Wednesday, December 23, 2020

How To Identify Deciduous Trees and Shrubs in Winter

Even after deciduous trees and shrubs shed their leaves, most can be easily identified using their winter characteristics. Buds, bark and overwintering fruits are reliable clues to identification. With a little practice, they're as useful as leaves to pin down a plant's name.

Look at Twigs

If they’re within easy reach, twigs have many features that are helpful for winter identification. The size, shape and arrangement of buds and leaf scars are reliable clues. Using green ash (Fraxinus pennsylvanica) as an example, here's what to look for. 














Depending on the species, buds may be covered with no scales, one scale, or several scales. Bud shape may be long and narrow, short and wide, or somewhere in between. Leaf scars and vascular bundle scars may be conspicuous or so small they are difficult to see without magnification. In any case, the combination of characteristics is unique for a species, as shown in the photographs below. 


Box Elder (Acer negundo) twigs are maroon in winter, often with a blue-gray, waxy coating. Surfaces with that coating are said be glaucous. The twigs turn green in spring but may still be glaucous.

Buds scales are covered with short gray hairs that are easiest to see with a magnifying lens. There are two buds per node, so Box Elder has opposite buds. If the twigs and buds are out of reach, look at the branching pattern. That, too, will be opposite.

Leaf scars are narrowly V-shaped. The opposing V’s meet on each side of the twig, forming a point.

Inside each leaf scar are three vascular bundle scars (arrows, left). One is directly below the bud and two are at the tips of the V, one on each side. 



The ends of Black Walnut (Juglans nigra) twigs are covered with short, gray or white hairs. Buds are similarly hairy, and they are longer than wide. Lateral buds are alternate – one per node.

Beneath each lateral bud is a heart-shaped or V-shaped leaf scar. Vascular bundle scars are dark brown. One semicircular bundle scar is at the bottom of the heart and two smaller bundle scars are at the top. The pattern reminds some of a face, with two eyes above and a mouth below.

The buds and leaf scars of butternut, Juglans cinerea, are similar, but the leaf scars are straight across the top instead of notched. 




Red Elderberry (Sambucus racemosa) doesn’t have twigs, but its coarse stems and branches are just as distinctive.

The lateral buds are opposite and covered with green, brown or red scales. Flower buds tend to be larger – almost as wide as they are long.

Below the buds are triangular or shield-shaped leaf scars with five large vascular bundle scars.

The stems and branches of Red Elderberry also have conspicuous “warts” on their bark. These raised areas are lenticels, eruptions of the bark that allow gas exchange with interior tissues. Many species have lenticels on their twigs or young stems, but few are as conspicuous as Red Elderberry. 



Look Closely at Bud Arrangement

Knowing the bud arrangement – opposite, subopposite, alternate or whorled – is especially helpful because it quickly narrows the choices for identification. If the twigs and buds are out of reach, look for the branching pattern. It will have the same arrangement as the buds. Be sure to look at several branches and twigs; there may be more than one kind of arrangement. Although that can be confusing, it’s useful information.

With the bud or branch arrangement determined, the next step is to find or recall a list of species that belong in that group. To remember species with an opposite arrangement, a helpful mnemonic is “MAD Cap Buck Horse.”

  • M is for maples, genus Acer.
  • A is for ashes, genus Fraxinus.
  • D is for dogwoods, genus Cornus.
  • Cap is for the family Caprifoliaceae, which includes honeysuckle shrubs, genera Lonicera and Diervilla.
  • Buck and Horse are for Buckeyes and Horsechestnuts, respectively. Both are trees in the genus Aesculus

Other trees and shrubs also have an opposite arrangement. To the list above add Viburnums (Viburnum species), Elderberries (Sambucus species), burning bushes (Euonymus species) and buffaloberries (Shepherdia species).  Amur Corktree (Phellodendron amurense), an invasive tree, also has opposite buds and branches. There are several more genera in this group. A thorough guidebook will point them out.

