Predicting Site Productivity of Mixed Conifer Stands in Northern Idaho from Soil andTopographic Variables1

H. G. BROWN AND H. LOEWENSTEIN*

ABSTRACTCharacteristics of 32 soils, consisting of an upper sequum in-

fluenced by volcanic ash over a buried sequum, and topographicvariables were used to develop prediction equations for site index,height, and total volume of mixed coniferous stands in northernIdaho. Soil and topographic variables explained 70% of the variationin site index. Ninety-four percent of the variation in height of the sitetrees, and 86% of the variation in total volume of the stand wereexplained by soil and topographic properties and age. Depth ofvolcanic ash influence, bulk density, P content, and elevation wereimportant soil and topographic variables.

Site index was negatively correlated with elevation, and positivelycorrelated with extractable Ca, exchange acidity, cation exchangecapacity, organic matter, total N, soil to rock ratio of the buried soils,and clay of the ash derived soils.

Additional Index Words: site quality, Douglas-fir trees (Pseudotsugamenziesii var. glauca (Beissn.) Franco), site and soil relationships,volcanic ash derived soils.Brown, H. G., and H. Loewenstein. 1978. Predicting site productivity ofmixed conifer stands in northern Idaho from soil and topographicvariables. Soil Sci. Soc. Am. J. 42:967-971.

SITE INDEX, the estimated height of dominant andcodominant trees at a given age, is often used to

estimate productivity of a forested site. However, manysites are either not stocked at all or only partially stocked,due to harvesting, fire, disease, insects, or other dis-turbances. On sites such as these, soil and topographiccharacteristics would be very useful in predicting potentialsite productivity. Many of these characteristics are quiteeasily determined in the field. Coile (8), for example, in asummary of literature up to 1951, found that aspect,exposure, topography, depth of soil, water table, and otherreadily measurable site properties were related to sitequality. Carmean (6), Gessel and Lloyd (12), and Stein-brenner (21) have also found similar relationships.

More recently, Mader (14), working with eastern whitepine (Pinus strobus L.), found that soil chemical propertiesaccounted for an additional 13% of the variation in sitequality not explained by topographic features or soilphysical properties. He also found that height (r2 = 0.88)and total volume (r2 = 0.65) could be predicted withgreater accuracy than site index.

Presently, site index must be used in conjunction withpreviously derived yield tables to obtain volume estimatesfor a specific area. Yield tables presume a fully stocked,even-aged stand, and are often not useful for unvegetatedsites. Since yield tables are used to obtain volume of one

'Contribution of the Univ. of Idaho, Forest, Wildlife, and Range Exp.Stn., Moscow, ID. Contributions no. 139. Supported in part by fundsprovided by the Mclntire-Stennis Cooperative Research Program (MS-16). Received 1 Dec. 1977. Approved 23 June 1978.2Graduate Assistant and Professor of Forestry, College of Forestry,Wildlife, and Range Sciences, Univ. of Idaho, Moscow, ID 83843;respectively. The senior author is presently the Forest Soils Specialist forthe State Department of Forestry, 2600 State Street, Salem, OR 97310.

species of tree on a specific site, they might not be as usefulin mixed stands as an estimate of the total volume in allspecies of trees. Prediction of total volume on a site fromsoil and topographic properties would be useful, if it weresuperior to yield tables.

The objective of this study was to provide predictionequations for site index and height of Douglas-fir trees(Pseudotsuga menziesii var. glauca (Beissn.) Franco) andtotal volume of stands in northern Idaho from soil andtopographic variables.

MATERIALS AND METHODS

Study AreaThe study area encompassed mixed coniferous stands in the

northern one-third of Idaho. Tree species present include: Doug-las-fir, grand fir (Abies grandis (Dougl.) Lindl.), western hem-lock (Tsuga heterophylla (Raf.) Sarg.), western red cedar (Thujaplicata Donn), western larch (Larix occidentalis Nutt.), westernwhite pine (Pinus monticola Dougl.), lodgepole pine (Pinuscontorta Dougl.), ponderosa pine (Pinus ponderosa Laws.),Englemann spruce (Picea engelmanii Parry), and subalpine fir(Abies lasiocarpa (Hook.) Nutt.). Although the stands areconsidered here to be mixed, at least 75% of the trees are grand firor Douglas-fir. Elevations of the sites range from approximately762 to 1,524 m. Precipitation, which falls mainly as snow in thewinter, varies between 64 and 127 cm/year. Mean annual airtemperature ranges from 1.0 to 8.0°C (22).

