Wood and Bark Specific Gravity of Small-DiameterPine-Site Hardwoods in the South

Floyd G. Manwiller

ABSTRACT. Ten amall-diameter treee from each of the 22 species (~ treee) were 88IDpledfrom throughout the southern United States. Mean SO was determined for stem wood and barkand the whole stem, for branch wood and bark and whole branches (to a minimum diameter ofO.M in.), and fortlee wood and bark and thewholetlee. Significantdift'erencee were determineda) among the speci. for each tree part measured and b) among the tree part8 of each SpeciM.The relationship between both stem-wood SO and stem-bark SO and height above ground isplotted by SpeclM. There was a significant inverse linear relationship between green MC andSO; with the exception of bark, the aahM contained lower ~t moisture than did speciM ofsimilar SO. Comparison of stem-wood SO values with Wood Handbook valuM indicates thatsmall-diameter pine-eite hardwoods are somewhat denser than larger trees of the same speciesmore typically meuuled.- -

that portion of each species range occurring irthe 11 southern states from Virginia to Arkansas and eastern Texas. Sampling was restrictec:to trees between 5.5 and 6.5 inches in diameteroutside bark, at breast height. Tree age vanec:from a species mean of27 years in yellow-popl8Jto 59 years in black tupelo, with an overalaverage of 39 years.

To sample the stem wood and bark irproportion to occurrence by volume, 2-inchthick disks were removed at 48-inch interval.along the stem- The lowest disk was taken at2-inches above ground, and the stem was considered to end at a point of branching abovtwhich a main stem could no longer be dietinguished. A 45-degree, pith-centered Wedg.measuring 1/2 inch along the grain W8Jremoved from each disk, and the wood and bar!separated at the cambium. SG values wer.determined from ovendry (OD) weight anigreen volume, the latter determined by wateimmersion after saturation. An average SG fostem wood (or for stem bark) was obtained b:adding the weights and the volumes of thindividual wedges of that stem to yield one 01weight and one green volume. Whole-stem S<was found by combining all the bark and wooweights and the volumes from stem wedges.

As REPORTED previoU8ly (1, 2, 3, 8), ~-sonnel at the Southern Foreet ExperimentStation have studied thoee properties of small-diameter hardwoods deemed important to theforest products industry. The hardwoodsstudied produce a substantial fraction of thetotal volume of fiber on sites capable of growingsouthern pine. However, they generally havenot been utilized because they lack a marketand because of their small size and poor form.An evaluation of their properties should aidprocess engineers in devjsing utilizationprocedures for this hardwood resource.

Specific gravity (SG) is an importantproperty because it is an indicator of otherproperties and of fiber yield per unit volume. Itis also relatively easy to measure. The objectiveof the study was to determine, for each of 22species, mean SG values for the entire tree; i.e.,wood plus bark, and for tree wood and barkseparately; and for the entire stem, and for stemwood and bark separately; and for the entirebranches, and for branch wood and barkseparately. A second objective was to deterDlinechanges in SG along the length of the stem foreach species. The 22 speci~ investigated (Table1) comprise nearly 90 percent of the hardwoodvolume growing on sites which support, or arecapable of supporting, pine stands.

The author is Professor of Foreetl'y, Iowa StatU niv., Am_, Iowa. This pa-Pel' was prep~ while hwas with the So. FOI'Mt Expl Sta., USDA Fo~S~ Pineville., La. Thia paper was received fopublication in July 1976.

ProcedureA total of220 trees, 10 of each species, were

cut from widely separated locations throughout

234 APRIL 191

along the entire length of the stem. However, adifference in sampling is probably not themajor reason for the higher valUM reportedhere. VariatioDS in SG with height in the stemare plotted in Figure 1 and thoee speciMexceeding the Wood Handbook values do so atall heights in the stem.VariatioD With Height in Stem

The stem-wood and stem-bark SG curvesplotted in Figures 1 and 2 are the average for the10 trees of each speciea. However, there is agreat deal of variability and in many speciesindividual trees often deviated from theaverage curve. Since not all stems are the samelength, each point is the average of 10 trees (90in red oaks) only up to a height of 22 or 26 feet.Above that height, the number of treesrepresented decreases w a minimum of either 4or 5 (20 in the red oaks).

