The Effects of Alien Shrub Invasions on Vegetation Structure and Fire Behaviour in South African Fynbos Shrublands: A Simulation Study

Abstract

(1) South African fynbos vegetation is fire-prone and susceptible to invasion by alien shrubs. Alien shrubs change the nature of the fuel bed and thus affect fire behaviour. (2) Changes in biomass, size and distribution of plant parts as fuel and plant moisture and energy contents were determined at two sites invaded by the important alien shrubs Hakea sericea Schrad. and Acacia saligna (Labill.) Wendl. (3) The data were used to define fuel models and to simulate fire behaviour using Rothermel's fire model. This simulation was used to test the hypothesis that invasion increases fire hazard through increasing fuel loads. (4) Invasion by H. sericea resulted in a 60% increase in fuel load and lowered the moisture content of live foliage from 155 to 110%. Simulated rates of fire spread and intensity were nonetheless lower than in fynbos due to a densely-packed fuel bed. (5) Invasion by A. saligna resulted in a 50% increase in fuel load. The high moisture content of foliage of this shrub (about 270%) effectively reduces the fuel load and fuel bed depth, resulting in low rates of fire spread and intensity in the simulation. (6) Shortcomings in Rothermel's model prevented the accurate simulation of high intensity fires which have occurred in invaded areas under extreme weather conditions. Such fires vigorously consume the increased biomass of shrub crowns, are difficult to content of foliage of this shrub (about 270%) effectively reduces the fuel load and fuel bed Under such conditions, the fire hazard will be increased by invasion.