The American Smoketree

Photo by Andrew Ward licensed under CC BY-NC 2.0

I am a sucker for smoketrees (Cotinus spp.). These members of the cashew family (Anacardiaceae) are a common sight around my town and really put on a dazzling show from late spring through fall. When I finally got around to putting a name to these trees, I was a little bit bummed to realize that all of the specimens in town are representatives of the Eurasian species, Cotinus coggygria, but it didn’t take me long to find out that North America has it’s own fascinating representative of the genus.

The American smoketree (Cotinus obovatus) is not terribly common in the wild or cultivation. Today, it exhibits a suffuse distribution through parts of southern North America, with disjunct populations occurring along the Ozark Plateau of Arkansas and Missouri, the Arkansas River in eastern Oklahoma, the Cumberland Plateau in northeastern Alabama, Tennessee, and Georgia, and the Edwards Plateau in west-central Texas. The major habitat feature that unites these populations is soil. All of them are said to grow on rocky, calcareous soils prone to drought.

Photo by Megan Hansen licensed under CC BY-SA 2.0

It is an interesting distribution to say the least. I haven’t found too much in the way of an explanation for why the American smoketree is limited to calcareous soils in the wild. Apparently it is fairly adaptable to different soil types in cultivation. Perhaps competition with other species limits this tree to harsh conditions. It isn’t a big species by most standards. The American smoketree generally produces multiple stems and only occasionally reaches heights of 30 feet (9 meters) or more in most circumstances. One phrase that gets repeated with some frequency is that the American smoketree likely represents a relictual species.

Though hard to prove without ample fossil evidence, it seems many experts believe that American smoketrees (and the genus Cotinus in general) were far more common and widespread in the past than they are today. Indeed, the fossil remains of a species named Cotinus cretaceus (sometimes C. cretacea) were found in Alaska and date back to the late Cretaceous. Given that the American smoketree’s closest living relatives are found throughout parts of Europe and Asia, such evidence suggests that this genus spread into North America during a period when land bridges connected the two continents and has since been reduced to scattered populations of this single North American species.

Photo by Andrey Zharkikh licensed under CC BY 2.0

European colonization of North America did not help the American smoketree either. American smoketree sap can be processed into a yellow dye, which was highly coveted during the American Civil War. Its rot-resistant wood was also widely used for fence posts. At least one source I found indicated that the tree was cut to near extirpation in many areas for these reasons. Luckily today, with harvesting pressures largely a thing of the past, the American smoketree has rebounded enough that it is currently considered a species of least concern.

The American smoketree has also benefited from some minor popularity in cultivation. Like its Eurasian cousins, the appeal of this species comes from its colorful foliage, wonderfully flaky bark, and billowy inflorescences. Its egg-shaped leaves emerge in spring and are silky and pink. As spring gives way to summer, the leaves gradually turn a pleasing shade of blueish-green. Come fall, the leaves paint the landscape in bright red until they are shed. Late spring is generally the blooming time for American smoketree.

Photo by geneva_wirth licensed under CC BY-NC 2.0

Photo by peganum licensed under CC BY-SA 2.0

Its tiny, inconspicuous flowers are borne on large, branching panicles. Each panicle is covered in tiny hairs that apparently continue to grow well after the flowers have been pollinated. This is where the name smoketree comes from. From afar, a tree covered in panicles looks as if it is billowing dense clouds of smoke from its canopy. The whole spectacle is stunning to say the least and I just wish this species was more popular than its cousins.

All in all, the American smoketree is a truly interesting species. From its fractured distribution and curious history to its status as an obscure native tree in cultivation, there are a lot of reasons to love this species. Though related to plants like poison ivy (Toxicodendron spp.), smoketrees only rarely cause dermatitis in particularly susceptible individuals. I hope I get the chance to see an American smoketree in the wild some day.

Further Reading: [1] [2] [3] [4] [5]

The Sinewy American Hornbeam

Photo by Richard Webb licensed by CC BY-SA 3.0

Winter is when I really start to notice trees. Admittedly, I am pretty poor when it comes to tree ID and taxonomy but there are a few species that really stand out. One of my all time favorite trees is Carpinus caroliniana.