Wood SO tended to remain relativelyconstant or w decrease with increasing heightin the stem (Fig. I), but the relationship variedconsiderably among species. In the ring-porousspecies SG d~ased with increasing height in

A similar procedure was followed in the top:each branch was sampled at 48-inch intervalsdown to a diameter of 0.5 inch, outside bark. Thefirst disk was taken at 24 inches from the pointof branching. For branch material, however,the entire disk, 1/2 inch along the grain, wasused rather than a wedge.

Mean SG for tt'ee wood was obtained bycombining the branch wood and stem woodweights, and the volumes. Stem wood valueswere weighted by a factor of eight since theywere taken from a 45-degree wedge rather thanfrom the entire disk. The same procedure wasfollowed for tree bark. Finally, whole-tree SGwas determined by combining the stem andbranch wood and bark weights, and thevolumes.

Statistical comparisons were made in twodays at the 0.05 significance level by Duncan'smultiple range test for: a) within-tree SG foreach species, and b) among-species SG by tissuetypes.

I determined the alcohol-benzene extrac.tive content of OD wood and bark andcalculated regression equations to determinewhether a linear relationship existed betweenextractive content and SG. The samplingprocedure was as described except that stemwedges and branch disks were 1 inch along thegrain. For each part (stem wood, stem bark,branch wood, and branch bark), the sampleswere combined and ground to pass a 4O-meshscreen. Extractives were removed with a 5-hourextraction of one volume 95 percent ethanol totwo volumes benzene, an alcohol wash, a5-hourextraction with 95 percent ethanol, and a water(not hot) wash.

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Figure 1. - Relationship between stem-wood SG ancheight aOOve ground in 6-inch (DBH) hardwood!growing on southern pine sit.. The red oak plot iIbased on data from !K) trees; the other plots represendata from 10 trees each.

Result.Hardwoods growing on pine sites are

apparently somewhat denser than larger treesof the same speciN more typically measured.The Wood Handbook (15) lists wood SG valUNfor 18 of the 22 species sampled. Stem-wood SG(Table 1) of most species was 0.01 to 0.04 abovetlte Wood Handbook value, witlt tlte meandifference for 18 species being 0.03. Sweetgumaveraged 0.01 below, while black oak, post oak,white oak, American elm, and southern red oakaveraged 0.06 to 0.09 above Wood HandbookValuN. An increase of 0.03 in SG wouldreprNent an additional 1.9 pounds of wood percubic foot, and a difference of 0.09 wouldrepresent an increase of-5.6 pounds.

It is unlikely that tlte Wood Handbookvalues were obtained by 8 systematic sampling

APRIL 197236

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-- I~_I t i ~ Ii - . R to to . . L

IE- -- - IFUT!Figure 2. - Relationship between stem-bark SO andheight above ground in 6-inch (DBH) hardwoodsgrowing on soutJ1em pine sites. The red oak plot isbased on data from 90 trees; the otJ1er plots representdata from 10 trees each.

(after an initial decrea8e) while in other specie!it decrea8ed or remained relatively constantKoch (5) found that red maple bark densit}increased to a height of 10 feet and the!:remained constant wherea8 the present resuluindicate a decrea8e above 6 feet. In 6- to 8-inclcdiameter at breast height (DBH) yellow-poplaJhe found that bark SG increased sharply 1<brea8t height, decrea8ed somewhat to a heigh!of 16 feet and then increased again (5). Iranother stand of 14- to 16-inch trees he fountthat Bark SG increa8ed steadily with height (6)The curves presented here for the red oaks arfsimilar to those for 8- to 10-inch northern reCoak, scarlet oak, and black oak (5). However,hifcurves for 14- to 16-inch red oak rise sharply to Eheight of 10 feet and then rise more slowly to Eheight of 40 feet.

