Looks cold, huh? (It was around 8 degrees today, and only reached 12 yesterday.) But here's the odd thing -- icicles form when liquid water runs down to a point, and then freezes. (You can see this by the spatters of ice below them, where some of the ice managed to fall before freezing.) So how are these forming, given it's too cold for the snow to melt in the first place?
First, they could be left over -- we had some warmer weather a few days ago. But the roof above these is clear, and our most recent snowfall was after that thaw.
The answer, of course, is that the combination of sunshine on the roof and heat from below is what's melting the ice. In fact, though, despite the cold weather, the sun alone would be enough to raise the temperature on that roof above freezing -- our garage is dripping right now, and it's not heated at all.
So why I am I going on about snow melting on roofs? Well, say you're a little caterpillar in the fall, looking for a place to spend the winter. The difference between a spot where the sun shines part of the day (the south side of a tree, for instance) and one where it doesn't (the north side of the same tree) could well mean the difference between surviving and dying. Even if it doesn't, the extra warmth in the spring could mean an earlier emergence and therefore more time to eat and grow come spring.
Ecologists have a term for this concept, naturally. We talk about microclimates -- a fancy way of saying that temperature, sunlight, humidity, and so on vary considerably across the landscape, down to the smallest scales we care to measure.
Another example is the old canard about finding your way in the woods by looking at the moss, since it only grows on the north side of trees. (Incidentally, this is a great way to get really lost!) The reasoning behind this statement is this: the north side of the tree doesn't get any direct sunlight, so it's going to stay cooler, and therefore water won't evaporate as quickly, and the moss can survive there, but not on the warmer, drier south side. Of course, in many forests, no part of the trunk gets direct sunlight, and in many others, water is plentiful enough that it doesn't matter. But the basic idea actually does make some sense.
Some species require (or at least use) very specific microclimates -- some lichens only grow on the bark of one genus of trees, while some plants will only grown on soil derived from certain types of bedrock. Specifically, in California, many invasive grassland species won't grow on serpentine soils, apparently because of low levels of N and P, although these soils have other unique characters that may play some part. (1) This, of course, means that to see the native California grasslands, it helps to have a map of soil types. Here in Illinois, most of the state's remaining prairie remnants are known as hill prairies, since they grow on steep west-facing slopes -- steep because they couldn't be plowed or effectively grazed, and west-facing because east-facing slopes stay cooler, and trees are able to grow on them.
It sometimes seems like biologists focus on tiny, irrelevant details. But the living world is entirely composed of those little details, and to a little lichen the difference between an oak tree's bark and a spruce tree's bark isn't irrelevant -- it's life or death!
(1) Huenneke, L. F., Hamburg, S. P., Koide, R., Mooney, H. A., & Vitousek, P. M. (1990). Effects of soil resources on plant invasion and community structure in Californian serpentine grassland. Ecology, 71(2), 478-491.
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