Where's the Colour?

Steven Katovich, USDA Forest Service, Bugwood.org
Fall color in northern Wisconsin, paper birch, and red maple.

"Why weren't the leaves as awesome this year?"

"My bonsai were kinda bleh this fall, I don't understand."

"The Japanese Maple in my front yard turns fantastic colours most years, but not this one. Why?"

"Where's the colour?!?"


From all quarters, there has been the same question this fall. "Where's the colour?" Over and over again that question repeated as autumn enthusists gazed over the yellows and browns, eagerly awaiting the vermillion foliage that typifies an East Coast fall. And over and over again, they were disappointed.

Oh, the golds and russets were lovely. But it just wasn't the same. Though there were a few die hard species in this area that, right at the end, put on a quiet, tired display that mimiced their reds of years past, here on Long Island we simply didn't see the autumn display that we usually do.

And we weren't the only ones.

So, what happened? Why was the colour muted this autumn? Why did the trees that turn yellow seem to be ok, but our glorious reds were, well, not so glorious.

For that we have to start with the science of why leaves change colour. During the late spring and summer, leaves are busy. Working hard. Making and utilizing chlorophyll, the green pigment which is where photosynthesis occurs. And therein lies the beginning of the entire cycle of energy on earth – plants take sunlight, water and carbon dioxide and create sugars and oxygen. Without this process, we wouldn’t even be here; there’d be no food, not enough free, breathable, oxygen. If this isn’t amazing enough for you, well, I don’t really know what else to say, and you should probably try a different blog.

Senescence in plants is the period of time between leaf maturity (where they are a sugar factory chugging out that beautiful oxygen by-product) and leaf death. A lot happens in that time, but the part that interests us here is the withdrawal and absorption of the chlorophyll. Triggered by cooling temperatures and shorter days, trees in temperate climates start to shuttle the energy and nutrient rich chlorophyll from their leaves, breaking them down in the process and returning those nutrients to their roots to store for the winter. Some of those nutrients and energy are what that tree is going to use come spring to start the whole process over again.

Now, yellow leaves are pretty straight forward. The yellow colour you are seeing is what is left behind when the more strongly pigmented chlorophyll is gone. As the chlorophyll is broken down and drawn back out during senescence, the yellow colour is unmasked. It was always there, we just couldn’t see it. These pigments are called carotenoids, an antioxidant specifically in place in the leaves to protect them from some of the damaging byproducts of photosynthesis. (For more about that, google ‘oxidation’ and ‘free radicals’ or just leave a note in the comments that you are interested in more SCIENCE in this blog, and I’ll be happy to oblige. For now though, we’re going to try to stay on topic with the science.)

Red leaves are a bit more complex. For the full story on just how complex you can see the original paper on it here: http://treephys.oxfordjournals.org/content/21/1/1.short. That’s just a link to the abstract, but you can get to the whole thing from there. “The physiological significance of anthocyanins during autumnal leaf senescence.” That’s a mouthful, isn’t it? Let’s see if we can break it down a bit and backtrack. A lot of that article is about the selective pressures, explaining WHY some trees tend to be red in the fall when others are yellow. It’s an interesting read, but I’m getting ahead of myself.

The red pigments that you see in autumn foliage are called anthocyanins. While carotenoids are simply left behind during senescence, anthocyanins are actually created by the plant as the chlorophyll is getting broken down. The mystery to scientists of course (which is part of the focus of the above paper), is why a plant would waste precious energy at the time of year where it needed to store every bit of it possible.

As it turns out, anthocyanins protect the leaf from damage from the sun directly. While the chlorophyll is being broken down, the leaves are more susceptible to damage, and would fall sooner, possibly before all of the chlorophyll had been absorbed by the tree. The anthocyanins protect the leaves, buying time and allowing the tree to absorb more of the chlorophyll and other nutrients before leaf death occurs. You might notice that many trees that turn red in the autumn turn a shade of darker burgundy before that brilliant crimson hue. That’s the point where there is enough of a mix of the unbroken down chlorophyll and newly created anthocyanins exist simultaneously in the leaf, creating that intermediate colour.

Trees that create anthocyanins also create carotenoids; the red pigment in the anthocyanins is just stronger than the carotenoids in these species, which is why red is the colour you see. Different species however will make different concentrations, which accounts for the breadth of red and orange hues available in the autumn palette.

There are theories about why some trees create anthocyanins and others do not. Many of those that do not are considered ‘pioneer’ species; species that are the first to colonize open land, and are already evolutionarily more adapted to high sunlight situations. Many of the trees that create them are those that evolved as secondary species, coming in after the colonizers. Many of these species start their lives as understory plants, growing in the shade of the first comers. Though they eventually outgrow their companions and take over a forest, the continued need for some protection from the sun may have led to the development of the anthocyanins. Trees that could tolerate shade in youth and then would be protected by the sun from internal sources at maturity would have naturally outperformed those that could not. This is mostly a concern in areas where fall days are sunny. It’s one of the reasons many native European species (not all, just quite a few) and those in the Pacific Northwest, tend toward the yellow end of the spectrum. In areas where autumns tend more toward the cool and cloudy, there is less biological pressure for those trees that begin life in the understory to protect their dying leaves.

This is all very nice and all. But why was the autumn colour this year so poor?

Many of the trees that normally are yellow in the fall seemed to do just fine. The problems came with our anthocyanin species.

There are many factors that will affect autumn foliage, and these can start as early as spring. Some known factors that will decrease leaf colour in the fall:

-A late spring.
- Summer drought.
-Uncharacteristic high temperatures in summer.

All of the above are likely to stress the tree and/or cause damage to the leaves. Though they do not directly affect the production of anthocyanins, they make the whole thing far more difficult and leave the trees with less resources to work with.

-Longer high temperatures in to autumn than usual, followed by a sudden drop and frost.
-Too warm nights in autumn.
-Not enough sunny days in late summer and autumn.
-Early autumn with cooler than usual days.

These four are situations that will direct impact the formation of anthocyanins. The best conditions for good autumn foliage? Typical (whatever that is for your area) spring and summer conditions, followed by warm, sunny autumn days and crisp, cool (but not freezing) autumn nights. During the warm days, the trees are producing sugars like mad, but the cool nights constrict the veins of the leaves and prevent many of these sugars from being absorbed again. Concentrated sugars and sun are part of what begin the process that spurs the tree to make anthocyanins.

What can we do about it, to bring the topic back around to our bonsai? Unfortunately, there isn’t a lot that we don’t already do. During periods of drought and high temperatures we can water and shade our trees properly, bringing them back out in to more sun again once the end of summer rolls around. The temperature and cloud cover are out of our control largely, and so far as I know, there is no information out there about using grow lamps during cloudy autumns to improve colour (if you try it, document it and document it well).

Here on the East Coast of the USA, our problems were definitely the summer drought and high temperatures of summer(112F days were killer), combined with a very rainy and cloudy late summer. We also kept our unseasonably warm temperatures later than usually, and had an exceedingly short period of properly cool nights with warm days. A lot of factors went in to this year’s autumn being dominated by the yellows and browns. While we can adjust the input to our bonsai far more than the general input to the plants in the landscape, we are still ultimately at the whims of local and global climate.