Rhode Islanders pay close attention to the changing foliage colors each fall and are justifiably proud of the picture-perfect show the region’s trees put on every year. But Keith Killingbeck, 67, probably pays closer attention than most. A recently retired professor of plant ecology and biology at the University of Rhode Island and the former assistant dean of the URI Graduate School, he has been studying the nutrient dynamics and energy flow in trees – the process that causes the leaves to change color – since his grad school days at the University of North Dakota. A native of Michigan who moved to Rhode Island in 1979, Killingbeck said that despite his 36 years teaching botany, he didn’t start out with a particularly strong interest in plants and foliage. “My passion is for nature and the outdoors and everything and anything that’s part of it.”
Q: Why does New England have the reputation for having the best fall foliage colors?
A: It has to do with the tree species we have and the climate we have. Often in New England we’ll have sunny days and cold nights in the fall, and the tree species that we have respond to
that with a number of chemical changes in their leaves
that yield the brilliant colors we see.
|Fall foliage in New England (Glen Russell)|
Q: What do trees do to produce those colors?
A: One of the most important things they do is break down compounds like chlorophyll to save some of the nutrients, especially nitrogen and phosphorous. Those nutrients are being shunted out of the leaves and moved back into the trees to be saved to be used in the next growing season. It’s essentially a conservation strategy.
Q: Why do different trees exhibit different colors?
A: There’s no easy answer to that. But if we look at birches, they turn yellow. Yellow pigments are unmasked as the green chlorophyll molecules are broken down, and as some of the nutrients are moved out, you have these xanthophylls – accessory pigments in tree leaves -- that show up as bright yellow. Tupelos turn a red-maroonish color, and those are from anthocyanins, a whole different kind of chemical, which are being built up in the fall rather than unmasked. They protect some of the photosynthetic systems in the leaves. It’s complicated.
Q: Do you have a favorite tree for its fall foliage?
A: I’ll give you three. My favorite tree in New England is sugar maple for its peachy color. This is an oddball, but I also like poison sumac, which has this beautiful orange color to its leaves. The third one is Virginia creeper, a vine that has beautiful maroon leaflets in fall.
Q: What factors influence the timing of when leaves change color?
A: The amount of daylight, or photoperiod, is one of the main kickers in all this. Photoperiod is constant from year to year to year, so it’s a good trigger for plants and animals for the timing of anything they do. Within that time period, though, everything from daytime versus nighttime temperatures, precipitation, drought, all those things really impinge on and impact the leaf changing color. The chemistry that’s going on during that time is impacted by all those factors, but photoperiod is a constant predictor of where we are on a year’s cycle.
Q: It doesn’t appear that the leaves all change at the same time. Why not?
A: They certainly don’t. In mid-September, many birches have yellow leaves already, Virginia creeper has maroon leaves already, but most of the leaves on other species are still green – oaks have not turned, most of maples have not turned. Individual species change at different times. In southern Rhode Island, for example, the peak of red maple color is probably around the second or third week in October. Oaks are going to be later, others earlier. It’s species by species and also individual site by individual site. Red maples in very wet areas are going to have their leaves turn earlier than red maples on a much drier site.
Q: How will the gypsy moth defoliation affect the fall foliage?
A: In those areas hardest hit, virtually all the leaves were eaten. Some of those trees that didn’t die reflushed some leaves, but as I’ve looked at some of those trees, the new leaves that were produced after the defoliation were much smaller and there are many fewer of them. So even if those individual leaves turn bright colors, there’s not enough leaf mass to show much color. And, of course, those trees that died, you’ll get no color out of them.
Q: What other factors affect foliage from one year to the next?
A: Ideally, to bring out the most vibrant colors, what works the best is bright sunny days when there is a lot of photosynthesis – they’re producing sugars during photosynthesis – and cool nights for depressing the rates of metabolism so those sugars being produced in daytime are not used up as rapidly as they would be on warm nights. So it’s those sunny days for good photosynthesis and chilly nights. That’s the perfect recipe for vibrant leaves.
Q: Based on those factors, do you have a prediction for what this year’s foliage is going to be like?
A: It’s certainly going to be at the usual time, not knowing what the upcoming weather is going to be, because the variation in timing from year to year is pretty small. I’ve been following leaf color change in individual trees for many years, and the timing changes only by a few days from year to year. The amount of rain we get, when we get rain, the temperatures at night all make a difference, but it’s very subtle. This year the thing that’s going to impact our color most is whether trees were defoliated in spring and early summer. But other than pockets of trees that were defoliated, I think the colors are going to be great again this year.
Q: Why do some individual trees change color especially early, like in August?
A: I’ve observed that also, and in fact I have a single red maple tree that I call my early red maple that I see when I drive to campus every day. It turns earlier than any of the other red maples around it. I think it’s genetics. The genetic makeup of the individual. The environmental conditions are the same versus trees meters away – whether it be soil conditions, rainfall, sunlight, anything else – so the difference has to be attributed to genetics.
Q: Is climate change likely to have an effect on foliage colors or timing in the coming years?
A: The simple answer is yes, as far as we can tell. There are certainly phenological studies around the world that show plants budding earlier in the spring than normal, extending the growing season on the front end, and it appears that leaves are being held onto later in the fall, at least for some species, extending the growing season on the tail end. So it certainly seems that the warming of the planet is having an effect and will continue to have an effect.
This article first appeared in the Newport Mercury on October 11, 2017.