Effects on Maples of Prolonged Exposure by Artificial Girdling Roots¹

Materials and Methods

In a block of 360 maples, planted in 1964 in the Shade Tree Laboratories’ Hadley Research Nursery as part of the Maple Decline Project (McIntire-Stennis #2), two transects were later assigned to this study. Each transect comprised: 5 sugar maples (Acer saccharum Marsh.); 5 silver maples (A. saccharinum L.); 1 red maple (A. rubrum L.); 5 Norway maples (A. platanoides L.); 1 Schwedler Norway maple (A. plantanoides cv. ‘Schwedleri’); and, 1 Crimson King Norway maple (A. plantanoides cv. ‘Crimson King’). Each tree was within a row of 20 trees of the same kind. The trees were 2.4 m (8 ft) apart in each direction. At first, the small trees did not compete for light or water; grass between them was mowed regularly. Later, crowns expanded so that deep shade killed all grass and also the slower-growing trees.

A tree with natural girdling roots, is always pushing against itself. Trees can, and commonly do, easily shove aside objects that threaten to girdle them on only one side (example: the lifting of adjacent sidewalks by root pressure). The creation of artificial girdling on one side of a tree could not be achieved by the juxtaposition of an unattached heavy object. We had to pit the tree against itself, like an isometric exercise.

In nature, girdling roots ordinarily occur around or against larger roots and around or against the trunk, either below, at, or slightly above ground. We placed our devices on the ground surface near the area of natural girdling roots.

Type I

Initially we tried to girdle in a limited way by means of a cable, attached at each end to one or other end of a horizontal angle iron that passed and touched the other side of the tree (Fig. 1 a). The device was applied on May 31, 1966, to 18 trees and 18 similar trees were assigned as untreated controls.

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Figure 1.

Diagrams to illustrate installation of synthetic girdling roots.

The Type I devices failed to yield even limited injury. By the second year silver maple growth filled the two little triangles between each bar and its cable. These trees become entirely girdled and in every case their above-ground portions died. We had to design a device that could not be filled for many years. However, from these preliminary results we realized that the angle iron girdled as effectively as the cable.

Results

Type II

For 16 years we watched the growth and health of the trees treated with our Type II girdling roots, and their control trees. The entire plantation grew taller and the foliar canopy merged. One control tree and one treated tree (Fig. 2) lagged behind the others in growth early in the study. Both were shaded and soon died. But the others, surprisingly, showed almost no differences in growth rate. Norway and sugar maples, 2” dbh in 1965, now are becoming large enough to fill the rectangle of angle irons plus bolts. Many died in the last year or two, until now 11 girdle-treated and 10 untreated (“control”) trees are alive in each original group of 18. However, the Norway maples are likely to engulf the girdles by self-grafting: their trunk tissues now have met outside the angle irons.

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Figure 2.

Photograph of a synthetic girdling root on a tree trunk that was removed from the experiment six years after installation. The device presumably did not kill this tree, since at this time only one of the 18 treated trees and one of the 18 untreated trees died.

Average dbh’s (diameters at breast height), in inches, at the end of the experiment, for girdled vs. nongirdled trees, and number (n) of survivors, were: Norway maple, 6.7 (4.8 to 9.5, n = 5) vs. 4.8 (3.8 to 5.3, n = 2); Silver maple, 9.1 (6.5 to 11.5, n = 4) vs. 9.9 (8.5 to 10.5, n = 4), Sugar maple 5.3 (n = 1) vs. 4.2 (2.8 to 5.0, n = 2); most sugar maples were cut by an unknown person several years ago), red maple 4.0 vs. 5.5 (only one red maple replicate in the experiment). The bases of all the girdled trunks were enlarged (through accumulation of translocated food above the blockage) and became greatly distorted over the 16 years (Fig. 2).

