Back to all articles

March/April 2024 | Angel Goldsmith

Disc harrowing for fuel load management in eucalyptus stands

Fire management

Fuel load management

Placement of firebreaks to counteract wildfires spread or for containment of fire within the plantation have become a standard practice in South Africa. However, silvicultural operations like thinning, pruning, and coppice reduction tend to increase the amount of available fuel within compartments. Therefore, under canopy burning is conducted in Pinus patula stands to reduce understory fuels. However, the latter presents practicality limitations in Eucalyptus stands due to eucalypts being fire sensitive species.

What are the effects of disc harrowing?
The implementation of slash burn incorporated with repeated disc harrowing in Eucalyptus plantations is practiced as an alternative to reduce stand fuel loading. This practice creates a break in fuel horizontal continuity to reduce rate of spread of surface fires as well as to reduce fuel loading on the disc swathes, thereby reducing fire intensity.

How is it done?
Disc harrowing is done using an agricultural tractor pulling a disc harrow to break up and incorporate slash and forest floor material into the soil. This is commonly done in swathes between tree rows, after every seven rows of trees. This tillage operation is commonly done repeatedly throughout the stand rotation. It should be noted that the strategic placement of disked swathes may vary from site to site, depending on the degree of fire risk and vulnerability of the compartment.

Benefits: In a fence line study of adjacent experimental plots, repeated disking (BD) was contrasted with non-disking (B0) treatments. Repeated disc harrowing significantly reduced the average oven dried fuel loading: The difference between the two treatments for humus was 19.0 t ha-1; for 10 hour fuels, 4.3 t ha-1; for 1 hour fuels (bark and leaf fraction), 7.3 t ha-1; and for 1 hour fuels (branch fraction), the difference was 2.7 t ha-1. The mean difference in total forest floor mass was 32.58 t ha-1 (Figure 1).
Figure 1: Combined fuel loading per size class across treatments
(Aa) Letters with different cases indicate significant differences among the treatments at p<0.05

Repeated disc harrowing significantly increased topsoil exchangeable cation quantities, S-value, and reduced bulk density. Topsoil exchangeable K, Ca, Mg, Na, and S-Value increased by 0.04, 0.34, 0.12, 0.01 and 0.51 cmolc kg-1 respectively following repeated disking. The topsoil pHKCl, extractable P, total N and C were not significantly different among the two treatments.

Stand productivity was similar among treatments, with basal area, volume, and plant biomass in the B0 treatment being 24.6 m2 ha-1, 212.5 m3 ha-1 and 134.4 t ha-1 versus 23.5 m2 ha-1, 202.6 m3 ha-1 and 127.5 t ha-1 for the BD treatment. Using a profile wall root study method, B0 treatment was observed to have a non-significantly higher root count of 30% on the top 10 cm soil depth when contrasted to BD treatment, which was 22% (percentage of the total root count on a 1 x 1 m vertical profile wall). All the differences observed on the tree growth and stand productivity parameters among the two treatments were not significant at (p<0.05). The negligible growth reduction in repeatedly disked treatment is surpassed by the significant fuel load reduction and reduced wildfire risk.
Figure 2: Example of a disked area

For more information on this research your can access the full thesis titled The combined effect of slash burning and repeated disc harrowing on changes in fuel loading, soil properties, root growth and stand productivity of Eucalypts in Mpumalanga: South Africa