The effects of a prescribed summer burn on the native grass Andropogon virginicus on a subtropical ranchland

Background/Question/Methods

Although ranchlands contain planted pastures, they also hold a mosaic of less altered native grasslands and other natural community types. While remnant native grasslands include many native species, the dominant grasses are often less productive and provide less nutrition for grazing animals than planted pastures. Ecologically, native grasses are important to maintain biodiversity, while from a production perspective native grasses are often more tolerant to drought and pests, and provide forage during times when planted grasses are dormant.  To encourage maintenance of native grasslands on ranches it is important to examine how management of native grasses could improve their use for grazing. One potential management tool is fire, a natural disturbance in Florida. Andropogon virginicus, a common tall-prairie bunch grass native to the Eastern United States, is widely distributed throughout Florida ranchlands, and has low utilization by grazing cattle.  We examined how A.virginicus responded to a late summer burn, coinciding with both the historical fire season and the time of year when the nutritional value is declining. Growth characteristics of 30 individuals in a burned and control treatment representing three diameter based size classes were recorded in August (pre-burn) and October (post-burn) 2012, and an additional 60 plants were clipped pre and post burn for tissue analysis of crude protein content and digestibility.

Results/Conclusions

The burn was less intense than expected and followed by a flooding event.  Under these conditions we found that size class, burn and month significantly affected plant growth characteristics and nutritive value. Following the burn, rosette diameter decreased in the burned area across all size classes. Live stem biomass significantly decreased after the burn while increasing in the un-burned area. Average live leaf biomass was significantly lower in both burned and un-burned areas in October, but the response was stronger in the burned area. Following the burn, both the percent crude protein content and digestibility of the live biomass and the average leaf to stem ratio were significantly higher in burned areas.

Although the burn decreased the quantity of biomass available for grazing, there was an increase in the quality, suggesting that for a rancher a late summer burn might be a valuable way to increase utilization of Andropogon dominated pastures. From a conservation perspective, decreasing the rosette diameter of the clumps may create spaces in the landscape for a greater diversity of species.