A Shrub and Its Buffer Zone

Confession: I can be really bad at recognizing patterns. It's not my strong point. As such, I tend to remain skeptical of what I think I am seeing. Sometimes I am right, though. A recent stroke of luck came while I was hiking through some scrubby habitat in northern Florida. I walked out into a clearing to get a better view of a pond when I saw some interesting shrubs up on the banks. The area was largely covered in tufts of warm season grasses but the shrubs seemed to be ringed by barren, white sand. My botanist friend was kind enough to point out that the shrubs I was looking at were none other than sandhill rosemary (Ceratiola ericoides). With a name attached to these species, I could dive into the literature on this plant to see if I was actually seeing a true pattern or not.

Before we get to the meat of the article, it would be nice to first introduce you to the sandhill rosemary. C. ericoides is not a true rosemary at all. Its resemblance to the culinary mint is purely superficial. C. ericoides is a heath (family Ericaceae) and is the only member of its genus. It can be found growing in dry, scrubby habitats throughout southeastern North America. It is dioecious meaning each shrub is either male or female. Unlike its showier cousins, this heath is not pollinated by insects. Instead, it relies on wind. Because of this, C. ericoides flowers are highly reduced structures produced in the axils of leaves near the tips of its branches.

The scrubby habitats it calls home are challenging places for plants to live. The sandy soils drain water rapidly and are prone to shifting with the winds. The stout, needle like leaves are a fantastic adaptation for minimizing water loss during the hottest, driest months of the year. Also, regular fires are the norm. Burning is vital to the health of this region, however, C. ericoides does not seem to be very well adapted to cope with it. Instead, these shrubs are killed by fire, relying on the seed bank for regeneration. But that isn't the only thing this species has evolved in order to cope with a regular fire regime. As it turns out, C. ericoides may be utilizing chemical warfare to increase its chances of survival.

C. ericoides is what we call "allelopathic." Allelopathy can be defined as "the direct or indirect harmful or stimulatory effect of one plant on another through the production and release of chemical compounds" and is "an important form of plant-to-plant interference in natural and agricultural settings" (Rice 1984). In other words, allelopathic plants utilize chemicals that are toxic to other plants in order to gain an upper hand when it comes to acquiring space to grow. For C. ericoides, this may also mean keeping itself safe from fire.

As anyone who has tried to light a fire knows, a little fuel goes a long way. In the wild, plant materials make up the fuel load. The more plant material lying around, the more fuel the fire has to burn through. Much of the understory of these habitats are filled with fire adapted plant species. Grasses are possibly the most fire tolerant of them all. What's more, grasses often release specialized compounds when they burn that actually increase the temperature of the fire. This is often bad news for less fire adapted plants in the vicinity. If grasses and other fire adapted species were to be growing near C. ericoides, they would not only increase the chances of fire reaching the shrub but also increase the intensity of the flames. This is where the allelopathy comes in.

Evidence from greenhouse experiments has shown that the allelopathic compounds from C. ericoides inhibit the germination and growth of other plant species. This is especially true for fire adapted grasses. Although these chemicals are abundant in the leaves of C. ericoides, research also shows that they are produced in the roots as well. As the leaves fall off, they decompose and release their chemical cocktails into the soil immediately surrounding the shrub. This keeps plants at bay in the immediate vicinity while the roots go a bit further. Since the roots of C. ericoides branch outwards from the plant, the effectiveness of its chemical warfare is increased by a few meters radius around the shrub. Indeed, it is believed that C. ericoides is actually keeping the surrounding area clear of most fire adapted vegetation. In doing so, the shrubs are creating a fire-free buffer zone.

Although more work needs to be done in order to understand the degree to which these effects occur in the wild, this is nonetheless tantalizing evidence that such plant interactions are shaping the landscape in ways we don't fully understand. On a personal note, it was exciting to know that there really was something to the barren ring patterns I observed around each shrub.

Further Reading: [1] [2] [3]