Carpinus caroliniana goes by a handful of common names including ironwood, musclewood, and American hornbeam. All of these names have been applied to other trees so I'll stick with its scientific name. Finding C. caroliniana is rather easy. All you have to do is look for its unmistakable bark.

Photo by Rob Duval licensed by CC BY-SA 3.0

With smooth, sinewy striations and ridges, it is no wonder how this tree got the name "musclewood." The wood is extremely close-grained and is therefore very hard, earning it another nickname of "ironwood."They are generally small trees, rarely exceeding a few meters in height, though records have shown that some individuals can grow to upwards of 20 meters in rare circumstances. I hope that someday I will be able to meet one of these rare giants.

Carpinus caroliniana is also an indicator of fairly rich soils. Due to their high tolerance for shade, they are often a tree of the mixed hardwood understory. Their foliage resembles that of the family in which they belong, the birch family (Betulaceae).

Photo by Katja Schulz licensed by CC BY 2.0

The caterpillar of the io moth (Automeris io)

An adult io moth (Automeris io). Photo by Andy Reago & Chrissy McClarren licensed by CC BY 2.0

A multitude of insect species utilize C. caroliniana as a larval food source including the famed io moth. In the spring, male and female catkins are born on the same tree and, after fertilization, they are replaced by interesting looking nutlets covered by leaf-like involucres. The seeds are an important food source for a variety of birds, mammals, and insects alike.

The male flowers of Carpinus caroliniana. Photo by Philip Bouchard licensed by CC BY-NC-ND 2.0

Carpinus caroliniana is a tree I could never get bored with. Not only does it have immense ecological value, it is aesthetically pleasing too. Its small size and shade tolerance also makes it a great landscape tree in areas too cramped for something larger. Why this species isn't more popular in native landscaping is beyond me.

Photo Credits: [2] [3] [4] [5] [6] [7] [8]

Further Reading: [1] [2] [3]

The Curious Case of the Yellowwood Tree

Photo by Plant Image Library licensed under CC BY-SA 2.0

The immense beauty and grace of the yellowwood (Cladrastis kentukea) is inversely proportional to its abundance. This unique legume is endemic to the eastern United States and enjoys a strangely patchy distribution. Its ability to perform well when planted far outside of its natural range only deepens the mystery of the yellowwood.

The natural range of the yellowwood leaves a lot of room for speculation. It hits its highest abundances in the Appalachian and Ozark highlands where it tends to grow on shaded slopes in calcareous soils. Scattered populations can be found as far west as Oklahoma and as far north as southern Indiana but nowhere is this tree considered a common component of the flora.

Though the nature of its oddball distribution pattern iscurious to say the least, it is likely that its current status is the result of repeated glaciation events and a dash of stochasticity. The presence of multiple Cladrastis species in China and Japan and only one here in North America is a pattern shared by multiple taxa that once grew throughout each continent. A combination of geography, topography, and repeated glaciation events has since fragmented the ranges of many genera and perhaps Cladrastis is yet another example.

The fact that yellowwood seems to perform great as a specimen tree well outside of its natural range says to me that this species was probably once far more wide spread in North America than it is today. It may have been pushed south by the ebb and flow of the Laurentide Ice Sheet and, due to the stochastic nuances of seed dispersal, never had a chance to recolonize the ground it had lost. Again, this is all open to speculation as this point.

Despite being a member of the pea family, yellowwood is not a nitrogen fixer. It does not produce nodules on its roots that house rhizobium. As such, this species may be more restricted by soil type than other legumes. Perhaps its inability to fix nitrogen is part of the reason it tends to favor richer soils. It may also have played a part in its failure to recolonize land scraped clean by the glaciers.

Yellowwood's rarity in nature only makes finding this tree all the more special. It truly is a sight to behold. It isn't a large tree by any standards but what it lacks in height it makes up for in looks. Its multi-branched trunk exhibits smooth, gray bark reminiscent of beech trees. Each limb is decked out in large, compound leaves that turn bright yellow in autumn.

