Improving^Hardwood

Timber StandsExtension Bulletin E-1578, April 1982

\

By Russell P. Kidd_ar\d, Melvin R.Department of Forestry

ioelling

ore than 10.4 million of Michigan'* 18.9 million\ acye^pf forest land are owned by non-mdustrial pri-

vate landowners. There are estimated to be more than\ "iriOjOOO MrTnT'i individuals whose ownership objec-

tives range from wildlife and recreation to aestheticsto fuelwood production. Commercial timber produc-tion has not been a major objective of most owners.However, as the costs of land ownership continue toincrease and the value and demand for forest productsimproves, many forest owners are looking at timberproduction as a possible source of income. This bulle-tin offers suggestions and guidelines for improving andmanaging hardwood forest stands to produce com-mercial forest products.

In Michigan and elsewhere throughout the country,most privately-owned hardwood stands have beenneglected, with little or no thought given to produc-ing commercial forest crops (Fig. 1). In some cases,this lack of management is the result of the landownernot knowing what to do and how to do it, and notrealizing that his forest can be a source of income.

Most hardwood timber stands will benefit from atimber stand improvement treatment. Timber stand im-provement (TSI) is a conscious effort made in a foreststand to improve future growth and quality. This prac-tice is applicable to woodlots which have grown pastthe sapling stage with trees more than 4 inches in dia-meter at breast height (dbh). After a TSI program,the growth potential of a given site will be concen-trated on trees of potentially high quality and value.TSI can also have positive wildlife and environmentaleffects. However, the principal objective is to en-courage a stand of high quality trees which will growmore rapidly than if left untreated.

Fig. 1. Many hardwood timber stands in Michigan have notbeen managed as a renewable crop and are in need of an im-provement and/or thinning operation.

Objectives of Timber Stand Improvement

The general purpose of a TSI treatment is to im-prove productivity of the forest. Three specific ob-jectives are as follows:

1. To develop a quality forest stand by removingdefective, damaged and otherwise undesirable trees,thus concentrating growth on fewer, high quality trees.

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Defective, damaged, diseased or deformed trees havelittle, if any, value for wood products other thanfuelwood (Fig. 2). They do not have the potential ofdeveloping into high value products regardless of howlong they are permitted to grow. However, these treesoccupy growing space and compete with the bettertrees for moisture, sunlight and soil nutrients. Remov-ing them from the stand will allow desirable trees togrow more rapidly.

2. To promote growth of high value trees by re-moving species of low or no commercial (economic)value. Following removal of defective trees, the nextstep is to select and mark those trees which are of adesirable species and form. Most hardwood woodlotscontain an adequate number of trees, although not allare equal for producing high value products. Thefollowing species have high commercial value:

American basswoodBlack walnutWhite ashYellow birchSugar mapleRed mapleBlack cherryYellow-poplarWhite oakRed oakBlack oak

Species with low commercial value which should gen-erally be discriminated against are listed below:

IronwoodSassafrasHickoriesElmCottonwood (except for pulpwood)Pin cherryAmerican beechAspen (except for pulpwood)White birch (except for pulpwood)

3. To regulate stocking (number and distribution oftrees) to encourage optimum growth of the residualstand. Many woodlots contain too many trees for max-imum growth. This condition may be present follow-ing removal of the defective and low value trees, or inyoung, even-aged second growth stands. Although thestand may be composed of desirable species, compe-tition among the trees for available growing space canbe severe. In such stands, thinning is needed to regu-late spacing among residual trees so maximum growthcan occur. Growing space requirements increase astrees increase in size; therefore, more than one thin-ning may be necessary in the life of the stand.

Timber Stand Improvement MethodsCareful planning of a timber stand improvement

operation is essential for the best possible return fortime and effort expended. Where possible, selling thematerial removed will reduce the cost of the opera-tion and may provide some income. Pulpwood andfuelwood are common products removed in TSI. Insome areas, low grade logs may be sold to pallet orblocking mills.

Most hardwood timber stands will need more thanone TSI operation in the life of the stand. To trans-form an unmanaged woodlot containing large num-bers of competing desirable and undesirable stemsinto a vigorous stand of high quality and properlyspaced trees is impossible in a single operation. Rather,a TSI treatment must be repeated over time to achievea well-managed woodlot. In subsequent TSI opera-tions in the same stand, products of increasingly higherquality and value will be removed. Eventually, TSIwill be phased into regular harvesting operations.

Two basic methods are suggested for performing aTSI operation. Each will achieve about the same resultalthough different approaches are followed.

