Glenmere Park, Greeley, Colorado. Photo by MKM Kennedy, Pixabay
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Although Greeley, Colorado is located in the high, dry, sagebrush plains of Northern Colorado, it has earned the designation of a “Tree City USA” from the Arbor Day Foundation for each of the past forty years. Within the city limits of Greeley over 100,000 trees are cared for by the city’s Department of Forestry. The current director of the city’s forestry department estimates that there are “several hundred” species of trees growing in the city. Glenmere Park over near the university alone has over 100 species of trees and many of these have arboretum labels. On the campus of Northern Colorado University there are either 106 or 110 different tree species (both figures are listed in the same publication). These trees are labeled and organized on a set of maps that are presented in an on-line pamphlet and self-guided walking tour.
So, although many of the familiar trees of Pennsylvania are not well represented here in Greeley, there are, though, quite a few trees to look at and enjoy!
As winter approaches both here in the high plains and also back in Pennsylvania, the leaves for all deciduous trees are shed primarily to help the trees to withstand the incredibly cold and dry conditions of the coming season. There are very well defined stages by which this leaf loss occurs:
First, in response to the length of the dark period of the day reaching a critical length, large numbers of cells are generated at the junction of the leaf’s stem to its branch. These cells greatly increase in number but not, at first, in their individual sizes. This layer of cells (the “abscission layer”) slowly starts to interfere with the flow of sugars out of the leaf and the flow of nutrients into the leaf.
Then, the lack of nutrients causes the leaf to stop synthesizing new chlorophyll molecules. Chlorophylls are, of course, the functional pigments of photosynthesis and also the pigments that give plant leaves their characteristic green colors. Initial cessation of chlorophyll production makes the leaves appear a bit paler and less intensely green than they were during the height of summer. Continued loss of the chlorophylls, then, starts to unmask the other pigments (the “accessory” pigments of photosynthesis: the carotenoids and xanthophylls) that have been present in the leaves all summer long. As these pigments are “revealed” the leaves then “turn” orange (from the carotenoids) or yellow (from the xanthophylls) before they finally fall.
The persisting chlorophylls in the leaves continue to carry out photosynthesis. The sugars formed by this ongoing process, though, are not able to easily exit the leaf because of the abscission layer blockage. These sugars then stimulate the synthesis of anthocyanin pigments in the leaf. These pigments generate purple or bright red colors and are thought to possibly protect the leaf (and particularly next year’s delicate leaf buds) from insect damage.
Cottonwood tree in autumn. Photo by Famartin, Wikimedia Commons
Cottonwoods are one of the few tree species that grew in northern Colorado before the arrival of European settlers. They are large trees with massive trunks, thick branches and rough, deeply furrowed bark. Cottonwoods naturally grow along rivers and creeks and in low areas that retain moisture. They have adapted well, though, to the human controlled moisture delivery systems of towns and cities and are the largest trees growing in my immediate neighborhood.
The cottonwoods started to show leaf color change several weeks ago. The leaves at the ends of the crown branches started to turn a golden yellow first individually and then in larger and larger groups. Most of cottonwoods today have an outer rim of gold leaves around a central core of still deep green leaves, and a few of the smaller cottonwoods have crown foliage that is entirely yellow. Few of these leaves, though, have fallen in spite of the strong winds that have been an almost daily occurrence. I assume in the larger trees that the color change will work its way into the crown mass and then, slowly, the trees will lose their fight with the prevailing winds and let their leaves drop and blow away.
Honeylocust tree in Fall. Photo by Kevmin, Wikimedia Commons
Honey locust trees are also very tall trees in the yards around my neighborhood. Their trunks are much more slender than the cottonwoods and are covered with a scaly bark marked by light and dark longitudinal ridges. The delicate leaflets of the locust leaves have just started to change from a shiny green to a crisp, pale yellow. The pattern of the color change is extremely random. One or two clusters of leaflets on several, scattered branches have turned yellow with their immediately adjacent leaflets retaining their green color. Leaflets, though, have been regularly falling from the locusts ever since we arrived here two months ago. Green and yellow leaflets accumulate on the ground under the locusts and then seem to almost disappear as they dry out and work their way into the grass and thatch of the lawn.
I expect that there will be a sudden change in the honey locusts when the very cold, pre-winter nights hit us in another couple of weeks. The turning of the leaves and their fall en-mass will be a major sign of the changing season.
Quaking aspens in the Fall. Photo by Famartin, Wikimedia Commons
Quaking aspens are not terribly abundant around my neighborhood, but their pale, smooth bark and graceful, often leaning trunks stand out from the other types of trees and shrubs that surround them. Even light breezes can send the rounded, drip-tipped leaves of the aspen into their eponymous, shaking tremors. Over the past few weeks the quaking aspen leaves have become a very pale green and seem to be on the verge of sudden, whole tree change. Last year my daughter, Marian, and I sat out on her front porch with her then newborn son, Ari (now a robust one-year old!) and watched an aspen tree across the street from her house change quite suddenly from green to gold. The next day, that aspen tree dropped all of its leaves in a gust of wind! Autumn is a short season here in Northern Colorado!
The aspens are the major Fall draw for leaf color aficionados to the foothills of the Rockies. News reports, in fact, describe that this weekend (the last weekend of September) as the peak of aspen color up in the nearby mountains. If we want to see the aspens in their fall glory we had better get out today. There is a cold front with wind and much needed rainfall coming in tomorrow! Many of the trees will be bare by week’s end.
“Painted” maple. Photo by B.R.Marshall. Geograph
In my neighborhood there are also Norway maples (an unfortunate alien invasive species) whose leaves are turning yellow, and Rocky Mountain maples (a native species) whose leaves are turning red. There are also painted maples (a hearty version of the Japanese maple (also an alien species)) that have the very interesting pattern of turning their outer layer of crown leaves red and the inner layer of crown leaves yellow.
So we are surrounded by a combination of native and alien tree species, and here at the end of September they all seem to be just starting to show their fall colors. There does not seem to be a sudden color convergence in these trees, though. The change is scattered and proceeds at very individualized paces. I remember feeling impatient back in Pennsylvania for the fall change to begin and show itself. The sugar maples and red maples up and down the hillsides near my old home seemed to all turn gold and red in one great wave. I am not sure why, though, I am always so eager to see these signs of the senescence and the end of the growth and life of summer. I guess that I want to get to winter to get it over with so that we can get back to the re-birth of spring.