smith 1986 respuesta de arboles a los tratamientos de raleo y podas

8/8/2019 Smith 1986 Respuesta de Arboles a Los Tratamientos de Raleo y Podas

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48 Part 2 / Tending and Intermediate Cutting

by cutting a competing tree, any prompt acceleration of growth is largely from an increasein water and nutrients supplied by the roots. The amount of foliage does not increase until

there has been time for the crown to enlarge, although this delayed effect is ultimately themost important. Not all units of leaf surface are equally efficient in photosynthesis. Leavethat are severely exposed to sun and wind produce somewhat less than slightly protectedones; heavily shaded leaves do little more than supply themselves. The part of theabove the point of horizontal crown closure, especially the upper middle portion, producesmuch more than anything below and more than the foliage at the very top of the t

The roots extend horizontally more widely and swiftly than the crown. Since they donot have to support themselves like aerial branches, their extension is not limited bystructural necessities. They may extend throughout the soil even if this means interminglingwith or going around the root systems of other trees. The root systems of healthy trees much wider than the crowns. Many species, but not all, form intraspecific root grafts, soit is possible for a tree to incorporate part of the root system of an adjacent cut tree. Infact, in some single-species stands, the trees that are individuals above ground share somesemblance of a common root system. There is some evidence from the tropics that closelyrelated species can develop functional interspecific root grafts as well.


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Chapter 3 / The Response of Individual Trees to Thinning and Pruning

rich get richer." Trees that have sufficient carbohydrate after meeting basic respiratoryneeds are able to put more energy into larger crowns and root systems, stronger ste

and the production of secondary compounds that protect the foliage from herbivory andphloem from beetle attack. These factors all result in even greater photosynthesis, and theeffect snowballs.

The extent to which a tree increases mechanical support depends on the vigor of thetree and the amount of carbohydrate that remains after provision for more vital functions.The part of the growth that is of greatest economic concern for timber production is thus

low enoughon the scale of biological priority that it is subject to great variation; anythingthat reduces the amount of photosynthesis will result in a reduction in stem growthfor thisreason that growth in diameter is so readily controllable by thinning or other meansofregulating stand density and tree vigor.

Stem Form

The tree stem as a whole tends to develop as a mechanical structure designed to beavertically acting load of its own weight and the horizontally acting load of wind It se


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50 Part 2 / TendIng and IntermedIate Cutting

It. 6 B . A" T = = - -

I. 11" X D.B.R ~ I Figure 3.1 The relationship between annual increment in basal areas ( ~ B . A . ) and

thickness ( ~ r ) of growth rings. The annular rings of two successive yearstraightened out into rectangles to show the large increase in basal area increment that is necessary to maintain constant ring width.

of response seem to be involved to some degree and are, in either case, governed bcomplex hormonal systems (Larson, 1963).

Even if the volume of wood laid down is constant along the stem, the thickness oh l i d b l h li h di f h b l i


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Both factors together result in the widest rings being near the base of the live crownin all trees. In vigorous trees, a second widening occurs near the base, but in trees of pvigor the thinnest rings will be near the ground. Figure 3.2 shows the difference in ringwidths between very vigorous and less vigorous trees. When this pattern continues overtime, dominant trees develop stems that are quite tapered (and very strong); trees that areovertoppedhave very cylindrical (and weaker) stems.

The general effect of thinning is that the crown expands. The lower branches livelonger and become thicker in diameter, which degrades the value of lumber produced in

the stem, but the increased foliage also produces much more carbohydrate. This in tumprovides the structural material that is laid down as increased diameter growth. However,the diameter growth is increased much more in the lower than in the upper parts of thestem, so that it becomes more tapering.

The wide variations that can be induced in diameter and basal area growth in the zoneofbutt-swell can give exaggerated impressions of increases or decreases in growth of stem

volume. Except in small trees, this zone definitely includes the breast-height level. Incrementcores taken at breast height give good qualitative indications of growth trends butdonot give good quantitative estimates of changes in volume without knowing somethingabout the changes in stem taper. It is unfortunate that trees are tall and people are shortb i i li bl di hi h i


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height growth of the leading trees of a stand is, in fact, so independent of stand densityand so closely controlled by the totality of growth-supporting factors of the site that it used as an integrated expression of site quality. Thus site index, the total height oleading trees at 50 years or some other standardized age, is, in this context, a manifestationof the fact that thinning affects height growth very little.

The height growth of dominants is not absolutely and always unaffected by standdensity or thinning. Trees that have become exposed, isolated, or open-grown may becomesomewhat stunted. The reasons are not clear but may involve growth of branche

lower bole at the expense of height. Very high stand density often causes reduced growthof trees in all dimensions, including height, probably because of the high respiration associated with a poor ratio of photosynthetic to aphotosynthetic tissue in each tree. Thinningthese crowded stands might result in increased height growth if the trees are able to respondto the increase in growing space.

