The effect of fire season, fire frequency, rainfall and management on fire intensity in savanna vegetation in South Africa

Abstract

Summary 1.Fire is important for the maintenance and conservation of African savanna ecosystems. Despite the importance of fire intensity as a key element of the fire regime, it is seldom measured or included in fire records. 2. We estimated fire intensity in the Kruger National Park, South Africa, by documenting fuel loads, fuel moisture contents, rates of fire spread and the heat yields of fuel in 956 experimental plot burns over 21 years. 3. Individual fires were conducted in five different months (February, April, August, October and December) and at five different return intervals (1, 2, 3, 4 and 6 years). Estimated fire intensities ranged from 28 to 17 905 kW m ); they were associated with a threefold difference between the mean moisture content of grass fuels in winter (28%) and summer (88%). 4. Mean fuel loads increased with post-fire age, from 2964 kg ha − 1 on annually burnt plots to 3972 kg ha − 1 on biennial, triennial and quadrennial burnt plots (which did not differ significantly), but decreased to 2881 kg ha − 1 on sexennial burnt plots. Fuel loads also increased with increasing rainfall over the previous 2 years. 5. Mean fire intensities showed no significant differences between annual burns and burns in the biennial, triennial and quadrennial categories, despite lower fuel loads in annual burns, suggesting that seasonal fuel moisture effects overrode those of fuel load. Mean fire intensity in sexennial burns was less than half that of other burns (638 vs. 1969 kW m − 1 ). 6. We used relationships between season of fire, fuel loads and fire intensity in conjunction with the park's fire records to reconstruct broad fire intensity regimes. Changes in management from regular prescribed burning to 'natural' fires over the past four decades have resulted in a decrease in moderate-intensity fires and an increase in high-intensity fires. 7. The highest fire intensities measured in our study (11 000 -> 17 500 kW m − 1 ) were significantly higher than those previously reported for African savannas, but were similar to those in South American cerrado vegetation. The mean fire intensity for late dry season (winter) fires in our study was less than half that reported for late dry season fires in savannas in northern Australia. 8. Synthesis and applications . Fire intensity has important effects on savanna vegetation, especially on the dynamics of the tree layer. Fire intensity varies with season (because of differences in fuel moisture) as well as with fuel load. Managers of African savannas can manipulate fire intensity by choosing the season of fire, and further by burning in years with higher or lower fuel loads. The basic relationships described here can also be used

Frequent burning promotes invasions of alien plants into a mesic African savanna

“…Without disturbances such as fire or grazing, tree cover can increase at the expense of grass cover and dominate these mesic savannas (Higgins et al 2000;Van Langevelde et al 2003). Because tree-grass coexistence is essential for savanna functioning, managers of these savannas use fire to control bush encroachment (Govender et al 2006). The use of fire to manage savannas is justified by managers on the grounds that savanna vegetation evolved in the presence of fire and is thus adapted to, or tolerant of, fire (Bond and Midgley 2001).…”

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

“…Fire-prone grassland and savanna ecosystems in southern Africa are often managed through a prescribed burning system based on a diverse fire regime that comprises three basic elements (Govender et al 2006;Smith et al 2013): frequency, season and intensity, which largely determine the effects on the biota (Mucina and Rutherford 2006). Furthermore, fires affect the net primary productivity of plants and their vegetative cover (Blair 1997;Carbutt et al 2011), soil nutrient composition and susceptibility to soil erosion (White et al 2006).…”

Elephant-mediated habitat modifications and changes in herbivore species assemblages in Sabi Sand, South Africa

“…4 The results of the hierarchical partitioning as depicted by the total contribution (independent and its conjoint contribution±SD) of each of the predictors in a set of regression equations as averaged for all browser (N=5; giraffe, steenbok, duiker, bushbuck, kudu), grazer (N=6; white rhino, buffalo, hippo, warthog, zebra, waterbuck), and mixed feeder species (N=2; nyala, impala), excluding elephants. The used predictor variables were elephant density, year, rainfall, tree density (all trees and only <1 m), and the total biomass consumption by all other browser and grazers species (Browser C, Grazer C) to represent the competition pressure from these two species groups stem density and the removal of the larger trees in particular might not only increase the frequency and intensity of fires (Govender et al 2006), but also decrease the facilitative role of trees on grasses by decreasing the role of large trees as nutrient pumps (Treydte et al 2007). This could be one of the mechanisms that stimulate growth of nutrient poor grasses that can only be used by megagrazers and are of too poor a quality for mesograzers (Olff et al 2002).…”

“…This could be one of the mechanisms that stimulate growth of nutrient poor grasses that can only be used by megagrazers and are of too poor a quality for mesograzers (Olff et al 2002). Fires certainly play a big role in structuring African savannas (Higgins et al 2000;Govender et al 2006) and contribute to the decline in small trees through fire-related mortality. So, the underlying mechanisms explaining the differences in the herbivore community might, besides the direct competition for food or the dispersal after the removal of fences, also be explained by the indirect effects of elephants on the grass layer which indirectly influences the fire regime and food quality.…”