Why are the Trees Losing their Leaves?

All over Iowa, it seems that some deciduous trees are celebrating fall about two months early. Since early August, premature fall coloration, browning of leaves, and heavy defoliation have been common on hackberry, oak, ash, and sycamore, among others. What's going on? Early senescence is usually a response to stress; 1988, the worst drought year, was the last time many species of trees lost their leaves so early. In 1992, the likely culprit is residual effects of the November 1991 freeze. That extraordinarily early cold snap caught trees before they were fully hardened off, and may have injured the sensitive cambial tissue of many trees.

Since cambium makes the sapwood for the next season, trees may have struggled by in 1992 with less than the normal complement of water-conducting tissue. Fortunately, perhaps because of our cool, wet summer weather, the leaves held on until August. At this point, they have served almost all of thier normal function for the tree, so defoliation will probably do little significant harm to the tree's health. With a few exceptions (resulting from diseases such as Dutch elm disease), defoliated trees that I have seen are not dead or dying, and have already formed buds for next year. With luck, and a little cooperation from the weather, most should get back on a normal schedule in 1993 .

This article originally appeared in the August 26, 1992 issue, p. 142.


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