By PAUL HETZLER
As the trees begin to turn, we can read a bit of their stories in how the leaves change.
The art of reading tea leaves, known as tasseomancy or tassseography, has a long tradition of providing intrigue and amusement, but by most accounts it imparts very little useful information. On the other hand, the science of reading tree leaves yields quite a bit of data. Sadly, it doesn't get its own cool word. This is an unfortunate oversight, especially considering that people are drawn to careers in science in part because they get to use lots of cool words, thus appearing smarter than they actually are. Let's call leaf-reading 'arborography.'
Altogether the leaves of a mature shade tree can add up to thousands, even tens of thousands, of square feet of surface area. Try reading any other technical document of that size and good luck finishing it in the next month, but you should be able to leaf through this kind in just a few minutes.
First, budding arborographers, check the margins. We all know that important stuff sometimes gets written there. The next time you're walking down a street, look at the trees growing in the narrow strip between sidewalk and road. Very often by midsummer the leaves are brown at the edges. This is called margin scorch, and it's a sign of water stress. Even in a year with average rainfall, street trees may exhibit margin scorch. This is because road salt gets plowed up onto the root zone (such as it is), and makes it hard for roots to take up water even when there’s plenty of it.
Color is an important aspect of arborography. When you’re driving in the countryside, look up from your smart phone screen long enough to check out the trees in the fencerows and on the forest edges. You'll probably notice that a fair percentage of them are not the deep green they usually are. Many have lemon-lime-colored leaves; others, quite yellow. Scientists call that 'chlorosis,' because it sounds a lot better than 'yellowishness.' Chlorosis is a nonspecific sign of stress, but you can guess what's bothering them this year. Trees that have other stressors will become chlorotic before vigorous ones will.
It’s always with mixed emotion that we gaze upon the first significant color change in late summer. As much as we appreciate the beauty, it’s a reminder that winter’s coming. (Parents of young children, though, universally love to see that first “back-to-school tree.”) This year, trees started turning color earlier than usual. Trees are sugar factories which need air, light, soil and water to operate. When they run short of water, production basically stops. Since a tree has to use energy to keep its leaves around even when they're not able to earn their keep, it may decide it's cheaper to close down early.
When leaves turn color before dropping, that usually means the tree was able to withdraw stored sugars from them. It also means the tree sealed the places where they were attached with a wax called 'suberin.' This is called 'forming an abscission zone.' It's essential; otherwise the vessels that served the leaves would remain open and be a highway, or 'an infection court,' for diseases to get in, and moisture to escape.
Again, trees under chronic stress turn the earliest, so if one of your yard trees is in color in late August/ early September, read the writing. You may notice one side of a tree only, or even a single branch, turning early. This is when the roots that nourish that sector of tree are under more stress than the remainder of the root system, such as when there's a driveway on one side.
Going east along the seaway you find limestone soil, which is extra drought-prone, and a lot of brown trees. When leaves turn brown and remain on the tree, there's no abscission zone, and the trees dry out even more, further reducing their chances for survival.
Size matters—sometimes, at least. Trees under chronic stress will produce smaller leaves, or in the case of conifers, shorter needles. But since this year’s leaves developed under adequate moisture, they’re mostly full size. It will be interesting to see next year how full the crowns are and how big the leaves are.
Ornamentation is the last heading in arbography. Galls and odd growths on the leaves are nearly always a cosmetic issue only, and no cause for concern. The spindle-shaped galls are made by mites; the larger round, ones by tiny wasps.
Right now you can see many silken caterpillar nests adorning the ends of branches. These are not tent caterpillars, nor are they anywhere near as destructive. They're fall webworms, and can be controlled by the “clip-'n-stomp” method (where that's feasible), or with a light dose of carbaryl (Sevin). Halloween's not that far off, though, and these gauzy nests will fit right in with any spooky décor. Then after Thanksgiving, maybe a touch of red and green spray paint on those nests, some glitter...why not get some mileage out of them?
Feel free to share your new-found skill, and practice saying things like “abscission zone” and “chlorosis” in casual conversation. In context, of course.
Paul Hetzler is a forester and a horticulture and natural resources educator for Cornell Cooperative Extension of St. Lawrence County.