Meet Pokeweed's Tree-Like Cousin

Photo by Roberto Fiadone licensed under CC BY-SA 4.0

There is more than one way to build a tree. For that reason and more, “tree” is not a taxonomic designation. Arborescence has evolved independently throughout the botanical world and many herbaceous plants have tree-like relatives. I was shocked to learn recently of a plant native to the Pampa region of South America commonly referred to as ombú. At first glance it looks like some sort of fig, with its smooth bark and sinuous, buttressed roots. Deeper investigation revealed that this was not a fig. Ombú is actually an arborescent cousin of pokeweed!

Photo by Dick Culbert licensed under CC BY 2.0

The scientific name of ombú is Phytolacca dioica. As its specific epithet suggests, plants are dioecious meaning individuals are either male or female. Unlike its smaller, herbaceous cousins, ombú is an evergreen perennial. Because they can grow all year, these plants can reach bewildering proportions. Heights upwards of 60 ft. (18 m.) are not unheard of and the crowns of more robust specimens can easily attain diameters of 40 to 50 ft. (12 - 18 m.)! What makes such sizes all the more impressive is the way in which ombú is able to achieve such growth.

Photo by Lanntaron licensed under CC BY-SA 3.0

Ombú is thought to have evolved from an herbaceous ancestor. Cut into the trunk of one of these trees and you will see that this phylogenetic history has left its mark. Ombú do not produce what we think of as wood. Instead, much of the support for branches and stems comes from turgor pressure. Also, the way in which these trees grow is not akin to what you would see from something like an oak or a maple. Whereas woody trees undergo secondary growth in which the cambium layer differentiates into xylem and phloem, thus thickening stems and roots, ombú exhibits a unique form of stem and root thickening called “anomalous secondary thickening.”

Essentially what this means is that instead of a single layer of cambium forming xylem and phloem, ombú cambium exhibits unidirectional thickening of the cambium layer. There are a lot of nitty gritty details to this kind of growth and I must admit I don’t have a firm grasp on the mechanics of it all. The point of the matter is that anomalous secondary thickening does not produce wood as we know it and instead leads to rapid growth of weak and spongy tissues. This is why turgor pressure is so important to the structural integrity of these trees. It has been estimated that the trunk and branches of an ombú is 80% water.

A cross section of an ombú limb showing harder xylem tissues separated by spongy parenchyma that has since disintegrated. Photo by Tony Rodd licensed under CC BY-NC-SA 2.0

Like all members of this genus, ombú is plenty toxic. Despite this, ombú appears to have been embraced and is widely planted as a specimen tree in parks, along sidewalks, and in gardens in South America and elswhere. In fact, it is so widely planted in Africa that some consider it to be a growing invasive issue. All in all I was shocked to learn of this species. It caused me to rethink some of the assumptions I hold onto with some lineages I only know from temperate regions. It is amazing what natural selection has done to this genus and I am excited to explore more arborescent anomalies from largely herbaceous groups.

Photo Credits: [1] [2]

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

How Trees Fight Disease

Plants do not have immune systems like animals. Instead, they have evolved an entirely different way of dealing with infections. In trees, this process is known as the "compartmentalization of decay in trees" or "CODIT." CODIT is a fascinating process and many of us will recognize its physical manifestations.

In order to understand CODIT, one must know a little something about how trees grow. Trees have an amazing ability to generate new cells. However, they do not have the ability to repair damage. Instead, trees respond to disease and injury  by walling it off from their living tissues. This involves three distinct processes. The first of these has to do with minimizing the spread of damage. Trees accomplish this by strengthening the walls between cells. Essentially this begins the process of isolating whatever may be harming the living tissues.

This is done via chemical means. In the living sapwood, it is the result of changes in chemical environment within each cell. In heartwood, enzymatic changes work on the structure of the already deceased cells. Though the process is still poorly understood, these chemical changes are surprisingly similar to the process of tanning leather. Compounds like tannic and gallic acids are created, which protect tissues from further decay. They also result in a discoloration of the surrounding wood. 

