In my Plainfield neighborhood, we have quite a few large deciduous trees, and they are looking glorious right now! When I walk my kid to school, the other parents and myself have commented to that fact, “Wow, the trees look extra colorful this year!”
I wondered though, is that a figment of my imagination or are the trees actually more vibrant this year than in year’s past? Maybe after escaping a pandemic, things just look brighter nowadays?!
To answer that, I went right to the experts, our friends at the Morton Arboretum! I asked if our observations on a more vibrant fall are accurate, if leaves can actually be brighter in some years (or are they always the same, no matter what), and if that’s the case, what factors play a roll in the vibrancy of Fall leaves.
This was the answer from the Morton Arb’s forest ecologist, Dr. Christy Rollinson:
“Yes, it does seem to be a particularly vibrant year for fall color, but we don’t have actual quantitative data to support that yet. (Right now that requires large-scale photographic imagery that we just don’t have large networks for yet.) In general, good fall color is a sign of healthy trees and a good growing season. This year had pretty moderate weather conditions — few periods of extreme heat or drought and lots of sun in early fall, which is good for tree growth. The Plant Clinic has gotten fewer reports of pests and pathogens on trees in the region this year compared to other years, which is also a sign it’s been a good year for trees in our region.In particular, I’ve noticed a lot more red this year, including on species like white oak not normally known for red color, which is a sign of sun and energy available to produce that specific pigment as nitrogen is re-absorbed from leaves. The yellows and browns are always there, but also seem to be brighter this year anecdotally.The wind earlier in the week really began the transition to late falls and many of the bright early- and mid-fall species like maples have dropped their leaves or are in the process of doing so. Oaks, which turn later in the fall, are really coming into their own and there’s still lots of great fall color out there, particularly in natural areas like The Morton Arboretum’s east woods.”
There’s some truly amazing color out, particularly on maples that get lots of light like in our Maple Collection and forest edges, but oaks still have a ways to go as does the forest interior (see last photo). This weekend is going to be great for getting out and seeing color… pic.twitter.com/2D6mY5bySB
— Christy Rollinson (@RolliEcology) October 21, 2022
“I was unaware that the same tree may have different color leaves from year to year. What causes that?”
There are a couple of contributing factors, but the overall health of the tree (e.g. stress due to things like drought or pests/pathogens) can have a big impact on overall vibrancy, but how sunny the days are throughout the fall really has a strong influence on how much red there is and how bright it is. Colors like yellow are always present in the leaf, and are revealed when trees break down green chlorophyll in the fall to recover nitrogen for next year’s use. Red is different and produced by some (but not all) kinds of trees when there’s a lot of sun to help with the process of getting nitrogen out of leaves. So lots of sun as trees start changing color means more and brighter red.
“What weather phenomenon has the most impact on when trees begin to turn colors? Is it temperature? Humidity? Wind?”
This is a big source of debate in the scientific community right now. There’s no one single factor that clearly controls the timing, unlike spring where warming temperatures clearly play the dominant role. In general, cold nighttime temperatures at night play a big role for determining when leaves turn color and how quickly, but daylength and moisture availability are also important. There’s also emerging evidence in the research community that how much the tree was able to grow earlier in the season plays may make fall color occur earlier, but there’s a lot of debate around that and consensus has not been reached. Wind like we had earlier in the week is typically what defines the end of the fall season as it can really causes leaves to fall en masse.