Values of average height calculated for all trees in stands are inversely related

stand density because the height growth of trees of the lower crown classes is stunted bycompetition. The greater the number of trees being submerged through competition withthe crown canopy, the lower is the average height.

The common opinion that trees grow taller in dense stands is, in general, incorrect.


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Chapter 3 / The Response of IndIVidual Trees to ThInnIng and PrunIng

The loss of trees with small crowns after thinning may be caused by insect attack,sunscald, or even the cutting of other trees that had formerly nourished the unthriftythrough root grafts (Eis, 1972). The most important cause may be merely the increase inrespiration induced by the sudden increase in temperature caused by exposure. (Respirationabout doubles for every 10 D C increase in temperature.) I f the respiratory demand is greenough, very little carbohydrate will be left for renewal of vital tissues. However, unthriftytrees are usually eliminated by bark beetles or other biotic agencies before they actuallystarve to death. Such difficulties are avoided by selecting crop trees from the dominant

and codominant classes with acceptable live crown ratios.

BIologicalBasis fo r Quantifying Tree DeYelopmentIt is easy to demonstrate that the accelerated crown expansion induced by thinning increases diameter growth, but difficult to develop precise quantitative relationships necessary to predict how various thinning programs would affect the growth of individual ctrees. When such relationships have been established, it becomes possible to simulatealternative thinning regimes by computer. Calibrating a model requires data derived fromthinning trials or other long-term observations necessitating decades of measurements.Most efforts to explain tree growth in quantitative terms seem to work best if tree diameter


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,dl~ GlI ~

I'c:

II

I tii J ....\ 11~ \ I ~\ I ~

\ /q,.'"

Figure 3.3 A two-dimensional representation of the relationship between the cumulati

volumes of stemwood and space occupied by the foliage of a tree duringof its previous development. The present size of the crown is enclosed wisolid lines and the space that has been successively filled with previous foliage is depicted by the dashed lines projected from the widest part


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Chapter 3 / The Response of Individual Trees to Thinningand Pruning

When a conifer is young and short, the loads placed on it are small and it usuallyproduces the weak juvenile or core wood. However, during its period of active increas

in size and height, the specific gravity and strength of wood laid down tend to increase.Whena tree of almost any species reaches maturity and ceases to increase much in heightor size of crown, the annual rings added to the bole become very thin and of low strerepresenting little more than enough xylem to renew the water-conducting system. As atree slows down in height growth and foliage expansion, because of the high endemands of maintaining a large size, the lack of increase in size means that the mechanic

stresseson the stem no longer increase; therefore, additional strength is not necessary forsurvival. Juvenile wood is normally produced only in those parts of tree stems that arestillclothed with living branches.

Growth Rate-Not a Causatlve FactorOneerroneous view that long persisted was that the strength of wood is directly controlby the rate of growth. According to this lore, rapid growth was reputed to produce strongwood in ring-porous hardwoods, weak wood in conifers, and no appreciable difference indiffuse-porous hardwoods The main practical consequence of these ideas has been the


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at that time. Thinning in conifer stands in droughty areas can prolong the time of diamgrowth later into the summer because water does not become limiting as quickly (Zahnerand Whitmore, 1960). The amount of early-wood can be very similar between thinned andunthinned stands, and most of the additional ring width is late-wood. Because late-woodis denser and stronger, wide rings formed well below the base of the live crown will formwood that is heavier and stronger than wood with narrow rings from the same stemposition.

ControHlng Wood Properties by ThinningThe structure and anatomy of xylem vary tremendously between and within species aswell as within tree stems; there are many important variations more subtle than thoseconsidered here (Haygreen and Bowyer, 1982). The choices that can be effected betweengood trees and poor ones in thinning improve the ultimate utility of the wood much more

than they might be impaired by any vaguely suspected baneful effects of making the treesgrow faster. In general, the larger the trees grow and the straighter their stems, the greateris the usefulness of the wood, at least until the heart-rots of old age start to causedeterioration.


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Chapter 3 I The Response of Individual Trees to Thinning and Pruning

amount of clear material is to be grown without artificial pruning, thinning should bedelayed until natural pruning has proceeded to the extent ultimately desired. Regardlessofhow the pruning is accomplished, it is prudent to set some realistic goal as to the length

ofbranch-free bole that will be developed; the remaining upper portion of the stem shouldordinarily be kept clothed with living branches. The contemplated length of clear boleshould be greater the better the site and the longer the rotation because it depends on theultimate height and live crown ratio. I f one plans to grow trees 100 feet (=33 m) tall a live crown ratio of 40 percent, it is important to plan for the proper development of the6O-foot (=13 m) length below the live crown.

