristics of any currently recognized soil series. TheClymer series, fine-loamy, mixed, mesic Typic Haplu-dults, is similar to this soil except that it has greaterdepth to bedrock and a finer family particle size class.The soil differs significantly from Clymer series in itsuse and/or management because of its moderately deepprofile, and thus was classified as a variant of the Cly-mer series. The Clymer variant soil is not as well de-veloped pedogenetically as the Nolo loam and Cook-port taxadjunct soils because surface soil creeprestricted the illuviation of clay into the argillic fromthe surface horizons. High surface runoff on the steepslopes where this soil occurs also results in reducedleaching of water through the profile. The disconti-nuity between the subsoil weathered in residuum andthe downslope creeping surface soil is indicated in thehorizon designations of the Clymer variant (Table 3).This soil does not have a fragipan, is well drained,and generally high in coarse fragments throughout,particularly in the partially weathered C horizons.Depth to bedrock is somewhat shallow (usually < 90cm) and forest stands are especially susceptible towindthrow on the steep slopes where this shallow soiloccurs. Black cherry (Prunus serotina L.), a shade-in-tolerant species, is quite abundant on this soil becausefrequent canopy openings associated with blow downshave allowed its establishment. On slopes with north-northeast aspects, virtually pure stands of hemlock(90-95%) have become established on the Clymer var-iant soil.

The soils formed in parent material weatheredchiefly from sandstone material are loamy-skeletal,mixed, mesic Typic Dystrochrepts (Fig. 4). The chem-ical and physical properties of this soil are within therange in characteristics of the Hazelton series. The tex-tures are very sandy and contain low proportions of

Table 4. Physical and chemical data of the Hazelton soil—loamy-skeletal, mixed, mesic Typic Dystrochrepts.


Depth

cm3 + -0

0-1919-2424-4040-5959-8080-9898-140 +

Horizon

OaEBEBwlBw2Bw3BCC

Sand

_72605966727367

Silt

_25463530222227

. Clay

..3464656

by vol.

1030403540456080

CEC

OM

i58.00.65.23.11.30.70.30.5

pH

3.43.63.54.24.54.64.54.5


4.20.67.13.61.41.01.01.1

- cmol ( + ) kg'1

-4.1

35.419.110.15.15.15.1

Effective

7.60.77.53.71.51.01.11.1

Base saturation

cations Effective

45.42.7 16.11.0 4.90.5 3.50.7 4.61.0 5.01.2 5.41.2 5.2


Table 5. Physical and chemical data of the Ernest soil—fine-loamy, mixed, mesic Aquic Fragiudults.

Depthcm

2 + -00-88-12

12-2323-4646-9696-124 +

Horizon

OaEBE2BT12Bt22Bxl2Bx2


Sand

-252018242739

Silt/o

_575052504746

Clay

183030262615

BulkdensityMglrn'

-1.451.101.111.331.711.75

Coarsefragments

by vol.

———— %<5

58

10102010

CEC

OM

)

74.02.75.33.71.80.71.0

PH

3.53.43.64.04.34.55.4

Exch. Al(KC1 ex tract.)

