Author: Bill Ward
A “new” population of exotic trees at Cibolo Nature Center took me by surprise last Friday. Where I expected to find a few tree of heaven (Ailanthus altissima) saplings, we found 147 plants and some of the trees were huge.
Last fall during our annual survey of trees and bushes in the riparian zone, we noticed several tree of heaven saplings and made plans for the Riparian-Woody-Plants Team to remove them this May. Every spring, we remove invasive exotic trees which are trying to take over the natural woods along Cibolo Creek at the CNC. This is part of an effort to restore the native habitat by removal of plants, instead of by re-introduction of plants.
So earlier last week I was bragging how we were going to eliminate this “recent” little stand of tree of heaven before it ever had a chance to be a problem. The place where these were growing is on a quarter-mile-long island between the main Cibolo Creek and an offshoot channel, way downstream just above the confluence of Menger and Cibolo Creeks. Park visitors rarely have occasion to go there.
After we began cutting trees, I was a little embarrassed that a count of tree rings showed that a couple of those trees of heaven sprouted there in 1979. Not every tree of heaven in that area was recent by any means!
In past years I must have failed to look up high enough into the canopy where these big trees stood; I didn’t know they were there. I hadn’t even seen just how many saplings had sprouted recently. A part of the island was destined to be a jungle of trees of heaven.
Why did tree of heaven apparently begin to grow here in 1979? Chuck Janzow reminded us that during August of 1978, this part of the Hill Country saw one the most devastating and deadly floods ever recorded. Most likely this island of alluvium in the Cibolo Creek was shaped or altered by that flood, and most vegetation was swept away. After the flood, tree of heaven seed from trees growing in Boerne or elsewhere upstream must have germinated on this large bar of alluvial sediment.
How ever they got there, why would we worry about removing tree of heaven at Cibolo Nature Center? Wouldn’t this tropical-looking tree with its large compound leaves and stalks of yellow-green to reddish spring flowers look pretty nice along Cibolo Creek? It might look good, but it wouldn’t be good.
Invasive exotic plants are not friendly to our natural ecosystem. They wouldn’t be invasive (i.e., overtake and replace the native plants) if they had to abide by the checks and balances which keep our natural ecosystem healthy. Who knows how many ecological checks and balances are destroyed when even one exotic plant species is introduced?
For example, native plants undoubtedly have developed a special symbiotic relationship with the soil biosphere, and probably the soil biology has evolved in response to the local native vegetation. The roots and roothairs of most plants are interwoven with the hyphae of certain fungi in a symbiotic association (mycorrhiza) that promotes the nutrient intake of that plant.
Introducing a non-native plant with its exotic mycorrhiza could upset the soil biosphere in ways advantageous to the exotic species, but detrimental to native plants. If so, this also could affect the above-ground food chain.
Many tiny critters at the bottom of the food chain depend on food and shelter provided by certain native plants. Certain larger animals such as insects feed on certain of these smaller organisms. Certain insects, in turn, provide nourishment for certain fish, frogs, lizards, birds, and mammals.
And so it goes up the food chain, all evolved into a natural balance that provides checks and balances to sustain the ecosystem. One species of exotic plant could upset the balance all the way from the soil zone up, and thus change the natural system, probably not for the better.
Tree of heaven is one of the most-invasive exotic trees in the US, and it has had time to invade from coast to coast. This Chinese tree was introduced into Philadelphia gardens in 1784 and also was brought into California by Chinese immigrants during the mid-1800s. Some people I know can remember the 1940s best-selling novel and popular movie called “A Tree Grows in Brooklyn”. The “title tree” was tree of heaven. It’s all over the place!
Along with the tree of heaven on the alluvial island in Cibolo Creek, we also removed 136 Chinese tallow trees. The day before that, we removed 332 Chinese tallow from another quarter-mile stretch on the other side of Cibolo Creek. Chinese tallow is such a destructive and costly invasive exotic which the Texas legislature officially added it to the list of Texas noxious weeds. That plant is not supposed to be sold in nurseries any longer.
It will take a long-term diligent effort to keep the Cibolo Creek riparian ecosystem healthy. A major element in this effort will be the continued removal of invasive exotic plant species.
