Friday, September 28, 2007

Fall is here

Nature Watch
September 28, 2007

By Susan Benson
Director of Education
Cable Natural History Museum

After the glory of autumn color comes the inevitable bare-limbed look of our forests as leaves fall from hardwood trees. Ever wonder what triggers those leaves to fall?

It’s due to a chemical called auxin, a growth-regulating substance produced by plants. When leaves develop in the spring, each new leaf makes large amounts of auxin, which among other things, signals the leaf stem to grip its branch tightly. As the leaves age, they produce less auxin. As summer wears on, two thin layers of cells called the abscission layer grow across the base of the leaf stem where it is connected to the branch; this is likely initiated by the decreasing daily light period.

As the supply of auxin dwindles, the cells in the abscission layer separate from one another, and the leaf stem’s hold on the branch grows weaker. A hard rain or a gust of wind then blows the leaf free and it falls to the ground.

Even though we’ve had frost, there is still some harvest to be had among gardens and orchards. October is harvest time for animals as well as humans; many animals now are very active storing and hoarding food. From chickadees to voles to red squirrels to beaver, an animal’s success at finding and caching food will likely mean the difference between starvation and survival in the upcoming winter.

Storing up food is a full-time job. A pair of adult beavers typically stockpiles half a ton of tree branches to provide for their families through the winter. Red squirrels have been known to stash 15,000 cones along with hundreds of mushrooms in tree cavities and holes to tide them over.

Black bears also are quite active now, packing in the calories in preparation for upcoming hibernation. During the fall, bears are in a food frenzy—they eat constantly and may gain up to a third of their body weight in this season. In the fall, a hungry black bear might eat more than 20,000 calories per day. Compare that to the average human’s daily intake of 2,000-2,500 calories!

Throughout most of their range, black bears crawl into their winter dens between mid-October and mid-November. Individual bears choose denning times depending on food availability in the fall as well as their own physical condition. Female black bears, with or without cubs, tend to den up before males.

Become a phenologist! The cooler weather and occasional frost this time of year mean biting insects are not much of a bother—it’s an ideal time to hike or explore in the woods, and notice how plants and animals are preparing for the coming winter.

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at to learn more about exhibits and programs.

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