Photo by Abrahami licensed under CC BY-SA 2.5

I love being humbled by plant ID. Confusion usually means I am going to end up learning something new. This happened quite recently during a trip through The Morton Arboretum. Admittedly trees are not my forte but I had spotted something that seemed off and needed further inspection. I was greeted by a small tree with leaves that screamed "birch family" (Betulaceae) yet they were opposite. Members of the birch family should have alternately arranged leaves. What the heck was I looking at?

It didn't take long for me to find the ID tag. As a plant obsessed person, the information on the tag gave me quite the thrill. What I was looking at was possible the strangest maple on the planet. This, my friends, was my first introduction to Acer carpinifolium a.k.a the hornbeam maple.

Photo (c)2006 Derek Ramsey (Ram-Man) licensed under CC BY-SA 2.5

The hornbeam maple is endemic to Japan where it can be found growing in mountainous woodlands and alongside streams. Maxing out around 30 feet (9 m) in height, the hornbeam maple is by no means a large tree. It would appear that it has a similar place in its native ecology as other smaller understory maples do here in North America. It blooms in spring and its fruits are the typical samaras one comes to expect from the genus.

It probably goes without saying that the thing I find most fascinating about the hornbeam maple are its leaves. As both its common and scientific names tell you, they more closely resemble that of a hornbeam (Carpinus spp.) than a maple. Unlike the lobed, palmately veined leaves of its cousins, the hornbeam maple produces simple, unlobed leaves with pinnate venation and serrated margins. They challenge everything I have come to expect out of a maple. Indeed, the hornbeam maple is one of only a handful of species in the genus Acer that break the mold for leaf shape. However, compared to the rest, I think its safe to say that the hornbeam maple is the most aberrant of them all.

Photo by Qwert1234 licensed under CC BY-SA 3.0

Not a lot of phylogenetic work has been done on the relationship between the hornbeam maple and the rest of its Acer cousins. At least one study suggests that it is most closely related to the mountain maple of neartheastern North America. More scrutiny will be needed before anyone can make this claim with certainty. Still, from an anecdotal standpoint, it seems like a reasonable leap to make considering just how shallow the lobes are on mountain maple leaves.

Regardless of who it is related to, running into this tree was truly a thrilling experience. I love it when species challenge long held expectations of large groups of plants. Hornbeam maple has gone from a place of complete mystery to me to being one of my favorite maples of all time. I hope you too will get a chance to meet this species if you haven't already!

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

Further Reading: [1] [2]

Photos by SNappa2006 (CC BY 2.0), nutmeg66 (CC BY-NC-ND 2.0), Eli Sagor (CC BY-NC 2.0), and Randy McRoberts (CC BY 2.0)

Say "tree bark" and everyone knows what you're talking about. We learn at an early age that bark is something trees have. But what is bark? What is its purpose and why are there so many different kinds? Indeed, there would seem to be as many different types of bark as there are trees. It can even be used as a diagnostic feature, allowing tree enthusiasts to tease apart what kind of tree they are looking at. Bark is not only fascinating, it serves a serious adaptive purpose as well. To begin to understand bark, we must first look at how it is formed.

To start out, bark isn't a very technical term. Bark isn't even a single type of tissue. Instead, bark encompasses several different kinds of tissues. If you remember back to Plant Growth 101, you may have heard the word "cambium" get thrown around. Cambium is a layer of actively dividing tissue sandwiched between the xylem and the phloem in the stems and roots of plants. As this layer grows and divides, the inside cells become the xylem whereas the outside cells become the phloem.

Successive divisions produce what is known as secondary phloem. This is where the bark begins. On the outside of this secondary phloem are three rings of tissues collectively referred to as the "periderm." It is the periderm which is responsible for the distinctive bark patterns we see. As a layer of cells called the "cork cambium" divides, the outer layer becomes cork. These cells die as soon as they are fully developed. This layer is most obvious in smooth bark species such as beech.

Similar to insect growth, however, the growth of the insides of a tree will eventually outpace the bark. When this happens, the periderm begins to split and cracks will begin to appear in the bark. This phenomenon is most readily visible in trees like red oaks. When this starts to happen, cells within the secondary phloem begin to divide. This forms a new periderm underneath the old one. The cumulative result of this results in alternating layers of old periderm tissue referred to as "rhytidome."

This gives trees like black cherry their scaly appearance or, if the rhytidome consists of tight layers, the characteristic ridges of white ash and white oak. Essentially, the distribution and growth pattern of the periderm gives the tree its bark characteristics. But why do trees do this? Why is bark there in the first place?

The dominant role of bark is protection. Without it, vital vascular tissues risk being damaged and the tree would rapidly loose water. It also protects the tree from pests and pathogens. The cell walls of cork contain high amounts of suberin, a waxy substance that protects against desiccation, insect attack, as well as fungal and bacterial infection. Thick bark can also insulate trees from fire.

Countless aspects of the environment have influenced the evolution of tree bark. In some species such as aspen or sycamore, the trunk and stems function as additional photosynthetic organs. In these species, cork layers are thin and often flaky. Shedding these thin layers of bark ensures that buildup of mosses, lichens, and other epiphytes doesn't interfere with photosynthesis. The white substance on paper birch bark not only inhibits fungal growth, it also helps prevent desiccation while at the same time making it distasteful for browsing insects and mammals alike.

When you consider all the different roles that bark can play, it is no wonder then that there are so many different kinds. This is only the tip of the ice berg. Entire scientific careers have been devoted to understanding this group of tissues. For now, winter is an excellent time to start noticing bark. Take some time and get to know the trees around you for their bark rather than their leaves.

Photo Credits: Eli Sagor (bit.ly/1OTnA8H), Randy McRoberts (bit.ly/1PgzH35), Lotus Johnson (bit.ly/1JyVt1E), SNappa2006 (bit.ly/1TkjHil), and nutmeg66 (bit.ly/1QwyZQ8)

Further Reading:
http://www.botgard.ucla.edu/