EVOLVING FIRE MANAGEMENT STRATEGIES AND THEIR IMPACT ON THE OCCURRENCE AND SPATIAL EXTENT OF UNPLANNED WILDFIRES IN A LARGE AFRICAN SAVANNA PARK* EVOLUÇÃO DAS ESTRATÉGIAS DE GESTÃO DE INCÊNDIOS E SEU IMPACTE NA OCORRÊNCIA E EXTENSÃO ESPACIAL DOS INCÊNDIOS FLORESTAIS ACIDENTAIS NUM GRANDE PARQUE DA SAVANA AFRICANA

Savannas cover approximately 20 % of the global land surface. In African savannas, fire is an important agent for controlling these ecosystems. Kruger National Park (KNP) is a large African savanna park which has implemented a variety of fire management strategies over the years. Using KNP’s recorded fire history (from 1941-2017), we examined the occurrence and spatial extent of accidental wildfires in KNP in relation to adaptations in the fire management strategies over time. From 1941 to 2017 fires were a regular, almost annual occurrence in KNP. However, fuel loads accumulate over time when fires are extinguished or controlled burning in these landscapes is prevented, and the result is a substantial amount of combustible material to support large unplanned wildfires. Therefore, fire management strategies influence the occurrence and spatial extent of unplanned wildfires in African savannas. Prescribed burning is a critical management tool which should be used in fire-prone landscapes, however, research is needed to determine the appropriate fire regime needed to manage a fire-driven system.


Introduction
Nearly 20 % of the earth's land surface is covered by Savannas which are characterized by a balanced mix of woodland and grassland (scholes & Archer, 1997;Sankaran, Ratnam, & Hanan, 2004). savannas are found in south America, India and Australia whilst the largest proportion is found in Africa (van Wilgen, 2009). These dynamic systems support a large proportion of the world's human population along with their rangelands, livestock and wildlife (Scholes & Archer, 1997;Sankaran et al., 2005). African savannas are driven by fluctuations in rainfall, herbivory, nutrients and fires (Walker & Noy-Meir, 1982;Sankaran et al., 2005;Archibald & Hempson, 2016). The impact of these various drivers in controlling savanna ecosystem functioning and processes differs according to the persistence and frequency of the disturbance. Fire ignition sources in these African savannas are due to either human origins, whether accidentally or purposefully as arson, or natural origins through lightning (Archibald, Roy, Van Wilgen, & scholes, 2009). Lightning fires are less common and often do not burn through large areas of woodland (Walter, 1971;Van Wilgen, Biggs, O'regan, & Mare, 2000).
Fire is an important agent in these African savannas where it has been part of this system for thousands of years (Scott, 1970). As such, the fauna and flora in these fire-prone ecosystems have co-evolved with fire and ultimately, resulted in a resilient fire-adapted system whereby savanna vegetation consists of many fire-adapted plant species (Bond & Keeley, 2005;Furley, Rees, Ryan, & saiz, 2008). It is believed that, in south Africa, humans have been using and controlling fires to manipulate their environment for roughly 55 000 years (Smith, 2007). Humans would have learnt to use fires to manage their environments for agricultural purposes, to cycle soil nutrients and to control herbivore movements to facilitate easy hunting of wildlife (Whitlock, colombaroli, conedera & Tinner, 2018). As such, the presence of people in a large African savanna park like Kruger National Park (KNP) in South Africa, would have promoted the occurrence of fires in this fire-driven ecosystem. According to Scott (1970), early authors such as Kanthack (1907) and Thompson (1936) documented records of early Portuguese explorers who referred to the interior of south Africa as "Terra dos fumos", meaning the land of smoke and fire. This further confirms the large-scale occurrence of fires in this African landscape.

Park (KNP)
Since the official proclamation of KNP in 1926, the first park Warden, Colonel James Stevenson-Hamilton, implemented a general ban on any deliberate burning as he believed that fires had a detrimental impact of vegetation and wildlife (Pienaar, 2012). Roughly a decade later in 1935, the Parks Board had decided that fires should not be explicitly banned but rather controlled, and in 1937 stevenson-Hamilton had suggested a burn policy whereby the bush would be burnt every second year to avoid the accumulation of moribund material (Joubert, 2007;Pienaar, 2012). In 1947, stevenson-Hamilton had suggested that half the park should be burnt every year between February and April in order to promote low intensity fires while vegetation is still green, and that areas to be burnt will alternate between years.
Due to the limited resources available during this period, most of these suggested policies were nearly impossible to implement as an official fire management strategy. In addition, there are scarce and limited records available on spatial extent and causes of fires during this time.
Therefore, the period between 1926 and 1947 may be considered as a time in which KNP did not have a clear and, more importantly, an achievable strategy towards fire management. Notwithstanding, limited fires were still applied in order to provide improved grazing for animals (Van Wilgen et al., 2000;Govender, Mutanga & Ntsala, 2012).
However, in the 1940s, philosophies on prescribed burning changed with the declaration of the soil

