Invasive Exotic or Horticultural Racist?

Peter Latz Landscaftspark ailanthus altissima tree of heaven bosque planting

The formal Ailanthus bosque at Landschaftspark

Every once in a while the ranting about invasive exotic plant species achieves a pitch that starts calling up images of the Third Reich in Germany and the Japanese detention camps in the US, so I get it when people want to go completely the other direction and embrace every new bit of horticultural bling that appears. Peter Latz’s Landschaftspark in Duisberg Nord, Germany is brilliant- an amazing space.  My irk with the park: Latz threw a formal grid of trees, a fancy bosque that calls up sites like Versailles and the Tuilleries, into the park and they were all Ailanthus altissima.  This is not a tree known for its neighborly, get-along-with-the-surrounding-vegetation attitude.

Invasive vs Exotic

Is there a middle ground? I think so.  An exotic plant is something that isn’t native to somewhere.  The trouble with that definition is the pesky fourth dimension: time.  Everything travels around, so is it native if it was here when Thomas Jefferson was President? (A LOT of imported plants include Jefferson in their ‘how I came to America’ story.) Or does it have to pre-date Columbus?  What about the mammoths?  Does it count if the seeds clung to mammoth fur in order to cross the Bering Land “Bridge”? (Which could more accurately be called the “Bering Subcontinent”, it was so wide.)

juniperus virginiana eastern red cedar

Controlled burns on the prairie keep the eastern red cedars in check.

Eastern red cedar not only isn’t a cedar (it’s a juniper, Juniperus virginiana), it’s not even from the east coast, let alone Virginia.**  The golden rain tree (Koelreuteria paniculata) that the National Arbor Day Foundation hands out to encourage people to plant native trees across the United States isn’t native to the US at all; either Jefferson brought it here or one of his friends sent him a sample thinking he’d like it.  (He did, I don’t- I find it gaudy.)

Food is even more world weary than trees: tomatoes aren’t from Italy, potatoes aren’t from Ireland, Mulligatawny is not an English soup, and okra is from Africa not the Caribbean.

So things move around. Asia sends North America some azaleas, North America sends Europe the black locust tree.  (We’re pretty happy about this exchange, Europe is less than thrilled.)  Exotic exshmotic.

campsis radicans trumpet creeper bignoniaThe concept of native can be highly regional too. Plants don’t care about political boundaries, their interest is climate and habitat, so plants that are deeply indigenous to the Pacific Northwest may be deeply problematic on the eastern seaboard but technically still “native to America”.  But even in their home terrain, it’s not like our natives all behave themselves either.  Trumpet vine (Campsus radicans) will be amongst the triumphant in the final Battle of the Invasives, and the beautiful black locust (Robinia pseudoacacia) will stand its own too.  Although ineligible for ‘invasive’ status where I live, they are certainly aggressive.

So why not pitch the whole ‘invasive exotic’ concept?  Well, the critters, for one.  Plants have always moved, creatures have always moved.  They both used to do this slowly enough that they could adapt to each other, though some niches remain untapped even after all this time, hence the existence of driftwood.   Right now, a sweeping number of changes are happening with electrifying speed, and the birds and the bees are having trouble keeping up.

But it goes deeper than that. My working definition says a plant as invasive if, instead of joining the extant ecology, it changes the structure or function of the ecosystem.

The Structure or Function of an Ecosystem

acer platanoides acer saccharum

Norway maple (left) vs. sugar maple (right)

Structure: the east coast has a swath of woodland wildflowers known as the spring ephemerals.  Before the trees leaf out each year, a slow but joyful riot of flowers seeps across the forest floors.  Spring beauties, bloodroots, mayapples, troutlillies… they show up, do their thing and by summer they are completely gone again.  As the Norway maples (Acer platanoides) seed themselves into the woods, something awkward happens.  The Norway maple leafs out earlier, casts a denser shade, and holds its leaves further into winter than any other maple.  The spring ephemerals had plans for that sunshine; around these maples, they show up and are shaded out or never get triggered to show up at all.

Function: a flood plain can be tremendously helpful.  Just when the river is deepest and moving more water at a greater speed, an outlet valve gets opened, and the flood waters spread out over an enormous area.  The water slows down, drops some of the sentiment and debris load the faster speeds had allowed it to carry.  This keeps the bottom of the river from being scoured clean of plant life, and keeps the bigger logs from wreaking more havoc downstream.

Quite a number of floodplains are having trouble functioning because they have filled up with feral roses and burning bushes (Euonymus alatus). It used to be that the plants that grew in a flood plain either didn’t expect to be there long and thus had a quick and dirty life cycle, or expected to lose some limbs on a regular basis (leading to a notorious dislike of flood plain trees in near houses and cars- guys like box elder [Acer negundo] will throw down a limb just cuz it’s Tuesday).

euonymus alatus burning bush

Euonymus seeding itself into a woodland.

The new shrubbery, however, stays put.  It bends over some, but it also thwarts the water quite a bit.  More of the flood stays in the channel, keeping the flood volume high, the speed high, the debris carrying capacity high, the bottom scouring and bank eroding high…  It’s only a matter of time before some downstream town loses a bridge to a flood and sues an upstream town for failure to maintain their flood plains.

Changing the structure or function of an ecosystem is the end result.  Earlier red flags include high reproduction rates and rapid growth. It’s like they call “shotgun!” on every temporarily vacant seat in nature and then never give it up. Eventually, they end up driving the whole car and taking us somewhere else entirely.

Lots of plants are new to where they are growing, but only a small handful can’t seem to get along with the others.  This important distinction makes exotic and invasive two wildly different concepts.

**(2/1/12 update: oops.  Got that wrong.  I’d misunderstood something Peter Hatch [head of landscape at Monticello] said.  Eastern red cedars hate growing in the shade, so when Virginia was heavily forested there weren’t many.  As the tobacco trade swept the trees from the state, lo and behold, all these little cedars started popping up.  At the time, apples were a drinking fruit- hard cider was like  tea to the Brits.  Clear the land, plant the apple trees and for 10 years you’ve got a pristine Eden of a spot to grow apples.  

Trouble is, after about 10 years, the “cedars” are old enough to start spitting out pollen, et voila– apple cedar rust.  Jefferson(‘s slaves) clearing the land (at his command) gave the cedars space to invade, but he didn’t (have his slaves) plant them intentionally.  [I put those bits in parenthesis to point out those bits casually dropped (erased) from most stories about life back then and there.])

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