The second step in the CODIT process involves the construction of new walls around the damaged area. This is where the real compartmentalization process begins. The cambium layer changes the types of cells it produces around the area so that it blocks that compartment off from the surrounding vascular tissues. These new cells also exhibit highly altered metabolisms so that they begin to produce even more compounds that help resist and hopefully stave off the spread of whatever microbes may be causing the injury. Many of the defects we see in wood products are the result of these changes.

The third response the tree undergoes is to keep growing. New tissues grow around the infected compartment and, if the tree is healthy enough, will outpace further infection. You see, whether its bacteria, fungi, or a virus, microbes need living tissues to survive. By walling off the affected area and pumping it full of compounds that kill living tissues, the tree essentially cuts off the food supply to the disease-causing organism. Only if the tree is weakened will the infection outpace its ability to cope.

Of course, CODIT is not 100% effective. Many a tree falls victim to disease. If a tree is not killed outright, it can face years or even decades of repeated infection. This is why we see wounds on trees like perennial cankers. Even if the tree is able to successfully fight these repeat infections over a series of years, the buildup of scar tissues can effectively girdle the tree if they are severe enough.

CODIT is a well appreciated phenomenon. It has set the foundation for better tree management, especially as it relates to pruning. It is even helping us develop better controls against deadly invasive pathogens. Still, many of the underlying processes involved in this response are poorly understood. This is an area begging for deeper understanding.

Photo Credits: kaydubsthehikingscientist & Alex Shigo

Further Reading: [1]

Why Trees Have Rings (and why they are so useful)

Dendrochronology is a field of study that focuses on tree rings. Though it may not be obvious, the amount of information we gain from looking at these rings is astounding. This research goes far deeper than simply finding out how old a tree was when it died. Dendrochronological data can be used to investigate paleoclimates, paleoecologies, and the archaeological dating of buildings and artwork. It is amazing how a practiced eye can look back in time. To date, we have an unbroken dendrochronological record for the northern hemisphere dating back some 12,000+ years!

All of this would not be possible if it were not for tree rings. But what exactly are they and how do they form? The answer is physiological. Essentially tree rings result from patterns in vascular tissues. Early in the spring, before the leaves start to grow, a layer of tissue just under the bark called the cambium begins to divide. In this cool, water-laden time of the growing season the vessels that are produced are large and less dense. This is the beginning of the spring or early wood. Although they are not as strong as vessels that are produced later in the season, they sure can move a lot of water. Things are a bit different for conifers. Because they do not produce vessel elements in their wood, this large cell growth is initiated instead by large amounts of a growth hormone called auxin that is produced by the new buds. This causes the cells of the early wood in conifers to grow large in a similar way to that of the hardwoods. 

As summer heats up, things start to change. The cambium starts producing smaller, thicker cells. The vessels that result from this are much stronger than those of the early wood. This late wood as it is called gives trees much of their rigidity and strength. Late wood is also resistant to what is called cavitation, a process in which water within the tree can literally vaporize, causing a damaging embolism during the hottest months of summer. In conifers, bud growth stops by mid to late summer and with it much of the production of auxin. This results in smaller vessels as well. 

In temperate regions, this cycle of growth occurs over the course of a growing season. As such, each ring demarcates a year in that trees life. Because so much of a trees growth is determined by environmental conditions, the size and shape of the rings can tell a lot about the conditions in which that tree was growing. That is why dendrochronology is such a useful tool. By looking at tree rings from all over the world, researchers can tell what was going on at that point in time. And, though it was long thought that this was a phenomenon restricted to seasonal forests, we are finding that even some tropical trees produce annual growth rings. This is especially true in regions that have a measurable dry season. It just goes to show you that data comes in many shapes, sizes, and forms.

LEARN MORE ABOUT DENDROCHRONOLOGY IN EPISODE 247 OF THE IN DEFENSE OF PLANTS PODCAST

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