If the plant doesn’t have opposite buds, then there are three other categories to choose from. Most of the remaining plants will have an alternate arrangement. Woody plants with a whorled arrangement are unusual. Catalpa (Catalpa speciosa), an introduced tree, has three buds and leaf scars per node. Native Bog Laurel (Kalmia polifolia), a low shrub of bogs, can have either an opposite or a whorled arrangement. Subopposite buds are unusual, too. Common Buckthorn (Rhamnus cathartica) has buds that can be subopposite, opposite or alternate.

Look at Bark

The bark of many trees and shrubs is distinctive enough to identify the species. The tricky thing is that bark changes with age. It often starts out smooth but develops more texture as the tree or shrub ages. Despite that change, the appearance of bark is a useful characteristic. The following questions highlight some of the features to look for.  

  •  Is the mature bark ridged and furrowed? If so, how deep are the furrows? Do the ridges and furrows form any kind of pattern?
  • Instead of ridges, does the mature bark have flat-topped plates, shaggy strips, or scales?
  • Are lenticels present on younger bark? If so, are they round, linear, or both? If they are linear, what direction do they run?
  • What color is the inner bark, the layer just below the surface? The brightly colored inner bark of some species provides a clue to their identification.

As with twigs, bark shows a lot of variety, both within and among species. The photographs below hint at some of that diversity.

Green Ash bark is brown to red when young, with white lenticels. Older bark is ridged and furrowed in a tight, honeycomb or diamond-shaped pattern.

The young stem on the far left is about 1 inch in diameter. The trunk of the mature tree is about 14 inches in diameter.




Young stems of Common Buckthorn (Rhamnus cathartica) have smooth, gray or brown bark with prominent, horizontal lenticels. Older bark is dark brown, rough and flaky. The photo at far left shows a clump of buckthorn stems at various ages. The largest one is about 3 inches in diameter.

One quick way to identify Common Buckthorn is by its bright orange inner bark. 



Black Cherry (Prunus serotina) has bark that changes drastically with age. On young trees, the bark is red-brown and smooth with horizontal white lenticels. The trunk at far left is about 4 inches in diameter.

In contrast, the bark of mature Black Cherry trees is scaly. Some say it looks like burned potato chips. The trunk at left is about 15 inches in diameter.




Look for Fruits

The fruits of most woody plants are shed before winter, but those of a few species tend to hang on. If they are present, their size, shape, color and texture (dry vs. juicy) and their placement on branches are important to note. As shown below, fruits can be easy and important characteristics for identification.

Bittersweet vines (Celastrus species) are easiest to identify by their persistent winter fruits. Invasive Oriental Bittersweet (C. orbiculatus), far left, has yellow capsules that open to expose fleshy orange arils. The clusters are found along the length of the stems.  In contrast, American Bittersweet (C. scandens), left, has orange capsules and red arils. Its clusters are found only at the ends of the stems.


Winged fruits are called samaras. Some samaras are in pairs, as in Amur Maple (Acer ginnala), far left. Other maples also have paired samaras.

Some samaras are single, as in Green Ash (Fraxinus pennsylvanica), left. Other ash species are similar. Some call the samaras keys, because a cluster of them resembles a bunch of keys on a ring.


Berries and berry-like fruits are typically juicy and either spherical or oblong.  Common Buckthorn (Rhamnus cathartica) has dark blue, almost black, berry-like fruits clustered at the nodes (far left). Inside each are 3-4 hard, seed-like stones (arrows, left). This type of fruit – juicy or fleshy with one or a few stones – is called a drupe. Berries are similar, but they lack stones and are juicy throughout.


Look for a Guidebook

Using a combination of buds, leaf scars, bark and fruits, anyone can identify trees, shrubs and woody vines in winter. All that’s needed is a guidebook, a reference that matches observed characteristics to names of species. Guides dedicated to winter identification are few, but there are some resources online and in bookstores that may be helpful.

  • The LEAF Program from UW-Stevens Point is a K-12 forestry education initiative that offers many online resources. Under the link for Curriculum & Resources, choose LEAF Tree Identification Tools. The LEAF Winter Tree ID Key is available there as a downloadable PDF.
  • Winter Botany: An identification guide to native trees and shrubs, by William Trelease. Dover, 1967. ISBN 0-486-21800-7.
  • Fruit Key and Twig Key to Trees and Shrubs, by William M. Harlow. Dover, 1959. ISBN 0-486-20511-8.
  • Winter Tree Finder: A Manual for Identifying Deciduous Trees in Winter, by May T. Watts and Tom Watts. Nature Study Guild Publishers, 1970.