Crop Tree SelectionIndividual trees are selected based on species, form

and distribution. The number selected per acre willvary depending on tree diameter and species mixture.Undesirable trees 4 inches dbh and larger are removedto permit maximum growth to occur on the selectedtrees. The selected trees are "crop trees" which will bemanaged for the production of high quality products,primarily sawlogs and veneer. Increased growth oncrop trees after TSI is often 40 to 60 percent greaterthan before. Increases in tree value will be rapid sincethe growth is being concentrated on the selected, po-tentially high value trees. The earlier TSI occurs, thegreater the opportunity to produce larger amounts ofhigher value products from the crop trees.

Selecting Crop Trees. Selection of crop trees shouldbe started as soon as individual trees of potentiallyhigh quality can be recognized. In most unimprovedforest stands this will not occur before the averagetree is 3 to 4 inches dbh. Selection should be madebefore any cutting or improvement operations arestarted.

In selecting crop trees, look first for species of thehighest value and of most desirable form. Most wood-lots contain an adequate number of such trees. Croptrees should possess a relatively straight bole, largecrown, and have no cull or other apparent defects.They should have the potential of occupying a dom-inant position in the future stand.

A stand is adequately stocked when there are at

Fig. 2. Low value, cull trees should be removed to allowdesirable trees to grow more rapidly. This removed materialis an excellent source of firewood.

least 40 crop trees per acre, although in most standsconsiderably more will be selected. This low stockinglevel is particularly acceptable if high-value species,such as black cherry, black walnut or yellow birch arepresent. The maximum number of selected crop treeswill vary for each stand depending on the size of thetrees present, site conditions and potential market forproducts removed in improvement cuttings and thin-nings. For example, fewer crop trees will be selectedin a stand with an average diameter of 12 inches thanin a stand with a 6-inch average diameter. Likewise,more trees can be selected per acre if the stand isgrowing on well-drained, productive soil than if grow-ing on droughty sites or infertile soils. If a market isavailable for products removed in improvement cut-tings and thinnings, a few extra crop trees may beretained when initial selections are made. In lateryears, these can be removed in successive thinnings.In such cuts, efforts are directed toward increasingthe quality of the residual stand.

Spacing Crop Trees. The total number of crop treeson each acre will vary depending on the size of thetrees present and other conditions of the site. Sincemore growing space is required as a tree increases insize, larger diameter trees must be spaced fartherapart than trees of smaller diameters. One approachfor determining correct spacing is to use a constant-multiplier factor which is related to tree diameter. Touse this approach, the dbh of the tree is multiplied by1.67 to determine the proper radius in feet aroundthe tree within which competing trees should be cut.As an example, a six-inch diameter tree should bespaced approximately 10 feet (6 X 1-67 = 10.02) fromadjacent trees for maximum growth to occur. Sincetrees of varying sizes are present in most stands, dif-ferent spacing requirements are necessary. Determinethe average diameter for all trees in the stand andthen compute an average spacing distance. For ex-ample, if 6-inch, 8-inch and 10-inch trees are present,the average spacing distance would be 13.4 feet [thesum of (6 X 1-67) + (8 X 1.67) + (10 X 1.67)divided by 3 or (10.0 + 13.4 + 16.7)/3 = 13.4] (Fig.2). Some adjustments within the woodlot may benecessary if groups of larger and/or smaller treesare present.

The following table has been prepared to indicateproper spacing and numbers of trees per acre forforest stands with trees of varying diameters.

Table 1. Recommended distances between trees and num-bers of trees per acre for trees of various dia-meters.

Diameter(inches)

3

4

5

6

7

8

9

10

11

12

13

14

15

Distance BetweenAny Two Trees (feet)

5.0

6.7

8.4

10.0

11.7

13.4

15.0

16.7

18.4

20.0

21.7

23.4

25.0

Number of TreesPer Acre

1742

970

617

435

318

243

194

156

129

109

93

80

70

These values represent ideal spacing which in prac-tice may be difficult to achieve. Individual tree char-acteristics and position within the stand will probablynot permit exactly this distribution. However, this isnot a serious problem. Remember that growth willoccur following TSI or thinning, and trees will in-

Wood or Cardboard Block,

Wood Rod or Dowel

Fig. 3. A simple angle gauge can be easily constructed to determine estimates of basal area.

crease in size. Within a few years, depending on howfast the trees respond, it will be necessary to thin thestand again. In this and later thinnings, spacing ad-justments can be made.

