By the time this Nature Nut goes to press, a lot of leaves will have dropped from trees in Rochester over the past month. Some of the early ones to lose leaves might have been from weak trees, but for the rest, it is a natural process that unfolds differently for different species.
I am not sure if it was because I was more observant or it was an unusual year, but I seemed to notice a lot more about the leaf "falls" then I normally do. I recall biking through a mat of all yellow maple(?) leaves covering a yard and the adjacent sidewalk on along 14th Street Northeast near Silver Lake.
A few days later, while walking with Elise and her two Chihuahuas in southwest Rochester, I recall a similar mat of maple leaves that were all red. In both cases, there were still leaves yet to fall before completely denuding the trees, something that often takes a week or more for many species, including maples. But, on the same walk, I noted a gingko tree that was full of leaves just a day earlier had apparently almost simultaneously dropped all its leaves.
I think I could remember back to some previous educational experience where I learned most trees that lose all their leaves in winter do not just simply let them fall, but instead spend days, or a few weeks, growing a kind of ‘scab’ where the leaf attaches to the stem. Once the scab grows enough cells, they push the leaves off their stems, essentially severing the connection of leaf to stem. For gingkos, the scabs are formed simultaneously and wait for a hard frost to all drop off at the same time, sometimes a golden yellow and sometimes still green.
This process that initiates the release of leaves from deciduous trees is triggered by hormones that sense colder temps and less daylight. These trees have broad leaves to maximize photosynthesis during warm months, which also requires large amounts of water to be transported from soil roots to the leaves. But with freezing temperatures, these very thin leaves would freeze and be rendered useless, and fall anyway. So, the tree holds water only in its roots and protected woody stems for the winter, relying on the energy produced and stored during summer to keep it alive.
Which brings us to the other kinds of trees, the evergreens, most which don’t appear to ever drop their leaves, that we usually call needles, even though they perform the same function as deciduous leaves. Also called conifer, for the cones they bear, they supposedly evolved a quarter billion years ago when the Earth began cooling. Related to conifers, many, like pines, do drop a fraction of their needles each year, while some, like tamaracks, drop all their needles each fall.
Evergreen leaves are mostly skinny needles that have a thick protective cuticle layer that protects them and keeps water in. Staying green all winter and moving up at least some water, evergreens can still carry out photosynthesis. It still puzzles me as to how they survive winter with any amount of water in their needles when subjected to sub-zero temps, or covered with ice and snow.
Observing a bit more closely just before submitting this column I realized the many varieties of deciduous trees found in Rochester are all over the board related to when they drop their leaves, with some trees like the weeping willows hanging over the North shore of Silver Lake still full of green leaves, while many other varieties are totally barren, and some in between.
I for one will try to keep enjoying the trees and leaves with any color, whether on branches, or the ground, as the next chapter is all white, except for the tough conifers.