6.84.98.76.64.64.80.4

Sum ofcations

iOl( + )kg--

16.238.327.217.213.79.2

Effective

9.65.19.06.74.75.53.7

Base saturationSum ofcations

-0.90.80.71.05.2

34.9

Effective% —————

62.912.810.725.451.088.666.2

silt and clay-sized particles (Table 4). Coarse fragmentcontent is high throughout the profile. The fairly uni-form increase in coarse fragment content with depthindicates that the soil has weathered from residuum.The Hazelton soil is well drained and has a permea-bility ranging from moderately rapid to rapid. It oc-curs on the nearly level to very steep slopes associatedwith resistant sandstone structural benches and on thelevel plateau tops where sandstone outcrops occur nearthe ground surface (Fig. 6). The ground surface onthese soils is excessively stony. The Hazelton soil ispedogenetically considered to be very young, althoughit may be among the oldest in the study area. The lackof an argillic horizon below the eluvial horizon placesthe soil in the Inceptisol order. The chemical analysesof the Hazelton soil indicate that the two horizonsimmediately below the eluvial horizon are high inamorphous hydroxides of Al metallo-organic com-plexes (Table 4). The vegetation on the Hazelton soilreflects the well-drained, coarse-textured nature of thissoil. Hemlock is not abundant on this soil, insteadbeech and sugar maple (Acer saccharum L.) comprisethe major portion (> 90%) of the stands. In general,the canopy of the stands is less dense than that oc-curring on the other soils because individual speciesmust occupy greater soil volumes in order to obtainthe soil moisture which they need. The root systemsof most of the trees observed on the Hazelton soilextend deep into the subsoil, as well as laterally forseveral meters. During drier periods, the soil can be-come too droughty for most tree species.

Soils Formed in ColluviumThe footslope and toeslope landscape positions in

the Tionesta areas are dominated by somewhat poorlydrained and poorly drained soils derived from me-dium to fine textured colluvial material (Fig. 6). Thesoils developed in these finer textured deposits havestrongly developed argillic horizons, indicating thatthese colluvial materials have been fairly stable in re-cent geologic time. It is probable that the bulk of thesecolluvial materials moved downslope to the footslopeareas during the last glacial advance, when ice waspresent a few miles northwest of the Tionesta area, asthey appear to be quite stable in the present environ-ment. The two major soils developed in the colluvialdeposits are fine-loamy, mixed, mesic Aquic Fragiu-dults and fine-silty, mixed, mesic Typic Fragiaqualfs(Fig. 4).

The profile characteristics of the moderately well-drained Aquic Fragiudult soil formed in colluvium arewithin the range of the Ernest series. This soil is high

% Al SATURATION BASED UPON 'Effective CEC'0 50 100 0 50 100 0 50 100

10

30

50

70

90COoc£110up 130z111oS 10

Cookporttaxadjunct

0 50 100

£j 30

Q50

70

90

110

130

Nolo loam0 50 100

Clymer variant0 50 100

Hazelton Ernest Brinkerton

Fig. 5. Distribution with depth of exchangeable aluminum in thesoils at the Tionesta area based upon the "effective CEC".

in clay and averages over 50% silt in the particle-sizecontrol section (Table 5). The most important phys-ical property of the Ernest soil is the presence of anextremely dense fragipan occurring at about 46 to 48cm below the ground surface. The dense, slowlypermeable fragipan perches water causing severe sur-face runoff and ground surface erosion in these foot-slope areas.

On the Ernest soil, hemlock and beech stands areusually encountered. Sugar maple occurs less fre-quently on these soils and black cherry is rarely ob-served. Yellow birch (Betula alleghaniensis L.) is notoften found in the upper footslope areas, but is morefrequently observed in the wetter, lower footslopelandscapes.

The characteristics of the poorly drained soil in col-luvium (fine-silty, mixed, mesic Typic Fragiaqualfs)are within the range of the Brinkerton series. The mostimportant characteristic of this soil is that it is suffi-ciently saturated with bases in the subsoil to classifyas an Alfisol (Table 6). Surface wash and subsurfacewater action have resulted in the accumulation of sol-uble bases from the higher elevations at the lower foot-slope and toeslope landscape positions where this soil


Table 6. Physical and chemical data of the Brinkerton soil—fine-silty, mixed, mesic Typic Fragiaqualf s.

Particle size distribution

Depth

cm3 + -0

0-1010-2626-5252-7676-115

115-130 +

Horizon

OaEBtglBtg22Bxgl2Bxg22Bx

( < •& mm material)

Sand

-181611272124

Silt

-574756474748

Clay

-253733263228

Bulkdensity

Mg/m"-

1.591.661.671.751.801.87

fragmentsby vol.

7t

<5<5

510153015

CEC

Exch. Al Sum ofOM

55.03.42.20.70.60.40.4

pH

3.73.94.24.75.05.35.4


4.26.07.05.33.00.93.5

cmol( + )kg-'-

16.822.915.814.114.914.0

Effective

11.16.98.07.26.17.8

10.5

Base saturation!