Within-Tree ComparisonsOf the within-tree comparisons possible

the following seemed of interest: a) stem wood.branch wood, b) stem wood-stem-branch wood.branch-tree wood-tree, c) stem bark-stem woodd) branch bark-branch wood, e) stem bark-treEbark, and t) stem bark-branch bark.

In 20 species there Was no significan1difference in SG between stem wood and branchwood. The two exceptions were laurel oak ancJblackjack oak, and in both, stem-wood SG waflower than that of branch wood (Table 1). Thesfresults agreed in general with those of TayloJ(13) who, in comparing stem and branch woo(of seven of these species, found that in onlyonf(southern red oak) was there a statisticall}significant difference between the two.

Because of the preponderance of wood-8~percent of the above-stump green weight wherspecies were averaged-significant differenCe!among stem wood, whole stem, branch woodwhole branch, tree wood, and whole trefoccurred in only six species. In post and whit«oaks and in American and winged elms, branctSG was significantly below that of the otheJfive categories. In addition, winged elm wholestem and whole-tree density, averaging 0.576Was significantly below that of the stem woodbranch wood, and tree wood, averaging 0.618Laurel oak and blackjack oak were the tw<species in which branch-wood SG exceeded tha1of stem wood, by 0.042 and 0.008, respectivelyIn laurel oak, branch-wood density also exceeded that of the wood; and in blackjack oakbranch wood was the densest tissue in the tree

Stem-bark SG averaged lower than that 0:stem wood in the two white oaks samples (pos1~

hickory, remained constant to a height of 30 feetbefore increasing in the red oaks, and steadilyincreased in green ash. In the diffuse-porousspecies, wood SG generally was high near thebase of the tree, decreased sharply to a heigh t of6 feet, then remained relatively constant to aheight of about 30 feet. Above that height SGusually increased and then decreased.

There have been some other studies thatmeasured the relationship between SG andheight in stem for these species. Therelationship reported here for yellow-poplar isvery similar to that found by Koch, Brauner,and Kulow (7), Taylor (12), Gilmore (4), andSchroeder and Phillips (11). The relationshipobserved for sweetgum is similar - but lower atall heights - to the results obtained by bothWebb (14) and McElwee, Tobias, and Gregory(10). The curve the latter plotted for white ashfalls between those we obtained for white ashand green ash.

The relationship between bark SG andheight in the stem (Fig. 2) also varied amongspecies. SG generally increased with height inthe ashes and in black tupelo and sweetgum

23'WOOD SCIENCE VoL 11, No.4

TABLE 3. - Weight ~ cubic foo' (grwlt ~) of (N8Aly felled 6-iM1a ~ 8IOIIIiII6 on piM aiIa.

S-.-ie. Stem St8aI Branch Bruchr-- wood bark St8aI wood bark Brandl ~ -

Po53.2M.O56.851.050.769.252.654.956.865.384.662.963.963.2-.163.063.866.063.384.158.056.8

&3.359.7&6.861.1~7M.O62.&54.866.8~.o84.182.&83.882.98&.882.863.284.782.983.7M.7&6.4

.. ...

68.2

66.3

58.8

41.7

44.7

43.6

M.O

56.646.463.6«).057.469.861.957.6«).562.661.246.359.737.646.1

&6.0&6.8&8.751.651.767.651.451.858.288.9..388.081.082.08&.2..458.381.358.881.356.761.9

52.660.856.663.651.662.3&2.254.8&9.367.2..864.764.863.469.463.664.1..068.166.&64.458.7

Red mapleHickory, trueHackberryWhite uhGrwn aahS-tcuiDYellow.poplarS~ayBlack tupeloWhite oakScarlltoakSouutem ftd oakCherrybark oakLaurel oakBlackjack oakWat.' oakN ortbem ftd oakShwnard oakPoet oak

