Photo by USFS. Public Domain
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From my “sit spot” at my writing desk in my old house in Pennsylvania, I could look out the window and study a young, spindly, black oak, pole tree. This tree and a number of other black, scarlet and white oaks scattered around my yard had sprung up after a 2006 thunderstorm obliteration of my yard’s large, shading blue spruces. The black oak had grown rapidly after the demise of the spruces and was, in 2020, 20 to 25 feet tall and starting to fill out. One very notable thing about this tree was that it kept a large percentage of its leaves all winter. They trembled and rattled in the passing cold winds and didn’t fall off of the tree en masse until its spring leaf buds began to open. This persistence of leaves on a deciduous tree through the winter is a phenomenon called “marcescence.”
In most tree species, the cells in between the leaf petiole and its woody stem (the “abscission layer”) secrete enzymes in the autumn that break the attachment of the leaf to its branch. A sudden, early frost or freeze may reduce the production or action of these abscission enzymes preventing the release of the leaves from their tree. In years of early frosts, then, a large number of trees may be marcescent.
There are, though, some species of trees in which these abscission enzymes are not immediately active at all. These include many types of oak, witch hazel, hornbeam and hophornbeam and American beech. These trees, then, especially their smaller/younger individuals, keep their leaves throughout the winter.
Photo by S. Shebs, Wikimedia Commons
What is the benefit of marcescence? The attached, rattling leaves may discourage deer browsing. It is also possible that the dry leaves themselves may be unpalatable because of their high lignin content or their sharp, pointy edges. The persistence of these “browsing barriers” on the tree branches, then, may discourage winter-browsing by deer, elk or moose. Also, in a moisture-stressed environment the lingering leaves may accumulate snow which then falls and melts directly under the marcescent tree thus adding precious moisture to the tree’s immediate soil volume.
Marcescence may also give the leaves a winter period of photodegradation that makes their eventual spring and summer decomposition more efficient. It may also keep the leaves from being decomposed during the winter (the non-growth period for the tree) and thus make the leaf nutrients more directly available to the growing tree in the spring and summer.
Based on these possible models, then, one might predict that trees in arid environments (especially ones in which winter snow is a major source of its annual precipitation) and that are also under heavy browsing pressures would be likely to exhibit marcescence.
Which gets us to Greeley, Colorado!
Greeley, as I have written before (Signs of Fall 3, October 8, 2020), is an island of trees out on the dry plains of Northern Colorado. My neighborhood, like most of the neighborhoods in Greeley, is rich with trees! There are Ponderosa pines, cottonwoods, honey locusts, quaking aspens, blue spruces, junipers and red cedars. There are also three types of maples: two are non-native species (Norway maple and Japanese maple) and one is native (Rocky Mountain maple).
Photo by D. Sillman
From the window next to my new writing desk (my new, Colorado “sit spot”) I can see pines, spruces, cedars, junipers, locusts, aspens and cottonwoods scattered about in my yard and the yards of my nearby neighbors. I can also see two Rocky Mountain maples. The Rocky Mountain maples really stand out here in the mid-winter landscape because they are the only nearby deciduous trees that are marcescent!
Rocky Mountain maples (Acer glabrum) are a native tree/shrub of western North America. As its common name indicates, it is primarily found in mountainous regions. Its geographic range extends from New Mexico and Arizona north through Colorado, Utah, Wyoming, Montana and Idaho, across the Canadian border into Alberta and British Columbia all the way up to southeastern Alaska. Many of this tree’s alternative common names reflect its broad, geographic range (“New Mexico maple,” “California mountain maple,” “Sierra maple”) and also its predilection for mountainous habitats and its frequent association with Douglas fir trees (“Mountain maple,” “rock maple,” “Douglas maple”).
Photo by Xomenka, Wikimedia Commons
Its appearance is quite variable depending on site features, soil fertility, moisture, crowding, amount of available sunlight and, possibly, genetics. It may form dense lines or patches of many-stemmed shrubs that are 4 to 6 feet tall (reflecting another alternative common name, the “dwarf maple”), or it may grow into single-trunked trees that reach 30 to 33 feet in height.
Rocky Mountain maples are often densely planted on mountain slopes and hillsides to help stabilize the highly erodible soil. This type of planting generates a shrub-patch growth form which can be a very favorable habitat for many species of birds and small mammals. This maple is also a tree that grows well in urban and sub-urban areas (hence its occurrence right here in my neighborhood in Greeley!).
Photo by S. Siegmund, Wikimedia Commons
The above ground portions of the Rocky Mountain maple both in its shrub and in its tree growth forms are quite susceptible to wildfires. Burned areas, though, often rebound into dense shrub-patches of Rocky Mountain maple because of the tree’s ability to vigorously root-sprout.
The branches and leaves of the Rocky Mountain maple are important browse for mule deer, white-tailed deer, moose and elk especially in the winter (although it is browsed to a lesser degree throughout the year). Possibly this susceptibility to browsing is a reason for the species’ observed tendency for marcescence.
The Rocky Mountain maple is a long-lived, shade tolerant species that frequently forms the understory layer beneath stands of Douglas fir and several other fir and spruce species. It is dioecious (in other words, individual trees have exclusively either male or female flowers). The tree flowers in the spring simultaneously with the emergence of its new leaves.
Photo by S. Siegmund, Wikimedia Commons
This maple is an abundant producer of seeds (although not every year is a bumper seed crop year), and its samaras are released to the wind in the late summer or early fall. I could find no record of the use of these seeds as food by birds or small mammals, but assume that they make up a part of the autumn diet of many animal species. The seeds require six months of cold before dormancy is released, and they are viable for only a single season. Only a small percentage of the seeds germinate, though, and germination and seedling growth most often occurs under partial shade.
Photo by ninjatacoshell, Wikimedia Commons
Right now my neighborhood Rocky Mountain maples are covered with brown, tightly curled up leaves. I have one of these leaves sitting next to my computer. It is thick and rigid and prominently veined with a long, robust petiole. In the Fall, though, the leaves of this maple were bright red and very distinctive!
I have many ideas for experiments on these leaves and these trees! How palatable or unpalatable are the leaves to vertebrates or invertebrates? How quickly do fall abscised leaves decompose as opposed to spring abscised leaves? Do the Rocky Mountain maples growing up in the mountains display marcescence? Do the trees that display marcescence grow taller or faster? Do Rocky Mountain maples that display marcescence tend to produce trees that also display marcescence?
Sounds like it’s time for a field trip!