20 BULLETIN 1495, U. S. DEPARTMENT OF AGRICULTURE 
the upper timber belts is undoubtedly a factor of prime importance 
in preventing the start of fires during the spring and early summer. 
The number of lightning fires is but 55 per cent as high in the brush- 
field type as in the western yellow pine and mixed conifer group. 
Since the brush type cuts across the entire timber belt and thus is 
subjected to the same lightning risk, the difference in rate is attribu- 
table to scarcity of trees in the brush fields. Tall trees are the objects 
commonly struck by lightning. The relative number of lightning 
fires from month to month in the timber types and in brush is fairly 
constant. 
TABLE 8.—Average seasonal distribution of lightning fires by types, 1911-1920, all 
forests 
[Number of fires per 100,000 acres] 
Type May | June | July | August pecba oe Total | * Ratio 
iIWieStermibyellow pees e sae suge na 0. 90 4.48 | 12.59 | 13.35 2. 62 05405 | 3453405 eee 
Mixed) conifers saeuae. Nana a eS nas 1,19 5.21 | 11.831) 17: 33 3. 20 S59 827 ee eee 
Douglas ire eee sees Cui A eile Wes ah - 96 2.97 8.91 9, 43 a Op ke ibaa 22. 97 63 
SULA PIMe My e MLN Ma ee ee RAL ie - 42 1.90 6.74 | 13.89 1. 47 21 24 5G3n| eee 
LO peut CCR IR AGS eS Sipe Re WEA a . 24 1. 40 9.67 | 16.85 2. 68 BO ||) tilly 1b ee £ 
GEASS CRD ie EDU BNI YR Se 2 YA Wi RN . 36 1. 62 2.16 4. 51 BEST ON en eS OOO | ess eres 
Obra parallel . 50 1. 24 1. 49 1.98 252 On| eae (ae: sl Vig sie 
VW O.O GLUE 0 CURE SP eee AEE RA CIN RL 1.47 4.10 3. 89 All -11 G(8) pe wee 
BW ef DES a Lut oe ie OLR A ti A ese 45 2.14 6. 61 9. 38 1. 38 . 09 20. 05 55 
UNV OT Os Aelia da an yee aismtye sag pan UI cs Aaya ~t2 3. 37 9.55 | 12.47 Pas) Ol he bret tayo ice a 
SUMMARY OF TYPE GROUPS 
Western yellow pine mixed conifer 
TT OUUTO apes se aan a PRR ck PAN SC 1.02 4.78 | 12.28] 14.98 2. 85 0.44 | 36.35 100 
Sugar pine-fir, fir group____-_____--_-- . 28 1. 51 9:01 | 16.19 2.41 6 9 29. 68 82 
Woodland, grass, chaparral group-_-__-- 21 1.47 2. 98 3. 67 . 84 - 05 9, 22 25 
In the woodland, grass, and chaparral groups the total number of 
lightning fires is but 25 per cent as great as in the western yellow pine 
and mixed conifer group, and but 46 per cent as great as in the brush 
fields, indicating a relative scarcity of lightning storms. The fact is 
that most of these storms originate above the upper timber belts and 
do not generally extend over the untimbered foothill region. 
NUMBER OF MAN-CAUSED FIRES 
The seasonal total of man-caused fires (Table 9 and fig. 6) is highest 
in the chaparral, followed in order by mixed conifer, western yellow 
pine, Douglas fir, brush, grass, woodland, sugar pine-fir, and fir. 
An interesting contrast is found in the fact that the figures for man- 
caused fires in western yellow pine (33.53) and mixed conifer (39.85) 
are practically identical with the lightning figures in the same types 
(34.34 and 39.27), whereas in the chaparral the 48.14 man-caused 
fires per 100,000 acres is the highest and the 7.44 lightning fires per 
100,000 acres, is the lowest of all. 
