Can the Decline in the Diversity of Fall Leaf Colors be a Proxy for Climate Change?
In the high latitudes, the effects of climate change are not subtle. A friend of mine who has lived in Valdez Alaska for forty years says that the changes there since he moved to Alaska have been so stark that Valdez doesn’t even seem like the same place that he knew when he first came there. But at the lower latitudes, here in the upper Mississippi Valley at 42-43 degrees, the changes so far are too subtle to positively establish much change, except for one thing: We don’t get much fall leaf color anymore. My daughter, born about 35 years ago, is not quite old enough to remember a good year for fall leaf color.
This was not always the case. My wife and I were married in the fall of 1971 and in that year and every year for the first ten years of our marriage, we would take a half day off work every October, so that we could drive up to a stretch of the Mississippi between Dubuque and Prairie Du Chien to see the glorious leaf show. The first frost would come about Oct 5th and a week later the leaf color would be at its peak. For Iowans in the 1940s, 50s, 60s, and 70s, this was an annual ritual. The car traffic on the roads along the river would be a bumper to bumper gridlock for a hundred miles. And the newspapers and TV stations would compete in giving advice as to the exact day when the leaf color would be at its peak. But few people bother to do this anymore. The last few times we took our leaf trip, we didn’t even take the camera out of the case—there was nothing to see. Sometime after 1980, it all changed.
To have a diverse palate of fall leaf color requires an exact confluence of events. You need a warm enough and moist enough August and September so that when the frost comes, the trees are still fully foliated, and the leaves are all healthy and turgid. Then you need a single, sharp frost. The temperature must drop to about 26 degrees F and stay there for 5 or 6 hours. And then the weather must warm back up to a pleasant and sunny 65 or 70 degrees Fahrenheit, and stay that way for a few days. When all this happens, the chemical changes which occur in the leaves of deciduous hardwoods will show a maximum diversity of colors. But for maximum effect, all these elements must fit together like the parts of a watch. The slightest change in this pattern of inputs will show a dramatic change in outputs; and perhaps this could give us a sensitive instrument of climate measurement--if we knew how to use it.
In the years when leaf color was good, what made it exceptionally good was the fact that this river valley is a zone of extreme diversity in species and sub-species of temperate hardwoods. Some years ago, a study found over 40 kinds of oak in just a two acre plot. And that was just the oak. There were also many different kinds of maple, birch, ash, hickory, elm, walnut, chestnut, poplar, sumac, cottonwood, box elder, and dozens of others. And each species responds slightly differently to frost, yielding a slightly different color. And in the river gorges cut by small streams flowing into the Mississippi, there are steep hillsides; and different elevations respond to frost differently. The tops of the highest trees will be colored slightly differently than the trees down in the valley, even if they are the same species. But we still have this same diversity of species, and the same topography. The only thing that could have changed is the climate.
The photo shown above was included, not to show an example of diverse leaf color, but an example of the lack of it. The picture was taken on Oct 20 of this year, at Backbone State Park. This is one of the places I used to go, 40 years ago, to take pictures of fall leaf color. It’s a pretty little place. But 40 years ago, at that time of year, there would have been a brilliant splash of color—bright crimsons and golds, almost fluorescent chartreuse, and almost every conceivable color except blue, which was amply supplied by the renaissance blue of the October sky reflected in the waters. I still enjoy going there, but as you can see, half the trees have already lost their leaves, and what is left has little color.
I suppose that if we were to use this effect to detect climate change, then it would be required that in the decades of bright leaf color, this color would have to have been documented in some systematic way. Did anyone do this? No; not that I know of. We all took pictures, but not in any systematic way. We easily could have. But how were we to know that a phenomenon which we had always taken for granted was about to disappear forever?
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