Photo by Elektryczne jabłko licensed under CC BY-SA 4.0

When mature, which can take upwards of ten years, yellowwood produces copious amounts of pendulous inflorescences. Each inflorescence sports bright white flowers with a dash of yellow on the petals. In some instances, even pink flowers are produced! It doesn't appear that any formal pollination work has been done on this tree but surely bees and butterflies alike visit the blooms. The name yellowwood comes from the yellow coloration of its heartwood, which has been used to make furniture and gunstocks in the past.

Whether growing in the forest or in your landscape, yellowwood is one of the more stunning trees you will find in eastern North America. Its peculiar natural history only lends to its allure.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]

Taxonomic Discoveries: My Version of the Butterfly Effect

Photo by Siga licensed under CC BY-SA 3.0

Witnessing a giant swallowtail (Papilio cresphontes) in flight is an incredible experience. It is the largest species of butterfly found in the US and Canada and with its yellow and black wings, it is impossible not to take pause and watch it flutter around the canopy. I will never forget the first time I saw one as a child. It was one of those moments that solidified my obsession with the natural world. Fast forward a few decades and now I can't help but ponder what kind of gardening I would need to do to attract these incredible insects to my yard. What I discovered surprised me to say the least. I had to plant something in the citrus family. 

We are all familiar with the fruits of various Rutaceae. This family contains the genus Citrus, providing humanity with oranges (C. × sinensis), lemons (C. × limon), grapefruits (C. × paradisi), and limes (mostly C. aurantifolia). These are largely tropical and subtropical trees, struggling to hang on anywhere temperatures dip below freezing regularly. How on Earth was a butterfly whose larva specialize on this family flitting around in temperate North America? What's more, reports place this species as far north as southern Quebec. I was obviously out of the loop on the taxonomic affinities of this family.

A little detective work turned up some surprising results. Temperate North America does in fact have some representatives of the citrus family. They are a far cry from an orange tree but they are nonetheless relatives. This inquiry actually solved a bit of trouble I was having with some riparian trees in my neck of the woods. As some of you probably know, trees are not a strong point of mine. I had encountered a few small woody things with compound leaves of three and dense clusters of greenish flowers. At first I thought I had found a rather robust poison ivy specimen but closer inspection revealed that wasn't the case.

Instead I had stumbled across something new for me - a common hoptree (Ptelea trifoliata). This cool looking tree is one of the giant swallowtails larval host trees, making it a member of -(you guessed it)- the citrus family. More often this small tree grows like a shrub with its tangle of multiple branches but they can reach some impressive heights, relatively speaking of course. Trees topping out at a height of 5 meters are not unheard of. Another common name of this tree - wafer ash - hints at its superficial similarity to a Fraxinus. Its compound leaves and wafer-like samaras are a bit of a curve ball for northerners like myself. It has a rather wide and patchy distribution throughout North America, and many subspecies/varieties have been named.

Common Hoptree (Ptelea trifoliata )

The other bit of this taxonomic journey involves another small tree, although this time I was better acquainted. Another host for the giant swallowtail is the prickly ash (Zanthoxylum americanum). It is interesting to note that both of these northern host trees superficially resemble ashes but I digress. The prickly ash is also small in stature and is most often found in thickets consisting of its own kind. As its common name suggests, you wouldn't want to go barreling through said thickets unless you wanted to donate some blood. It is well defended by sharp prickles on its stems. It does produce fruit but they are rather small and berry-like (technically follicles) and are distributed far and wide by birds.

Prickly Ash ( Zanthoxylum americanum ). Photo by manuel m. v. licensed under CC BY-SA 2.0

Both trees are rather aromatic. They produce volatile oily compounds like most of the family, making them smell quite pleasant. Their small size makes them interesting specimen trees for anyone looking for something unique to put in a native landscape. What's more, they host a variety of other larvae as well, including those of the spicebush swallowtail butterfly (P. troilus).

Together, these two species are the most northerly representatives of the citrus family, making them quite special indeed. I am happy that my interest in attracting giant swallowtails to my property resulted in a fascinating dive into the geography of this interesting family.