Thecriteria of quality for pulp depend on the pulping process and the characteristicsdesired in the product. I f the paper needs to be strong, the procedures necessary to producestrong, dense wood are appropriate. Fibers that are long, strong, and narrow interlace toproduce strong paper. However, if the paper must be smooth, it is necessary to have someshort fibers to nestle in the gaps between the stronger ones. These differing requirements

are better met by mixing fibers of different species than by trying to make spruce fiberslike those of maple and vice versa. In general, the farther the wood is from the pith, thegreater arethe length, density, strength, cellulose content, and anatomical structuralqualityofthe fibers. I f the wood is of high density, the volume that must be processed for a given


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extent that they act as infection courts for rotting fungi, their continued presence does notthreaten tree survival. However, dead knots are more serious timber defects than ones fromliving branches and dead branches are unsightly on ornamentals.

Natural Pruning

Most of the branch pruning that takes place in forests is caused by physical and bioticagencies of the environment and is called natural or self-pruning. In most situationsbranches die from lack of light resulting from shading by higher branches. Wind cacause the breakage and death of branches. As trees become taller, the crown edges hadjoining crowns with increasing force during wind sway. This results in crown sha phenomenon which leaves spaces between the crowns of adjacent trees. In alpinethe buds are destroyed by wind and ice crystals leading to the stunted krumholz tree form.

A similar process occurs near oceans where the buds are killed by salt spray. Crowns canalso die back from water stress during drought periods or from the induced water stressafter sudden increases in light levels.

In closed stands, natural pruning proceeds from the ground upward and starts with


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Ramicorns can also form if the terminal bud dies or merely if some hormonal abnormacauses it to fail to assert apical dominance over the branch buds.

One common occurrence in some conifers is lammas growth, the sudden expanand growthfrom a bud late in the growing season. I f the terminal bu d of the leader expathesingle stem is maintained. However, if the leader remains dormant and a branchbelow has this additional growth, the branch may contest the leader for apical control thenextgrowing season. I f the battle is even the stem will fork, but more often the battlelastfor several growing seasons and the losing branch will form a ramicorn.

In most natural stands, the branches of surviving trees tend to be larger in the miand upper parts of the bole than in the lower. This is because inter-crown competitionusually severe before differentiation into crown classes takes place. Trees on poorusually have small branches but the vertical distance between them is short.

The final step in natural pruning is the occlusion or covering with new tissuesshort stub left by the dead branch. Whether the pruning is natural or artificial, this procecan be likened to the submergence of a post set in a rising stream of water. The rsubmergence depends on the rate at which the stream rises; that is, the faster the rgrowth, the sooner the wound heals, but the diameter of the branch makes remarklittle difference As with the post the upstream edge of the top that is the upper s


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Discontinuity

Figure 3.4 Radial section through a pine branch that persisted for many years after itsdeath. The branch is thickest at the point where it joined the stem at time ofdeath. Note that the annual rings laid down around the branch after death ofthe tree tum inward without joining the wood of the branch. Even whe


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62 Part 2 I Tending and Intermediate Cutting

possible. The most common tool is a saw or clipper on a long pole, which is effective toabout 18 feet (=6 meters) above ground but wobbles too much to enable one to prunhigher. A variation is a power saw on the end of a long pole. The engine is at the end ofthe pole held by the operator. Many operators avoid such saws because they tend to beheavy and unwieldy, but improvements in design are making them more useful. It is alsopossible to use hand saws from ladders, but the moving of ladders is cumbersome enoughthat it is hard to prune higher with them also.

Several pruning machines have been developed. These machines power themselves

up the tree, cutting branches as they go. They are usually controlled by either a wire or aradio control. They work best on cylindrical trees with only minor bumps or other irregularities. When they work well, trees can be pruned very high up the stem. So far mostNorth American tree species have proven to have enough stem irregularities to render thesemachines of marginal utility. Expert tree climbers, with safety belts, can get to almost anyheight using climbing irons or ropes. As with the pruning machine, this method can be

carried to almost any height, but is almost always is too costly for use except in parks andintensively managed settings.

Ch t 3 / Th R f I diVid l T t Thi i d P i


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Chapter 3 / The Response o f IndiVidual Trees to Thinning and Pruning

postpone the first pruning at least until the terminal shoots have grown to the height ofthe first log, thus avoiding the risk that forks or crooks may develop before that height isattained. In typical cases, pruning might be limited to 65-120 trees per acre (150-29hectare) in the diameter class of 5-10 inches (::::::13-25 em). I f the object is to proclear lumber, the clear shell must grow to at least 4 inches (::::::10 em) in radial thicktobeofany advantage. Pruning is cheap enough up to about 10 feet (=3 m) that somethe pruning of more trees may be justified, but the number usually must be reduced whenthe time comes to carry the pruning higher.

