Buckthorn and the oak savanna

When we began restoring our oak savannas at Pleasant Valley Conservancy, we had to deal with a huge amount of buckthorn (Rhamnus cathartica). We got rid of it, by brush cutter, chain saw, and Garlon 4 herbicide treatment (basal bark), and spent three or four years getting an herbaceous understory established in face of buckthorn allelopathy (the critter produces a potent toxin that stops the growth of other species). While we were doing this work, I did not stop to ask why the buckthorn infestation was much worse in our savannas than in our oak woods or prairie remnants.

Recently I ran across a paper by Apfelbaum and Haney, written in 1987, that explains the relationship pretty well. These researchers did quantitative measurements of buckthorn density in different kinds of oak habitats, from close oak woods to virtually open prairie. They found that buckthorn density was highest when the oak canopy had about 60-70% cover, was much less at low canopy densities, and was almost nonexistent in heavily shaded oak woods.

Although Apfelbaum and Haney did not explain this relationship, it seems reasonable to me that in the closed oak woods, it is too shady for significant buckthorn growth. In the open prairie, especially tall grass prairie, the shade from quick-growing prairie grasses would suppress the significantly slower-growing buckthorn. (If the prairies were burned, the significantly hotter fires might be an additional factor.)

These data fit nicely with our experience carrying out restoration work on oak savannas. Getting rid of buckthorn in the oak woods has been relatively simple. All it took was a program to carry out basal bark treatment with Garlon 4 in a single winter. The buckthorn were scattered and relatively easy to treat.

On the other hand, our oak savannas had very dense buckthorn stands. It took lots of effort to cut all the buckthorn, treat all the cut stems with herbicide, and build the large brush piles needed to get rid of all the biomass.

Another consequence of the development of these dense buckthorn stands is that the herbaceous ground layer is completely eliminated. This is probably partly because of allelopathy and partly because of the dense shade created by the buckthorn shrubs.

The Apfelbaum/Haney paper has apparently never been published, but is available on-line from the Applied Ecological Services web site: www.appliedeco.com/Projects/MgmtDegradedOakSav.pdf