Right now, the northern hemisphere is in the middle of one of the best nature shows of the year: the annual leaf-changing extravaganza that sends off the warm days in a blaze of color, before the trees drop their foliage to prepare for winter. People drive for miles to see the most spectacular forests, even though many of us have it pretty good right in our own neighborhoods. But the climate is changing, and it's likely to change even more, triggering wild responses from carefully calibrated ecological systems. Will autumn change too? How?
The short answers are one, yes; and two, science isn't sure. Researchers know a lot about how climate change may affect spring, but the natural processes of autumn are harder to tease out. Some things are easier to find out than others. For example, modern and historical naturalists like poet and essayist Henry David Thoreau have often kept careful records of their observations, and scientists can compare sometimes up to hundreds of years of data about a place to see how autumn has changed. Thanks to these records, we know that leaf change starts later, on average, than it used to.
The oranges, yellows, and flaming reds of fall happen when trees pull their green chlorophyll out of their leaves for winter storage, revealing other pigments that were previously masked. Trees start changing in response to a combination of day length and temperature, and they can adjust when they do it. If the temperature stays warm later than usual, trees can delay their fall transition. Science doesn't understand exactly how these two factors, and possibly others, work together to trigger trees to start changing, so it's hard to say what will happen in the future. This inherent flexibility may allow some trees to better withstand the stresses of climate change, but other species may have more rigid requirements. As day length and fall temperatures get further out of sync, how will trees respond? Will they drop their leaves when the days get too short, or will they wait until the temperature gets too cold?
Another unknown is the fate of tree species with limited ranges. Climate change will likely raise annual average temperatures, and some species may need to move further north in order to stay comfortable in their climatic niches. Birds and other animals will do this more easily than plants, which often depend on animal agents to disperse their seeds, but wildlife of all kinds will be restricted by the scarcity of undeveloped land. Conservationists are exploring the idea of human-assisted migration to help mitigate range-shrinkage, but migration will only work if there are wild places (preferably long, wide stretches of connected habitat) for wildlife to relocate to. This means that some tree species may become much rarer as their historic ranges become inhospitable, leading to potentially less vibrant fall shows as certain species drop out. Lower species diversity means fewer colors and a shorter season as a small handful of hardy species start to dominate forests.
The most frustrating thing about climate change is how little is being done to address it. The second most frustrating thing about climate change is how little we can predict about what it will do to the world around us. In a climate-changed future, autumn is likely to be later, maybe shorter, possibly duller, and probably less spectacular as we lose tree species diversity. Some species will survive, and possibly thrive, but we could end up with a monochrome week or two instead of the months'-long parade of colors we love. Scientists only recently started addressing the gap in understanding how climate change may affect our forests in autumn. Unfortunately, by the time they're able to explain it to us, we might be seeing the evidence with our own eyes.