Friday, October 18, 2013

The unfortunate case of the butternut tree

Species name: Juglans cinerea

Common name: butternut, white walnut

Location: UWO campus

The butternut is a rather unfortunate tree, and a great example of how "world connectedness" is sometimes a bad thing. The butternut is native to the Carolinian forest, but is now completely absent from North and South Carolina in the United States, and almost completely absent from Ontario in Canada (the two extremes in the natural range of this species). Towards the middle of the range of this species in Ohio, numbers are dwindling quickly. Why is the population of butternut decreasing, you ask? Well, the saying "when in doubt blame it on a fungus" certainly applies here. In fact, when in doubt blame it on two fungi is even more appropriate.

Before we go blaming things on fungi, let's talk a bit about the butternut tree itself. It looks incredibly similar to the black walnut (which you can read all about HERE), but differs in two major ways. The first way that a butternut differs from a black walnut is in the shape of the leaves. Both butternuts and black walnuts have numerous leaflets in their compound leaf (and this number often depends on maturity of the leaf which determines the length of the petiole or the "leaf branch", rather than genetics determining number of leaflets in a compound leaf), but the butternut leaf ends in a terminal leaflet where the black walnut leaf ends in a terminal pair of leaflets. Unfortunately, the wind can shear off the terminal leaflet (pictured in the second image near the pink arrow) so the leaves end up looking identical. So when using leaves to identify species, make sure you look at more than just one! The second way to tell them apart can be seen in the third image above, but admittedly this is a terrible picture of the fruit (the pink arrow points to the fruit in the third image). On this entire tree there was only one single fruit produced this year, so I didn't have much to work with for angles to photograph. But in a nut shell (pun intended), the fruit (or nut) of the black walnut is spherical, but the fruit (or nut) of a butternut is shaped like a football. This can even be distinguished by looking at the shell casings on the ground, so take a poke around the base of the tree if you're trying to identify a butternut or black walnut. Like the black walnut, the meat on the inside of the nut is completely edible, and apparently tastes wonderful when roasted slightly in the oven or pan-fried a bit (I haven't ever had the pleasure of eating a butternut, so I don't speak from experience on this one; see below for why I DON'T recommend eating butternuts).

I guess there's no time like the presence to talk about an evil fungus. The steady decline of butternuts isn't just due to habitat loss (and there is quite a bit of that happening in the Carolinian deciduous forest in Eastern North America!). It's actually because of a disease called Butternut Canker, which is caused by a fungus. :( I know, sad face. But, of course, that's a boring story so I wouldn't just tell you about a single fungus. Recent evidence shows that it's not just one fungus that's responsible for the butternut canker disease, it's actually two. The first one comes in and attacks the tree, causing a localized disease that, theoretically, the tree should be able to overcome. Unfortunately, this causes open wounds in the bark of the tree, exposing the underlying wood and the juicy vascular tissue. This vascular tissue is critical for the survival of the tree, and is the only living part out of all of the wood in the trunk in the tree. Because it is located just under the bark, this is one of the reasons why cracking bark is a critical step in so many fungal infections (sometimes even insect infections or bacterial infections) of trees. Once this vascular tissue has been exposed, the second fungus comes in and completely kills the tree. It will start off being one or two limbs of the tree near the crown (a fancy word for the top), and once the branch loses all of its leaves it won't grow them back the next year. Even pruning these dead branches won't save the tree; once limb death starts it's only a matter of time before the rest of the tree is killed. Because it's a combination of two different fungi that cause the death of the tree, it is exceedingly difficult to come up with a solution to prevent infection. The first fungus that attacks the tree (it has a ridiculously long name: Sirococcus clavigignenti-juglandacearum) is carried by potentially hundreds of different vectors, from the wind to other plant species to insects, and nearly all of these are impossible to prevent from coming in contact with the tree. You might think that lone trees would be most susceptible to this disease, but actually that's not the case. The canopy of the forest traps a lot of pathogens in the circulating air within the forest, so it actually increases the incidence of butternut canker. Lone trees often survive much longer without the disease. This fungus, an invasive fungus introduced from Asia, has managed to infect nearly 90% of all butternut trees left in existence. It won't be long before this species goes extinct, as there is no way of preventing the disease. It's an incredibly unfortunate set of circumstances for butternut trees, and for the Carolinian forest in general. We're about to lose one of the most majestic trees in the deciduous forest of eastern North America. Because this species is so critically endangered, do the world a favour: if you happen to find an intact butternut fruit, don't eat it! Instead, hold it in your hands, do a little "rain and good health to butternuts" dance, and lovingly plant it in the ground. Cover it with some chicken wire (I recommend making a box, and burying it at least 4 inches into the ground over where you planted the seed), and hope for the best. These are one of the preferred food sources for deer and squirrels, so not many seeds manage to escape being eaten (especially considering how few are left). I'm not going to suggest you take a seed home and try to grow it in a pot; that would be illegal in Canada according to our Species At Risk Act (SARA). Even if you plant the seedling back exactly where you found the nut, it would still be against Canadian endangered species laws, and much the same regulations exist in the United States. You might ask who enforces these laws and how common these people are in butternut territory...

Instead of ending on a sour note, let's end on a happy note! There is currently an effort underway in both Canada and the United States to try to protect the butternut in some way from disease. The method developed currently will not save the trees that are dying, but will instead protect the new generation of butternut trees. When butternuts are hybridized with the Asian walnut, the tree has almost complete resistance to the butternut canker. This is great, but unfortunately it dilutes the genetic pool of butternuts with Asian walnut genes. To try to combat this (since hybrids don't actually have any status according to any endangered species act anywhere in the world), hybridizing this hybrid with a different butternut tree causes the tree to have characteristics that are nearly identical to the butternut, but still have the disease resistance of the Asian walnut. This process is known as "back-crossing," and is commonly done with agricultural plant species to introduce disease resistance. This still doesn't make a "true butternut," but the resulting tree is nearly identical to a genetically pure butternut. There might come a day when we have forests containing this "butternut" (technically called a "Butter-Buart" which to me just sounds strange) in abundance once again. I wonder if anyone has tested the edibility of the butternuts of this backcrossed hybrid to determine tastiness... :)