Soil Temperatures During Burning Related to Postfire Seedbeds on Woodland Range

Abstract

AFTER WOODLAND RANGE iS burned, land managers need some way to recognize the suitability of the postfire seedbed for artificial seed-ing. Seedbed conditions are likely to vary widely because of variations in fuel and burning conditions. Where sufficient fuel is burned and the soil surface becomes hot enough to kill seeds of plants that would compete with sown forage species, sowing will have a good chance of success. Where there is little fuel or it is not consumed by a hot fire and seeds of competing plants are not killed, artificial seeding will be a waste of money. Range managers have learned that brush burns which leave a conspicuous whitish ash over most of the burned area usually destroy most seed of competing plants. But the ash may disappear by the time of sowing. Or it may cover only part of the ground surface. To provide a wider range of criteria than •he presence or absence of white ash, we classified seedbeds in four groups by the appearance of the soil surface after burning. One is the .grass-burn seedbed, which is found where fire burned through grassy openings. The other three are brush-burn seedbeds, defined as follows: Black-ash, where leaf litter was thoroughly charred but not consumed. Bare-soil, where litter was consumed. White-ash, where heavy pieces of wood were consumed. Then we measured soil temperature during burning of various fuels on woodland range in the Sierra Nevada foothills. By relat-emaintained at Berkeley, California, by the Forest Service, U.S. Department of Agriculture, in cooperation with the University of California. Mr. Bentley is range conservationist and Mr. Fenner is former range conser-vationist at the station. ing the temperatures to postfire seedbed classes, we were able to redefine the seedbed classes and set up criteria which should be widely useful. Methods The test was conducted in a controlled burn at the San Joaquin Experimental Range in September 1952. The 200-acre burn was in open woodland type with interspersed thickets of trees and shrubs. Some of the shrubs had been smashed with a bulldozer in preparation for burning. The her-baceous cover in the openings was composed of annual plants. Records of soil temperatures during burning were made at 19 locations inside the fire perimeter. The locations were selected to include the range of fuels which typically occur within the woodland type. At each two or more pyrometer records were made. A total of 20 records was obtained under the fuels found in grassy openings between brush thickets. These light fuels varied from thin to thick stands of grasses and forbs. Most of the litter on the soil surface was from herbaceous plants, but leaves from trees or shrubs were included at some stations. A total of 24 records was obtained under fuels found within stands of trees and shrubs. These fuels varied greatly in quantity, from small shrubs to heavy logs. At some stations the shrubs were upright, at others they had been smashed flat. There was a layer. of leaf litter at most stations. The soil pyrometer used in this test 2 was designed by Fenner to •Fenner, R. L., and J. I•. Bentley. A simple fusion pyrometer for measuring maximum soil temperatures during wilcL land fires. (Manuscript submitted to Forest Science.) 737 record the maximum depth at which any one of 10 specific temperatures occurred within the surface litter and upper 2 to 3 inches of mineral soil. The pyrometer consisted of a very thin mica plate, 2 inches wide and 3 inches high, on which 10 different fusible compounds had been painted, each in a single vertical strip. The specific melting points for the different compounds, in degrees Fahrenheit were: 150, 200, 250, 350, 450, 550, 650, 750, 950, and 1,150. The face of the mica sheet, on which the strips were painted, was covered with a thin piece of asbestos paper. The pyrom-eter was inserted into the ground with a special tool so that the back of the mica sheet was packed tightly against an undisturbed soil profile. The top of the sheet was placed approximately flush with the surface of the litter layer, or flush with the mineral soil surface if there was no litter. Physical appearance of the area surrounding each pyrometer station was described and photographed before and after the fire, and any factors which might influence the temperature of the soil during burning were noted. After the fire, a 1-foot-square quadrat surrounding the pyrometer was classified according to seedbed condition. Before each pyrometer was removed from the soil, the level of the mineral soil surface was marked as closely as possible on the mica plate. The maximum depth below mineral soil at which each of the 10 temperatures had been recorded, as indicated by melting of the fusible compounds, was measured on each mica plate. In January 1953, notes were made on germination of herbaceous plants in the square-foot quadrats at each py-rometer station.