As the beautiful fall foliage of New England spreads across the landscape, you may be wondering a few things. Such as, why don’t all the trees change color at the same time? Or, why do trees in my neighborhood look different than trees in downtown Boston?
There are many possible answers to these questions, although it’s perhaps not as straightforward a matter as one might think. For starters, you may know that certain species change before others. For example, the red trees dotting the landscape of Harvard Forest here
are mostly maples. These trees consistently change color before the oaks that fill the rest of the landscape with their still-green foliage.
So is it a matter of certain species being hard-wired to change color in fall, or leaf out in spring, at different times than other species? To some extent, the answer is yes, as shown in the picture above. However this may not explain all the variation in leaf phenology across landscapes. Temperature is another contributor to the timing of leaf life cycle events. While temperature differences have been shown to contribute to tree phenology differences on large spatial scales, such as across the United States, or through changes in elevation of hundreds of meters, temperature effects due to microclimate variation on smaller scales may also make a significant contribution.
In my research at Harvard Forest, I’ve used a small unmanned aircraft to get aerial photography of the forest landscape and determine the timing of tree phenology in spring and fall. There is visible variation across the forest, as shown in the following picture, where we can see that in late autumn, deciduous trees in the south of the scene still hang on to their leaves, while trees to the north have already lost them.
Using a species map of this area, I conducted analysis that determined that species composition only explained less than half of the spatial variation of leaf phenology in spring and autumn. Now, based on preliminary data, I am investigating the role that microclimate variation in temperature might play. This research may show a new spatial scale at which climate affects phenology. So now you know a possible reason why trees in different parts of your neighborhood, or back yard, don’t all change color at the same time. But what about trees in downtown Boston versus those in outlying areas? I’ll leave that for a future post!