CHAPTER 2 LITERATURE REVIEW
2.5 Driving factors of bush encroachment
2.5.4 Fire
Bush encroachment is affected by different aspects of fire, which include (1) the management of fire, (2) types of fires, (3) intensity, seasonality, and frequency of fires, and (4) the response of woody vegetation to fire.
Management strategies
Fire is one of the principal drivers of the structure of savanna ecosystems (Scholes and Archer, 1997) and a principal tool in managing savannas (Bond & Archibald, 2003; Hudak & Fairbanks, 2004; Gordijn, 2010; Smit et al., 2010; Naudé, 2018). Fires are typically used on savannas as a management tool to suppress woody cover (Parr & Andersen, 2006, in Devine et al., 2017). Fire
may also manipulate both livestock and wildlife herbivory patterns (Archibald et al., 2005; Gordijn, 2010). The green growth in a burnt area may attract grazing herbivores and focus the impacts of these herbivores on the burnt area (Archibald et al., 2005; Gordijn, 2010).
If the grass fuel biomass allows for hot and frequent fires, it increases the capability of controlling woody plant densities by destroying their aboveground biomass (topkill) (Bond and Keeley, 2005;
Lohmann et al., 2014). Intense and frequent burning regimes limit seedling and sapling establishment, and trees are often suppressed from reaching a reproductive stage (Bond &
Midgley, 2000, in Devine et al., 2017). As encroachment progresses, there may also be a ‘tipping point’ as tree canopy cover increases over 45-50%, above which fires rarely occur. According to Scholes (2009) “to keep the trees in check, fires need to be sufficiently intense (> 3000 kW/m) and sufficiently often (< 5 years) to prevent young tree saplings from escaping the ‘fire trap’”.
Once an area heavily encroaches, the fire regime cannot be reversed, and only chemical and mechanical methods can be used for bush clearing (Scholes, 2009). Fire is seen as a risky management regime, as owners fear the risk of damaging infrastructure and/or livestock, as well as removing grass (livestock fodder) (Lohmann et al., 2014).
Types of fire
Fire-type is also a factor that may influence bush encroachment. Surface fires (most common) occur below tall tree canopies, whereas crown fires can occur in the canopies of tall trees (Snyman, 2002; Trollope et al., 2002; Gordijn, 2010). Head burns and back burns are two types of surface fires that have been used to manage bush encroachment (Trollope et al., 2002). Back fires move towards the general wind direction, while head fires move with the wind (Trollope et al., 2002). Back fires burn close to the ground and do the most damage to the herbaceous layer (Snyman, 2002). Head fires have a less burning effect on the herbaceous layer and a higher impact on the topkill of trees. Head fires have a longer flame length, resulting in more intense fires and higher damage to woody plants (Trollope et al., 2002).
Intensity, seasonality, and frequency of fires
Fire intensity refers to the energy released when burning and may be practically measured by flame length and rate of spread (Bond & Keeley, 2005). Another term is “fire severity”, which is also another measure that describes the impact of fires on ecosystems (Gordijn, 2010). High- intensity fires played a crucial role in maintaining open savannas in the past, but with the introduction of livestock farming, veld fires were suppressed (De Klerk, 2004). Fire intensity can decrease woody plants (Smit, 2004; Walters et al., 2004; Govender et al., 2006) and is positively related to the mortality of woody plant seedlings (Trollope & Tainton, 1986; Gordijn, 2010). The
mortality of woody plants due to fire alone however, is low (Trollope & Tainton, 1986; Midgley et al., 2010). Studies suggest that increased fire intensities are correlated with greater topkill of woody plants (Trollope & Tainton, 1986). Taller plants with increased stem diameters however, are less vulnerable to stem death, compared to shorter plants (Balfour & Midgley, 2006). Trees with a height ≥ 4 m, show fewer amounts of topkill and are considered to have “escaped” the effects of fire (Trollope & Tainton, 1986; Higgins et al., 2000). Environmental factors, such as wind speed, slope, fuel load (grass biomass), type of fire, humidity, temperature, and season of burn may affect fire intensity (Trollope et al., 2002). Humidity can decrease fire intensity, whereas wind speed, temperature, and fuel load can increase fire intensity. A down slope would decrease the spread rate and intensity of the fire (Trollope et al., 2002). Fires in the wet season have a lower fuel load and higher relative humidity. Therefore, the season may also influence the fire intensity (Trollope et al., 2002).
