21 8
R. DAUBENMIRE
burned in western Africa, soil surface temperatures rose as high as
720°C, whereas at a depth of 2 cm soil temperature was elevated only
14°C as a maximum (Masson, 1954). These data indicate that when
African grasslands burn, soil surfaces become considerably hotter than
elsewhere. Even here, however, the duration of maximal values is so
brief that direct influence of grass tircs on soil would appoar negIigible.
The data reviewed above show that, the height of maximum heat
level when a grass cover burns is often well above the ground surface,
sometimes as high as 20 cm. With the highest temperatures so elevated,
short unburned stubble often remains after a fire has passed, and in
consequence shallowly placed buds and seeds escape damage.
The highest temperature recorded during the burning of a grass cover
is 720"C, with the next highest reports for different fies being 715, 669
and 600"C, and all of these were in subtropical areas with relatively
large amounts of fuel being consumed. Even where the litter load was
artificially raised to 1 -2 kg/m2 ground surface, air temperatures rose no
higher than 877°C (Iwanami and Iizumi, 1966b). Thus grass fires are
not nearly as hot aa forest fires, where temperatures may reach 1150°C.
Not only do temperatures rise much higher when woody plants burn,
the high levels persist longer. In Venezuelan savanna this tends to kill
grass in circular patches centered on isolated trees, and before grass
can reinvade seedlings of new woody plants may join sprouts from the
old ones to initiate a small grove. Woody-plant expansion in savanna
may thus be favored by fire (Vareschi, 1962). Usually, however, the
trend is the reverse, i.e., for forest in all climates to give way to grassIt is interesting that Marchuntia, Funaria and Pyronerna, oharacteristic invaders of forest soils where fires have been very hot, appear
not to have been reported on grassland soils that have been recently
burned. This may well reflect a lesser degree of alteration of the biotic
and/or abiotic characters of the soil surface in consequence of its lower
degree of heating as grass burns.
land if burning is frequent.
-
IV. SUBSEQUENT EFFECTS O N ENVIRONMENT
A. TEMPERATURE
In all t,he comparisons drawn in this and in subsequent sections, it is
to be understood that burned areas are being contrasted with comparable nearby unburned areas. Most of these published temperature data
represent instantaneous measurements, and the time of day when the
observations were made was often not, stated. This lack of uniformity
undoubtedly accounts for some of the dishawony among the reports.
R. DAUBENMIRE
burned in western Africa, soil surface temperatures rose as high as
720°C, whereas at a depth of 2 cm soil temperature was elevated only
14°C as a maximum (Masson, 1954). These data indicate that when
African grasslands burn, soil surfaces become considerably hotter than
elsewhere. Even here, however, the duration of maximal values is so
brief that direct influence of grass tircs on soil would appoar negIigible.
The data reviewed above show that, the height of maximum heat
level when a grass cover burns is often well above the ground surface,
sometimes as high as 20 cm. With the highest temperatures so elevated,
short unburned stubble often remains after a fire has passed, and in
consequence shallowly placed buds and seeds escape damage.
The highest temperature recorded during the burning of a grass cover
is 720"C, with the next highest reports for different fies being 715, 669
and 600"C, and all of these were in subtropical areas with relatively
large amounts of fuel being consumed. Even where the litter load was
artificially raised to 1 -2 kg/m2 ground surface, air temperatures rose no
higher than 877°C (Iwanami and Iizumi, 1966b). Thus grass fires are
not nearly as hot aa forest fires, where temperatures may reach 1150°C.
Not only do temperatures rise much higher when woody plants burn,
the high levels persist longer. In Venezuelan savanna this tends to kill
grass in circular patches centered on isolated trees, and before grass
can reinvade seedlings of new woody plants may join sprouts from the
old ones to initiate a small grove. Woody-plant expansion in savanna
may thus be favored by fire (Vareschi, 1962). Usually, however, the
trend is the reverse, i.e., for forest in all climates to give way to grassIt is interesting that Marchuntia, Funaria and Pyronerna, oharacteristic invaders of forest soils where fires have been very hot, appear
not to have been reported on grassland soils that have been recently
burned. This may well reflect a lesser degree of alteration of the biotic
and/or abiotic characters of the soil surface in consequence of its lower
degree of heating as grass burns.
land if burning is frequent.
-
IV. SUBSEQUENT EFFECTS O N ENVIRONMENT
A. TEMPERATURE
In all t,he comparisons drawn in this and in subsequent sections, it is
to be understood that burned areas are being contrasted with comparable nearby unburned areas. Most of these published temperature data
represent instantaneous measurements, and the time of day when the
observations were made was often not, stated. This lack of uniformity
undoubtedly accounts for some of the dishawony among the reports.