Sum ofcations

5.23.7

11.522.146.4t49.9T

Effective

70

62.912.810.725.451.088.666.2

t Base saturation > 35% within a depth of 75 cm below the upper boundary of the fragipan horizon.

occurs. The bulk density is high throughout and therooting depth is limited to the upper 50 to 60 cm ofthe profile. The subsoil is saturated with waterthroughout most of the year and gleying occurs within10 cm of the soil surface.

Hemlock is the most abundant species occurring onthe Brinkerton soil. Yellow birch is frequently en-countered on this poorly drained soil. White ash(Fraxinus americana L.), although not often observedon the other soil types, commonly grows on this soil.

Soils Formed in Mixed Alluvium and ColluviumThe southwest corner of the Tionesta study area,

along East Fork Run (Fig. 6 and 4), contains a smallunit of soils derived from mixed alluvium and col-luvium. Alternating episodes of sediment depositionby the stream during high discharge periods and col-luvium from the steep slopes adjacent to the streamhave resulted in a highly heterogenous reworked col-luvium. These soils were not characterized becausethe parent material varies greatly. The drainage of thesesoils varies substantially within short distances. Thesoil directly adjacent to the stream channel, domi-nated by coarse-textured alluvium, is better drainedthan the soil between the stream channel and thefootslope landscapes.

The vegetation on these soils is similar to that oc-curring on the Brinkerton soil and is dominated byhemlock and beech stands. Yellow birch is an impor-tant component of the stands and some white ash alsooccurs.

Chemical CharacteristicsThe most important chemical characteristics of the

soils in the Tionesta area are low cation exchange ca-pacities, low base saturationxvalues, and extremely highexchangeable aluiriinunUevels (Table 1-6). Low CECand base saturation ^values reflect the abundance ofquartz and other lo,w' base primary minerals which arehighly resistant tox weathering. This is further substan-tiated by high,'values of exchangeable Al. Aluminumtends to accumulate in soils as weathering proceedsand the mbre soluble cations are removed from thesoil matrix.

For/acid soils that have little pH-dependent charge,such'as montmorillonitic soils low in organic matter,standard methods for determining cation exchange ca-pacity are adequate. However, for other acid soils

'which exhibit a significant amount of variable charge,these methods can significantly overestimate the ex-change capacity. A better estimate of the CEC of thesetypes of soils can be obtained by leaching the soil witha neutral, unbuffered salt such as KC1 at the pH of the

, soil (Sanchez, 1976). This estimate of CEC, termedthe "effective CEC," was determined for the soils in'the study area in addition to the standard measure-•ments of CEC. The effective CEC values (effective CEC. = exchangeable Al + sum of basic cations extractedby 0.25 M BaCl2-0.25 M triethanolamine, pH 8.2)more accurately reflect the exchange capacity in thesesoils because only those cations exchangeable at thelower pH values associated with the soils are consid-ered. The low effective CEC values of the soils reflect

SOILBrinkerton Very Stony Silt Loam,

0 to 8% SlopesCookport Loam, Coarse-Loamy Taxadjunct,

0 to 15% SlopesClymer Stony Loam, Coarse-Loamy Variant,

3 to > 25% SlopesErnest Very Stony Loam, 0 to > 25% Slopes

•• Hazelton Very Stony Loam, 0 to 25% Slopes

Nolo Loam, 0 to 8% Slopes

mm Mixed Alluvium & Colluvium, 0 to 5% Slopes

EastFork Run

Symbols* Wet Spot* Swamp or Marshv Rock Outcrop

Boundaries>-^f~ Soil Boundaries- - - Area Boundary

0 500 m 1 km

Fig. 6. Detailed soil map of the Tionesta Research Natural Area.


the relative absence of weatherable primary mineralsand the accumulation of secondary clay minerals oflow exchange capacity as a result of weathering pro-cesses. Base saturation values obtained from the stan-dard CEC values are underestimated making the soilsappear more acid than they actually are. A more ac-curate measurement of the base saturation of the soilshas been determined from the effective CEC (Table1-6).