Reviews of these references or suggestions for others are welcome. Please use the contact form at right to offer comments. Upcoming posts will feature winter identification of select groups of plants. 




Thursday, December 3, 2020

Sunset for Kentucky Coffee Tree?

Its numbers are declining in nature. A look into its distant past could explain why.

 

A female Kentucky coffee tree, Gymnocladus dioicus, in November 2020.

Kentucky coffee tree (Gymnocladus dioicus) cuts a striking silhouette in the landscape. Its stout branches, plated bark, and chunky, persistent pods stand out, especially in winter. Nothing else looks like this.

And nothing else has quite its combination of puzzling traits. Its range today is strangely limited, its fruits should attract herbivores but don’t, and natural distribution of its seeds, heavy and nonbuoyant, is mostly by water. Ecologically, the tree doesn’t make sense. That wasn’t always so.

A Glimpse Into the Past


Kentucky coffee tree is thought to have arrived here sometime during the Miocene, an epoch of geologic time that extended from 23 million to 5 million years ago (Zaya & Howe, 2009, citing Tiffney & Manchester, 2001). Grasslands and savannas were widespread then, and so were many herbivores. In fact, North America then is thought to have resembled the African savanna today, at least in terms of the diversity of animals and the structure of their communities (MacFadden, 2000, citing Webb, 1997, 1983).  

Important to the story of Kentucky coffee tree, many of those herbivores were huge. They were the megafauna, and they included North American rhinos, camels and elephant-like animals called gomphotheres (MacFadden, 2000; Zaya & Howe, 2009, citing Webb, 1983, and Janis et al. 2004).  Long after the Miocene, land bridges brought additional large mammals to North America, including giant sloths and armadillos from South America and mammoths and bison from Eurasia (MacFadden, 2000).

The exact diets of the megafauna aren’t known, but it’s speculated that they included the large fruits of plants like Kentucky coffee tree. The big animals would have been tall enough to reach the tough pods and strong enough to open them, enticed, perhaps, by the sweet green pulp inside. The extremely hard seeds could have withstood their forceful bites and been passed through their digestive systems intact, arriving, finally, in a pile of dung, there to begin another generation (Zaya & Howe, 2009).

Hints of a Different Life

Kentucky coffee tree thus could have spread wherever its herbivores roamed. The tree itself hints that it was once more abundant and widely distributed. Although the natural range of the tree today is confined mostly to floodplain terraces (Smith, 2008, 2018), at one time it was likely more common in open, early successional (colonizing or re-colonizing) habitats. Evidence comes in part from its growth habits: It reproduces vegetatively, and vigorously, from root sprouts, its seedlings don’t tolerate shade, and the tree tolerates drought, a combination of traits that would suit it for life in disturbed uplands (Zaya & Howe, 2009, citing Huxley & Griffiths, 1992).

The flowers, too, hint that Kentucky coffee tree was once more abundant. Most trees produce either male (pollen producing) or female (fruit producing) flowers on separate plants. Separation ensures outcrossing and offers the potential benefits of genetic mixing – an advantage in changing environments – but it would have been a disadvantage if opposite individuals had been few and far between. That disadvantage could be overcome if the flowers were pollinated by specialists. Insects dedicated to Kentucky coffee tree would gather pollen only from those flowers, and they would travel some distance to do so. That isn’t the case with this tree, however. Its flowers are likely pollinated by generalists, insects that gather pollen from a variety of sources and are unlikely to go far (Zaya & Howe, 2009).

Range map from USDA PLANTS database, 
December 2020. 
Like the flowers, the seeds plant suspicions of a wider distribution. In its modern floodplain habitats, the seeds are spread primarily by water. That’s unexpected, judging from their heft. Water-borne seeds tend to be small, light and buoyant; those of Kentucky coffee tree are large and heavy and they sink quickly. By all appearances, they aren’t adapted for spread by water. They are adapted for spread by animals. Why would they be enclosed in a sweet, green goo – a lure and reward for herbivores – if they’re intended to be carried passively, and inanimately, by water?