Fire typically occurs in regions with between 450 and 1 800 mm of annual precipitation, particularly in dry seasons, because fire requires a source of ignition, as well as fuel to sustain spread across a landscape (Turpie et al., 2019). In southern Africa, the main burning season is usually from March to November (Archibald et al., 2008). The duration of the dry season will determine the amount of time that the fuel is dry and available to burn (Archibald et al., 2008).
Weather conditions on the day of burning however will also determine the fuel moisture (Russell- Smith et al., 2007; Archibald et al., 2008). A longer dry season will decrease the relative humidity, directly decreasing the fuel moisture and increasing the chances of fire (Archibald et al., 2008).
Smit et al. (2010) found that dry-season fires reduced woody plant cover more than wet-season fires.
Fire frequency measures the occurrence of fires in a specific area for a specific amount of time (Bond & Keeley, 2005). The effect of fire frequency is determined by “event-dependent” and
“interval-dependent” factors (Bond & van Wilgen, 1996; Gordijn, 2010). “Event-dependent” effects are determined by factors at the time of the fire, i.e. fire type, fire intensity, and fuel load. “Interval- dependent” factors are mostly influenced by growing conditions between consecutive fires.
Previous studies suggest that the response of woody plants to fire frequency is difficult to predict and dissimilar results have been found (Hoffmann, 1999; Higgins et al., 2007). Hoffmann (1999) observed that tree density decreased in the Brazilian Cerrado vegetation type with an increase in fire frequency, while Higgins et al. (2007) found that fire frequency had no influence on tree density in semi-arid and mesic savannas (Gordijn, 2010). Higgins et al. (2007) however, found that woody plant structure was influenced by fire frequency, with the dominance of smaller woody plants (≤ 2 m) increasing in annual and biennial fire treatments. Triennial treatments indicated the increase in the dominance of taller trees (≥ 2 m) (Higgins et al., 2007). Smit et al. (2010)
found that more frequent fires reduced woody plants more than less frequent fires. “Event- dependent” and “interval-dependent” factors are important to consider when controlling bush encroachment, as they may complicate the responses of woody plants to fire frequency (Pratt &
Knight, 1971; Higgins et al., 2007; Munkert, 2009; Gordijn, 2010).
Woody vegetation responses to fire
Fire is an important factor in regulating and changing the dynamics of a savanna–woodland ecosystem (Bond & Keeley, 2005; Gordijn, 2010). Previous studies suggest that savanna trees must have adaptations that allow them to survive frequent and intense fires (Balfour & Midgley, 2006; Gordijn, 2010). Bark and branch thickness is an example of woody plants adapting to survive intense fires (Hoffman et al., 2003). Studies suggest that woody plants with thicker bark and branches have a higher survival rate (Hoffman et al., 2003; Hoffmann & Solbrig, 2003).
In addition to causing woody plant mortalities, fire also reduces the height of woody plants (Hoffmann & Solbrig, 2003). Fire may cause woody plants to be in a “gulliver” state (Bond & van Wilgen, 1996) until a disturbance occurs that enables the escaping of the “fire trap” (Bond &
Midgley, 2000; Gordijn, 2010). It is recorded that savanna trees generally mature at shorter heights compared to forest trees (Hoffman et al., 2003). This allows savanna trees to reach maturity faster and reproduce in the presence of frequent and intense fires (Hoffman et al., 2003).