The role of fire in the California pine forests

Abstract

In forest regions where fire commonly results Ia complete destruction of the standing timber the role played by fire is definitely recognized and serious efforts are made to control this menace In the Calif ornia pine region, however, and in other localities where fires run through the forest conipaiatively lightly and only occasionally assume the dimensions of a crown fire, injury is less, obvious and the consequences of fire both in mature and in growing timber often go unrecs .ized. Physical cons tions in the pine forests of California have led to the frequent recurrence of fires for centuries, but the fact that magmflcent forests still cover large areas and give the appearance of wellstocked, vigorous stands has blinded the public to the harm that fires have done and are steadily working throughout the whole region Were it possible for the observer to visualize the entire area upon which pine has growti, and to behold it truly fully stocked, he.w.ould thea see by comparison that the present (Jahfornia sine forests represent broken, patchy, understocked stands, worn down by the attrition of repeated light fires. The t,rtie rOle fire has played in this region .can. J)e appreciated to-day only by careful scrutiny of the less obvious forms of dam-, age. True, the thousands of acres of waste land or unproductive brush fields, the small stands of mature timber so isolated by brush as to be inaccessible for logging, the areas on which the forest type is changing to less valuablespecies, the failure of eut-over lands to reproduceall 'these are apparent enough, and tell an eloquent tale conditions and the "harmless" forest' fh'eare uuaily not kiiow'n or are o'verooked, so gradual is the cumulative effect. Fire scars and heat killing begin the destructive work directhr to burning down of trees or susceptibtht.y to wind insect infestation, to decay from fungous diseases. Each &eeb. eratea the progress and effects of the next; each successive4i1dd its often imperceptible weight to the force of attrition that thins the stand, weedis out the finest individual trees, and gradimily rfxl1ces the forest m quality and quantity to the point where the returns will not justify the cost of logging. Where logging has been in progress, the broadcast slash fire sweeps over with perhaps little apparent permanent dmage; but in its path reproduction, th haisof the new forest, is gone and will return only after many decades. The few seed trees left are weakened and are prey to the brat strong wuid. Brush sprouts from the unburned roots and gains the mastery. The next fire will blaze as fiercely as the slash burn. If any tree seedlings are still striving to regain the land for timber, they will be wiped out completely At the bor*jers of the brush field, the skirmish line of timber and brush, the fire will attack the vanguard of the forest with all the strength of a crown llre,itnd the forest will gIvi ay again before it. These are the links in the chain. Once they are seen and un4oi stood, fire in the California pine region can no longer be regailed. as the friend of man or a negligible foe. Its role of destroyer is as unmistakable here as elsewhere. The habit of thinking of the forest as an inexhaustible source of material, to be mined rather than grown as a crop, has been ai elistacle to a general appreciation of the destructiveness of fire. Another obstacle has bcen the difficulty of obtaining the facts regarding damage, and the consequent ease with which the. cse could be overstated or underst&ted. Although the importance of fire dam e was recognized by the earliest administrative and investigtive oh cars of the Forest Service working in 'California, and. although estimates of fire damage have been made a part of each report ever since systematic fire protection was begun in that. State, the Ugires compiled have proved to be m general very much below the atna dam (\ 2 llorsrius1y misleading these figures iiay be is shown bya 4jiie of five large fires made after a considerable lapse of time. Tb's critical survey disclosed a direct loss of 2,000 board feet to the acre.,. liz contrast with the orginal estimates, made directly after ibeflr, of only 525 board feet to the acre. Thus with few definite faofa to wor with, much misstatement of the situation, and in some cases lflrClir able data, it has in the past been difficult completeness the fire history of a given area. The most complete reóords bearing on fire scars for California were obtained by Boyce (3) in his study of dry rot' in incense cedar; For each tree studied the dates of all injuries were determined with unusual care, since it was essential to know how long the decay had been working in a particular tree in order to determine the age of infection, the rate of growth, and the rate of sprea4. of the fungus. This study was conducted on six areas extending from the Kiamath National Forest near