Response Of Forest-Grown Trees To Topping

Literature Review

The term “topping” is used to describe the drastic cutting back of tops and limbs (Peace 1962, Pirone 1972). Pirone points out that topping may disfigure a tree by removing the normal canopy and may eventually lead to serious branch decay and sunscalding; however, this practice may be necessary as an invigorating measure, or to eliminate interference with overhead wires. Heavy topping should be done with extreme care and with certain precautions, if the tree is not to be severely damaged.

Pirone (1972) outlines these rules to follow during topping. Branches should be removed with a slanting cut, starting just above a vigorous bud or shoot, and running back across the limb at a 45° angle. A right-angle cut to the long axis of the branch is not likely to heal; those at too sharp an angle will weaken the elbow. All wounds, regardless of size, should be painted with a dressing (McQuilkin 1950). In actual practice, only wounds 5.08 cm (2 in) or more in diameter are so treated, but for low-vigor trees, where healing is slow, decay may develop rapidly even in small branch wounds.

Wounds made in early spring heal most rapidly. Some species, however, such as maples and birches, bleed so profusely when cut at this season that pruning of these trees should be delayed until summer when sap runs less freely. As a rule, American basswood, elms and black locust can be cut-back severely at any time of the year without seriously reducing vigor (Kramer and Kozlowski 1960).

When drastic topping is necessary, careful consideration must be given to species. Although most species when young can tolerate heavy topping, many when older (birch, sycamore, ash and beech) make weak recovery. Mature elms, on the other hand, can withstand heavy topping. Drastic topping of conifers is risky; they should be topped so that at least two whorls of actively-growing branches are left. Those broadleaved trees which produce strong coppice shoots from the stump when felled, such as quaking aspen and yellowpoplar, usually recover from heavy topping (Peace 1962).

Not all portions of a tree crown are equally important in terms of photosynthesis. Lower crown branches, for instance, usually have relatively little leaf tissue and a large respiring surface. The natural death of lower branches is preceeded by an extended period of retarded branch growth (Kramer and Kozlowski 1960). Pruning live branches decreases the amount of photosynthetic surface, and also the respiratory surface. Because many shaded basal branches have a small photosynthetic surface, they may consume most of the carbohydrates in respiration which they produce in photosynthesis. Consequently they do not contribute to stem growth (Labyak and Schumacher 1954).

Location and Description of Study Areas

Calvert Cliff Line, Maryland. This transmission corridor contains two 500 kV lines, and was constructed by Baltimore Gas and Electric Company between 1970 and 1972. The corridor extends from the Calvert Cliff thermonuclear plant in southern Maryland north to Baltimore, a distance of 128 km (80 mi). The section of corridor used in this study is located in Anne Arundel County.

The major hardwood forest type on mesic and xeric sites within the study area is white oaknorthern red oak-hickory. Associates of these stands include red maple, American beech, chestnut oak and black gum. On upland sites wherever abandoned fields have recently been invaded by trees, young stands of Virginia pine often form the forest cover. On hydric sites the river birch-sycamore type is the predominant cover. Black gum, sweetgum and red maple are associated species.

In the spring of 1972, the power company selectively cleared the study area with topping of many “leave” trees. Where possible, trees were topped rather than felled. Topping ranged from light, removing 10 percent or less of the total crown length, to severe, where up to 70 percent of the crown length was removed.

Plymouth Meeting House-Conowingo Dam Line, Pennsylvania. This transmission corridor contains two 230 kV lines. The section of corridor used for this study is located in Montgomery County, Pennsylvania, and is owned and operated by the Philadelphia Electric Company.

The major forest type on hydric sites is the black ash-red maple-American elm type. Red maple is the most abundant component of this type. On mesic sites the yellow-poplar-white oak-northern red oak type is the dominant cover. Black oak is a common associate of this type. On xeric sites the white oak-northern red oak-hickory type is the predominant cover, and chestnut oak is an abundant associate.

The power company initially cleared this corridor in 1927; however, they did not top the trees in the immediately adjacent stand until 1956. Topping was done to taper the profile of the adjacent stand towards the corridor edge, and thus eliminate the pronounced vertical line of the corridor margin. The company also did some topping in 1968. A later topping, which included all trees within 30 m (100 ft) of the corridor margin, was completed in January 1973.

Field Procedure

On the two study corridors the most abundant topped tree species were determined by sampling. From this tally the three most abundant species on the Pennsylvania study corridor were chosen for study. Because of the greater species diversity on the Maryland corridor, six species were selected for study.

Study plot locations on each corridor were established by selecting six potential study locations based on species diversity and accessibility. Using random numbers, three of these corridor sites were selected as study plots.