Figure one locates the study sites on a general geologic map ofthe study area. There are three major geological formations; theprecambrian Belt Supergroup which consists of a number ofsubordinate formations ranging in composition from slightlymetamorphosed siltstones to highly metamorphosed gneisses,schists, and quartzites; the cretaceous (age) Idaho Batholith whichis made up primarily of quartz monzonite (a granite type of rockcontaining a lower percentage of quartz and K-feldspars); theColumbia River formation, a typical fine grained, dark-coloredbasalt, which is overlain by a very thick deposit of loess that haseffectively prevented any contribution of the basalt to soilformation at any of the study sites. The entire area is covered witha relatively thick mantle of volcanic ash (up to 75 cm at site no. 7)which originated from Mt. Mazama (Crater Lake) 6,700 years ago(Powers and Wilcox, 18).

Habitat types are a union of myrtle boxleaf (Pachistimamyrsinites Raf.) understory associated with one of three climaxoverstory species—western hemlock, western red cedar, or grandfir (Daubenmire and Daubenmire, 9). Topography varies betweensteep mountain slopes at the higher elevations to more gentle tolevel slopes at lower elevation sites. Topographic variablesinclude aspect (along with various transformations), slope, andelevation.

Study SitesThe control plots of 32 previously established fertilization sites

were utilized for this study. DF (Douglas Fir) was studied on 14sites and GF (grand fir) was investigated on 14 other sites. BothDF and GF shared the four remaining sites, providing a total of 32different sites and 36 opportunities for site index and heightmeasurements, 18 in DF and 18 in GF. Volume measurementswere restricted to 32 observations, since volume calculations werebased on all trees on the sites not just DF or GF.

967

968 SOIL. sci. soc. AM. j., VOL. 42, 1978

LEGENDIdaho Batholith (Quartz Monzonite)

Belt Supergroup (Metasediments)

U Columbia River Basalt

10° Study Site

Fig. 1.—Geologic map of northern Idaho showing locations of studysites (geology from AAPG, 1).Aspect, elevation, and slope were measured. Habitat type was

recorded for each site but not used in the analysis because thethree types described were not evenly distributed among the 36sites.

Tree and Stand MeasurementsAverage total age of each site was found by adding 10 years to

the average breast height age which had been determined fromincrement borings of 10 dominant trees. Volume (Stage, 20) wascalculated using all trees on the site. Site indices for DF and GFwere based on the average height and age of 10 dominant sitetrees, and calculated from formulas developed by Brickell (5).Since site indices of 18 sites were obtained from DF and 18 fromGF, it was necessary to relate site indices to one species in orderto construct a regression equation for all 36 sites. Grand fir siteindex was expressed in terms of Douglas-fir site index by using aregression equation developed from similar stands in anotherresearch project at the Univ. of Idaho:3

DF(SI) = 13.34+ 0.4518GF(SI)

3K. Stoszek, H. L. Osbourne, J. Moore, and P. Mika. Relationship ofsite and stand attributes and management practices to Douglas-fir TussockMoth epidemics. Personal communication.

where SI = site index, base age 50 years, height in meters. Heightof GF was also put in terms of DF using data obtained byDeitschman and Green (10).

SoilsOne pit was dug on one control plot at each site. Soil horizons

were identified and sampled (Soil Survey Staff, 19). The sampleswere returned to the lab, air dried, and sieved. The > 2-mmportion was weighed and recorded. The < 2-mm fraction wasanalyzed in the following manner: pH by glass electrode in a 1:1paste; Ca, Mg, Na, and K by atomic absorptions after extractionwith neutral IN NH4OAc; CEC by ammonium saturation,washing with 95% ethanol, and distillation of the ammonia(Chapman, 7); exchangeable acidity by barium chloride-triethano-lamine (Peech, 17); percent organic matter by the Walkley-Blackmethod (13); percent sand, silt, and clay by Bouyoucos hy-drometer (23); total N by the Kjeldahl method (Bremner, 4);soluble P extracted with dilute hydrochloric and sulfuric acid(Nelson, 16); P sorbed determined from P-sorption isotherms (Foxand Kamprath, 11); P in both methods analyzed using theprocedures of Murphy and Riley (15); bulk density by the clodmethod (Blake, 3); percent moisture at 1/3 and 15 atm by pressureplate extraction; base saturation percent by summing the cations,dividing by CEC, and multiplying by 100.