the Oregon line through the Sierras to the Sierra National Forest near the southern end of the great belt of the California pine region. It was to be anticipated that these investigatioi would disclose with reasonable completeness the fire history of the pine region within the period represented by the age of the trees studied. RECURRENCE OF FIRE YEARS In analyzing the data from fire Ccars each area was treated as a unit and the dates of all fire injuries were plotted on cross-section paper. From these charts it was at once evident that certain years in the record were characterized by a large number of scars and might safely be consMered fire years Even with the slight errors that are likely to occur m counting the ringserrors that no doubt account in part for a sprinkling of fire scars m the years between the principal peaksthere was no reason to doubt that in the past the forests have been subjected to periodically recurrmg fires. The earliest date of past fires found on any of the trees studied was 1530, and for more than a century after that date cqmparatively few scars were observed, for the reason, no doitbt, that of the trees growing in the sixteenth and. seventeen,th centuries oply a few have survived to the present. There is the fpither ac already explamed that repeated fires in a particular woulid tend to - obliterate the evidence of earlier fires, even if they do not destroy the tree From about the year 1700 on the frequency of the scars is such that the fire hiqtory of most of the areas studied can be stated with a fair degree of precision. During the past three centuries the years 1'685, 1690, 1699, 1702, 1708, 1719, 1726, 1735, 1743, 1747, 1750, 1757, 1766, 1786, 1796, 1804 1809, 1815, 1822, 1829, 1837, 1843, 1851, 1856, 1q65, 1870, 1879, 1889 are indicated cleary as years of extensive fires Naturally enough the latter part of the record is marked by a greater number of scars than the earlier part, os'ing to the gradual elimination of the older and more heavily scarred trees During the two centuries for which the data can be regarded as fairly complete 25 clearly marked fire years are found and it is a fact of high sigmficance that tins general average periodicity of eight years hold. true for all areas studied The shortest period between fire years is 3 years and the longest 11 years. EFFECT OF CROWN INJURIES ON GROWTH One of the important secondary injuries resulting from fires is tha reduction of the rate of growth as a result of crown injuries. On the Kiamath National Forest a study of the effect of surface fires on rate of growth was very instructive. In 1910 a fire ran through a mixed stand of western yellow pine, sugar pine, and Douglas fir, partially killing the crowns of nearly all trees but failing to kill any tree outright. In the fall of 1915, when the rate of growth for the previous decade was determined by measuring the annual rings, the atand failed to show noticeable external damage from fire. Many trees which had badly scorched crowns in 1910 had recovered, and oniy one tree had succumbed to fire injuries during the five years. the growth at breAstheight was measured separately for the fiveyear periods before and after the fire It was found that 16 per cent of all the trees measured grew -at the same rate for both periods, while 68 per cent grew more slowly and 16 per cent grew more rapidly after the fire. The growth of one tree fell off 69 per cent, while none of those increasing made a greater gain than 14 per cent. Averaging all trees, the growth in basal area after the fire was oni 83.6 per cent of the growth before the fire, while the volume growt was correspondingly reduced by about 25 per cent. The stand on this particular plot was 43,230 board feet to the acre, and the vol.. ume growth for the five-year period before the fire was 1,945 board feet to the acre, or 390 board feet each year. The reduction of 25 per cent means aloss of nearly 500 board feet-per acre for the fiveyear period. Checks on adjoining unburned areas showed no change m rate for. the two half decades. In 1919 a similar study was -made INDIRECT PHYSICAL DAMAGE TO MATURE TIMBER INSWr IN3URV Certain other secondary injuries from fire are not immediately evident or simple of proof and,, indeed, can be exactly determined only after detailed study over a period of years. Chief among these are losses from insects and woo d-dstroying fungi. Of the insects which cause the greatest losses to pines the genus Dendroctonus is the most important and most prevalent. Though ,a light infestation is practically always present in the pine forets5 turnbérmen and foresters are chiefly concerned with this and other forest insects only when an epidemic develops. Insect epidemics have been known for many years, but the reasons for the sudden outbreaks btve been exceedingly obscure. Only recently has it bei discovered that insect epidemics are frequently precipitated or intensified by forest fires.