For each topped tree the following information was recorded: species, diameter breast high, height to base of live crown, height growth or dieback since topping, diameter growth before and after topping (from increment cores), and vigor of present crown (a subjective evaluation based on color and density of foliage). From the study plots on each corridor approximately 30 topped trees of each species were studied with the exception of sycamore, which proved scarcer on corridor study plots than the original tally had indicated.

In addition, for each species an equal number of untopped control trees was studied. Untopped trees were located in the woods immediately adjacent to the corridor. Control tree measurements included: species, total height, height to lowest live branch, relative crown position (dominant, codominant, intermediate or overtopped), diameter growth since corridor trees were topped (from increment cores), and vigor.

Results

Table 1 presents a list of species used in this study, the number of study and control trees, location, and average diameter breast high. Table 2 gives annual height growth for each species since the topping operation. Annual height growth is presented by crown classes to illustrate the effect of relative crown position on subsequent height growth. Table 3 presents the basal area growth by species for the five growing seasons before and after topping for the Maryland study area. For comparisons, this table also contains average basal area growth for control trees during the same time periods. Table 4 presents the basal area growth for the four growing seasons before and after topping for the Pennsylvania study area, and basal area growth of control trees during the same intervals.

To determine the influence of various degrees of topping on basal area growth, regression analyses were made for each species using the model:

where:

  G = basal area growth since topping,

  D = diameter breast high,

  H = percent of original crown length remaining after topping.

Table 5 presents the results of these multiple regression analyses.

Discussion

It was not possible to obtain precise data on mortality of topped trees because some of the smaller trees on the Maryland study area had been felled subsequent to the time of selective clearing and topping. The small diameters of these stumps suggested that these trees had died from exposure rather than from topping. Trees of the size indicated by these stumps would have been from the intermediate and overtopped crown classes; trees of these crown classes were rarely tall enough to require topping. Of the topped tree’s present at the time of this study, only one was dead. This was a co-dominant Virginia pine that had been topped so severely that only two whorls of branches remained.

Annual height growth since topping varied with species (Table 2). Dominant and co-dominant trees generally recovered from topping more rapidly than intermediate or overtopped trees.

Virginia pine, a relatively short-lived tree, exhibited the least annual height growth of the nine study species. Northern red oak, chestnut oak and yellow-poplar, species on the Pennsylvania study area, had the largest annual height growth after topping. This was attributed to the better site conditions along this corridor. Sweetgum and sycamore, fast-growing species on the hydric sites on the Maryland corridor, recovered very rapidly from topping.

For the Maryland study area the basal area growth for all species combined indicates that topped trees grew only slightly less than they had in the period prior to topping (Table 3). The greater basal area growth of sycamore, when compared with other Maryland species, is attributed to the fact that all of the sycamore study trees had much larger diameters than those for other study species (Table 1). It has been shown that trees of larger diameters produce a larger basal area increment than do trees of smaller diameters (Loetsch et al. 1973).

The influence of topping on basal area growth is less apparent in the Pennsylvania study tree data (Table 4). Northern red oaks increased less in basal area after topping than they had in the preceding four growing seasons. Their growth after topping was less than that of the controls. Basal area growth of topped chestnut oaks actually increased in the years after topping and was superior to the basal area growth of controls during this period.

Multiple regression analyses to identify the relationships between diameter, percentage of crown remaining after topping, and basal area growth since topping showed a strong relationship between diameter and basal area growth (Table 5). Surprisingly, for most species there was no significant correlation between percentage of crown remaining after topping and subsequent basal area growth. This relationship, however, was significant for chestnut oak and yellow-poplar. The fact that few of the study trees had received such severe topping that one-third or less of their original crown length was left intact, may explain why the relationship between percentage of crown remaining after topping and subsequent basal area growth was not significant for all species.

Conclusions

Mortality of topped trees, four and five years after partial crown removal, was low. Few of the topped trees on the study areas, however, had more than 60 percent of their crown length removed in the topping operations. Only one of the 231 topped trees died from excessive crown removal. Several overtopped and intermediate crown class trees died from exposure after the protection of overhead trees had been removed.

Height growth of topped trees was rapid. Dominant trees grew at an annual rate of 0.77 m (2.54 ft) and co-dominants at an annual rate of 0.68 (2.22 ft). Height growth of intermediate and overtopped trees was much less. Chestnut oak, northern red oak, sweetgum, sycamore and yellow-poplar grew most rapidly in height. American beech and white oak were intermediate in rate of height growth. Hickory and Virginia pine grew comparatively slowly.

Unless topping removes the major part of the crown, basal area growth is not seriously affected by topping. Basal area growth of topped trees during the five years after topping was only slightly less than that during the five years prior to topping.

For many of the species included in this study, there was no significant relationship between the intensity of topping and subsequent basal area growth. This relationship, however, was significant for chestnut oak and yellow-poplar. Evidently topping of the intensity used on the two study areas (averaging no more than 60 percent crown removal per species) is not severe enough to cause a major depression in basal area and diameter growth.