The fact that natural fynbos vegetation were replaced by pine plantations in the southern Cape, and that it was subsequently invaded by the nearby invasive pine trees and it considerably increased the severity of the 2017 Knysna wildfires, reports one of the many findings of a study published recently in the journal Fire Ecology by a research team from the DST-NRF Centre of Excellence for Invasion Biology (CIB) at Stellenbosch University, Nelson Mandela University, SANParks, and the CSIR. The study wanted to analyze the climatic, weather and fuel factors that led to one of the region’s worst fires ever recorded.
Over a period of four days in June 2017, the Knysna fires burnt almost 15000 hectares, killing of seven people and causing damage more than 5000 hectares of commercial pine plantations and more than 800 buildings.
For the study, researchers used satellite imagery so that they could the landscape before and after the fire, including the type of vegetation that covered and still covers the different areas. This data helped them predict the amount of biomass consumed by the 2017 fire.
One of the main findings of the study is that the severity of the fire was considerably higher in plantations of invasive alien trees and in fynbos invaded by alien trees, as compared to that in uninvaded fynbos. And although the weather conditions were extreme, they weren’t unprecedented, as similar conditions have occurred in the past at a rate of nearly one day every three years. The severity of the 18-24 month drought that had occurred before the fires, on the other hand, was higher than what has ever been recorded in the historical weather record, and this contributed significantly to the impact of the fire.
Professor Brian van Wilgen, who is a fire ecologist with the CIB and one of the co-authors, says that large tracts of natural vegetation in the southern Cape have been systematically replaced with plantations of Pinus and Eucalyptus species, which has increased above-ground biomass from about four to 20 tonnes per hectare- “Given that more than two-thirds of the area that burned was in one of these altered conditions, our findings demonstrate clearly that fuel loads have substantially increased compared to earlier situations when the landscape would have been dominated by regularly burned uninvaded natural vegetation.”
Scientists have estimated that pine trees have invaded over 90% of the Garden Route National Park’s fynbos vegetation at many densities. More invasions by Australian Acacia and Eucalyptus species cover a further 29% and 14% respectively: “By increasing the amount of fuel available to burn, the fires become more intense and more difficult to control,” he explains. But Van Wilgen warns that such events can become more frequent as the climate of the southern Cape becomes more hot and dry, it leads to the increase in extent of invasions. “The conditions that exacerbated the severity of the 2017 Knysna fires will occur again. People need to stay vigilant and implement fire-wise practices, and, more importantly, steer away from placing developments in high-risk areas in the long inter-fire periods.”
“Our study underscores the need to implement effective programs to control the spread of invasive alien plants, and to re-examine the economic and ecological sustainability of commercial planting of invasive alien trees in fire-prone areas.”