Wound Closure Rates on Trees

Materials and Methods

The ten species used in this study were English elm (Ulmus procera), green ash (Fraxinus pennsylvanica var. lanceolata,) hackberry (Celtis occidentalls), honeylocust (Gleditsia triacanthos var. inermls), littleleaf linden (Tilia cordata), Norway maple (Acer platanoides), pin oak (Quercus palustris), sycamore (Platanus occidentalis), tulip tree (Llriodendron tulipifera), and white birch (Betula papyrifera). The trees were growing in the Illinois Natural History Survey arboretum located 1 mile south of Urbana, Illinois. Species, year of planting, plant spacing, and size of tree when the study began in 1981 are given in Table 1.

The wounds inflicted on the trunks of the trees were ellipses with pointed ends and of two sizes: small, 25 mm wide by 50 mm tall, and large, 50 mm wide by 100 mm tall. A template was used to mark the wound margin, and curved and straight wood chisels were used to remove the bark and expose the wood. When bark adhered to the wood, the wood face was scraped clean.

Four wounds, two small and two large, were cut into the trunk of each of 20 trees of each species for 4 consecutive years, 1981-1984. Wounds were cut in the cardinal directions; large wounds were cut north and south in 1981 and 1983 and east and west in 1982 and 1984. Within each year, wounds were at approximately the same height on the tree and within 0.5-2.0 m of the ground line. Cuts were made the week of 14 May 1981, 3 May 1982, 2 May 1983, and 16 April 1984. After the 4 years, each tree had been wounded 16 times.

Initial trunk diameter and subsequent diameter growth for 5 years were measured with a steel tape at a line painted on the trunk of each tree approximately 1 m above the ground. The initial measurement was made in April of 1981 (Table 1), and subsequent measurements were taken in late September or early October of 1981-1985.

Closure of wounds was determined annually through horizontal measurements at the greatest width of wood remaining exposed in late September or early October of 1981-1985. Since dieback can occur after wounding and since errors in wound preparation may be present, all wounds made in 1981 and 1983 were remeasured during the first week of May.

Results

The growth in trunk diameter of the ten species of trees exceeded 8 mm/yr during 1981-1984 (Table 2). Growth in 1983 and 1984 was less than in 1981 and 1982 in ash, birch, honeylocust, maple, and tulip tree. These differences may have resulted from drought in June 1983 and July 1984 or from increasing competition between closely spaced plants. In addition, some birches were attacked by bronze birch borers in 1984. The honeylocust, linden, pin oak, and tulip trees averaged annual trunk diameter increases greater than 15 mm. These four species were in plots of widely spaced trees (Table 1).

First-year closure of wounds that were originally 50 mm wide was greater than that of wounds that were originally 25 mm wide in every year and on every species (Fig. 1) with the single exception of maple in 1982. The average increase in callus growth due to wound size ranged from 8% in maple to 27% in sycamore. Closure of wounds in 1983 and 1984 was substantially less than in 1981 and 1982 on birch, honeylocust, and tulip tree. The mean extent of closure on birch and honeylocust was less than one-half that on elm, maple, pin oak, and tulip tree. First year closure was greater than diameter growth on all species except birch and honeylocust.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1.

Diameter trunk growth and first year closure of 50 and 25 mm wide wounds on ten species of trees (average from 20 trees).

Many of the 50-mm wounds failed to close during the second year after afflication. On pin oak, most of the wounds closed in mid-season of the second year and were not subsequently measured. On ash, honeylocust, elm, maple, tulip tree, sycamore and hackberry, many wounds remained open throughout the second year. Data collected the second year after wounding confirmed data collected the first year (Fig. 2).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

Diameter trunk growth and second year closure of 50 mm wide wounds on seven species of trees (average of 12-20 trees).

Wound closure correlated linearly with growth in trunk diameter in both the first year and second year after wounding. The results of linear regression analyses to estimate closure using growth in trunk diameter as the independent variable for 25- and 50-mm elliptical wounds on the trunks of the ten species of trees studied are given in Table 3. Growth and closure were positively correlated; the slope for each year and for each wound size on all species was positive (with the exception of the 50-mm wounds on sycamore in 1983 and on ash in 1984). With the exception of the 50 mm wounds on ash in 1984, all of the Y intercepts were positive, a finding which taken in conjunction with a high correlation coefficient indicates that wound closure per unit of diameter growth is greatest for slow-growing trees.