Black oak

Winged elm

Am~ elm

3). The inverse relationahip between MC andSG was investigated by linear regression.Equationa for each of the 8even ti88ues werederived in two ways: using all 220 obeervationafrom individual trees, and using the 22 speciesmeana. In plotting the data it became evidentthat, with the exception of stem bark andbranch bark, white and green ash contain 1e88moisture (percentage-ba8i8) than do otherspecies of similar SG. When the equations werecomputed again, with the ashes omitted, thecoefficient of determination was significantlyimproved. The relation8hips, presented inTable 4, were significant at the 0.01 level. Thecoefficient of determination was improvedusing specl~ meana since variability wasreduced.Specific gravity-Extractive ContentRelationships

The addition of extractives to a givenvolume of wood increases its SG and it wasdecided to determine whether there was a linearrelationahip between extractive percent (un-published) and SG. Individual tree valUM wereused for 8tem wood, branch wood, stem bark,and branch bark to determine the four~uationa for each SpeclM. Of the 88 ~uationsobtained, only 8 were significant at the 0.05significance level and 6 had a negative slope.This indicates that at the concentration8obtained in thMe speclM (ranging from 3.0% in

presented here apply only in a general waysince groups overlap and since significantdifferences occur within groups.

Because tree bark is composed primarily ofstem bark, species ranking and significantdifferences are essentially the same for both.Winged elm and sweetgum had the lowest SG(0.34 and 0.37). Yellow-poplar, American elm,the ashes, black tupelo, and sweetbay were nextwith values ranging from 0.39 to 0.44. In anintermediate position were hickory, red maple,and the two white oaks, ranging from 0.49 to0.54. The nine red oaks and hackberry were thedensest group, ranging from 0.60 to 0.64.

Yellow-poplar branch bark was least densewith SO of 0.34, followed by the two elmsaveraging 0.36 and by sweetbay and 8weetgumaveraging 0.40. Intermediate were black tupelo,post oak, the ashes, hickory, and red maple,ranging from 0.43 to 0.47, averaging 0.45. Theoaks and hackberry, ranging from 0.49 to 0.55,were densest with an average SO of 0.52.

Specific Gravity-Moisture ContentRelationships

In a previous paper (9), moisture content(MC) of the same trees, freshly-felled, wasdetermined for seven of the nine tissuesreported here; it was not determined for treewood and tree bark. Using the SG and MC data,it was po88ible to calculate the weight per cubicfoot of each b'ee part at the time of felling (Table

139WOOD SCIENCE Vol II, No. 4

Ide pel' cgbi.56.056.256.461.061.058.862.062.667.762.961.861.162.362.970.261.6M.362.464.$63.061.364.2

'oat M.4

63.858.051.152.263.749.048.850.951.763.352.4M.667.263.456.0M.45&.045.156.041.444.0

TABLE 4. - &Iommhip ~ JlC and sa 0( 6-illC/ahardwood. 6rowing ora pine .ite.'

-

~coefficient,

(6)

Regre88ionintercept

(0)t-ticmintree

Literature Cited1. CARTBR, F. L.. T. L. AMBUBGEY. and F. G. ~

1976. Re8iatan~of22~uthem hardw0CMi8 to wood-decayfaD8i and 8U~ t8nDit8. Wood Sci. 8(4)~~

2. CHOONG. E. T., F. O. Taoao. and F. G. ~1974. Permeability of twenty.two 8mall diameterhardwooda crowing on ~uthern pine sit.. Wood andFiller 6(1):91.101.

3. . and F. G. MANWn.I.&a. 1976. DimeD8ionalaDd reJative hy..-~iiic Pru.-ti- of hardwoods from~uu-n pine .i_. Wood sa. 9(1):39-43.

4. GlLMou, A. R. 1971. Specific gravity of plantation.Innm yellow-poplar i8 not reJated to Gte. Wood and Fiber~):182-183.

