Effects of fire on woody vegetation structure in African savanna

Abstract

Despite the importance of fire in shaping savannas, it remains poorly understood how the frequency, seasonality, and intensity of fire interact to influence woody vegetation structure, which is a key determinant of savanna biodiversity. We provide a comprehensive analysis of vertical and horizontal woody vegetation structure across one of the oldest savanna fire experiments, using new airborne Light Detection and Ranging (LiDAR) technology. We developed and compared high-resolution woody vegetation height surfaces for a series of large experimental burn plots in the Kruger National Park, South Africa. These 7-ha plots (total area approximately 1500 ha) have been subjected to fire in different seasons and at different frequencies, as well as no-burn areas, for 54 years. Long-term exposure to fire caused a reduction in woody vegetation up to the 5.0-7.5 m height class, although most reduction was observed up to 4 m. Average fire intensity was positively correlated with changes in woody vegetation structure. More frequent fires reduced woody vegetation cover more than less frequent fires, and dry-season fires reduced woody vegetation more than wet-season fires. Spring fires from the late dry season reduced woody vegetation cover the most, and summer fires from the wet season reduced it the least. Fire had a large effect on structure in the densely wooded granitic landscapes as compared to the more open basaltic landscapes, although proportionally, the woody vegetation was more reduced in the drier than in the wetter landscapes. We show that fire frequency and fire season influence patterns of vegetation three-dimensional structure, which may have cascading consequences for biodiversity. Managers of savannas can therefore use fire frequency and season in concert to achieve specific vegetation structural objectives.

Post-fire dynamics of the woody vegetation of a savanna forest (Cerradão) in the Cerrado-Amazon transition zone

“…In other words, wildfires have a greater impact on closed formations in comparison with more open ones, as observed in African savannas (Smit et al 2010). …”

“…In the savannas, by contrast, fire is an intrinsic and regularly recurring phenomenon (Miranda et al 2002;Staver et al 2011). While Brazilian savanna vegetation is fire-adapted (Coutinho 1990;Walter & Ribeiro 2010), the frequency and intensity of fires can lead to changes in its floristic composition, structure, and vegetation dynamics (Hoffmann 1999;Miranda et al 2002;Smit et al 2010). Frequent fires may also reduce the density of woody plant species (Hoffmann 1999;Moreira 2000;Lima et al 2009), alter growth rates and reproductive success (Frost & Robertson 1985;Hoffmann 1999), as well as decrease species richness (Coutinho 1990;Ribeiro et al 2012), due to the exclusion of fire-sensitive species (Moreira 2000).…”

“…The study compared the characteristics of sample plots burnt during a wildfire in 2008 with neighboring plots that survived the fire, as well as comparing both types of plot over a four-year period, between 2008 and 2012. We tested the hypothesis that community dynamics differed significantly between burned and unburned savanna forest over the study period due to the alterations caused by wildfires on species composition, diversity, and the structure of savanna vegetation, in particular through modifications of growth and mortality rates (Hoffmann 1999;Miranda et al 2002;Smit et al 2010). …”

Effects of a fast-burning spring fire on the ground-dwelling spider assemblages (Arachnida: Araneae) in a central South African grassland habitat

“…Effects on plants depend on the amount, vertical level and rate at which heat energy is released (Trollope et al 2002), which varies between habitats of different vegetation structure and composition (e.g. Smit et al 2010;Levick et al 2012), influencing other organisms directly.…”

Potential aboveground biomass in drought‐prone forest used for rangeland pastoralism

“…The findings suggest that the mulga dry forest is a resilient ecosystem, livestock grazing has a variable but overall small impact on stand biomass, and that only particular and unlikely combinations of management (i.e., wet weather and grazing protection) can generate substantial increases in tree densities, which are then vulnerable to collapse in subsequent droughts. The resilience of the mulga dry forest to human-induced disturbance is consistent with some forests of similar climatic regimes (Tefera et al 2007), but contrasts sharply with other dry forests where fire is an important disturbance agent (Smit et al 2010). …”