Neither intraspecific nor interspecific growth was uniform, and growth also varied from year to year. As a result, comparisons of closure rates between species are difficult. Table 4 compares closure between species at increments of diameter growth within the range of data obtained in this study. Data are averages from 3 or more years and are based on the lines of regressions for those years. A significant difference in closure between species was found at increments of diameter growth of 6, 12, and 18 mm. Birch, honeylocust, and sycamore had the lowest closure rate per unit of diameter growth; elm, maple, and oak had the highest.

Wound closure rates for individual plants in a species propagated from seed are variable and this generality proved true in this study. Selection within a population for individual plants that exhibit rapid wound closure would, therefore, be a desirable practice. Trees with closure rates consistently greater than the standard deviation of the population are those most likely to be selected for additional study. The deviations from the lines of correlation for each wound size on each tree for each species for each of the 4 years were determined. The number of trees within a species with wound closure rates greater than the standard deviation of the population are shown in Table 5. The likelihood of finding trees within a species with rapid closure rates appears to be best in honeylocust, pin oak, ash, and hackberry.

Discussion

Radial growth in tree stems results from three phases of cambial activity-division, enlargement, and differentiation. Each phase is regulated by a separate set of controls (9). All phases are modified by such environmental conditions as temperature and availability of moisture, but specific regulation is through the basipetal flow of products that are synthesized in the crown (leaves) of the plant, largely photosynthate and growth regulators (auxin, gibberellin, and inhibitors) (2, 5). The specific roles of individual components in influencing the thickening of the cambial sheath have yet to be determined, but both the amount and type of annual xylem increment as well as its distribution along the stems are influenced by the physiological activity of the crown in a predictable way (2). High correlations exist between cambial growth of trees and seasonal increase in leaf production, and these correlations vary among species (2).

Trees have an inherent capacity to regenerate lost or damaged parts. Parenchyma cells in the various secondary tissues, especially those of the cambial zone and its derivatives, proliferate rapidly. Ray and vertical parenchyma cells in the phloem regenerate more rapidly than do those in the xylem by de-differentiating to become meristematic. The pattern of healing on stems depends on where the separation occurred, the time of wounding and on which tissues were exposed to the environment. When the separation leaves the vascular cambium attached to the xylem and the exposed surface is kept moist by covering it with foil, plastic, or the removed bark, a relatively uniform wound callus rapidly forms across the entire surface of wood (10). Occasionally, relatively rapid and uniform formation of callus will occur even when the cambium is attached to the removed phloem if sufficient proliferating ray cells are present in the xylem. When the newly exposed parenchyma cells of the xylem remain uncovered and die of desiccation or when they are mature and have lost the potential to regenerate, the pattern of closure is different. Under these conditions, the callus originates in the existing cambium around the outer periphery of the wound, and the newly formed cambium forms adjacent to and in continuity with the existing undisturbed cambium (10). With yearly centripetal growth, the callus completely covers the exposed xylem.

The mechanism that regulates growth of meristematic tissue at the periphery of a wound versus the growth on the same plant a short distance from the wound has not been established and may be physiological, anatomical, or genetic. This study suggests that this mechanism varies among tree species. Because all growth uses the photosynthate produced by the leaves as an energy source and building material (9), the amount of thickening probably relates to the distribution of photosynthate. Wounds can be considered partial girdles, and Kozlowski (2) observed stimulated cambial growth above stem girdles and inhibited cambial growth below them.

Wounds can also be viewed as obstructions in the phloem pipline (9). Mass flow of photosynthate around the obstruction and through the narrower channels would be more rapid than that above or below the obstruction. Sway causes redistribution of cambial activity in the tree stem (9). In a similar manner, wounds may serve as sinks that utilize more photosynthate than would normally be used. Differences in anatomical structures among tree species as related to wound closure have yet to be studied. This study supports the hypothesis that growth across the wound face occurs at a rate more rapid than that of the vascular cambium and that the increase is proportionately greater on slow-growing trees.