Saturday, November 3, 2012

Happy Fall! Does it seem late?

I don't know about you, but this week I'm ready to think about anything other than the election...or hurricanes...So how about celebrating fall? Ah, raking leaves, the crisp air, and cider donuts.

Yet, does it seem like fall is (forgive me) falling later than it used to? My wife, who grew up in upstate NY, recalls the chore of doing the final leaf-rake before she could go Trick-or-Treating for Halloween. Right now, though,my trees in Niskayuna are at most only half-way through dropping their leaves. Is it our imagination, or is fall getting later?

Yes, fall is really getting later. There are two explanations: Climate change, and a shift toward new species.

First, climate change. Our warmer climate - already at least 0.5 degrees C warmer than early in the 20th Century - is causing plants and animals to adjust the timing of major annual events such as migration, emergence, or senescence. That means that our springs are starting earlier and our falls are ending later. Since 1970, New York's average temperature has warmed by 2 degrees C warmer; our winters are 5 degrees C warmer!

Japanese barberry - still green while the rest of the forest has dropped its leaves
That's not the only explanation for more green left in our forests in November. The other reason is that our forests are changing. Invasive species like Japanese barberry, autumn olive, or honeysuckle are now common in the forests of the northeast. A study published in Nature by Syracuse University's Jason Fridley has demonstrated that such invasive species have extended their growing season by four weeks in the fall! This extra time to photosynthesize and acquire resources may be a key element to their success compared to our native flora.

What does this mean for our forest? Well, many things.

First, some invasive plants such as honeysuckle may actually be crucial food sources for native birds. If we could wave a magic "remove invasive species" wand, it would likely have many unintended consequences unless we could simultaneously address many other ways that forest landscapes have been altered. Furthermore, scientists and habitat managers are becoming more and more aware that even invaded habitats may provide essential "ecosystem services" that benefit humans such as maintaining biodiversity, filtering pollutants in waterways, and storing carbon.

Yet, these later falls serve as one more reminder that nothing that happens in nature is free from the ever-present hand of human activities. No longer can we think of large tracts of land - even those as large as the Adirondacks or the Amazon Basin - as wilderness. We have changed the climate, we have changed the species composition, and we have changed the physical landscape. Our impacts now transcend every spatial or temporal scale we could think of.

Geologists have proposed a term for this new human-dominated Earth - the anthropocene. If future intelligent life-forms dig through fossilized deposits that include the present period, they will see the unmistakable effects of humans on biodiversity, on global atmospheric chemistry, and on land use across the globe.

Far from making me feel powerful, that makes me feel guilty.

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