An interesting response of sumac to periodic burns

Sumac can often be a serious invader in prairies and savannas. Although it is regularly top-killed by fire, it reprouts immediately. In addition, sumac root suckers readily, so fire can often appear to stimulate sumac growth.

Recently I learned of a long-term fire study in oak woodlands of the Missouri Ozarks where sumac greatly increased with a four-year burn interval, whereas there was no sumac in unburned or annually burned plots. What is going on here?

The study area is at the University Forest Conservation Area in Butler County in southeastern Missouri. There were three treatments: unburned, burned annually, and burned on a 4-year interval. This study began in the early 1950s and has been continued up to the present (see reference).

Knapp and co-workers made extensive measurements of the understory and overstory in these experimental woodlands. There were a number of interesting observations, but the focus here is on  sumac growth.

Neither the unburned controls or the annually burned plots had any significant sumac.However, sumac was the dominant shrub in the plots burned every four  years (see photo). The species of sumac was Rhus copollina.

Photo courtesy of Benjamin O. Kanpp

How should we explain this?

First, it is not surprising that the unburned plots have no sumac, since sumac does not grow in shade.

Second, it is not surprising that the plots burned annually do not have any sumac, since it would be top-killed by fire. In fact, there is virtually no understory shrub layer in the plots burned annually.

But how do we explain sumac thriving in the plots burned every four  years? Most likely, the fire is unable to carry through the sumac thickets. The stem density of a sumac clone is high, and the leaves will prevent oak leaves, the principal fuel, from reaching the ground.

Stem density of  a sumac clone. The ground is essentially bare. Fire would probably not carry through

It appears that once sumac gets started, there is no stopping it!

This is not the first time I have heard about 4-year burn cycles not being sufficient to control brush. See the paper from Konza Prairie by Briggs, cited below.

Knapp, B.O., Stephan, K. and Hubbart, J.A. 2015. Structure and composition of an oak-hickory forest after over 60 years of repeated prescribed  burning in Missouri, U.S.A.. Forest Ecology and Management 344 (2015) 95-109.

.Briggs, J. M., A. K. Knapp, and B. L. Brock. 2002. Expansion of woody plants in tallgrass prairie: a fifteen-year study of fire and fire-grazing interactions. American Midland Naturalist 147:287–294