Furthermore, all selected crop trees within a foreststand will not be of the same size, nor will they re-main in the same size class in relationship to eachother after their selection. Differences in species andsite conditions will result in variations in growth rateand, consequently, tree size. This is a desirable con-dition since most mixed and northern hardwoods re-spond well to management under an uneven-aged,mixed-size forest condition. In addition to favoringreproduction and early seedling growth, uneven-agedmanagement provides the landowner an opportunityfor periodic harvests as opposed to less frequent har-vests at longer intervals with even-aged stands.

To achieve an uneven-aged distribution in hard-wood stands, trees of varying sizes should be selected.As the trees increase in size, subsequent cuttings willbe necessary to maintain proper growing space amongcrop trees. These cuttings will do much to achieve anuneven-aged stand through regulation of stand density,size distribution, and encouragement of desirable re-production.

For hardwood management on larger commercialor industrial tracts, even-aged management may bepractical. This results from a uniform distribution oftree sizes in second-growth stands, allowing for in-creased efficiency in harvesting. Such operations areusually arranged so that use of small size and lowquality material is obtained. Particular emphasis mustbe given to promoting regeneration of desirable speciesin even-aged management when the mature stand isharvested.

Basal Area MethodThe concept of basal area is another approach

commonly used to regulate stocking in hardwoodwoodlots. Basal area refers to the cross sectional areaof an individual tree measured at dbh. For example,if a given tree has a dbh of 10 inches, its basal area(actual stem cross-sectional area) is 78.54 square inchesor 0.545 square feet. While basal area can be deter-mined and calculated for an individual tree, it ismore commonly expressed on a per acre basis for alltrees 4 inches dbh and larger. For second growthhardwood timber stands on good sites in Michigan,a total basal area of 100 square feet per acre andabove is fairly common.

The concept of basal area as an expression of standdensity and its use in regulating tree stocking isfounded on the assumption that individual tree spac-ing and distribution are not as important as total treestocking per acre. Even though this method is basedon total tree stocking per acre, individual trees arestill important and should be distributed optimally.Accordingly, each acre of forest is managed to obtainoptimal growing conditions (spatial distribution) whileproducing maximum volumes per acre. Optimal basalarea stocking levels will vary depending on the typeof forest present and the favorableness of the growingsite. Also, total basal area of a particular forest standchanges during the life of the forest as trees increasein size. Therefore, repeated thinning and/or harvestoperations are necessary at periodic intervals to main-tain optimal stocking levels.

Determination of Basal Area. Basal area estimatesare commonly determined using a point sampling pro-cedure and an angle gauge or wedge prism. Point

Fig. 4. To estimate basal area using anangle gauge, the observer turns a com-plete circle over the sampling point andcounts the total number of trees largerthan the width of the block.

sampling is a quick method of estimating tree basalarea per acre without actually laying out sample(area) plots, measuring tree diameters and convertingto basal area. Rather, only representative sampling"points" or plot centers are selected in a woodlot.Estimates of basal area, using an angle gauge or wedgeprism are taken directly above the established points.To obtain an accurate estimate of basal area per acre,however, it is important that several uniformly scat-tered "points" or plot centers be established and sam-ple estimates taken with the results averaged.

An angle gauge is a simple wooden rod with a peepsight at one end and a small block of a predeterminedwidth at the other end (Fig. 3). The length of therod and width of the block will vary depending onthe multiplying factor used. For most hardwood tim-ber stands, a ten-factor angle gauge is adequate formeasuring basal area and can be simply constructedusing a 33-inch wood rod with a 1-inch wide blockat one end and a %-inch screw eye at the other end.

f

The angle gauge is used by standing at a representa-tive sampling point within the woodlot and viewingthe dbh of each tree in the vicinity of the point throughthe peep sight. With the sight held close to the eye,each tree which appears larger than the width of theblock at the end of the rod is counted (Fig. 4). Theobserver turns a complete circle (directly over thesampling point) looking at all trees 4 inches dbh andlarger. The total number of trees tallied is multipliedby 10 to arrive at an estimate of total basal area peracre. If a wedge prism (10-factor) is used, the sameprocedure is followed; all trees which appear partiallyoffset when viewed through the prism (Fig. 5) aretallied, and the total number is multiplied by 10.

Use of Basal Area in Stand Management. Once totalbasal area per acre has been determined, it shouldbe compared with guidelines obtained from researchstudies. These have been prepared for similar foresttypes making allowances for site variances. This com-

n Fig. 5. A 10-factor wedge prismcan be used to determine basal

Table 2. Recommended stocking levels for several Mich-igan hardwood forest types.

Forest Type Recommended Stocking Level(Basal Area/Acre - sq. ft.)

AspenLowland hardwoodsOak-HickoryNorthern hardwoods

10090-10070-80

90

Table 3. A comparison of number of trees per acre, dis-tances among trees and basal area per acre fortrees of different diameters.

