City Creek Nature Notes – Salt Lake City

September 22, 2016

September 21st

Filed under: Astronomy, Box Elder Tree, Cottonwood tree, Maple tree, Plants, River birch, Seasons, Tree — canopus56 @ 2:24 pm

Summer Ends and Fall Begins When?

6:00 p.m. Last day of astronomical summer. It is a dry, cool, wonderful end-of-summer day. Spurred on by the change of seasons, this evening, the lower canyon is full of strolling couples enjoying the annual display of colorful Fall leaves. On August 31st, I concluded that the canyon had peak plant productivity, and on September 13th, I noted that the maximum of the annual turning of leave colors had been reached. Today, almost all of the maples have completely turned, and on a gross level, the annual display is well past its maximum. But there will be another peak of leaves changing color in the future because the date of peak leaf-change is not monolithic.

The time when tree leaves change color varies between different tree types and within the same species, between differing trees based on their location. Soil and water availability varies the times at which the tree’s internal genetics turns the celluar switches on that tell an individual tree to go to sleep for six months. The primary tree of the lower canyon’s woodland is Gambel’s oak, and those oaks are still green and leafy. The cottonwood trees also still have their leaves, and their branches hang heavy with ripe catkins of the cottonwoods’ helicopter-like seeds. Amongst the Gambel’s oaks, most of their leaves show signs of heat stress: the borders of the leaves were turned brown and curled by the summer Sun, but the interior of the leaves are a healthy green. In the first third of a mile, a few individual Gambel’s oaks have turned red-orange. At mile 1.7, a small group has also turned color. Otherwise, these oaks are still green. River birches in the first third of a mile are still a leafy green, but above that point, the leafs of all river birches have turned.

There will be another peak in the changing of the leaves: the one that occurs when the Gambel’s oak and cottonwood trees change color.

Although it is astronomically the last day of summer, when fall starts has, like civil twilight, has a commonplace definition. When Fall starts is commonly determined from when the leaves of most trees have changed color. While jogging here on August 27th, as I passed a strolling couple, the woman said, “I love the canyon during the Fall,” although on that date astronomical fall was more than three weeks away, and on that date most of the leaves has changed color.

The astronomical division of Summer and Fall is not useful local indicator. This is because there is a lag between the date of solstices and equinoxes and the time of maximum and average temperatures that varies by latitude. For example, at the equator, the date of maximum temperature and the date of the summer solstice coincide, but at this Salt Lake City canyon at 40 North latitude, the date of maximum temperatures was in August, about three or four weeks after the date of maximum sunlight on the July 22nd summer solstice. Similarly, the effects of astronomical Fall that begins tomorrow will not be fully felt into mid-October, and the stretch of coldest over-night lows does not happen until early January, a couple of weeks after the winter solstice.

When do we perceive that summer ends and fall begin? Is it when most leaves have changed last week? Is it when the Earth reaches a particular point in its annual orbit at the end of today? Is it when the last predominate tree, like our Gambel’s oaks, change color, in in two more weeks?


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