FOREST FIRE 



pass before another crop of pine seed 

 is produced. Meanwhile, hardwood 

 sprouts and herbaceous vegetation 

 produce a new mantle of litter that 

 retards pine-seed germination. 



On the other hand, fires before 

 seedfall may favor pine establishment 

 if they do not occur too early in the 

 growing season. Hardwood stems 

 killed after early August sprout very 

 little until the following spring. Con- 

 sequently, fires in late summer before 

 seedfall not only increase favorable 

 seedbed conditions by consuming 

 slash and undisturbed litter, but also 

 give pine seedlings an even start 

 with hardwood sprouts and seedlings 

 the following spring. The earlier that 

 fires occur in the summer, the more 

 nearly the sprout and seedbed con- 

 ditions approach those following 

 dormant-season fires after harvest 

 cutting. 



Effect of Frequency — While occa- 

 sional fires favor loblolly pine re- 

 generation, fires at intervals of less 

 than 10 years eventually eliminate 

 loblolly pine. Frequent fires repeat- 

 edly destroy the pine reproduction, 

 while hardwood stems are multiplied 

 by seedling establishment, sprouting, 

 and suckering. The process becomes 

 relatively rapid when the dominant 

 pine stand is clear-cut or otherwise 

 destroyed. Continued frequent burn- 

 ing may ultimately result in a vegeta- 

 tional type dominated by grasses. 



Effects of Fire on Soil 



Fire heats the soil only very shal- 

 lowly, but it affects the surface soil 

 both physically and chemically. 

 Physical effects range from none or 

 negligible to measurable. 



The texture of the surface soil may 

 be a factor. Thus, in the coastal 

 plain of South Carolina, a fire-effects 

 study showed no reduction in bulk 

 density, total porosity, or percolation 

 rate down to a depth of four inches, 

 even after ten annual fires. Yet, in 

 other coastal-plain locations, investi- 



gators have remarked on the com- 

 pactness and reduced permeability 

 of the soil surface after burning. 



It is logical to expect a greater 

 effect on soils of heavier texture. A 

 difference in effect on plant growth 

 attributable to soil texture and drain- 

 age characteristics was found in a 

 study in northeastern North Carolina, 

 where the area occupied by hardwood 

 sprouts increased rapidly for three 

 years after logging and site prepara- 

 tion and then grew more slowly. On 

 soils with poor surface drainage and 

 plastic subsoils, hardwood reached 

 85 percent ground coverage in five 

 years without burning but occupied 

 only 65 percent in burned areas. On 

 soils with good surface drainage, 

 burning had the opposite effect: hard- 

 wood cover increased faster on 

 burned than on unburned areas, 

 reaching about 10 percent greater 

 coverage after five years. 



Erosion following fire is not a con- 

 cern in the flat coastal plain but may 

 be a danger farther inland on more 

 pronounced topography. However, 

 instances of serious erosion following 

 fire in the loblolly pine range have 

 not been reported. It may be that 

 the vegetation grows back fast 

 enough to protect the soil surface 

 when the high-intensity summer 

 storms occur. 



Fertility — Burning usually results 

 in an increase in organic matter in- 

 corporated in the surface soil. Fire- 

 charred material filters into the upper 



layer. In addition, the ; mount 



of herbaceous vegetation that usually 

 follows burning, especially grasses 

 with their abundant fine roots, may 

 be a source of increased organic mat- 

 ter. These observations are from 

 coastal plain locations. On sloping 

 land, water would tend to carry ash 

 and charred material from the site 

 and organic matter increases might 

 not be so pronounced; organic matter 

 might even be reduced on heavier 

 textured soils. 



Soil chemical properties are usually 

 improved for plant growth after fire. 

 Calcium is increased appreciably, 

 with an accompanying decrease in 

 acidity. Other mineral elements may 

 be increased slightly through release 

 from the litter. (See Figure IX-14) 

 Nitrogen is increased, apparently 

 from several sources. Burning re- 

 leases much of the nitrogen in the 

 litter but some remains in fire-charred 

 material, which is concentrated close 

 to the soil surface. The increase in 

 herbaceous plants includes not only 

 grasses but also legumes, possibly 

 providing a source of nitrogen. How- 

 ever, an observed annual increase of 

 23 kilograms per hectare in annually 

 burned loblolly pine stands in the 

 lower coastal plains of South Carolina 

 could not be accounted for by the 

 transfer of nitrogen from the litter to 

 the mineral soil. 



Soil organisms in the litter and 

 humus layers are destroyed by burn- 

 ing, but the effects have not yet been 

 comprehensively investigated. Ap- 



Figure IX-14 — QUANTITIES OF NUTRIENTS RELEASED 

 BY BURNING TROPICAL VEGETATION 



Tropical rain forest 

 (forty years old) 



Savanna woodland 



PHOSPHATE POTASSIUM CALCIUM MAGNESIUM 

 112 731 2,254 309 



7 



41 



31 



23 



The table gives an estimate, in terms of pounds per acre, of several nutrients that 

 are released to the soil by burning vegetation in two different tropical regions. 



309 



