Carolina Buckthorn Recovery

The Carolina Buckthorn seems to be recovering from its earlier infestation of Asian Soybean Aphids.  Those plants that hosted just a few aphids appear almost normal.

Plants that were hardest hit still show the curled leaves that afforded shelter to the aphid colonies. 

Despite their odd appearance, the damaged leaves remain alive and green.  I assume they are still performing their photosynthetic duties, but at a much less efficient level.

New leaves are developing in a normal manner.  It’s encouraging to see that they aren’t yet showing any signs of drought stress.  These plants are growing on an extremely dry site and most of the other plant species in the area are looking distinctly wilted.

Presence of the Asian Soybean Aphids on the Carolina Buckthorns is a fairly recent phenomenon.  I imagine now that they are here, the aphids will continue to be an annual event.  Being at the extreme northern edge of their range already puts the buckthorns under a certain amount of stress.  Dealing with annual aphid damage will certainly add to that stress.  I’m wondering how long it will be before I begin seeing a dieoff of Carolina Buckthorn.

I took this as a bit of positive news.  A few of the plants are producing a bit of fruit.  Of course I’m also aware of the fact that stress can act as a stimulus to produce more fruit before death occurs.  I guess I’ll just have to wait a few years to see how things are trending.

Asian Soybean Aphid

The Carolina Buckthorns are once again under attack by Asian Soybean Aphids, Aphis glycines.  I first encountered the aphids last year when they were found on a few Carolina Buckthorns.  This plant that I found yesterday is typical of the worst condition I found in July of 2011.  There are some yellowed leaves and a few tightly curled leaves.

The aphid damage is much more severe and widespread this year.  Entire branches are showing curled leaves.  Carolina Buckthorn is an uncommon plant in Ohio with a range that just catches the southern edge of the state.  Its primary threat has always been extreme low winter temperatures.  I frequently monitor the plants to see how they are growing, so I am certain that the aphids were not a problem before last year.

This is what’s inside the curled leaf.  Aphids busily feed and reproduce within the protective folds of leaf.  Ants visiting the aphids to collect honeydew, provide another measure of security.  Asian Soybean Aphids are a non native species that was first discovered in the United States about 12 years ago.  They became a species of concern because of their use of Soybeans as a summer host plant.  Large populations of these aphids can cause severe damage to soybean plants and devastate yields. 

Soybeans are annual plants that only exist for a limited time each year.  In order to sustain themselves during the non soybean season, the aphids utilize a second host for the winter.  Non-native Buckthorns, Rhamnus species, were the original preferred winter host.  It became a case of one invasive species damaging another.  Now it’s obvious that at least one native Buckthorn is a suitable host for the Asian Soybean Aphid.

Aphids feeding on the buckthorn cause the distorted, curled leaf formation.  Judging by what I found, it appears that the leaves curl prior to being invaded by the aphids.  The leaves remain green even after intensive feeding has taken place, so the leaf functions may continue to provide nutrients to the plant.

Aphids on the buckthorn come from eggs laid the previous fall.  These aphids are all females that reproduce by giving live birth.  The youngsters begin producing their own young when about a week old, so it doesn’t take long for a population explosion to occur.  Eventually, winged females are produced and these fly off to find soybean plants on which to start a summer colony.

The aphid spends most of its time attached to leaf veins by way of its tubular mouth.  Consuming sap and producing babies is the life of the aphid.  After producing many generations of aphids on soybean plants, male and female winged adults search out buckthorns where mating occurs and eggs are laid.  Aphids in the wind may travel hundreds of miles, so the proximity of soybean fields is not an indicator of whether or not the buckthorns will be found by aphids.  The eggs overwinter and the whole cycle begins anew.

The winged individuals apparently don’t all develop on the same schedule.  This population looks to have everything from winged adult to newborn infant.  I’ll have to wait to see what level of survival threat the aphid poses to the Carolina Buckthorn.  Since the aphids will leave the buckthorn in early summer, the buckthorn may have time to regrow with minimal ill effects.  What I’m expecting next is to find the aphids on local native legumes.  I’m afraid that won’t be a fun discovery.

Dead Carolina Buckthorn

It’s sad to witness the death of a tree that you’ve come to know and enjoy. I’ve been watching this Carolina Buckthorn for the last 15 years. I’ve seen it fully loaded with leaves and admired its production of bright fruit. It’s had its setbacks over the years, but it has always managed to pull through.

It stands on the lower portion of a north facing slope at the edge of a mixed stand of cedars and oaks. Carolina Buckthorn is an understory tree and does best when lightly shaded by its neighbors. This location sheltered the tree from drying winter winds and seemed the ideal place for this type of buckthorn to grow.

The first signs of trouble were early loss of leaves and poor fruit production. The next season began with several dead branches and undersized leaves. Leaves again dropped early and no fruit was produced. It was obvious that the tree was on its way out. Growth during the last year was in the form of spindly shoots coming up from near the base. These died before the summer was over and that was the end of the tree.

The trunk and larger branches are still strong and are being colonized by lichens. Carolina Buckthorns are uncommon in Ohio and it’s that fact that initially caused me to be interested in this tree. I soon learned that Carolina Buckthorn was quite an interesting species in itself. It would have been worth studying this tree even if it were the most common plant around. There are still plenty of Carolina Buckthorns growing at Blue Jay Barrens, but none that I know as well as I knew this guy.

I noticed swellings and long fissures at the upper end of the three year old branches. I wonder if these had anything to do with the death of the tree. I said that I knew this tree well, but I don’t remember seeing this condition when the tree was still alive.