Dbh(in.)

456789

101112131415

No. ofCrop Treesper Acre

970617435318243194156129109938070

Distance BetweenAny Two Trees

(ft.)

6.78.4

10.011.713.415.016.718.420.021.723.425.0

Basal Areaper Acre(sq.ft.)

84.684.185.485.084.885.785.185.185.685.785.585.9

parison will indicate if the stand is over- or under-stocked. Suggested stocking levels for different foresttypes present in Michigan are presented in Table 2.If overstocked, the differential between the suggestedstocking guideline and the amount actually present canbe used to determine the amount of TSI and/or thin-ning necessary to obtain ideal stocking. The sameguidelines as suggested for crop tree spacing, e.g.,removal of defective trees and low value species toprovide additional growing space, will apply in usingthis method. Although the approach is different, ap-plying TSI and thinning in a stand using either thecrop tree selection method or basal area method willproduce similar results. A comparison of basal areavalues, number of trees per acre and distance amongtrees is illustrated in Table 3.

Basal area can also be used to estimate the boardfoot volume of wood and/or number of standardcords of pulpwood present per acre. This may be de-sirable if a rapid estimate of the volume of timber ina woodlot is needed. For example, estimates of boardfoot volumes in trees 10 to 12 inches in diameter andup can be determined and averaged for each plot. Bymultiplying the average number of trees per plot by600, an estimate of total board feet per acre can beobtained. Moreover, to obtain an estimate of the num-ber of standard cords present, multiply the total num-

ber of 8-foot logs or 100-inch "sticks" present in alltrees 4 inches dbh and larger on the average plot by0.6. If a combined sawlog-pulpwood volume is desired,trees in the 4- to 11-inch dbh classes should be meas-ured for pulpwood and trees 12 inches dbh and larger

jhould be counted for sawtimber.

Carrying Out TSI and Thinning OperationsImplementing a TSI program requires putting the

recommended suggestions into practice. Admittedly,these suggestions represent ideal conditions that maybe difficult to achieve due to variations in forest type,stocking, site quality and other factors. In general,some hardwood timber stands, especially those wherea large initial number of desirable trees are present,will respond better than will lower quality stands.However, all woodlots will benefit from a TSI pro-gram. With subsequent thinnings, woodlots contain-ing a large number of trees of poor form and qualitywill be gradually improved over time, if a TSI pro-gram is conscientiously applied throughout each thin-ning operation.

Regardless of the approach chosen, the first step isto mark the selected crop trees. One convenient meth-od of temporarily marking crop trees is to tap selectedtrees with an old cotton sock filled with powderedlime. Vinyl flagging tape, tree marking paint, or sim-ilar commercial products can also be used. However,the use of powdered lime or flagging tape is oftenpreferred since either can be easily removed from atree if adjustments in spacing or crop tree selectionare necessary in planning the cutting operation. Oncethe crop trees have been identified and the properspacing determined, selected trees should be per-manently marked. Moreover, it is recommended thatmost individual owners undertake TSI in only a smallportion of a woodlot at a time (3 to 5 acres in size)to prevent the operation from becoming an unman-ageable task.

Removing Unwanted TreesOnce the desirable trees have been identified, the

remaining unwanted trees should be eliminated usingthe most economical, effective and safest method.Often, much of the material removed in TSI opera-tions can be sold either as pulpwood, fuelwood, orlow grade block or pallet material. If a market isnot available or the cut material is not saleable, it maybe desirable and more economical to kill the treeswithout removing them from the forest stand. Severalmethods are available including girdling, cutting,and herbicides.

A girdle must sever the cambium layer (tissue

Fig. 6. A chain saw girdle is a simple way to remove un-wanted trees from a woodlot.

located just beneath the bark) completely around thetree to insure a complete kill. A girdled tree dies slow-ly, and later, comes down in pieces, thereby doing lessdamage to the residual stand. However, girdled, deadstanding trees may pose a hazard to people usingthe woodlot for hunting, hiking or other recreationalpurposes. Stump-sprouting from girdled trees is nota serious problem for northern hardwood species, es-pecially trees over 10 inches in diameter. Girdlingmay be accomplished by using an ax or chain saw.

Ax girdling is done by cutting a double-hack notchcompletely around the circumference of the tree stem.If a chain saw is used to girdle unwanted trees, caremust be taken to make a complete girdle; otherwisethe tree will continue to live for many years. Thebest method of girdling with a chain saw is to cuttwo saw rings around the tree, approximately 2 to 3inches apart on the trunk (Fig. 6).

