2006-07-15 / Columnists

Drought Is Cause for Beech Tree Leaves Falling Early, Not Disease

By Patricia Martin

Last week a friend asked me if I had seen any sign of Beech Bark Disease. Beech Bark Disease is caused by a fungus that is carried by an insect. One of the characteristics of the disease, after it has progressed quite a way, is the dropping of the leaves of the American beech, and this disease has the potential to wipe out beech forests. When he asked, I remembered that the day before, while I was riding on Partridge Trail, I did notice that a number of beech tree leaves had fallen. This is, of course, the wrong time of year to expect to see leaves of the beech on the ground. Thankfully, the State Park naturalist went out and found no evidence of the insect or its scale on the bark, and is of the opinion that it's the lack of moisture that is causing these leaves to drop. Under water stress, beech are one of the first trees to lose leaves. I've had reports of other trees beginning the same process of leaf loss, including Norway maples. If you note the areas where the leaves seem to be dropping, they appear to be along roads or open areas or in higher spots, where they're more exposed to drying by wind and sun. Some of these areas also don't have the amount of leaf litter, which keeps the moisture in the soil, and some are where roots have been compacted, near roads in particular.

Most places in Michigan are not having a drought. According to the government, for the period of April 1 to July 2, the northwestern, northeastern, central, and southeastern Lower Peninsula are all above normal in their precipitation amounts this year. The southwestern and south central portions of the Lower Peninsula are below average, with most of the deficit occurring in the last month. The Upper Peninsula is below the average by less than two inches of rain, and that lack of water has occurred in June, but that is certainly not drought conditions, as the U.P. in general has received about nine inches of rain. Even Lake Huron is up about three inches. But the Straits Region of St. Ignace, Mackinac Island, and Mackinaw City has received considerably less. I think you could classify us as having a localized drought. The rainstorms keep going north or south of us. Just take a look at the grass that hasn't been watered, the dust in the turnouts, or the water in the storm-fed ponds or seeps. There is no water there.

Plants can't live without water. Everyone knows that, of course. Plants need water for every part of their life cycle. Seeds require moisture to germinate and will remain dormant if they fall on dry soil, even if all other conditions are correct.

Moisture is required for growth, because with inadequate moisture, gas exchange is drastically reduced. Plant leaves regularly take in carbon dioxide and use it and water in the presence of sunlight to build simple sugars and release oxygen into the air. It is these simple sugars that provide the energy for growth. Plants try to prevent too much water loss by developing a waxy layer on the surface of the leaf, known as the cuticle, which doesn't let water pass through, but it also prevents carbon dioxide and water from passing, as well. To get around this problem, plants have developed openings in the leaves known as stoma, which only remain open if there is enough water present; therefore, if there isn't enough water, gas exchange doesn't occur and sugars are not produced. Adequate moisture is also important to plants, since they rely on the transpiration stream to transport nutrients from the roots to the leaves. Movement of water and nutrients from the roots to the leaves promotes uptake of both from the rhizomes, which in turn causes water and its solutes to move from the soil into the rhizomes. As long as there is more water in the soil than in the rhizomes, water will continually flow into the plant. If there is inadequate soil moisture, no more water will flow into the rhizomes and the plant will have a water deficit.

In herbaceous plants, those with no wood to hold them up, water also supplies turgor pressure, which keeps them upright. If the water pressure is decreased, they wilt. If the lack of water is not severe, they will come back from their wilt if watered. Woody plants don't wilt, though their fleshy parts may, such as the leaves.

Most plants that live in areas with drying periods have some methods of dealing with the problem. The ability to close down the stomata in the afternoon heat to prevent water loss is one such adaptation. In many of our evergreens, pines, spruces, northern white cedars, their cuticles are very impermeable. This really reduces water loss so that they can survive winters with frozen ground, where there is no moisture available. In addition, they have additional lipids in the cuticle that help slow cuticular transpiration. Other plants have different adaptations to deal with periodic water deprivation. Some plants increase the extent of the root system, shorten the shoot size, thicken their leaves, or have smaller leaf cells or thicker cell walls. Some plants develop smaller stomata and crowd them together more densely on the leaves or have the stoma sunken more deeply into the leaf. The presence of leaf hairs helps keep moisture higher in the air around the leaves and therefore reduces the rate of water loss from the leaves.

There are also physiological changes, which help plants adapt to low water levels. Some herbaceous plants have adapted their life cycle. For instance, our spring ephemerals like the trillium and trout-lily send up leaves, then flower, fruit, and die back before the annual drying of August occurs. Many plants in our area, especially flowering trees, flower and begin setting fruit before the height of the summer, when problems of summer drought usually occur. Even the losing of leaves, like we've seen in the beech, is an adaptation. The leaves are where the sugars are produced, but with the lack of water, not much of that is going on. In addition, the leaves are where most of the water is lost. They're not being useful and so, to conserve water, they're lost. This doesn't mean the tree will die. If low precipitation levels continue over several years, the tree might die eventually, but the good news is that if we get some good rains, these beeches may send out new leaves this summer and most certainly will next spring.

If any of you know a good rain dance, or any other way of getting some precipitation in our area, I'd suggest you commence to dance or sing or shout or whatever. If I could place my order, I would like a nice steady rain that would start about midnight and continue until about six a.m. It would be nice if that could happen every other day or so for awhile, until the water levels are restored.

Trish Martin is a year-around resident of Mackinac Island, has earned a master's degree in botany from Central Michigan University, and owns Bogan Lane Inn.

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