Northland Nature: Ironwoods keep leaves all winter
Retired teacher Larry Weber, a Barnum resident, is the author of several books, including “Butterflies of the North Woods,” “Spiders of the North Woods,” “Webwood” and “In a Patch of Goldenrods.” Contact him via Katie Rohman at email@example.com.
It was a little over three months ago, mid-October, that the vast majority of leaves fell from the local deciduous trees. The outstanding colors — reds in September, mixed with the dominant yellows that gave way to nearly all yellows in early October — preceded this arboreal event of the leaf drop.
We experience this each autumn and after a few days of wind and rain, the leaves that have been with us since last May and given so much color in fall, depart from the trees. Suddenly, the yards and forest floors are covered with the fallen leaves.
As often happens in nature, the event does not take place with all the trees at once. Within our native forests, it is unlikely that we’ll see leaves staying on branches past October. But many of the non-natives that thrive in the region, along with some late natives, will continue to hold leaves until early November. I find this most with silver maples, small swamp willows, large weeping willows and others of the yards and parks such as lilacs, forsythias and buckthorns.
And out in the wetlands, the second half of October is lit up by the tamaracks — our only conifer to drop all its needles in preparation for winter. No matter — native or not, by the time we reach the end of November, the deciduous trees are mostly bare.
Now, with a snow cover in January, it is easy to see as we pass by the winter woods that some trees are still holding leaves, though they are brown, curled and dead. They are present on the tree branches.
A closer look reveals three species of trees that continue to carry dead leaves throughout the winter, a condition known as marcescent (also called “everciduous”): sugar maple, red oak and ironwood.
While it is only some sugar maples and red oaks, usually the smaller or younger ones, that hold leaves all winter, this condition is more widespread with ironwoods. In some places, mostly to the south and east of our region, pin oaks also are marcescent. Further east, beach trees exhibit it, too.
Ironwood (Ostrya virginiana) is a small- to medium-sized tree that grows well in the shade of the larger forest deciduous trees. Toothed leaves are much like that of elms and the wood is hard. Male catkins also stay on trees in winter, much like what is seen on birches.
Catkins develop in spring to pollinate the female and produce a scaly nut known as a hop. (Ironwood is also called eastern hophornbeam.) The hops drop off in fall and if it were not for the long-lasting leaves in winter, we would never realize how common these trees are in the area forests.
The main reason that deciduous trees drop their leaves in fall is an adaptation to winter. In the arid air of winter, broad leaves would lose moisture and cause desiccation. Such dryness could harm the trees — best to drop the food-producing leaves for the cold season and grow new ones in spring. Also, such leaves would be dangerously burdened by the weight of snow on the branches.
One of the reasons proposed as to why ironwood keeps leaves all the way to the new growth in spring, is that dead leaves can also help from drying. Another reason may be that dead leaves can discourage browsers from eating the small buds. Or maybe, it is just a genetic variation.
Whatever the reason, ironwoods let us know how common they are by keeping leaves all winter.