In addition to being more hazardous than girdling,felling trees with an ax or chain saw and leaving themto decay may be undesirable because of heavy slashaccumulations. This is especially true in dense sap-ling or pole size stands. However, material of thissize is excellent for firewood. Felling is advantageousfor trees under three to four inches dbh, since theycan usually be cut faster and cheaper than they canbe girdled.

The use of herbicides to kill unwanted trees prob-ably represents the fastest and easiest way to improvehardwood timber stands. Chemical thinning is ac-

Fig. 7. Herbicides can be injected directly into the tree throughuse of appropriate specialized equipment.

complished using commercial herbicide formulationsor "brush killers," depending on the species to be con-trolled. Contact a local county extension agent, chem-ical company salesperson, or Michigan State Univer-sity forestry specialist for proper herbicide recom-mendations.

Foliar sprays of certain herbicides are effective onbrush and younger trees. On larger trees up to fourinches in diameter, basal sprays may be used. Theentire circumference of the lower 12 to 18 inches ofthe trunk and root collar is sprayed to the point ofrun-off. Basal sprays can also be used effectively totreat fresh cut stumps and prevent sprouting if thestump is sprayed immediately after cutting.

On trees larger than four inches in diameter, herbi-cides may be applied in frills cut around the circum-ference of the tree. Hand-operated equipment is avail-able to create the frill and apply the herbicide in onesimultaneous operation (Fig. 7). Control may also beobtained by applying herbicide solution in ax or chainsaw frills.

Be sure to read the label and follow the directionscarefully whenever working with herbicides. Herbi-cides should not be used where wind drift or vola-tilized chemical will come in contact with trees thatare to be kept in the residual stand. Furthermore,spray equipment used for herbicides should not beused to spray insecticides, as it may be difficult tocompletely remove all traces of the herbicide fromthe sprayer.

Special ConsiderationsGenerally, trees originating from stump sprouts are

not as desirable as those originating as seedlings in aforest stand. Stump sprouts have a greater likelihoodof decay and poor form development. However, cer-tain high value species, such as basswood, maple andoak, characteristically reproduce by sprouts. In manystands it will be necessary to thin some of these clumpsto maintain proper spacing throughout the stand. Inthinning sprout clumps, only 2, or at most 3, stemsshould be kept. Preference is given to stems of thebest form which are widely spaced and have originatedfrom the root collar or positions low on the stump.Sprout clumps of other species should be removedentirely from most stands unless stems of larger di-ameters are present. Twins (low forks) should betreated as clumps, although they may be kept in thestand if needed for stocking purposes during the earlieryears of management. A general rule of thumb is toleave both or remove both.

To produce high quality sawlog and veneer logmaterial, all limbs should be pruned from the lowerbole of selected crop trees. Research studies indicatethat pruning the bole of the tree for a distance equalto 50 percent of its total height will improve the qual-ity of wood produced without reducing either heightor diameter growth. In hardwoods, pruning to 17 feetto produce a clear 16-foot log is usually recommended.

All pruning should be done with a pruning saw cut-ting flush with the stem of the tree (Fig. 8). Live-limbscars, less than one inch in diameter will normallyclose completely within two to three years. Little ifany new branch sprouting will result on pruned treeswhen they are thinned following the crop tree or basalarea method.

In some locations, hardwood timber stands are un-derstocked and not in need of a thinning operation.TSI, in these instances should be limited to removingdefective cull tree material in the overstory, and seed-lings of non-commercial species in the understory.This will allow desirable small trees to grow into theoverstory, thereby increasing stocking of the stand.

The suggested TSI practices are not difficult toimplement. Once the woodlot owner can properlyidentify tree species and select desirable trees withproper form, he can usually complete a TSI or thin-ning program on his own. Implementing such a pro-gram is an excellent means of obtaining fuelwoodwhile improving the condition of the woodlot. Where

Fig. 8. Careful pruning will encourage rapid healing and in-crease bole quality.

larger acreages are involved, or time or other con-straining factors are present, the use of consultingforesters or the services of landowner assistance pro-grams of some forest industries may be appropriate.Management information, and sources of harvestingand marketing assistance, can be obtained from localDepartment of Natural Resources field offices, SoilConservation Service offices or individual CountyCooperative Extension Offices.

If the TSI-thinning recommendations in this publi-cation are applied, the production of any treatedwoodlot will be greatly increased. The specific resultsobtained will depend on the original condition ofthe woodlot and on site factors such as moisture andfertility relationships. Obviously not all woodlots havethe same potential for production; however, applica-tion of appropriate TSI and thinning programs willbring each area to more nearly its potential produc-tion.

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