The effects of alien shrub invasions on vegetation structure and fire behaviour in South African fynbos shrublands: a simulation study.
South African fynbos vegetation, principally Protea repens and P. neriifolia, is fire-prone and susceptible to invasion by alien shrubs. Changes in biomass, size and distribution of plant parts as fuel and plant moisture and energy contents were determined at 2 sites at Vergelegen and Muizenberg invaded by Hakea sericea and Acacia saligna. 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 by increasing fuel loads. Invasion by H. sericea increased fuel load from 1.22 kg/m2 to 1.95 kg and lowered the moisture content of live foliage from 155 to 110%. Simulated rates of fire spread and intensity, however, were lower than in fynbos due to a densely-packed fuel bed. Invasion by A. saligna increased fuel load to 1.80 kg/m2. The high moisture content of foliage of this shrub (approx. 270%) effectively reduced the fuel load and fuel bed depth, resulting in low rates of fire spread and intensity in the simulation. 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 contain and are potentially more damaging to ecosystems. Under such conditions, the fire hazard will be increased by invasion.<new para>ADDITIONAL ABSTRACT:<new para>Data are presented on changes in biomass, size and distribution of plant parts as fuel and plant moisture and energy contents at two sites invaded by Hakea sericea and Acacia saligna. Although H. sericea increased fuel load by 60% and reduced m.c. of live foliage from 155% to 110%, simulated rates of fire spread and intensity were reduced as a result of the densely packed fuel bed. A saligna increased fuel load by 60% but m.c. of foliage (about 27%) effectively reduced fuel load resulting in low rates of simulated fire spread and intensity.