Vegetative Invaders, Part I

In my last post, I mentioned that I had more to say about invasive plants.  Well, I had originally intended to write a more comprehensive piece on the vegetative invaders in the southeastern United States, but after looking at the long, long list of exotic plant species present in Mississippi alone (from this site: https://www.gri.msstate.edu/ipams/MSExotics.php), I decided to focus my attention on the invasive trees, shrubs, and vines that I commonly see in my part of the state.  This first post covers the trees and shrubs, and a later post will tackle the invasive vines.  Be sure to check out some of the links!

Let’s start with trees.  A couple of quick definitions: although there are some exceptions, the general rule is that trees have a single woody stem or trunk, while shrubs have multiple woody stems/trunks.  Most of the trees that I regularly observe are native to my area, but, every so often, I will come across an invasive species or two—especially near urban settings.  Some commonly-seen cultivated plants, such as the ginkgo (an ancient, “living fossil” species that was wiped out across much of its range during the Pleistocene Ice Age) are basically innocuous, but others, such as the Bradford pear, can cause extensive damage to native ecosystems.  The Bradford pear is actually a cultivar of the species Pyrus calleryana, the Callery pear, which the United States Department of Agriculture introduced from Asia to the United States in 1916 (https://www.invasive.org/alien/pubs/midatlantic/pyca.htm) to combat blight in the common pear, Pyrus communis.  (Obviously, common pears, along with other species that are used for food, largely benefit humans, so no one is suggesting that they should not be grown.)  Bradford pears are incapable of reproducing with other Bradford pears, but (as any fan of Jurassic Park would likely guess) this doesn’t stop them from proliferating; they just do it by cross pollinating with other calleryana cultivars, spreading this invasive species far and wide.  

One of several Callery/Bradford pears in the neighborhood.  Actually, this looks like it might be a "colony" instead of just one tree.

Chinese tallowtree, or popcorntree, is another common invasive.  Introduced to the U.S. in 1850 (http://www.gri.msstate.edu/ipams/species.php?SName=Triadica%20sebifera), it is known for the waxy compounds found in its seeds, which have been used for candle- and soap-making (https://plants.usda.gov/plantguide/pdf/pg_trse6.pdf).  Because the plants are toxic, few animals will eat them, so there is next to no biological control of their populations.  The USDA states that eradication is “virtually impossible,“ that the trees may begin reproducing at the age of only three years, and that they can continue breeding for 100 years (https://plants.usda.gov/plantguide/pdf/pg_trse6.pdf)– all of which make this species a particularly bad choice for North American naturescaping.  Don't grow it here!

The tallowtree tends to have a nice shape, which explains why it's so frequently planted.

Tallowtrees have heart-shaped leaves.  The seeds, which aren't in this summer photo, resemble popcorn.

In one sense, shrubs may be the stumpier cousins of trees, but their lushness more than compensates for that.  Along with tree saplings, shrubs comprise the understory layers in forest ecosystems.  However, invasive species, such as privet, don't always need to grow within wooded areas; they frequently thrive wherever their seeds have been dropped.  While several introduced privet species range across North America, the most common in the mid-southeastern region (see http://www.gri.msstate.edu/ipams/species.php?SName=Ligustrum%20sinense) is Chinese privet.  There seems to be some conflicting information regarding when, exactly, Chinese privet was introduced to the United States—some sources say 1852, while others, including the previous link, say 1952.  The latter date might have been a typo, since the 1852 date shows up in sources such as the websites for the USDA (https://plants.usda.gov/plantguide/pdf/pg_lisi.pdf), the University of Florida’s Center for Aquatic and Invasive Plants (https://plants.ifas.ufl.edu/plant-directory/ligustrum-sinense/), and the Texas Invasive Species Institute (http://www.tsusinvasives.org/home/database/ligustrum-sinense).   

Chinese privet in the summer.

Privet leaves and unripe fruits.

At any rate, Chinese privet has become well established outside of its native range—so much that United States and regional noxious plant legislation isn’t even concerned with it and other privet species (http://www.gri.msstate.edu/ipams/species.php?SName=Ligustrum%20sinense).  Eradicating privet from the U.S. is basically a lost cause.  Its fruits are eagerly consumed and widely spread by birds and other animals; it possesses tough, hardy, evergreen foliage; it forms extremely dense thickets; and, if cut, it regenerates quickly from its extensive underground root systems.  Several privet hedges in the neighborhood where I live have been cut before, but that didn't stop them from coming back with a vengeance.  

A mockingbird and privet, photographed in Arkansas in 2013.  Birds are attracted to privet berries.

I have written about two bird nests—White-eyed Vireo and Northern Cardinal—that I observed in a thorny olive (or thorny elaeagnus) near the house.  As I mentioned in the Northern Cardinal post, the upper branches of thorny olives grow long and snake-like for the purpose of clinging to whatever is directly above them.  While this species—introduced in 1830 from Asia (https://www.fs.fed.us/database/feis/plants/shrub/elapun/all.html)—lags behind privet in terms of invasiveness, it’s still a problem in many areas, forming spiky, impenetrable barriers.  The website for the Geosystems Research Institute at Mississippi State University lists a number of methods used to control thorny olives (https://www.gri.msstate.edu/ipams/FactSheets/Thorny_olive.pdf), but describes these as “difficult,” “labor intensive,” “slow,” etc.  Essentially, even though the thorny olive does provide a certain amount of seclusion and protection for birds during the nesting season, there are much better options that don’t choke out all other vegetation in the ecosystem!

Long "tendrils" on the thorny olive.

The undersides of the thorny olive's leaves have silvery scales and sparse, tiny brown spots.

In general, all of these introduced shrubs and trees flourish in their new environments due, in part, to a lack of herbivory by insects.  Native insect species didn't evolve with these plants, so they generally lack the ability to effectively utilize them.  Many studies have shown that insect communities tend to decline in the presence of nonnative plants.  A paper written by Berghardt et al. (2010) and published in the scientific journal Ecosphere (https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.1890/ES10-00032.1) describes how Lepidopteran (that is, butterfly and moth) communities showed both lower population sizes and less species diversity on nonnative plants than on native ones.  Given the fact that many songbird species rely on caterpillars as a food source for their nestlings, the results of this study should definitely ring some alarm bells.  Fewer caterpillars mean less leaf damage, but also impoverished ecosystems.