When that first swatch of color hits a sugar maple tree every August somewhere in the Northland, there’s always someone ready to post a photo on social media.
“Looks like fall is coming early!” is often the response.
But of course, early fall color is not a sign of an early fall. Trees — like caterpillars or the Farmer’s Almanac — can’t possibly predict when or how autumn weather might start. Trees can only react to existing and previous conditions. And this fall, some Northland trees are reacting to drought.
In general, trees change color and drop their leaves pretty close to the same time each year because the process is driven by daylight and temperature: As both drop, so do the leaves.
But recent climatic conditions also impact trees, and that’s why some trees in the area have leaves already turning brown and dropping to the ground, skipping the usual color display entirely.
“Soil moisture is essential to tree health, obviously, so if there has not been enough moisture at fall color time, color could be delayed, just plain dull, or leaves can die early without changing color,’’ said Val Cervenka, forest health program coordinator for the Minnesota Department of Natural Resources. “Trees that are under stress usually show it first in their leaves. Since so much of a tree’s energy goes into producing leaves, the tree can conserve energy by simply dropping the leaves, like it does before it goes dormant in the winter.”
May and especially June were among the driest on record in Duluth, which was some 8 inches below normal annual rainfall at one point this summer, spurring a formally moderate drought. A wetter July helped quell some of the problem, but August came up an inch short of normal rainfall and — as of mid-week — Duluth remained more than 6 inches short for rainfall since Jan. 1.
But across the Northland, even across a city or a neighborhood, the impact of the drought isn't uniform. Some areas may have received a small but intense thunderstorm at just the right time. Other spots hold water in the soil better.
“Weather is what makes (the fall color) prediction difficult, and even if an area of the state has had little rainfall, some trees growing in ideal locations may still show great fall color, because more moisture was retained in the soil in a low-lying area, for example,’’ Cervenka noted.
It also depends on the type of tree. Sugar maples seem to be more vulnerable to moderate drought than oaks, for example. Yet, even some maples that are showing signs of stress are right next to others that remain vibrant and healthy with deep-green leaves.
In addition to drought, other stressors that can impact leaf color and leaf drop include disease, insects and human traffic and soil compaction nearby, such as construction.
Eric Otto, DNR forest health specialist based in Grand Rapids, says he’s seen some signs of drought-impacted trees, but nothing too severe.
“It appeared there was some leaf scorch occurring on some aspen trees in Itasca County," Otto said, noting the phenomenon occurs when the tree is unable to take up a sufficient amount of water when the conditions are hot and dry. “The water is leaving the tree faster than it can be replaced. It’s typically a top-down pattern of browning where the leaves on the top of the tree will brown first.”
Otto said a dry summer can also make leaf damage from insects like the blotch miner and birch leaf miner look worse, but that aspen and birch trees usually bounce back quickly from both.
In general, across most of Minnesota, Cervenka said adequate summer rainfall reports portend a good fall color season in most areas with very few floods and only small areas of lingering drought.
“I might go out on a limb and say that there will be beautiful color contrasts anywhere there are hills and valleys with mixed species of trees. According to the current U.S. Drought Monitor map, there are a few areas in the state where we have had a moderate, but not severe, drought this summer,’’ she said. “I think that although some individual trees may not have been healthy enough to produce great color this fall, there will be plenty of trees that do.”
What makes fall colors start?
If trees are healthy and well-watered, the onset of fall color is triggered by shortening daylight hours and the impact that change has on the chemistry of leaves. That's why the process starts at nearly the same time every year.
But temperature also plays a role, and cooler nights help bring out more color. It's that combination of shorter days and colder temperatures at night that cause the change in hormones in the leaf that trigger senescence, the breakdown of cells and the color change.
An unusually warm fall can slow things down. Experts say a light frost at the start of the color season actually helps produce vivid color. But a hard frost, with temperatures down into the lower 20s, can kill color fast. And of course, a big wind or heavy rainstorm as peak color approaches can ruin everything.
Chlorophyll gives leaves their green color, while anthocyanins, carotenoids and tannins produce the reds, golds and browns that come to the forefront in the fall, the DNR notes. Those shorter periods of daylight mean a closing off of the leaf veins that carry liquid sugar in and out of leaves. Sugars in the leaf permit the red and purple colors to develop.
Purple-like and red pigments are found in the leaves of maple and oak, some varieties of ash, and tall shrubs like cherry, sumac and viburnum. Yellow is always present in leaves all summer long, but the color is revealed when the green pigment in chlorophyll breaks down.
More recent research also shows that leaves have evolved to develop those bright colors as a sort of sunscreen against damage as the tree goes dormant, protecting the leaf as long as possible so the tree can keep drawing nutrients until the leaves fall off.
Over the hills
Naturalists remind folks to look down, not just up, to see bountiful fall color among grasses, wildflowers and ferns. And they have another suggestion you may have overlooked: If you want to see more color, look for hilly areas, and not just because you get longer vistas. In general there's more variety, more tree species, in hilly areas.
Flat areas generally tend to have fewer or even a single species. The more species, the better the chance you'll see one or more at its peak color at any one time.
Follow the color reports
The Minnesota DNR will again offer weekly fall color updates each Thursday starting Sept. 10 from state parks across the state. Go to dnr.state.mn.us/fall_colors.
For information on fall color status in Wisconsin, go to travelwisconsin.com/fall-color-report.