Due to the presence of volcanic ash in the upper horizons, 31 ofthe 32 soils (the soil at site no. 18 did not contain volcanic ash)were classified either in the Andept suborder or in the Andicsubgroup of the Ochrept suborder. Eighteen soils contained aburied sequence which had formed in silty material derived fromthe siltstone members of the Belt Supergroup or in the deep loessdeposits over basalt. In some instances fragipans formed in theburied sequence. Profile no. 11 is representative of those soilswhich contain fragipans. At 13 sites, where the parent material isderived from coarse-grained gneisses and quartz monzonite, thesoils contain a higher percentage of sand and coarser material, noburied sequence, and volcanic ash deeper in the profile.

Two representative profile descriptions are presented belowwith their corresponding chemical properties in Table 1.

SITE No. 10This soil was described at an elevation of 884 m on a 35% north

slope. The parent material is volcanic ash over highly weatheredmica rich schists. Mean annual temperature varies from5.0-6.1°C, mean annual precipitation from 79-84 cm. Conifervegetation is primarily grand fir and Douglas-fir with a smalleramount of western red cedar. It is classified in the medial overloamy, mixed family of Entic Cryandepts.Profile Description

O—2.5 cm to 0. Fresh and partially decomposed needles,leaves, cones, twigs, and pieces of wood; abrupt, wavy bound-ary.

527ir—0 to 15 cm. Brown (10YR 5/3) loam, dark brown(10YR 3/3) moist; weak fine granular structure; soft, very friable,slightly sticky and slightly plastic; abundant very fine and fine,plentiful medium and coarse roots; many microinterstitial andvery fine tubular pores; clear, wavy boundary.

B22ir—15 to 46 cm. Light yellowish brown (10YR 6/4) loam,dark yellowish brown (10YR 4/4) moist; weak fine and mediumsubangular blocky to weak fine granular structure; soft, veryfriable, slightly sticky, slightly plastic; plentiful very fine andfine, few medium roots; many microinterstitial and very finetubular pores; clear, irregular boundary.

UA'l—46 to 71 cm. Yellowish brown (10YR 5/4) loam, darkyellowish brown (10YR 3/4) moist; moderate coarse and mediumsubangular blocky structure; slightly hard, friable, slightly sticky,slightly plastic; abundant very fine, few fine and medium roots;many micro, common very fine and few fine tubular pores; veryfew thin clay films in pores; moderately micaceous; clear, brokenboundary.

BROWN & LOWENSTEIN: PREDICTING SITE PRODUCTIVITY OF MIXED CONIFER STANDS 969

Table 1—Selected chemical and physical properties of two northern Idaho inceptisols.

Extractable cations Basesatura-

Horizon Depth pH Ca Mg Na K H CEC tion OM N C/N

Phosphorus isothermst Cit-dith. extr.

P Intercept): Slope Fe Al Mn Sand SiltBulk

Clay density

B21irB22irIIA'lIIB'2t

-meq/100 g—————— ——— % ——— ppmProfile no. 10, Entic Cryandept, medial over loamy, mixed

g/cms

0-1515-4646-7171-107

6.3 6.46.4 2.66.2 4.36.0 4.0

1.7 0.05 0.6 20.2 24.1 360.5 0.07 0.4 18.9 16.0 220.7 0.07 0.3 7.3 9.4 570.6 0.04 0.1 1.6 6.6 72

6.2 0.20 15.3 0.42.6 0.11 13.7 0.51.0 0.05 21.1 2.90.1 0.01 5.3 2.4

2,313.5 1,555.71,930.5 952.3

312.8 234.0139.3 103.6

1.4 0.46 0.03 28 56 16 0.611.3 0.37 0.08 30 50 20 0.750.9 0.08 0.03 40 42 18 1.110.7 0.04 0.01 56 32 12 1.56

Profile no. 11, Andic Fragiochrept. medial, frigidB21irB22irB23irIIA'+B'xIIB'x+A'IIB'2tx

0-1010-2828-3838-6969-104

104-163

6.3 4.86.1 2.46.1 1.95.7 4.15.6 4.95.5 4.8

0.9 0.06 0.80.5 0.06 0.70.3 0.07 0.60.8 0.09 0.31.0 0.12 0.32.3 0.10 0.3

18.0 19.320.1 17.516.7 13.66.3 8.37.3 11.18.7 12.5

342120625760

3.3 0.10 19.1 4.52.1 0.10 12.3 7.01.3 0.06 12.5 4.40.2 0.03 4.2 7.70.1 0.03 1.9 7.80.2 0.05 2.6 4.3

