The role of fire in a tallgrass prairie

It’s not too early to think about next spring’s controlled burns. One point that arises is burn frequency.

Restoration should be guided by the goals of the project. Thus, if the focus is tallgrass prairie, a fire-dependent ecosystem, then annual burns are essential, especially if it is a prairie remnant. (Prairies planted on former ag fields are a different topic.)

My recent post which analyzed the extensive research by the Konza Prairie group showed that for brush control in tallgrass prairies annual or biennial burns are essential.

Brush control is only one of the important reasons for burning a prairie or savanna remnant. The other major reason is because the forbs and warm-season grasses demand it. There is a vast amount of research that shows that without fire the species that we most want will have difficulty thriving. One of fire's main accomplishments is to remove the thick prairie thatch so that the delicate new prairie plants can reach the light.

The thickness of the prairie thatch varies from year to year depending on the summer rainfall. Sometimes it is so thick that delicate plant seedlings can not find their way through it.

A thick prairie thatch from a single year's growth. This is the prairie grass from the previous season, being mowed to widen a fire break before burning. Last year’s growing conditions were very good, so the grasses are lush. Also, snow was light so that the plants remain upright, guaranteeing a hot fire. 

Concerning the prairie thatch, pioneering papers by Weaver in Nebraska and Rice in Oklahoma are well worth consulting. Rice in particular showed that one role of fire was to remove last year’s thatch so that sun could reach the soil and encourage the growth of warm-season plants. The graph below makes this case very well.

Rice, E.L. and Parenti, R.L. 1978. Amer. Jour. Botany 65: 1091-1097. Rice’s burns were done in the spring dormant season.

In our own simplified work we divided a well established prairie into two halves, burning one and leaving the other unburned. The result with Toby’s Prairie is shown in the photo below. We obtained a similar result with the Pocket Prairie.

Toby’s Prairie divided into two blocks. The block to the right was burned and the block to the left was unburned. The difference in growth of the prairie grasses (Indian grass and little bluestem) is striking. The burn was done on 5 April 2006. The photo was taken on 15 Feb 2007.

How does fire act on a prairie? The photo below shows fire backburning through a prairie consisting of Indian grass and little bluestem. After the ashes are blown away, bare soil will be exposed. In this photo, because of the heavy snows the previous winter, the grasses lay flat, thus ensuring that the flame heights are low.

The action of fire on dormant prairie grasses. In such a thick mat, delicate prairie plants have a hard time finding the light.

Suppose you don't burn? The prairie plants will eventually find their way through the thatch, probably two weeks later. Eventually a new, somewhat impoverished, prairie will form which will form a new layer of thatch. Some of last season's thatch will decompose but not all, so the thatch gets thicker, and in the next year even thicker. Eventually the prairie will reach a point at which most prairie plants cannot survive. But the woody plants can, and eventually the prairie will become some sort of wood lot.

Read my previous post for more details on the long-term future of a prairie that is not burned.