Photo Credits: [1] [2]

Further Reading: [1] [2]

Meet the Fringe Tree

The fringe tree (Chionanthus virginicus) Photo by Arthur T. LaBar licensed under CC BY-NC 2.0

Coming across a fringe tree in full bloom is a spectacular experience. Known scientifically as Chionanthus virginicus, some may surprised to realize that this is a native tree to eastern North America. Though it has found its way into the horticultural trade, it is still not terribly common. Today I would like to celebrate this interesting tree as well as bring to your attention some alarming facts that might threaten its existence in the wild. 

Fringe tree can be found growing wild in the understories and edges of forests throughout eastern North America. It tends to be quite a rarity on the edges of its range, hitting its densest distribution in a handful of the southeastern states. Individual trees are either male or female but both produce quite a floral display. They produce dense clusters of wispy white flowers, which do give off a slight fragrance but one has to get up close and personal with the branches to really appreciate it. 

The fringe tree hails from the same family as the ash trees - Oleaceae. Unfortunately, this taxonomic relationship may be bad news for the fringe tree in the long run. At least one study has shown that fringe trees can serve as hosts for the emerald ashborer. The sample size on this study was quite low, only 4 of 20 adult trees showed signs of completed larval development and adult emergence holes. Subsequent observations suggest that fringe trees are in fact viable hosts for this invasive pest, which is certainly cause for concern. Perhaps the one thing fringe tree has going for it are its sparse populations, making it harder to detect by these wood boring beetles. Only time and a lot of attention will tell. 

Regardless, I think this is a wonderfully underrated tree for a native eastern North America landscape. It is rather hardy and puts on quite a show every spring. As the Grumpy Gardener so eloquently put it, "It’s tougher than dogwood, more dependable than saucer magnolia, longer-lived than cherry, and smells better than stinky Bradford. And it’s beautiful." I couldn't agree more. Just make sure that if you know of wild fringe tree populations or have some growing on your property that you regularly monitor them for signs of emerald ashborer infestation.

Photo Credits: [1] [2] 

Further Reading: [1] [2]

Meet the Redbuds

I look forward to the blooming of the redbuds (Cercis spp.) every spring. They paint entire swaths of forest and roadside with a gentle pink haze. It’s this beauty that has led to their popularity as an ornamental tree in many temperate landscapes. Aside from their appeal as a specimen tree, their evolutionary history and ecology is quite fascinating. What follows is a brief introduction to this wonderful genus.

The redbuds belong to the genus Cercis, which resides in the legume family (Fabaceae). In total, there are about 10 species disjunctly distributed between eastern and western North America, southern Europe, and eastern Asia. The present day distribution of this genus is the result of vicariance or the geographic separation of a once continuous distribution. At one point in Earth’s history, the genus Cercis ranged from Eurasia to North America thanks to land bridges that once connected these continents. At some point during the Miocene, this continuous distribution began to break apart. As the climate changed, various Cercis began to diverge from one another, resulting in the range of species we know and love today.

All of them are relatively small trees with beautiful pink flowers. Interestingly enough, unlike the vast majority of leguminous species, redbuds are not known to form root nodules and therefore do not form symbiotic relationships with nitrogen-fixing bacteria called rhizobia. This might have something to do with their preference for rich, forest soils. With plenty of nitrogen available, why waste energy growing nodules? Until more work is done on the subject, its hard to say for sure why they don’t bother with nitrogen fixers.

One of the most interesting aspects of the redbuds are their flowers. We have already established that they are very beautiful but their development makes them even more interesting. You have probably noticed that they are not borne on the tips of branches as is the case in many flowering tree species. Instead, they arise directly from the trunks and branches. This is called "cauliflory," which literally translates to "stem-flower." In older specimens, the trunks and branches become riddled with bumps from years of flower and seed production.

It's difficult to make generalizations about this flowering strategy. What we do know is that it is most common in dense tropical forests. Some have suggests that producing flowers on trunks and stems makes them more available to small insects or other pollinators that are more common in forest understories. Others have suggested that it may have more to do with seed dispersal than pollination. Regardless of any potential fitness advantages cauliflory may incur, the appearance of a redbud covered in clusters of bright pink flowers is truly a sight to behold.

Further Reading: [1] [2] [3] [4] [5] [6] [7]