Fire Frequency, Area Burned, and Severity: A Quantitative Approach to Defining a Normal Fire Year

Fire Ecology

Table 1 Annual fire statistics for fires >40 ha in the Yosemite study area, 1984 to 2009, and comparisons among fire regime metrics. Area burned represents the area of Landsat pixels (0.09 ha) either within the reported fire perimeter or outside and adjacent to the reported perimeter with dNBR ≥ 150. The frequency, area burned, and severity of fires are classified by quartile (L = lowest quartile, M = middle 50 %, H = upper quartile). The years with the most extensive annual area burned are associated with high cumulative severity, and the years with low annual area burned are associated with low cumulative fire severity. However, the years with an annual area burned in the middle 50 % are associated with all levels of cumulative burn severity. The three metrics of frequency, area, and severity were consistent in 13 of 26 years (bold).

Year	No. fires >40 ha	Area fires >40 ha (ha)	Severity§	Comparison with middle 50% of years^†
Year	No. fires >40 ha	Area fires >40 ha (ha)	Severity§	No. fires	Area (ha)	Severity (pixel)
1984	6	795	0.20	M	L	L
1985	11	3 395	0.24	H	M	M
1986	4	1 789	0.25	M	M	M
1987	10	12 925	0.35	H	H	H
1988	15	6 550	0.25	H	H	M
1989	1	698	0.17	L	L	L
1990	8	10 909	0.36	H	H	H
1991	4	2 718	0.27	M	M	M
1992	4	538	0.21	M	L	L
1993	1	488	0.20	L	L	L
1994	4	2 412	0.21	M	M	L
1995	3	407	0.25	L	L	M
1996	6	24 812	0.34	M	H	H
1997	3	1 656	0.26	L	M	M
1998	3	1 008	0.21	L	L	L
1999	11	7 257	0.25	H	H	M
2000	1	144	0.16	L	L	L
2001	1	3 018	0.28	L	M	H
2002	5	2 348	0.22	M	M	M
2003	10	5 253	0.31	H	H	H
2004	4	3 846	0.29	M	M	H
2005	10	1 935	0.24	H	M	M
2006	7	3 124	0.25	M	M	M
2007	6	1 228	0.21	M	M	M
2008	6	2 933	0.28	M	M	M
2009	4	5 546	0.29	M	H	H

^† L = annual value lower than the middle 50 % of years, M = within the middle 50 % of years, H = annual value higher than the middle 50 % of years.
§ Severity metric (SM) calculated from the cumulative distribution of severity for each year as: \(SM = 1 - \sum\limits_{i = - 200}^{1200} {{{proportion\;of\;pixels\; \leq i} \over {1401}}}\)