The soils in the study area appear to have an ionexchange system intermediate between the pure layersilicates and pure oxides systems described by San-chez (1976). The effective CEC values, although low,indicate the permanent charge of the soils. The dif-ference between the exchange acidity estimated at pH8.2 (CEC sum of cations) and effective CEC can beconsidered the variable or pH-dependent charge of thesoils. Sanchez (1976) pointed out that in oxide-coatedlayer silicate systems, layer silicates are often partiallycoated by thin, monomolecular coats of Fe and Aloxides. The permanent CEC in these systems is con-siderably lower than that of pure layer silicates be-cause negative charges arising from isomorphous sub-stitution are balanced by net positive charges of theoxides. This is what appears to be happening in mostof the soils in the Tionesta area, as indicated by thelow cation exchange capacities and high exchangeableAl levels (Table 1-6, Fig. 5).

With the exception of the poorly drained Brinkertonand Nolp soils, the process of podzolization (spodic-like horizons) is well expressed in all the soils. Al-though none of the soils in the Tionesta area havespodic horizons as chemically denned (Soil SurveyStaff, 1975), the well-drained Hazelton and Clymervariant soils occurring on steep, rugged terrain withnorthern aspects and nearly pure hemlock stands havevery strongly expressed spodic morphology.

In the Tionesta area, it was observed that the ma-jority of the roots in the soil profiles were distributedin the uppermost organic enriched horizons. Theseorganic horizons have the highest cation exchange ca-pacity. Apparently, the vegetation and soils are at a"steady state" condition with most of the nutrient cy-cling occurring within the organic horizons. Indeed,the mineral horizons of all the soils were found to behighly saturated with exchangeable aluminum (Fig. 5).Such high levels of exchangeable Al would have det-rimental effect on the growth of the trees if a largeportion of the Al was actively involved in the nutrientcycle of the stands.

CONCLUSIONSThe topography of the Tionesta Research Natural

Area is typical of a mature plateau with flat uplands(plateau tops) and steep V-shaped stream valleys. Thegentle regional dip of the bedrock, 9 to 15 m in asouthwest direction, has had some influence on thelandscape geometry by causing the preferential ero-sion of the south-facing slopes in the two major streamchannel valleys. Structurally controlled benches oc-curring on the upper reaches of the valley slopes reflectthe contrasting erodibility of alternating sandstone,siltstone, and shale bedrock. Deposits of cplluvial sed-iments have produced convex profiles in the foot-

slopes and toeslopes of the stream valleys. Much ofthe colluvium may have been deposited under differ-ent environmental conditions during the period of theWisconsin Ice Age when the unglaciated AlleghenyHigh Plateau, including the study area, was experi-encing an intense periglacial climate. Much of the col-luvial material presently appears to be stable, as evi-denced by the strong pedogenetic development of thesoils in these deposits.

All of the soils in the uplands (plateau tops) are verystrongly acid to extremely acid. The colluvial soils onthe lower footslope and toeslope landscape positionshave somewhat higher pH values, reflecting the ac-cumulation of soluble bases from higher elevations.In all other soils, the exchange sites in the mineralcolloids are greater than 90% saturated with Al. Withthe exception of the Alfisol on the lower footslope-toeslope areas, the maintenance of the CEC appearsto be synonymous to the maintenance of organic mat-ter. The soils and vegetation in this climax forest ap-pear to be at a "quasi steady state" condition with themajority of the nutrient cycling occurring within theorganic matter-enriched horizons.

ACKNOWLEDGMENTSWe wish to acknowledge the financial support of this re-

search study by the USDA-Northeastern Forest ExperimentStation's Forestry Sciences Laboratory, Warren, PA. We alsoexpress our gratitude to Prof. Arthur L. Bloom, Dep. of Geo-logical Sciences, Cornell Univ., for his helpful discussionsand suggestions about geomorphic processes.

soil-landscape relationships of a climax forest in the allegheny high plateau, pennsylvania1

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