Discussion

It has been almost a fad recently to debunk various beliefs that have been held almost as matters of faith by earlier arborists and arboricultural scientists. Our study shows that the earlier tenets on girdling roots are not all wrong. Girdling all the way around finally does kill a tree, unless it engulfs the girdling object and completes a natural bridge graft over it on one side of the tree before too much interruption occurs in the upward flow of water and/or downward flow of nutrients at all other points.

Tate (1980, 1981), in Ann Arbor, Ml, also found no difference in growth rate, in this case between 336 Norway maple street trees with one or more “girdling” roots and 74 without such roots.

Even partial girdling, as it approaches completeness, reportedly may weaken a tree enough to entice attack by secondary insects and pathogens, although we did not encounter this effect in our experiment. On the other hand, the word “girdle” means something that goes all the way around an object. It seems fair to suggest that a single tangential “girdling” root is not actually girdling the tree and is not a serious threat to wellbeing. Such a root, that occurs only on one side and causes only partial girdling, might better be called an “embedded root.”

The wound that would be left by removal of such a very large embedded root in a large tree might be worse than leaving the embedded root in place. The injury through potential invasion of wood decaying organisms would be accentuated by the loss to the tree of the considerable root system that is naturally associated with such a root, as pointed out by Hudler (1981).

One caution: Our experience has been that after a single embedded root is removed, digging often reveals others at lower levels. The sum of the effects of several such roots may indeed be to girdle the tree in the true sense of all-the-wayaround!

Conclusions

The Shade Tree Laboratories now advise preventative steps against girdling by roots. The first step can be taken at the time of transplanting. Allow ample lateral space to spread the roots, to allow for symmetrical growth. The second step should be taken early in a tree’s development — after it has been in place no more than about a decade. With a trowel, gently (to avoid wounding large roots) turn back the turf over the roots system, out to at least half a meter from the trunk. Any small roots that branch at right angles to the main radiating root, and hence appear to lie in a position to become embedded once the trunk enlarges enough to reach them, or any small roots that cross over large radiating roots, can easily be snipped off with hand shears.

The discovery of a large embedded root also should lead to more digging. With trowel and whisk broom, explore underground all the way around the base of the trunk, and also along all the major buttress roots. If there are multiple embedded roots around the circumference, which would combine with the visible one to girdle the tree, surgery is unavoidable. But if we find no trouble except the one embedded root, and if most of the root system is intact, we would leave it alone — especially if there are no foliar symptoms of concern. Tate (1980, 1981) also concluded that removal of such roots was not cost-effective. Only 5% of his 336 cases were “girdled” more than halfway around the trunk, and 53% were less than ¼ “girdled.”

Girdling roots usually are diagnosed because they are visible above ground. Other times they are suspected because of crown dieback (“stagheading”) or, at an earlier stage, because of leaf “scorch” resulting from water deficiency. If exploration by careful digging shows many girdling roots, adding up to near-complete girdling, then their removal may offer hope for recovery.

If, however, only a single, tangential, embedded root is found, it could also be dangerous to delude oneself that the sole cause of the dieback has always been found. We no longer consider a single embedded root (on one side of a tree trunk) to be sufficient diagnosis for scorched foliage and/or for dead upper branches. Our study indicates that then, for maples at least, some other cause remains to be found and appropriately corrected before hope can be held out for restoration of the tree to vigor and thrift.

The fact that Norway maples engulfed and selfgrafted over the synthetic girdling roots should not be taken as conclusive evidence that they would necessarily accomplish this feat with natural girdling roots. After all, the synthetic devices did not grow any larger with time, whereas the tree’s own girdling roots enlarge every year. Tate (1980, 1981) did not report engulfing of such embedded roots on any of his 336 Norway maples.

This study does not purport to show a critical point below which girdling is of minor consequence and above which it is a threat to tree survival (either directly or by opening the way for secondary attacks). We have, however, modified our views at the Shade Tree Laboratories as to how much girdling a tree can stand without obvious detriment, apart from basal distortion of appearance as only a blemish requiring no remedial treatment.