1,280.7nd§

1,156.1262.8291.5322.2

886.6nd654.8183.8200.6214.1

1.1 0.50 0.05 221.1 0.51 0.04 401.0 0.36 0.03 301.0 0.06 0.03 181.0 0.06 0.03 161.3 0.08 0.04 20

624858585852

16 0.6012 0.6312 0.9524 1.7626 1.7228 1.85

t P sorption isotherms were calculated from this formula: y = a + 0 (log x) where, y = P sorbed (ng/g soil), a = intercept, 0 = slope, x = equilibrium solutionP(ppm).

t Amount of P sorbed in ng/g soil at an equilibrium solution P level of one ppm.§ nd = not determined.

IIB'2t—71 to 107 cm. Light yellowish brown (10YR 6/4)sandy loam, dark yellowish brown (10YR 4/4) moist; weak coarseprismatic and coarse subangular blocky structure; slightly hard,friable, slightly sticky, slightly plastic; very few fine roots;common micro, few very fine and fine tubular pores; commonthin clay films in pores and on ped faces; highly micaceous.

SITE No. nThis soil was described at an elevation of 853 m on a 10% north

slope. The parent material consists of volcanic ash over loess.Mean annual temperature varies from 5.6—6.7°C, mean annualprecipitation from 71-76 cm. Conifer vegetation is primarilygrand fir and Douglas-fir. It is classified in the medial, frigidfamily of Andic Fragiochrepts.Profile Description

O—4 cm to 0. Fresh and partially decomposed needles, leaves,cones, twigs, and pieces of wood; abrupt, wavy boundary.

B21ir—0 to 10 cm. Yellowish brown (10YR 5/4) silt loam,dark brown (7.SYR 4/4) moist; weak medium subangular blockyand weak fine granular structure; soft, very friable, slightly sticky,slightly plastic; abundant very fine, fine, medium, and few coarseroots; many microinterstitial and very fine tubular pores; clear,wavy boundary.

B22ir—10 to 28 cm. Light yellowish brown (10YR 4/4), darkbrown (7.SYR 4/4) moist; weak medium subangular blocky andweak fine granular structure; soft, very friable, slightly sticky,slightly plastic; abundant very fine, fine, medium, and few coarseroots; many microinterstitial and very fine tubular pores; clear,wavy boundary.

B23ir—28 to 38 cm. Light yellowish brown (10YR 6/4) siltloam, dark brown (7.SYR 4/4) moist; weak medium subangularblocky and weak fine granular structure; soft, very friable, slightlysticky and slightly plastic; plentiful very fine and fine, abundantmedium, and few coarse roots; many microinterstitial and veryfine tubular pores; abrupt, wavy boundary.

IIA'&B'x—38 to 69 cm. Light yellowish brown (10YR 6/4) siltloam, dark brown (10YR 4/3) moist; moderate coarse subangularblocky structure; hard, firm, slightly sticky, slightly plastic;plentiful very fine and few fine roots concentrated along pedfaces; many micro and few very fine tubular pores; heavyaccumulations of bleached silt grains in pockets, streaks and manyfine lenses; gradual, wavy boundary.

IIB'x&A'—69 to 104 cm. Light yellowish brown (10YR 6/4)silt loam, dark yellowish brown (10YR 4/4) moist; moderatecoarse subangular blocky structure; hard, firm, slightly sticky,slightly plastic; few very fine and fine roots concentrated alongped faces; many micro and common very fine tubular pores; few

Table 2—Explanation of variables used in regression equationsand their respective means (n = 32).

Abbreviation Explanation Mean

Physical soil characteristicsBulk density (g/cm8)—ash layer 0.92 ± 0.15Bulk density (g/cm*)—buried horizons 1.51 ± 0.24Soil to rock ratio—buried horizons 0.78 ± 0.23Percent clay—ash layer 0.17 ± 0.08Percent silt—ash layer 0.49 ± 0.14Depth of the ash layer in centimeters 41.9 ± 18.4