Survival in a Changed World

Together these anomalies point to a vastly different life. Times have changed, drastically. The megafauna that could have helped the tree spread began declining 15,000 years ago, victims of climate and habitat change, disease, overhunting or some combination of causes (MacFadden, 2000; Barlow, 2001). By 10,000 years ago at the latest, most of them were gone, and as far as Kentucky coffee tree is concerned, nothing has replaced them. Its fruits are poisonous to cows, sheep and other modern herbivores (Rowe & Geyer, undated). Even the largest plant eaters don’t have the voluminous digestive systems of megafauna, so they lack the greater diversity and number of intestinal microbes that are thought to have metabolized the toxins (Zaya & Howe, 2009, citing several studies). The fruits now fall and rot, uneaten, or the seeds germinate under the parent tree. Either way, distribution is severely limited. Even if the seeds are transported in streams, they won’t germinate there (Zaya & Howe, 2009, citing van der Pijl, 1982, and Murray, 1986), and their waterlogged journey would continue to confine them to lowland habitats.

Changes in its environment have made Kentucky coffee tree a rare find in the wild, so it is designated a species of special concern in Minnesota (Smith, 2018). It’s in no danger of extinction, however. Humans have been spreading its seeds, intentionally and unintentionally, for centuries. Early Native Americans used various parts of the plant for medicine and food, including a coffee-like drink made from roasted seeds (VanNatta, 2009; NAEB, 2020). In fact, the presence of Kentucky coffee tree on floodplains today may reflect the movement of Native Americans along stream corridors in the past (VanNatta, 2009). European immigrants also used the seeds as a coffee substitute and, like Native Americans, utilized the seeds as game pieces (Zaya & Howe ,2009; VanNatta 2009).

Today the tree is most often planted as an ornamental or shade tree, as a “rewilded” tree returned to its probable upland haunts, or as a curious link to the past. No one is certain what, if anything, ate its fruits. If nothing did, the tree’s investment in pods, pulp and seeds had no payoff. That seems unlikely. Surely this unique tree fed something besides our imaginations.

References

Barlow, C. (2001). Ghost Stories from the Ice Age. Natural History, 110(7), 62.

MacFadden, B.J. (2000). Cenozoic mammalian herbivores from the Americas: Reconstructing ancient diets and terrestrial communities. Annual Review of Ecology and Systematics 31, 35-59.

Native American Ethnobotany (NAEB) Database. Accessed November 28, 2020. http://naeb.brit.org/uses/search/?string=Gymnocladus+dioicus

Row, J.M., and Geyer, W. (n.d.). Plant Guide: Kentucky Coffeetree. USDA NRCS Plants Database. https://plants.usda.gov/plantguide/pdf/cs_gydi.pdf.

Smith. W.R. (2008). Trees and Shrubs of Minnesota. University of Minnesota Press.

Smith. W. (2018). Rare Species Guide: Gymnocladus dioica. Minnesota Department of Natural Resources. https://bit.ly/3mE5Ea2

USDA, NRCS. 2020. The PLANTS Database (http://plants.usda.gov, 3 December 2020). National Plant Data Team, Greensboro, NC 27401-4901 USA.

VanNatta, A. (2009). Ecological importance of Native Americans Culture to the Kentucky Coffee Tree (Gymnocladus dioicus). University of Wisconsin Stevens Point. https://www.uwsp.edu/forestry/StuJournals/Documents/NA/avannatta.pdf

Zaya, D.N., and Howe, H.F. (2009). The anomalous Kentucky coffeetree: megafaunal fruit sinking to extinction? Oecologia, 161, 221-226. https://doi.org/10.1007/s00442-009-1372-3.

 

 

Oriental Bittersweet in Winter

The yellow capsules and scarlet arils of Oriental Bittersweet stand out in winter. Oriental, Asian or Asiatic Bittersweet ( Celastrus orbicu...