2.3 Results
2.3.2 Objective two
Fire return interval: number of years between fires in the Kogelberg Nature Reserve
Analysis of all the data (from 1952 up to April 2006) was carried out to identify changes in fire frequency over the period of the study. Intervals of 1952-1980 and
1952-2006 were allocated respectively to the data.
To calculate the fire interval for all the fires that occurred in the Kogelberg Nature Reserve, GIS Arc view 3.3 and Excel were used to join multiple fire dates into polygons for a particular area using the method described by De Klerk et al. (2005)
2.2.3.3 Objective three Seasons of burn
Brown et al., (1991) describes the natural fire season for fynbos as November to April, with a peak between November and February. All the fires that burnt at the Kogelberg Nature Reserve in the two management periods of the study were analysed to determine in which months they burnt the total area that burnt during those months and if there were any shifts in the season of burn.
GraphPad Prism 5® (GraphPad Software, San Diego California USA), Arc view GIS 3.3 and Microsoft Excel were used to analyse the data. As the fire data were not normally distributed, the median was used as a more appropriate indication of the central tendency rather than the mean. Non parametric Mann–Whitney U tests (GraphPad Prism 5) were used for statistical analysis. The test was two-tailed and P- values ≤ 0.05 were considered significant.
Table 2.1: Fire distribution and area burnt at the Kogelberg Nature Reserve over the two periods of the study (1952 - 1980 and 1981 - 2006) (percentages in brackets)
Size Class (ha) Fire management periods
1952-1980 1981-2006 All fires
0 -100 4(1) 38(1) 42(1)
101-500 8(8) 35(12) 43(11)
501-1000 6(17) 3(3) 9(7)
1001-2000 6(32) 7(12) 13(17)
2001-3000 2(17) 3(9) 5(11)
3001-5000 2(25) 1(5) 4(11)
5001-10 000 0 4(38) 4(28)
10 001-20 000 0 1(20) 1(14)
Total number of fires
28(100) 92(100) 120(100)
Mean area burnt (ha)
971 836 867
Median area burnt (ha)
669 161 216
Total area burnt (ha)
27 176 76 891 104 067
Mean area burnt (ha) per year
937 2 848 1 858
Proportionally of all the fires that burnt, the largest area burnt was between 5 001-10 000 ha. Although the period 1981-2006 was shorter than the period 1952-1980, the total number of fires, total area burnt and area burnt per hectare were much higher than that of the period 1952-1980. However, the mean area burnt as well as the median of the period 1952-1980 were higher than that of 1981-2006 (Figure 2.1).
Based on the information shown in Figure 2.1, the total number of fires burnt increased from 28 fires to 92 fires in the period 1981-2006
0 20 40 60 80 100
1952-1980 1981-2006
N u m b er o f fi re s
Period (years)
Figure 2.1: Number of fires during the 1952-1980 and 1981-2006 periods.
The median of the total area burnt (835ha for 92 fires) for the period 1981-2006 was significantly smaller (p=0.0013) compared to the median of the total area burnt (970 ha for only 28 fires) for the period 1952-1980 (Figure 2.2).
1952-1980 1981-2006 0
1000 2000 3000 4000 5000 10000 15000
20000 P=0.0013 (Mann-Whitney U test)
Period
Area burnt (ha)
Figure 2.2: Statistical analysis for area burnt during the 1952-1980 and 1981-2006 periods. The black horizontal lines represent the median of area burnt and the dots represent each individual fire.
Mann–Whitney U test was used for comparison of the medians of area burnt during the 1952-1980 periods and the 1981-2006 periods (GraphPad Prism 5). The y-axis has been segmented into two, first segment showing major ticks interval of 1 000 and the second segment showing major tick intervals of 5 000
In summary, the total area burnt increased during the period 1981-2006, however, the individual fire sizes were smaller compared to the period 1952-1980. The steady increase in the number of fires is an indication of an increase in sources of ignition.
The fact that the fires were smaller in size could be an indication that either there was a quick response to extinguish them or there was less fuel to promote the spread of the fires due to the younger age of vegetation.
2.3.2 Objective two
Fire return interval: number of years between fires in the Kogelberg Nature Reserve
The fire returns intervals for the Kogelberg Nature Reserve from the beginning of 1952 until April, 2006 ranged from 0-54 years. Intervals were calculated as the time in days between fires divided by 365 and an integer taken. Therefore an interval of 0 indicates up to one year between fires.
The fire return interval graph (Figure 2.3) shows that the 50% cumulative probability of fire return with time was just more than 15 years for the entire study period. The recommended fire return interval to maintain biodiversity in fynbos is 15-20 years between fires at 50% probability (van Wilgen, 1982). Furthermore, data in Table 2.2 were interpolated using a graph in Figure 2.3. Table 2.2 shows a steady increase in the 50% probability of fire burning from 1952-2006 where a mean fire return interval of just above 15 years was achieved by 2006. Table 2.2 also shows the probability of 80% fires burning from the previous one, where there was an increase in the mean fire return interval from 8.11 years during the period 1952-1970 to 22.69 years during the period 1952-2006. At 30 % probability, an improvement to above 8 years in the last six years was noted as shown in Table 2.2. This improvement is desirable for a balanced biodiversity management point of view (van Wilgen and Scott, 2001).
Figure 2.3: Fire return interval for the period 1952-2006. The dotted lines indicate 50% cumulative probability of ignition with time in the Kogelberg Nature Reserve. Probabilities were derived from the intervals between fires dissected into 713 polygons using Arc View GIS 3.3.
Table 2.2: Interpolated cumulative probability of fire return interval Cumulative
Probability
entered 1952-1970 1952-1980 1952-1990 1952-2000 1952-2006
30 5.76 4.16 7.66 7.9 8.02
50 6.96 8.43 13.95 15.01 15.36
80 8.11 12.69 20.25 22.05 22.69
Figure 2.4 indicates that there was a significant increase in the fire return interval during the period 1952-2000 compared to the period 1952-1970 (p<0.05). There was also a significant increase in the fire return interval for the period 1952-2006 compared to the period 1952-1970 (p<0.05) and a significant increase in veld age during the periods 1952-2000 compared to periods 1952-1980 (p<0.05). A very significant increase in veld age was noted during the periods 1952-2006 compared to periods 1952-1980 (p<0.005). Overall, this means that the fire return interval at 50%
cumulative probability increased over the study periods to desirable levels in fynbos fire management.
1952-1970 1952-1980 1952-1990 1952-2000 1952-2006 0
20 40 60
Time period
Fire return Interval (years)
Figure 2.4: Statistical analysis of the 50% cumulative fire return interval for the study period 1952-2006. Box-and-Whisker plots represent the median (centre black line) of fire return interval for the periods 1952-1970, 1952-1980, 1952-1990, 1952-1995, 1952-200 and 1952-2006 of Kogelberg Nature Reserve. The Mann-Whitney U-test (GraphPad Prism 5) was used to compare medians of fire return intervals. P-values ≤ 0.05 were considered significant. (*p<0.05 and **p<0.005).
2.3.4 Objective three