We had exceptionally chill temperatures and heavy snow throughout February up and down the East Coast this year. Cold air that normally hangs over the north-west was pushed down to the Plains by the jet-stream, leaving Alaska feeling relatively balmy. As if that affront was not enough for one winter, we were also battered by a nor’easter in early February. Virginians with long memories tell us that not since 1980 can they remember a deeper winter in Williamsburg.
The melt always starts first on roofs and asphalt driveways because the dark colors absorb even the feeblest infra-red rays that penetrate the translucent snow cover. The next place for snow to go is on the compost pile, which shows that our microbial and fungal friends are not slumbering but can still generate a little heat. Snow starts its ground retreat from under bushes and trees and reaches open ground last, where there is plenty of solar radiation. Perhaps the snow that settles on evergreen foliage and boughs rarely falls to the base of the tree but melts in situ, disappearing more quickly on darker colors.
This is a good time for gazing at the bare skeletons of sleeping trees. The verticality of their trunks is more obvious as they snub gravity; their crowns are so marvelously balanced and finished with a tracery of fuzzy twigs. Any gaps caused by wind damage will be filled in the growing season by disproportionate new growth. Under the boughs, there is a litter of small branches and twigs among the fall remains of acorns, walnuts, and maple wing-nuts. The wood looks wasted like unlucky victims of storm damage, but this kind of pruning is so necessary because branches multiplying each year by compound interest would soon become overloaded. Shedding weaker twigs is a picture of natural selection in motion.
When Leonardo da Vinci mused about the shapes and dimensions of trees he recorded a curious fact. Irrespective of height above ground, the cross-sectional area of a branch equals the sum of the same dimensions in the branches it subtends. This rule applies at each transition from trunk to uttermost twig. The standard explanation is plumbing. We might expect to find this correspondence because the living cambium and conducting vessels continue from each branch to its daughters.
According to a recent paper in Physical Review Letters from a UCSD biophysicist there is another explanation. He formulated a mathematical model that closely fits Leonardo’s observations but suggests that this geometry gives the branches the best strain resistance to high winds. Perhaps we don’t have to choose one theory over another but can accept that both may be correct. Biology is neat and good at math.
The clearing patches of snow under trees are good places for wildlife that can’t migrate or hibernate. Small birds and rodents scratch for a living among the brown leaves between the green spears of daffodils while sapsuckers drill neat holes above for licking at the weeping sap. Bird feeders are the avian equivalent of soup kitchens: they can save lives in hard times. But they also offer easy pickings for predators like our pair of red-shouldered hawks unless the prey dashes into cover nearby. Despite the continuing grip of winter, lengthening days make spring brains. As soon as their breakfast is over, cardinals and Carolina wrens burst into song and a crow proudly carries a twig to an untidy matt in an old pine tree.
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