Thursday, December 12, 2013

The Problem With Exotic Plant Species

         As a naturalist, I often get questions about what people should plant in their yards.  They might ask me, “Well do you think hostas would work well here?” or they might say, “Oh I just love wisteria, it is so beautiful.  Will it grow in my yard?”  When I say that I wouldn’t plant either one, they look at me like I have bugs crawling out of my ears.  It’s not that I don’t think these plants aren’t nice, but I personally prefer native plants.  Oh and by the way, wisteria is a non-native invasive unless you just happen to find the native variety.  So many of our native species are so under-appreciated because they often aren't as showy as most ornamentals (hence the name), butmore often than not they are a better alternative than something exotic (non-native to this region) which can often be more of a problem than anything else.
A kudzu monoculture.  This is why they call it "the vine that ate the South".
            Let’s think about it for a minute and study a plant we all know:  kudzu.  I can’t tell you how many times I have been asked the question, “What is that plant that grows everywhere and climbs up in the trees?”  Some will even comment on how beautiful it is.  You can usually tell who isn’t from around here if they don’t know what kudzu is.  Kudzu was originally brought to this country from Asia in the late 1800’s and displayed as an ornamental at the Philadelphia Centennial Exposition in 1876.  It was later used as a forage crop and an erosion control plant along highway and railroad rights-of-way in the wake of the dust bowl.  No one would have ever imagined that such a useful plant would launch a takeover south of the Mason Dixon line that would rival Sherman’s March during the Civil War (or War of Northern Aggression if you are a true southern patriot).  Because kudzu comes from a latitude and climate similar to ours, it is well suited to grow here.  In its natural environment, it is not nearly so invasive and has natural predators and other limiting factors which prevent it from covering the world.   That is not the case here.  In our little part of the world, kudzu has no natural predators, at least none that can effectively keep it in check without destroying native species in the process.  This is not to say that things don’t feed on it because they do, but there is not enough consumption to control its growth.  Kudzu, to its credit, is high in protein.  In fact, among legumes it has among the highest protein potential.  Its root is high in starch and the flowers are a great source of sugar, hence kudzu jelly.  In Japan, kudzu is eaten in many different forms and is also used to make paper and other things.  So one might argue, “Well if it has all these good qualities, why is it so bad?”  The answer is that it inhibits biodiversity.  When kudzu becomes established to the point that it begins to climb trees, it begins to eliminate species in the area in which it grows.  Plants that were in the understory, from small trees to wildflower species, are suddenly cut off from their light source.  All the many vines as they blanket the ground and eventually climb into the treetops, take root and suck up the nutrient supply from the ground, depriving the native species of their underground food supply.  If a plant can’t photosynthesize because of no light, and if it can’t get nutrients from the ground, the plant dies.  This means whatever benefits from that plant is also affected, whether it’s another plant, or an animal.  This causes a dramatic change to that ecological community, limiting biodiversity and creating a monoculture of a non-native species.  This is the impact of invasive exotics.
The flower of kudzu (Pueraria montana)
            Now I’m certainly not saying that all exotics are bad because they are certainly not all invasive.  Look at the number of foods that we grow and eat.  Many of these are not native to our area.  What I am saying is that caution should be taken anytime an exotic species is planted, particularly ornamental species.  As climate change is occurring, many plants that have been held in check by our mild climate, could some day become a problem.  Many plants, under optimal conditions show a remarkable ability to adapt to subtle changes in their environment, particularly when predation is low or non-existent.  Kudzu is a perfect example.  Kudzu used to be limited to the southern states but it is slowly adapting and is now found as far north as southern New York, so it is no respecter of geographical boundaries.

            In an area as rich in biodiversity as Hickory Nut Gorge, the control of exotic invasives is very important.  Of course the threat that is receiving the most attention right now is the Hemlock Woolly Adelgid, which is not a plant (it’s a bug), but it severely impacts an important plant species, the hemlock.  But, the Hemlock Woolly Adelgid is a newcomer, and while its impact is tragic, there are other exotic, invasive species out there that are changing our ecosystem and are not getting the attention that they should be.  
Hemlock Woolly Adelgid (Adelges tsugae)
           Take for instance, oriental bittersweet: this stuff has practically taken over the Asheville area and is quickly spreading out from there.  Every year there are more infestations of this plant in the Gorge.  Up north, this plant is like kudzu.  Down south, it could potentially be worse than kudzu, for several reasons:  (1) It’s a climber, like kudzu; (2) it has a nasty talent for lying dormant until a tree blows over or some type of disturbance creates a light gap; (3) unlike kudzu, over 90% of its seeds are viable and will likely germinate under favorable conditions; (4) it is a whole lot harder to kill; (5) unlike kudzu, people like bittersweet because it has pretty berries in the winter time; and on, and on, and on. 
Oriental Bittersweet (Celastrus orbiculatus)
 Japanese knotweed is becoming more and more prevalent along stream banks.  Tree-of-heaven (more like “hell”) and princesstree are growing up in cutovers and powerline rights-of-way.  These are pests that have been here a long time but have escaped our attention because the results are much slower than those of something like hemlock woolly adelgid, however, the results and impact are the same and failure to manage these pests (because eradication is often not an option), will result in further impact to our native species and the stability of their ecological communities.
Japanese Knotweed (Fallopia japonica)
Tree-of-heaven (Ailanthus altissima)
Princesstree (Paulownia tomentosa)
Some advantages to going native are:  (1) less maintenance; (2) less watering; (3) less predation from deer, woodchucks, and muskrats; (4) you can choose a plant pallet for four seasons; (5) you can still attract hummingbirds and butterflies.  I could probably think of some more but I’m rather limited in space in this column.

The point to all my ramblings is that there is more to taking care of our environment than clean water and clean air.  The things we plant also have a great impact on our natural environment and we have to be careful about what we introduce.  We don’t know what the impact may be in the future.

To see some other exotic invasives that are impacting our natural communities, click on the link:
Exotic Invasive Species of Hickory Nut Gorge

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