If you’ve wondered what awful new malady has struck our oak trees this spring, resulting in shriveled, deformed and dead leaves, the answer is chilling. Literally; as in cold. A hard freeze on the night of May 17-18 happened at just the right – or wrong – time, catching oak foliage at a critically tender stage. Since trees can’t change their locations (to my knowledge, at least), I guess you could say that oaks were in the right place at the wrong time.

Periods of unusually warm temperatures between April 12-22, and again from May 6-13, enticed many trees to push out new growth quickly. This likely set the stage for more widespread harm than if the mid-May freeze had occurred in the midst of a slow, gradual
warming trend.

Other tree species that bud-out late such as hickory, butternut, and walnut were also affected to various degrees. Across the region, the extent of freeze injury varies from slight to severe, depending on things like slope aspect, genetic variation among individual oaks, whether trees were partly sheltered by the surrounding forest, and soil pH (bud-break is a bit earlier on acid soils). In many places, the damage looks harsh – even zipping along a highway, one can pick out which trees are oaks by the sparse canopies and unusual reddish cast to the foliage.

Young new growth is rosy because after that hard frost, oak trees now want to protect their leaves from sunlight. A class of chemicals called anthocyanins, which produce the red and purple color range in plants, act as UV protectants. It sounds bizarre to put sunscreen on leaves, which need sunshine to photosynthesize. But it turns out chlorophyll can be destroyed by intense UV rays at low temperatures. Eventually, the red will give way to green as anthocyanins break down and are not replenished (these large molecules cost plants a lot of energy to create, and while investing in them in springtime makes sense, no one has yet explained why trees make them in the fall after chlorophyll is gone).

In the spring of 1993, a similar thing took place where I was living in the Saranac Lake region. That time, a freeze occurred when beech trees were just leafing out, and I was flooded with calls about a new “beech-leaf blight.” And then in 2011, sugar maples in the St. Lawrence Valley got touched by frost when their leaves were still folded. As the leaves opened, long gaps appeared, and I was awash in questions about what was “eating” the maple leaves.

At this time, I see no need to despair about the future of oak trees, although feel free to do so if you really want. Many oaks are still in the process of pushing out a new crop of leaves, and even those that go through the season with minimal leaf-surface area have energy reserves on which to rely. Dry soil conditions can inhibit re-foliation, so as long as we get a reasonable complement of rainfall this summer and don’t have an outbreak of defoliators like spongy-moth or tent caterpillars, the oaks should be OK.

Photo of burr oak by Katja Schulz from Washington, D. C., CC BY 2.0, via Wikimedia Commons