Chemical soil characteristics

ABDBEDBSRRACLAYASILTADEP

AKBH

AOMBOMAPHOS

BPHOS

BNITEPOS

BBSATBCAACNRBCEC

ELEVSLOPEASPC

HTSI

TVAGE

Potassium (meq/100 g)—ash layer 0.37 ± 0.27Exchangeable acidity (meq/100 g)—

buried horizon 4.9 ± 2.6Percent organic matter—ash layer 2.1 ± 1.0Percent organic matter—buried horizons 0.36 ± 0.38Phosphorus (ppm) by double acid

method—ash layer 4.5 ± 7.1Phosphorus (ppm) by double acid

method—buried horizon 9.1 ± 21.1Percent N—buried horizons 0.03 ± 0.02Slope of P sorption isotherm curve-

buried horizon 207.6 ± 65.2Base saturation percent—buried horizons 55.5 ± 14.1Calcium (meq/100 g)—buried horizons 3.1 ± 2.3Carbon to N ratio—ash layer 16.2 ± 4.3Cation exchange capacity (meq/100 g)—

buried horizon 7.9 ± 5.1Topographic characteristics

Elevation (meters/100) 10.60 ± 1.98Slope in percent 28.1 ± 15.8(Aspect? x 45) - 45 4.2 ± 2.7

Stand characteristicsAverage height of the site trees in meters 15.4 ± 6.1Site index, base age 50 years, height

in meters 20.5 ± 3.1Initial volume in m'/ha 255.1 ± 133.5Average total age of site trees in years 42.1 ± 12.6

t Aspect was coded from one to eight beginning with N45E and con-tinuing in a clockwise direction using 45 ° increments to North.

thin clay films in pores; thin coatings of bleached silt grains onped faces and in pores; gradual, wavy boundary.

HB'2tx—104 to 163 cm. Light yellowish brown (10YR 6/4)silty clay loam, dark yellowish brown (10YR 4/4) moist;moderate very coarse and coarse subangular blocky structure;hard, firm, sticky, plastic; few very fine and fine roots con-centrated along ped faces; common micro and very fine tubularpores; many thin clay films in pores and on ped faces; thin and

970 SOIL. sci. soc. AM. j., VOL. 42, 1978

patchy areas of bleached silt grains on ped faces; abrupt, irregularboundary.

IIIR—163 cm. Basalt bedrock of the Columbia River for-mation.

Analysis of DataSince there were two control plots and only one soil pit per site,

means of the topographic and stand variables of the two plots wereused—making one observation for each site. In order to keep thesoil variables to a workable size, means of the volcanic ashderived upper horizons, and means of the buried horizons (thosebelow the influence of volcanic ash) were used instead ofindividual horizons. Site index, height, and volume were re-gressed in a stepwise procedure against 43 soil and four to-pographic characteristics including their various transformations.However, only those variables appearing in the regressionequations are listed in Table 2. The analysis was done on an IBM370 computer using a preprogrammed statistical analysis package(Barr et al., 2). The number of variables in the model was limitedto a maximum of 20% of the observations, unless the percentincrease in the coefficient of determination (r2), from addition ofmore variables, was greater than, the percent loss in degrees offreedom. • .

RESULTS AND DISCUSSIONSoil and topographic variables were used to explain

variation in site index, height, and volume. Although sitequality varied, the better sites were located on soils whichhad greater amounts of fines in the buried horizons. Lowerquality sites were located on soils developed from volcanicash and coarse-grained granitic and metamorphic rocks.Prediction equations for dependent stand characteristics arelisted in Table 3. These equations have not yet been testedon sites other than those used to develop them.

Site Index Regression EquationNine soil and topographic site characteristics were used

to explain 70% of the variation in site index (Table 3). Fourof the variables were soil physical properties, three weresoil chemical properties, and two were topographic siteproperties. Soil physical properties explained 36% of thevariation in site indices, soil chemical properties 23%, andtopographic characteristics 11%.

Soil properties significantly correlated with site index arelisted in Table 4, even though not all appeared in the

Table 3—Prediction equations for site index, height, and totalvolume (numerical coefficients are significant at p = 0.05

unless otherwise noted, r2 values are significantatp = 0.01).

EquationStandarddeviation r2

SI = 30.45 - 10.41(BBD) + 0.85(BH)- 0.22 (ELEV) + 0.06(ADEP)- 3.63(BOM) + 5.74(BSRR) - 8.67(AK)*+ 4.57(ABD)** - 0.23(ASPC)**

HT = -0.319 + 0.442(AGE) - 15.50(ASILT)+ 1.87IAOM) + O.OIS(BPOS)- 0.095(BPHOS) + 0.218(APHOS)- 0.183(ACNR)

TV = 47.19 + 9.20(AGE) + 294.52(BSRR)-4.0KBBSAT) + 3.59(SLOPE)-2.33(ADEP) - 1.59(BPHOS)-0.44(BPOS)

± 1.97m

± 1.63m

0.70

0.94

± 54.97 m'/ha 0.86

* and ** indicate significance atP = 0.10 and-P = 0.20, respectively.