5. KOCH, C. B. 1970. Variation in bark .pecific gravity ofselected Appalachian hardw00d8. Wood sa. 3(1):43-47.

6. . 1971.111icbI- and 8pecific gravity of innerand outer bark of recI oak and y8j0w-popiar. Wood sa.3(4);214-217.

7. , A. BRAUNER, and D. KULOW. 1968. Specific8l8vity variation within young yellow-poplar trees. We8tVL UDiv.~. Expt. Sta. Bul. 564T. 9 pp.

8. MANWILLBa. F. G. 1974. Filler len«th8 in .tems and~ of .maD hardw00d8 on ~uthem pine 8it8.Wood sa. 7(2):130-132.

9. . 1975. Wood and bark moietuft contents 018mall-diameter hardw00d8 growinc on _them pinEGt.. Wood Scl. 8(1):384-.188.

10. McELWEE. R. L. R. C. ToBIAS, and A. H. GUOORY. 1970Wood characteristics of three eouthern hardwood 8pecle1and their nlationahip to pulping properti88. TAPPI53(IO):1~2-1886.

11. ScHROEDER, J. G~ and D. R. PHI1.uP8. 1973. Somtphysical properties of yeUow-poplar wood and bark. Pan11. VariatioD8 with heilht in the tree. Wood sa. 6(2):1231-.

12. TAYLO8, F. W. 1968. Specific 8l8vity differen~ withirand amo~ yellow-poplar ~. Forwt PrOO. J. 18(3):7581.

13. . 1977. A note on the nlation8hip betweerbr8Dd1. and .temwuod prvperti8 of 8eleded hardwood!IIOwinc in the mid-South. Wood and Fiber8(4)~7-.1

14. Waa, C. D. 1~. Natural variation in specific gravityfiber length and intarlocked rrain of 8weetcum (Liquidambar .tyraci{lua L.) within a-, aJOOnl~. anIaIDODI l8O8TBphic -- in the South Atlantic Stat.Ph. D. dIe8ia. N. C. State Univ., Raleigh, N.C. 1~ pp.

15. U.S. FOUST PaoOucrl l.ABORA~Y. 1974. Wood Handbook:Wood .. an En,ineering Material. USDA Handbook 72, Rev.

~~

OBSERVA'nONS FROM D) mE~Stem wood 170.10 -188.90 O.mlStem bark 133." -125.$ .4445Branch wood 159.38 -182.11 .6812Branch bark 163.78 -179.14 .4756Stem 168.58 -171.15 .5667Branch 169.73 -183.10 .6696TIW 170.M -175.03 ~

O~ERV A'nONS FROM m> ~ -ASH~ OMI'ITED

Stem wood 174~ -171.78 0.8682Stem bark 140.93 -137.87 .4783Branch wood 166.07 -1..19 --Branch bark 166.37 -184.55 .4971Stem 177.72 -183.18 .7233Branch 175.74 -190.88 .7474Tree 178.87 -1M.32 .7414

OBSERV A 'nONS FROM 22 ~ MEANSStem wood 177.~ -182.69 0.5767StMD bark 138.&1 -lam .56498I'8DCh wood 1~ -1~ .838I'ancl1 bark 187.68 --.76 .8759StMD 177~ -187.07 .6229Branch 183.18~.53 .7598TIW 178.52 -190JWI .6438

O~V A 'nONS FROM m SPECIES MEANS -ASH~ OMlTrED

Stem Wood 181.03 -183.44 0.7894Stem bark 148.31 -151.65 ,«)12Branch wood 119.18 -192.44 .8133Branch bark 187. 71 ~.82 .6'766Stem 186.62 -1~.91 .8240Branch 1~ -213.74 .8577TIW 187.33 -3)1.31 .8:1'J7-

iModel: Moietn.. content. . + b (1Peciftc graVity)

sweetbay stem wood to 21.8% in green ashbranch bark) extractiVM do not systematicallyinfluence SG.

240 APRIL 197

Coefficientof

determination(R')