Tired Oaks

Life in nature's bomb shelter

Runner oak forms an abundant ground cover in the Guld coastal plains longleaf pine forests. It is rarely taller than calf-height. Drawing by WRT.

            A walk in the higher, drier areas of the piney flat woods of Florida often takes you through dense, ankle-high ground cover of a tough-leaved woody plant that, on closer inspection, bears acorns, in other words, is an oak. Now, most people would expect oaks to be trees, and would be surprised by the identity of this scruffy, creeping stuff. No mighty oak, this!  Or is it?

A dense carpet of runner oak, regrown three months after a fire.

            Pulling up some of the plants reveals more surprises, for all the plants in a patch, even when the patch is large, are connected by underground stems that sprout branches, roots and leafy, above-ground shoots at intervals.  The entire connected patch is probably a single individual, an oak clone. So, what we have here is a tree that has gone underground, a lazy tree that saw no advantage in putting a lot of expensive resources into a weighty trunk to create a lofty, shady canopy, and seems satisfied with shooting up short, dinky branches while lying down.

Left: a dense stand of runner oak. Right: pulling up some of the runner oak shows that all the above-ground vegetation arises from underground stems and roots. Stands arise largely as clones that spread through such underground stems.

            On closer examination, it begins to look less like laziness and more like good sense. Runner oaks (there are two species in the flatwoods) grow in a habitat that, under natural conditions, burned every two or three years, so it would be rare indeed for the tree to grow tall enough to avoid death in the next fire. So rather than evolving an escape from fire, runner oaks acquiesced to the inevitable and regularly sacrifice their above-ground parts to fire. This makes more sense once you realize that most of their biomass is underground, immune to fire, and the incineration of the stuff above ground merely leaves behind ash to fertilize the regrowth of new sprouts from the dense fabric of underground stems.

A ground fire has killed the above-ground vegetation. Under natural conditions, this happened every year or two.

            But look around--- runner oak isn't the only flatwoods plant that lives this way. Pull up a few gallberry, or shiny blueberry, or gopher apple, or lyonia, or backen fern, and you will see that they are all clones springing from a fabric of dense underground stems from which they regrow after fires. 

Gopher apple (left) is another cloning species that spreads by underground stems (right). It is more common in higher, drier sites.

Gallbery (left) spreads outward from wetland margins by means of underground stems (right). Also note the palmetto in the left image.

Shiny blueberry (left) clones by means of a dense mat of roots and underground stems (right). It produces small, sweet berries, but it is unlikely that the plant often starts from seeds.

These clones spread more like bacterial colonies, or mold on a jar of jam than seed plants, advancing on visible fronts like Napoleonic armies. When fire is as frequent as every one to three years, these armies cannot advance from their wetland margin bases, and the fast-recovering bunch grasses hold the field. Natural fires in the piney woods were started mostly in the spring “lightning season” when thunderstorms were frequent, but rain was not. These annual fires burned until the summer monsoon rains extinguished them.

But with the reduction of fire frequency under human management, the gallberry, fetterbush and their ilk have marched upslope from the wetland margins, while the shiny blueberry, gopher apple and runner oak have marched downslope to clash and intermingle along wide fronts. It is a slow-motion war, a slow conquest played out over decades, the main losers being the grasses.

A scene in the piney flatwoods of the Apalachicola National Forest three months after a burn in early March. The regenerative powers of flatwoods ground cover resides almost completely underground and gives rise to a very rapid regrowth.

            In a landscape that is scorched over and over, year after year, the only sensible thing to do is to retreat to the bomb shelter underground. In the flatwoods, everybody does it, and it is not laziness, it is the only hope for survival.