Reluctant Fall Leaves

Instead of turning their normal yellows and oranges, many of the deciduous trees in Alberta were caught in a freeze and died, turning the leaves brown.

Instead of turning their normal yellows and oranges, the leaves of many of the deciduous trees in Alberta were caught in a freeze and died, turning them brown.

It has been a strange autumn here in central Alberta. The leaves that normally turn colors and fall to the ground have been reluctant to do so. Now, Alberta is not especially noted for its fall colors but we do have them, mostly yellows and oranges with some red and purple thrown in. Of course, these come from our deciduous trees, mostly balsam (Populus balsamifera) and aspen poplar (Populus tremuloides); but also birch (Betula spp.), as well as shrubs such as willow (Salix spp.) and alder (Alnus spp.) among many others.

Because we are in the northern half of the northern hemisphere, we have short growing seasons, and normally trees start to turn to their fall colors beginning in late August to perhaps mid-September (central Alberta). The turning of the leaves is when the trees shut down the physiological processes that turn sunlight energy into food energy. This process, called photosynthesis, is accomplished via the green compound, chlorophyll, which dominates the color of most leaves during the growing season. Unlike most coniferous trees (spruce, pine) that maintain their green color and photosynthetic properties year around, deciduous trees shut these process down during the cold and short days of winter. The leaves (needles) on coniferous trees are able to protect themselves from freezing using an anti-freeze-like chemical, those of deciduous trees are not. So, the chlorophyll is allowed to degrade, revealing the other colors in the leaves before the leaves are dropped. But why are our deciduous trees so late in turning color this year?

I put that question to Dr. Janice Cooke of the University of Alberta’s Department of Biological Sciences. She reminded me that the change of color in trees is determined by day length and temperature. In other words, as the day length shortens, the trees begin to prepare to abandon their leaves. However, if the weather continues to be warm, the abandonment processes are suspended, which if you think about it, is a good strategy. Why shut down making food when you are not under the threat of freezing? Of course, temperatures do eventually fall and the colors change and the leaves drop. However, this year the delay in the drop is the longest in the memory of most people.

As described by Dr. Cooke, “We had a particularly warm September, right up to about Sept. 27. The warm days, but especially the warm overnight temperatures of mid-September, caused a delay in senescence (aging) and leaf color changing. Mainly, the trees were continuing to photosynthesize, and so they needed their chlorophyll. As such, the chlorophyll was not degraded, thus not revealing the colors of the other pigments.”

“Unfortunately for the deciduous trees,” she continued, “we had a wild swing in temperatures in a short time span.” In a nutshell, daytime temperatures in the 30s (C, 80s F) in late September suddenly dropped to -10°C and below on October 1. “This rapid change in conditions did not allow the plants much time to begin the autumn senescence process or even begin the cold acclimation process correctly. As a result, the hard freeze of 
-10°C and colder that we experienced in the first couple of weeks of October actually froze many of the leaves before they had assumed their fall colors.”

The fallen leaves include many green ones taking with them nutrients the trees need next spring.As a result, we have a situation where some of the trees have turned color and started dropping leaves, while others remained green until the freeze. Since that freeze, the leaves have thawed in more moderate weather and instead of turning yellow or orange, have turned brown and the autumn forest looks a lot duller than we are used to. Unfortunately for these trees, they were not able to recycle the nutrients in the leaves that they normally do during the color change process. Thus, in the coming spring, these trees will have less nutrients to begin their spring leaf production and other life processes, and could become quite stressed. To compound the situation according to Dr. Cooke, the drought that we have been experiencing has caused the trees to produce and store a lot less energy over the summer than they normally do.

The bottom line is that we have a lot of stressed trees overwintering this year and there could be some significant changes in the forest come spring.

So, what’s wrong with that?


About Don Meredith

I am a writer and biologist living in Alberta, Canada. I wrote a monthly column for the Alberta Outdoorsmen magazine, and have published articles for several other magazines.
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