By Monique Botha
I recently conducted a field experiment to test the responses of tree saplings to fire at the University of Pretoria experimental farm. This forms part of my post-doctoral research that I am conducting under the supervision of Prof. Sally Archibald and Dr. Michelle Greve.
My research mainly focusses on the mechanisms of forest encroachment in Buffelskloof Private Nature Reserve, an open savanna system near Lydenburg in Mpumalanga. Here, encroachment often occurs in the form of bush clumps, which start out as individual trees under which forest pioneer trees establish. With time, these bush clumps expand as forest climax species establish, to eventually form small forest patches, which result in the loss of grassy habitat. I am specifically interested in the survival strategies of tree saplings in their first few months of development in this encroachment process. While mature trees may be able to persist under environmental stresses such as fire in an open savanna, their establishment will be limited by the survival of their saplings. Investigating survival strategies at the sapling stage is an important step towards understanding the mechanisms of tree encroachment in open savannas.
We selected 17 tree species (15 individuals each), representing one of three successional stages in bush clump formation (savanna, forest pioneer and forest climax) for the fire treatment. The trees were planted in October 2017 and grown under 40% shade-netting in 3.57 L soil bags filled with a growth medium composed of a 1:1:1 ratio of topsoil, compost and river sand.
The day before the fire trial, the soil bags were placed in a 3 x 6 m trench in an open plot of land and covered with soil so that the tops of the soil bags were level with the soil surface. A uniform fuel bed of dried grass, approximately 20 cm deep, was then scattered around the base of the trees.
A headfire was created by igniting the grass in a single line on the upwind side of the plot.
After the burn, the average char height (the vertical distance from the soil surface to the highest point of blackening) for the trees were measured at 47 cm. The trees were then dug out and moved back to their original spot in the nursery, where they will be monitored on a monthly basis to record their survival rates. The number of resprouting buds as well as height and diameter growth rates of surviving individuals will also be recorded in response to the fire treatment.