Observed effects of burning alien invasive vegetation

​One of the Conservancy’s main activities is the removal of alien invasive vegetation. We practice three main methods to eradicate them, these are chemical, physical, and biological approaches, either alone or in conjunction. While, we utilize all methods in our area, this blog will concentrate on mechanical and chemical removal and some effects that have been identified in relation to fire. These techniques commence with the mechanical removal of the vegetation with the aid of machinery, followed by the application of herbicide on the resulting stumps. The remains are stacked and burnt to decrease the fuel load in the event of a wildfire.

Two types of burn intensity can be employed, a cold or hot burn. Both come with their advantages and drawbacks. In our experience we have found hot fires often occur and are necessary when removing large fuel loads in an area that has been cleared of woody alien vegetation. The alternative would be to remove the material from the property, this option is costly, time consuming, and often impractical as clearing usually occurs in areas that are inaccessible to vehicles.

However, there are disadvantages to hot burns. One of our landowners, Keith Moodie, observed the impacts of a hot burn on the rejuvenation of forest vegetation and shared some images from his experience. He hypothesised that a hot burn reduces subsequent regrowth of vegetation, damages the seed bank and the resulting biodiversity. This applies in areas that were originally natural forest. This concept is illustrated in the recovery of his land in these images. 

The area at the bottom of the picture was burnt in 2013 and the area where the cow is grazing received a hot burn. The area where the cow is grazing is dominated by grass and Rhu species. This illustrates the lack of regrowth post hot fire.

The grassy area in the top left of the picture received a hot burn. The area in the bottom of the picture was not burnt. You can see a variety of species growing through the stacked wattle.

Milkwood growing through wattle stacks

​The area that endured a hot burn was dominated by grass and Rhus species (Family Anacardiaceae), the unburnt land saw a variety of tree species growing through the unburnt wattle stacks. He acknowledges that the follow up herbicide application is often more challenging through the stacks of wattle but believes that fewer follow ups would be necessary if left unburnt.

An article written by (Holmes et al, 2000) corresponds with Keith’s observations. The article describes the technique of felling aliens and burning them as intrusive but explains that this technique is useful in removing alien invasive seeds and non-sprouting aliens. Our fire manager Twakkie (Goliath Highburg) agreed with this concept. He explained that forest species struggle to re-establish after a hot burn but other vegetation types such as the Fynbos biome requires a burn every 10 – 15 years.
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In conclusion, it is therefore very important to decipher the type of vegetation where you are working, the ultimate restoration goal (forest versus fynbos) before applying the correct fire regime. Our conservancy tries our utmost to combat alien invasive vegetation with minimal impact on the environment, while protecting our farms. We are constantly adapting our approaches to alien invasive removal, depending on the results of former removals and feedback from landowners. Do you agree with our approach? What have you observed? We welcome your feedback and comments below.
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Reference:
Holmes, P.M., Richardson, D.M., Van Wilgen, B.W. and Gelderblom, C., 2000. Recovery of South African fynbos vegetation following alien woody plant clearing and fire: implications for restoration. Austral Ecology, [PDF] 25, pp. 631-639. Available at: https://0-doi.org.wam.seals.ac.za/10.1111/j.1442-9993.2000.tb00069.x [Accessed 23 April 2020].