PROTECTION OF FORESTS FROM FIRE 



513 



gether, the tire has the nature of a sur- 

 face fire, intensitied by the burning 

 crowns. If the crowns meet, and there 

 is a more or less complete canopy, a 

 true crown fire is developed. 



A special class of brush fires are those 

 in the chaparral of the southwest. The 

 brush is dense and there are many spe- 

 cies with intiammable foliage. In many 

 places a thick layer of litter and humus 

 is formed on the ground, just as in a 

 dense forest. Fires in this type of 

 scrub forest are very fierce and de- 

 structive, and are analogous to fires in 

 dense stands of young conifers. 



GROUND FIRES 



This term is applied to the slow fires 

 that burn in the deep accumulations of 

 vegetable matter common in many of 

 our damp northern forests. Here the 

 fallen leaves, needles, and other off- 

 castings of the trees decompose very 

 slowly, and a deep layer of partially de- 

 cayed organic matter accumulates, often 

 to a depth of from two to three feet. 

 This material absorbs moisture with 

 avidity and retains it tenaciously. Con- 

 sequently, in most seasons it is not read- 

 ily ignited. In some seasons, however, 

 it becomes thoroughly dry and will 

 burn. A fire in this peaty substance 

 burns slowly, but with very intense 

 heat, and is exceedingly difiicult to ex- 

 tinguish. Ground fires in the Adiron- 

 dack's have been known to burn all win- 

 ter, creeping along under a deep layer 

 of snow. 



Ordinarily a ground fire will not 

 cover more than a few acres in a day. 

 Frequently, however, there is upon the 

 surface a large amount of dry debris 

 or small coniferous trees, so that there 

 accompanies the ground fire a surface 

 fire or a brush fire, or both, and occa- 

 sionally a crown fire. 



CROWN FIRES 



Crown fires are those which burn 

 through the crowns of the trees. (See 

 Plate II.) They almost invariably start 

 from surface fires. Occasionally, how- 

 ever, thev are started when lig-htninc 



strikes and ignites a dry stub or resinous 

 tree surrounded by a dense stand of 

 conifers. If the crowns are of such a 

 character that they will burn they may 

 be easily ignited by the fiames which 

 rise from a surface fire. Sometimes a 

 crown fire is started by the flames from 

 a burning clump of young growth, and 

 where the trees have exuded resin or 

 there is loose inflammable bark, a crown 

 fire may be started by the flame's run- 

 ning up the trunk. 



Crown fires occur when the woods 

 are very dry and when there is a high 

 wind. Without a strong wind a crown 

 fire is seldom started, and even if the 

 crown of an individual tree is ignited, a 

 fire does not usually spread and run 

 through the crowns on a still day. Be- 

 fore a high-wind, a crown fire spreads 

 with great velocity, taking at once a 

 V-shaped form with a distinct front or 

 head. This head may be only from fifty 

 to 100 feeet wide, but in the case of the 

 largest fires its width may be very great. 

 In the case of the larger fires the front 

 is generally carried forward by a se- 

 ries of heads. The head of the fire 

 burns very rapidly through the crowns, 

 and there follows closely a surface fire 

 burning with the same rapidity. There 

 are well developed wings, where the 

 fire runs through the crowns on each 

 side of the head. These, in turn, are 

 accompanied by surface fires, while 

 spreading out on the skirts are wider 

 surface fires, eating out diagonally with 

 the wind and covering a broader area 

 than the crown fire. 



The strong draft of heated air aris- 

 ing from the fire carries up with it an 

 immense quantity of burning cinders 

 and pieces of bark. The wind, in turn, 

 carries this material far in advance of 

 the main fire head, and thus innumer- 

 able new surface fires are started. This 

 gives rise to the popular idea of a spon- 

 taneous starting of fires in advance of 

 a crown fire. 



An ordinary crown fire does not run 

 more than two or three miles an hour, 

 although undoubtedly the great con- 

 flagrations of the north woods, such as 

 the famous Hinckley fire in Minnesota 

 in t8()4, are swept along at a much 