Whittling away some of the bark didn’t shed much light on the cause of the deformity. The wood was swollen and the bark was thickened. It looks as though this might have been a point of branching that died back. It seems that something else also had to happen for this growth to be formed. I guess I’ll have to check some other Carolina Buckthorns to see if they show any signs of the same affliction.

Curled Leaf

I hate to see plants suffering from afflictions that result in abnormal growth or deformities of the leaves. It’s especially disturbing when the plant in question is generally uncommon. From a distance I noticed the pale yellow coloring of the new growth on a patch of Carolina Buckthorn, a southern species that just barely makes its way into Southern Ohio.

Then I found a few older leaves that were tightly curled and displaying a mottled coloration. Most of the leaves still looked perfectly healthy, but I know how quickly that can change. Besides being curled, the leaves contained holes and various sized spots of dead leaf. The leaf petiole and portions of the branch contained similar discolorations. It’s not normal for Carolina Buckthorn to be showing this type of damage.

Ants are always foraging on the plants of Blue Jay Barrens, so it’s not odd to find them on the buckthorn. Their behavior can often give clues as to the reason they are present. As you might imagine, ants displaying a defensive posture usually have something to defend. That means there must be something here that is of interest to them. There doesn’t appear to be anything of significance on the outside of the leaf, so maybe it’s hidden in the curl.

The ants would have been hovering over these aphids if they had not all exited stage right to attack my hand as it held the leaf open enough to take a picture. Aphids produce honeydew which is collected by the ants for food. There are several aphid species that cause curling of the leaf as a result of their feeding activities. These aphids worry me because they look like the Asian Soybean Aphid, which is one of the new invasive species rapidly infesting the Midwest. They have become quite a pest in Soybean fields. So what are they doing on buckthorn?

The Asian Soybean Aphid is one of those species that uses multiple hosts during the year. During the summer it feeds on Soybean plants. In the fall, it lays its eggs on the invasive European Buckthorn. The eggs hatch in the spring and feed on the buckthorn. When winged adults are produced, they fly off to develop colonies on Soybeans. Carolina Buckthorn is in the same genus Rhamnus as the European Buckthorn. Having invasive insects attacking invasive plants doesn’t bother me. When the invasive insect begins to utilize native plants, I become concerned. Will the aphids next find a native legume to use in place of the Soybean? This situation deserves a little more study.

Carolina Buckthorn

Carolina Buckthorn, Rhamnus caroliniana, is one of my favorite shrubs. Part of the attraction is the sheer beauty of the plant itself, with its large glossy leaves. The second reason has to be the fact that the plant is a rarity in Ohio.

I normally find this shrub growing on the edges of the prairie openings or as an understory plant in the cedar thickets. In one area it grows on an open ridge top. There’s evidence that the ridge top was much more closed in the past. There are old tops from large cedars that were probably cut for logs about 30 years ago and several skeletons of Redbuds that must have lived along with the cedars. Small Carolina Buckthorn now grow along with the tall grasses.

There were a couple of large Carolina Buckthorns that lived on this ridge, but they were killed by extremely cold temperatures about 20 years ago. This shrub is a southern species and the populations here are at the extreme northern limit of their range. This ten foot specimen represents a large size for this area. I would try a couple of these as landscape shrubs around the house, but I know they will suffer the loss of their top growth every few years.

Where once there was one large shrub, there are now dozens of smaller plants. The short plants are usually protected by a covering of snow during the coldest part of the winter. Once they reach a height that exceeds the typical snow levels, the top growth suffers frequent die backs. Only a few of the hardier plants seem to survive to get any real height.

The dry, rocky conditions in which the Carolina Buckthorns grow doesn’t seem to have any adverse impacts on the plant. Even during the hottest and driest of summers, the shrubs present a perfectly healthy appearance.

Even though the population itself seems healthy, there are always some individuals that show signs of disease. Several of the little plants on this ridge top show signs of canker growth that is probably the result of some fungus infection. It makes for some odd shaped plants when you add disease damage to that already inflicted by the weather. Even under ideal conditions, these diseased individuals would probably never grow large enough to produce fruit.

Carolina Buckthorn

Autumn is an ideal time to locate and monitor certain trees and shrubs. Each species tends to make the transition from summer to winter in a unique fashion that makes them easily identifiable at a certain point. The leaves of the Carolina Buckthorn, Rhamnus caroliniana, maintain their waxy shine and begin to yellow late in the season. This is a southern shrub that has its northern limit in extreme southern Ohio.

A deer trail runs just to the left of this medium sized Buckthorn and a buck deer has found the trunk to be a nice place to rub. Carolina Buckthorns suffer a lot of hardships in this area. Extreme cold temperatures during the winter can kill the top growth on these shrubs. This little clump contains some big specimens compared to the others found at Blue Jay Barrens.

This group is growing beneath a couple of fairly large Black Walnut trees. Many shrubs would not be able to grow in this location because of the juglones produced by the walnut. Maybe the Carolina Buckthorn is a suitable companion plant to the walnut and enjoys less competition from other shrubs in this location.

Carolina Buckthorn leaves always look as though they’ve been waxed and polished. It would make a wonderful landscape plant if it weren’t so susceptible to winter-kill.

One of the key characteristics of this shrub is the naked bud. The tiny beginnings of next year’s leaves are exposed for all to see, instead of being covered by bud scales. The buds are a great help to identifying Carolina Buckthorn in the winter. Years ago I was walking near the creek when I came across a huge patch of buckthorn. I was getting ready to record this exciting find on my map, when I took a closer look at a twig and discovered I was looking at a cluster of little Pawpaws. About that time a Blue Jay started chattering in the tree above me and I’m sure it was laughing at my blunder. Pawpaws have terminal buds very similar to those of Carolina Buckthorn and I had made the classic mistake of making a rapid identification based on one quickly glimpsed character.