Plantations of pine trees in the background, and invasion by escaped pines on the Garcia Pass in the southern Cape. These invasions can substantially increase fuel loads, leading to more intense and damaging wildfires, say scientists.
Image: Brian van Wilgen

Invasive pine trees significantly intensified the June 2017 Garden Route fires‚ scientists said on Thursday.

Pines have invaded more than 90% of the Garden Route National Park’s fynbos vegetation at various densities‚ according to a study published in the journal Fire Ecology.

Acacias and blue gums contributed to the fuel load available for the fire‚ which burnt 15‚000ha‚ destroyed 800 buildings and left seven people dead‚ said co-author Professor Brian van Wilgen‚ of the Centre of Excellence for Invasion Biology at Stellenbosch University.

“By increasing the amount of fuel available to burn‚ the fires become more intense and more difficult to control‚” said Van Wilgen‚ whose research team included academics from Nelson Mandela University‚ South African National Parks and the CSIR.

They found that the severity of the fire‚ which claimed more than 5‚000ha of commercial pine plantations‚ was significantly higher in plantations of invasive alien trees and in fynbos invaded by alien trees‚ than in uninvaded fynbos.

The two-year drought that preceded the fires was the worst on record and contributed significantly to the impact of the disaster‚ they said.

A burned-out plantation near Harkerville shortly after the 2017 Knysna wildfire.
Image: Johan Baard

And they warned that similar fires could become more frequent as global warming takes its toll on the climate of the southern Cape.

The aim of the study was to assess the climate‚ weather and fuel factors that contributed to the four-day fires‚ and used satellite imagery to compare the landscape before and afterwards‚ including the type of vegetation covering different areas.

Van Wilgen said large tracts of natural vegetation in the southern Cape had been systematically replaced with pine and eucalyptus plantations‚ increasing biomass from about four tonnes per hectare to 20 tonnes.

“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‚” he said.

“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 programmes 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.”