Table 4—Correlation coefficients (r) of site index with selectedsoil and topographic properties, n = 36.

BSRR ELEV BCA BH ACLAY BCEC BOM BNIT

SI 0.513** -0.404* 0.427** 0.528** 0.423* 0.479** 0.351* 0.450*

** and * indicate significance atP = 0.01 andP = 0.05.

prediction equation. Calcium, cation exchange capacity,and percent N in the buried soils all are positivelycorrelated with site index. An increase in these soilcomponents probably improves the fertility of the site andpromotes higher growth rates. The positive correlation ofsite index with percent clay in the volcanic ash derivedhorizons would also improve the soil, since a higherpercentage of clay (which is typically low in these ashderived horizons) would enhance fertility by increasing thenumber of exchange sites and thus the total amount ofnutrients which could be retained in the soil, in addition toimproving tilth and increasing water-holding capacity.

Table 4 shows that soil to rock ratio (BSRR), percentorganic matter (BOM), and exchange acidity (BH) of theburied soils are significantly, positively correlated with siteindex. Since BSRR and BOM are instrumental in theretention of water in the soil, an increase in these twoproperties would probably lengthen the time in summer thatwater is available for growth in an area where mostprecipitation occurs in the winter. The fact that the bettersites are located at lower elevations, where the presence ofloess has increased BSRR (and undoubtably aided in theretention of organic matter as well as AL and H) is anexample of the positive correlation between SI and BSRR,BOM, and BH.

The negative correlation of site index with elevation(ELEV) is probably due to cooler temperatures and shortergrowing seasons reducing growth at high elevations.

One situation of note in Table 4 is the relationship ofexchange acidity (BH) to Ca (BCA). Since they are bothpositively correlated with site index (SI), a positive cor-relation could be implied between them. While suchimplications are not statistically sound, separate correla-tions did show that a positive correlation (r — .51**),rather than the more customary negative correlation, doesindeed exist between exchange acidity and calcium. Theexplanation for this lies in the fact that the number ofexchange sites changes with each of the 32 soils. In otherwords, if the number of exchange sites remains relativelyconstant as it would in one soil horizon, exchange aciditywould be expected to be negatively correlated with cal-cium. If, on the other hand, the number of exchange sitesvaries between soils as it does in this study, the amount ofexchange acidity and calcium would more likely bepositively correlated with each other, since both couldincrease as exchange capacity increased. Correlation coeffi-cients between exchange capacity and exchange acidity (r= 0.73**) and exchange capacity and Ca (r = 0.94**)demonstrate that each increases with increasing CEC.

Height Regression EquationNinety-four percent of the variation in height was

explained by six soil variables and age, which aloneaccounted for 71% of the variation. Percent silt in the ash

influenced horizons was the only soil physical propertyentered in the equation. It explained 10% of the variation inheight. Soil chemical properties explained the remaining13% of the variation. Topographic variables were noteffective in explaining variation in height.

Total Volume Regression EquationThe stands used in this study were of mixed composition,

and no single species of tree occupied more than 75% ofeach site. Thus, basing volume estimates on one specieswould not produce as precise an estimate as one which wasbased on all species. Therefore, volume estimates wereobtained from all trees on the site.

Eighty-six percent of the variation in total volume on the32 sites was accounted for by seven variables. Soil andtopographic variables accounted for 44% of the variationand age accounted for 42%. Soil to rock ratio and depth ofash were the only soil physical properties entered, ac-counting for 18% of the variation. Soil chemical propertiesaccounted for 17% and topographic variables for 9% of thevariation.

SUMMARYPrediction equations for dependent variables site index,

height, and total volume were derived from 43 soil and fourtopographic variables. Height of the site trees was predictedmost accurately—94% of the variation was explained bysix soil properties and age. Eighty-six percent of thevariation in total volume was accounted for by age and sixsoil properties. Seventy percent of the variation in siteindex was explained by seven soil properties and twotopographic variables. Site index was also significantlypositively correlated with extractable Ca, exchange acidity,cation exchange capacity, organic matter, and total N in theburied horizons and clay content of the ash derivedhorizons. Site index was negatively correlated with ele-vation.

Topographic variables were useful in explaining vari-ation in volume and site index but not variation in height.Soil physical and chemical properties were about of equalvalue in explaining variation. Age was the single mostimportant variable—accounting for 71% of the variation inheight and 42% of the variation in total volume.