Ordinarily, pruning is done after the branches are dead and in two or sometimes stages of height. Different tools, usually saws with handles of differing lengths, are usedforeach stage. The object is often to keep the knotty core of uniform thickness. Commonly,the problem is not that the lower part of the knotty core becomes too thick but that thelater stages of pruning are delayed so long that the upper part gets thicker than the lowerpart.

A very high proportion of the merchantable volume and value of a tree is oftenconcentrated in this zone anyhow. In many instances, however, it would be fine if thepruning could be carried higher; sometimes it is. The branches that develop somewhathigher on the stem are usually the largest; they form the poorest kinds of knots an


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Chapter 3 I The Response of Individual Trees to Thinning and Pruning


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ameter of the knotty core is too small to justify the work, and the practice may do moreharm than good. In some diffuse-porous hardwoods, for example, cutting into the branchcollar increases the amount of bacterially induced pathological heartwood (Shigo et1979).

Bud pruning is a method of very early green pruning in which the lateral budsrubbed or clipped off. The purpose is to produce logs with no knots at all. This canefficient system for producing small-diameter, short, plywood veneer bolts of high-hardwoods. It is important not to get carried away with the ease of this method be

growth reductions and the serious results of injuries to the terminal shoots can make treatment disappointing. The same effect, called "foxtailing,' can develop naturasome tropical pines planted where the dormant season is not long enough. Thewhorls that develop at the top or bottom of the long, straight, branch-free segment ,produce large knots or other deformities.

Pruning of Ornamental Trees

Artificial pr ning on open gro n shade trees is often desirable D d or broken bra

Chapter 3 I The R f Indi id al T to Thinningand Pr ning


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Chapter 3 I The Response of Individual Trees to Thinningand Pruning

Shearing and Pruning of Christmas Trees

Conifers being grown for use as Christmas trees often require special pruning techniques

tocorrect the effect of excessively rapid or asymmetrical growth and produce dense crownsof the proper shape. The most common practice, referred to as shearing, is the clofbothterminal and lateral shoots. The objective is to develop crowns that are narrow andglobose at the bottom and conical in the middle and upper portions. Shearing is usuallyconfined to the removal of part of the shoots of the current or most recent growing seasBuds formed just below the point of cutting must be depended upon for renewed elongatioof the decapitated shoots. Species that have dormant buds throughout the length of eachinternode, such as firs and spruce, can be sheared at almost any season except early summer. Pines rarely have any dormant buds along the internodes and are best sheared whenthe shoots are actively elongating. This stimulates the prompt development of vigorousadventitious buds just below the cut ends (Bramble and Byrnes, 1953; Brown, 1960).

Excessivelytall and rapidly grown trees can sometimes be converted into good Christmas trees in one operation simply by pruning off the lower three-quarters or more ocrown. The remaining tuft of crown at the top then grows much more slowly and masoon develop into a properly compact form without any shearing. This technique has been


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Page, A. C., and D. M. Smith. 1994. Returns from unrestricted growth of pruned eastern white pYale Univ. Sch. Forestry and Env. Studies Bul. 97. 24 pp.

Seymour, R. S., and D. M. Smith. 1987. A new stocking guide formulation applied to eastern whitepine. For. Sci. 33:469-484.

Shigo, A. L. 1989. A new tree biology: facts, photos" and philosophies on trees an d their proan d proper care. 2nd ed. Shigo and Trees Assoc., Durham, N.H. 618 pp.

Shigo, A. L., et aI. 1979. Internal defects associated with pruned and non-pruned branch stubs inblack walnut. USFS Res. Paper NE-440. 27 pp.

Stokes, B. J. 1992. An annotated bibliography of thinning literature. USFS Gen. Tech. Rept. 80178 pp.

Tomlinson, P. B., and M. H. Zimmermann (eds.). 1978. Tropical trees as living systems. CamUniv. Press, London. 675 pp.

Wilson, B. F. 1984. The growing tree. Rev. ed. Univ. Mass. Press, Amherst. 152 pp.Zahner, R. and F. W. Whitmore. 1960. Early growth of radically thinned loblolly pine.

5 8 : 6 2 8 ~ 3 4 .

Zimmermann, M. H., and C. L. Brown. 1971. Trees, structure an d function. Springer, New 505 pp.

smith 1986 respuesta de arboles a los tratamientos de raleo y podas

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