June 10 – There was an article in the paper this morning about the Army Corps of Engineers removing trees from levees that hold back river water during times of flooding. Their thinking is that the trees destabilize the levees. Apparently they’re encountering a certain degree of opposition in a number of communities. People love trees – you can see that in the Midwest as you drive past one overplanted yard after another. Trees give us such pleasure, and worrying about 100-year floods probably seems like overkill to most people. One individual quoted in the article said that just the opposite was true; that, in fact, tree roots stabilize levees. So which is it?
The little bit of knowledge I’ve managed to glean about trees and their roots being destabilized was acquired in North Carolina during Hurricane Fran. It rained for two weeks almost non-stop before Fran hit. All of that water, penetrating the ground over an extended period, loosened the hold tree roots had on the soil, and when the 70 mile-per-hour winds that swept central North Carolina shook those trees, they came down by the score. Any soil held in place by those roots would easily have been cleared away by the torrential rain. But hold on – don’t jump to conclusions. There are variables we have to consider.
The soil in North Carolina’s piedmont zone is heavy red clay. Tree roots have difficulty penetrating it, so many of the trees that grow there are unable to put down deep roots. Compounding this inherent instability is the fact that pine trees, of which there are millions in the Carolina’s, are shallow rooters by nature. (Then again, their wood is soft, so they can bend rather than break.) What should quickly become apparent is that, in this part of the country, the trees cannot be counted upon to stabilize the soil when the roots are stressed during abnormal conditions. The vast majority of the time, the trees are an absolute blessing: it gets very hot in that part of the world, and trees give shade. But what about during extreme weather events, which have become far more common in our global-warming world?
On the other hand, there are undoubtedly many places in this country where trees can put down deep roots. A tree’s ability to hold the soil in place will vary by species, of course. Some trees, in addition to the main branches of the root system, send out smaller and smaller roots from those branches, reaching out in all directions, forming an almost fibrous mat. Such trees are likelier to withstand violent weather events, like flooding, and remain standing. Others will be neither shallow rooted nor deep rooted, but fall somewhere in between.
It seems to me it would behoove the Corps to know something about the trees that grow in a given area before they cut them all down (the approach they are currently taking). Plainly shallow-rooted trees are a liability during a flood; they could well be the problem the Corps says they are. Just as plain, it seems to me, is the fact that deeply-rooted trees could be a tremendous asset during a flood, holding in place precious soil that would need to be replaced, were it washed away. Bottom line? One size does NOT fit all when assessing the value of trees and their roots’ ability to stabilize soil. The Corps needs to be willing to put in the time and effort necessary to make these vital evaluations. It is counterproductive to clear away deeply-rooted trees and then lament their loss. Evaluate first, then cut. It is not only the most environmentally-beneficial use of tax-payer dollars, it is the most cost-effective.