The Lilacs of Mackinac Island by Carl D. La Rue

Excerpt from American Midland Naturalist, University of Notre Dame, Notre Dame, Indiana, USA

The Lilacs of Mackinac Island emphasized in a moving picture taken on that historic island, have been widely known for their large size and great age, but it is doubtful whether many persons have realized how unusual they really are. I had observed their unusual size several times in the last ten years and wondered about their age and growth rate. On June 23, 1947, I had an opportunity to satisfy my curiosity, at least in part, by measuring a number of them. I was surprised to find that they were much larger than I had supposed and that some of them had diameters at the butt which would be significant to a lumberman.

By far the greater number of the old lilacs belong to the species Syringa vulgaris, the common lilac, but white lilacs, Syringa vulgaris var. alba are also common in the landscape. As elsewhere, the white lilacs are taller than the purple ones and have longer trunks, which, however, do not compare in girth with those of the purple type.

Lilacs, under usual conditions tend to be shrubs with many stems, but they respond readily to training into tree forms. Those of Mackinac Island are nearly all reduced to a few stems, either by training or by survival, but one large specimen had a vast number of stems, both ancient and recent. There are many of tree form which appear to have been trained to a single stem. These are true trees, since they exceed by far, the dimensions usually ascribed to shrubs. On most of them, the trunks are very short so diameters could not be measured at the standard breast height. Instead, measurements were made as near the ground level as possible, but above the root crown in any case.

Many trunks are gnarled and covered with knotty excrescences and remind one of the trunks of ancient olive trees. These irregularities made accurate measurements impossible, but the percentage of error involved in the measurements given in Table 1 is not great. Some of the largest trunks are smooth, so it is evident that the large sizes presented are not due merely to knots.

All of the trees examined were in remarkably fine condition. The leaves are smaller than those of young plants, but both twigs and leaves fail to show the depauperate appearance of old lilacs farther south. No scale infestation was seen, and no signs of mildew, though it was probably too early in the season for this to be visible. Some plants had lost large trunks, (the five-trunked tree at Astor House had lost two nearly as large as the survivors), but no dead trunks or large dead branches, and remarkably few small ones were seen on this inspection.

The trees appear to grow rather slowly. Rings were counted in a branch 40 inches in circumference which had been sawed off cleanly. A core 2î long was drilled with an increment borer in a dead trunk. The wood was drilled with an increment borer in a dead trunk. The wood was exceedingly hard, and threatened to break the instrument if the boring were continued. From the ring counts obtained, the annual rings appeared to average 10 to the inch, which means the diameters increased an inch in five years. At this rate the oldest tree measured would be about 118 years old. The trees on the grounds of the old Astor House appear not to have been planted during the time that original structure existed (1818-1829), but probably were set out some fifty years later. The data are too scanty to be reliable but they indicate that the trees are not remarkably old, and do not date back to the earliest days of the settlement. Even so, they must be among the oldest, as they surely are among the largest lilacs in the United States. I know of no records of size in lilacs. Unfortunately, while much is known of the life span of species of trees and many records exist of unusually large specimens, similar data have seldom been collected for shrubs.

In southern Michigan, and similar regions, the main trunks of lilacs do not live very long, although the roots may persist indefinitely. Although they are very tolerant of less satisfactory conditions, lilacs undoubtedly prefer a cool climate. The giants on Mackinac Island owe their statue to favorable growth conditions and a long period of undisturbed development.

For more dialogue about lilacs, please visit the International Lilac Society
For more articles about the history of botany in America's Middle West, please visit the American Midland Naturalist