222
R. DAUBENMIRE
stated that annual burning in March, only, had reduced the humus
content of the upper 762 cm of the profile from 1.77 to i.47yo (Anderson, 1964).
In the much drier steppe of eastern Idaho very hot single burns reduced humus in the upper 12 mm of the soil profile, but this effect had
disappeared by the fourteenth post-burn mason when a second test was
made (Blaisdell, 1953).
It was observed in southern Africa that the humus of the upper
38 mm of the soil wm the same on unburned plots as on those burned
annually for 6 years, but since the unburned plots carried 20 times as
much litter, this difference might eventually result in a difference in
humus content (Cook, 1939). In Kenya grassland protected from burning for 10 years or more, a distinct increase in humus was measurable
(Edwards, 1942). Thirty years of annual burning in Nigeria raised the
humus content of the upper 20 cm of the soil 17y0 where light burns
were made early in the dry season, but lowered the humus content by
12% where the fires were set late in the dry season and so were much
hottfer (Moore, 1960). This result may be interpreted in part by the
effect of early burning stimulating increased productivity, the effects
of which were eventually reflected in higher humus levels below the
soil surface.
While grassland fires are seldom hot enough to directly oxidize
organic matter that lies deeper than a few millimeters in the soil profile,
from the subsequent increase of direct insolation that warms the aoil
and stimulates microbial activity, coupled with at least a temporary
elimination of the source of humus, it might be anticipated that burning
would reduce the humus content of soil, especially if burning is frequently repeated. On the other hand, the decay of roots of plants killed
by fire, any replacement of perennials by annuals, or any increase in
primary productivity (which, as will be shown, is common), will tend
to have the opposite effect. The research reviewed above shows that in
some geographic areas burning at certain seasons increases humus
whereas under other circumstances it is reduced. There has been no
serious attempt to evaluate the interactions among the processes
involved.
c. EROSION
Long-term studies of the effects of regular burning of fire-maintained
grass vegetation in Mississippi failed to reveal any evidence of accelerated erosion (Wahlenberg P t al., 1939), and the absence of reference to
evident erosion on burned areas that have been studied in the midcontinental grasslands of North America show that there is no significant habitat deterioration of this type there either. But in the much
R. DAUBENMIRE
stated that annual burning in March, only, had reduced the humus
content of the upper 762 cm of the profile from 1.77 to i.47yo (Anderson, 1964).
In the much drier steppe of eastern Idaho very hot single burns reduced humus in the upper 12 mm of the soil profile, but this effect had
disappeared by the fourteenth post-burn mason when a second test was
made (Blaisdell, 1953).
It was observed in southern Africa that the humus of the upper
38 mm of the soil wm the same on unburned plots as on those burned
annually for 6 years, but since the unburned plots carried 20 times as
much litter, this difference might eventually result in a difference in
humus content (Cook, 1939). In Kenya grassland protected from burning for 10 years or more, a distinct increase in humus was measurable
(Edwards, 1942). Thirty years of annual burning in Nigeria raised the
humus content of the upper 20 cm of the soil 17y0 where light burns
were made early in the dry season, but lowered the humus content by
12% where the fires were set late in the dry season and so were much
hottfer (Moore, 1960). This result may be interpreted in part by the
effect of early burning stimulating increased productivity, the effects
of which were eventually reflected in higher humus levels below the
soil surface.
While grassland fires are seldom hot enough to directly oxidize
organic matter that lies deeper than a few millimeters in the soil profile,
from the subsequent increase of direct insolation that warms the aoil
and stimulates microbial activity, coupled with at least a temporary
elimination of the source of humus, it might be anticipated that burning
would reduce the humus content of soil, especially if burning is frequently repeated. On the other hand, the decay of roots of plants killed
by fire, any replacement of perennials by annuals, or any increase in
primary productivity (which, as will be shown, is common), will tend
to have the opposite effect. The research reviewed above shows that in
some geographic areas burning at certain seasons increases humus
whereas under other circumstances it is reduced. There has been no
serious attempt to evaluate the interactions among the processes
involved.
c. EROSION
Long-term studies of the effects of regular burning of fire-maintained
grass vegetation in Mississippi failed to reveal any evidence of accelerated erosion (Wahlenberg P t al., 1939), and the absence of reference to
evident erosion on burned areas that have been studied in the midcontinental grasslands of North America show that there is no significant habitat deterioration of this type there either. But in the much