Conservation Act No. 45 of 1946 whereby the South
African Government had promoted the establishment of soil conservation District committees who would oversee soil conservation schemes enforced upon landowners (Rabie, 1974). one aspect of these soil conservation schemes was the prohibiting of prescribed burning which would have resulted in expropriation of land if landowners contravened this legislation (Rabie, 1974).
These early ideas were based on the perceived negative effects of fires on land degradation and soil erosion, thus resulting in complete prohibition of prescribed burning in KNP during 1948-1956 known as the Fire Suppression or Protection Era (Trollope, 1984;van Wilgen, 2009 (Joubert, 2007). colonel Sandenbergh's statement provides further insight into the philosophy of park management during that period whereby anthropogenic ignition sources were not considered as "natural" and part of the ecosystem.
During the Fire suppression period in the 1950s, a major programme aimed at grading a network of firebreaks across the park was initiated in order to gain control of wildfires (Joubert, 2007).
By the time, KNP management had adopted and implemented a Fixed Prescribed Burning strategy , the graded firebreak network resulted in more than 400 burn blocks ranging between 50 and 24 000 ha (Van Wilgen et al., 2000). This prescribed burning strategy led to a fixed fire regime whereby fires were applied every three years in Spring (after the first rains) in each burn block (Brynard, 1971;Joubert, 2007;Govender et al., 2012). In 1981, this rigid burning programme was declared unsuitable and adapted so as to allow for seasonal variation in the timing of prescribed burns, whilst retaining the three year block rotation (van Wilgen, Govender, smit & MacFadyen, 2014). This fire management strategy lasted until 1990 and was known as the Flexible Prescribed Burning period. In 1991, park management had shifted their fire strategy towards a "Natural" Fire Policy KNP has adapted its fire management strategy at least seven times since proclamation in 1926. This is evident of an adaptive management strategy which is consistently informed by the best available information and knowledge at the time.

Study Area
The study area is a large conservation area, known as to open plains with low shrubs (Venter & Gertenbach, 1986;Mabunda, Pienaar & Verhoef, 2003).
The regional climate is driven by anticyclonic systems travelling over southern Africa in a westerly direction (Venter & Gertenbach, 1986). summers are wet and hot, with a daily average temperature of 34°C during November to February, whilst the winters are characterized by dry conditions with mild temperatures averaging at a maximum of 27°C during June to August   (Kennedy & Potgieter, 2003). The southern and central

regions of KNP receives a Mean Annual Rainfall (MAR)
of 500-700 mm whilst the northern more arid parts of KNP receives 300 -500 mm MAR (Venter et al., 2003).
The beginning of the rainy season is characterized by thunderstorms with extreme lightning events at the end of the dry season. KNP's distinct wet and dry periods provides a conducive environment with ideal conditions for fires to occur (Kennedy & Potgieter, 2003). The wet summer period allows for the accumulation of biomass and increases available fuel loads for dry winter fires.
Besides fuel load, fuel moisture content and topography, certain weather conditions promote the spread of fires, i.e. temperature, relative humidity and wind speed.  (Justice et al., 2002;schroeder et al., 2008).
Beyond those minor disadvantages, MODIS still provides some of the more reliable satellite imagery available to date, and with recent launches such as sentinel-2 and -3, fire detection and monitoring will significantly improve in the next few years.
In this paper, we used digitized hand-drawn maps and  1945, 1954, 1966, 1971, 1984-1984, 1992-1994, 1998, 2016-2017 and 622 466 ha the following year. It is interesting to note that the year in which the greatest fire extent had been recorded, i.e. 1946, these fires were all as a result of unplanned fires recorded as being "Accidental".
Unfortunately, the exact causes of most of these fires during this "No Strategy" phases are unknown, as this was not a major concern during this period of KNP's history. It is conceivable that lightning fires may have occurred but the spatial extent is unknown, although it is believed to be a minimal proportion of the overall area burnt. Lightning fires are known to not be a regular ignition source and do not often burn through large areas of woodland (Walter, 1971;Van Wilgen et al., 2000). Fire occurrence, its extent and its perceived negative impacts on land degradation and erosion was more important than the actual fire cause (Rabie, 1974;Trollope, 1984),    Throughout KNP's fire history and its various fire management strategies, unplanned wildfires have been a frequent occurrence. Particular strategies where fires were either actively suppressed or prescribed burning prohibited, resulted in situations where wildfires were more frequent and burnt through much larger areas. This has been shown in other fire-prone landscapes such as conifer forests in North America where inappropriate fire management strategies were ineffective in limiting large, uncontrollable wildfires (van Wagtendonk, 2007). Fires should be promoted as a natural ecosystem process and land management practices need to be designed in order to ensure the role of fires (Hann & Bunnell, 2001).
The use of prescribed burning will assist in the creation of a patchy mosaic of recently-burnt areas with lower fuel loads which, in turn, could limit the spread of wildfires through large tracts of bush. These breaks in fuel load will assist park management in controlling wildfires, if they occur. Fortunately, KNP has an extensive road network that act as firebreaks which, depending on the intensity of the fire and weather conditions, can be used to stop the spread of a wildfire. Prescribed fires should be promoted and tolerated in fire-prone landscapes whilst ensuring safety to people and properties. The avoidance and suppression of fires will increase the risk of large-scale uncontrollable wildfires which will require additional fire-fighting support in order to contain and extinguish these wildfires.

Conclusion
Fires are inevitable in these fire-driven savanna landscapes and will occur on an annual basis in various parts of KNP, except in extremely low rainfall years.
Fire management strategies influence the occurrence and spatial extent of unplanned wildfires over time.
An inappropriate strategy such as banning prescribed burning in these ecosystems, will lead to extensive areas being burnt by inevitable wildfires due to the accumulation of fuel over time. This will increase the potential risk of the occurrence and spread of devastating wildfires which will threaten loss of life and damage to property. Therefore, prescribed burns by park management should continue to be implemented in order to limit the occurrence and spread of unplanned and uncontrollable wildfires.