Agriculture Reference
In-Depth Information
when all anthropogenic fires were suppressed. This contrasts with 44.7% during
the preceding period of prescribed burning (1975-1985) and 16.6% during a
period of active fire suppression (1951-1974). The mean area burned during these
three periods was similar (7907 ha natural, 9015 ha prescribed, 5612 suppression).
Thus, at least for this region of the fynbos biome, lightning regularly causes fires
and lightning-ignited fires account for similar areas burned to those ignited by
people (Seydack
et al.
2007
).
In summary, we have an incomplete knowledge of what determines fire regimes
in fynbos. Fire return intervals are usually longer than the fuel accumulation
threshold of
4-6 yrs before fires will spread under typical summer conditions
of wind and humidity. Large fires are associated with prolonged summer droughts
but these are quite rare, occurring at mean intervals of 5-8 yrs in one study
(Richardson & Kruger
1990
). Given the combined constraints of sufficient fuel,
and weather conditions promoting high fuel continuity, return intervals for large
fires would be a decade or more, depending on ignition. Fynbos has a much lower
fire frequency than more mesic C
4
grassy biomes. The different fire frequencies
cannot be attributed to slow biomass accumulation - the rates are similar at
2.5 Mg
1
ha
1
yr
1
(Kruger
1977
; O'Connor & Bredenkamp
1997
). But fuel
structure (see
Chapter 2
) is very different. Annual retention of green foliage and
slow accumulation of dead fuels in fynbos limits fire starts in early seral stages and
the nutrient-poor soils further limit fuels in early stages due to the near lack of
annual species. By comparison, fuel structure is very different in C
4
grasslands,
where the annual turnover of living foliage in herbaceous perennial grasses gener-
ates highly flammable fuel loads on an annual basis.
∼
Human Impacts
Humans evolved in Africa. The oldest human artifacts in the Cape region are
those of the earlier Stone Age dating back more than a million years (Deacon
1992
). Some of the earliest anatomically modern human fossils, dating back
∼
120 000 yrs BP, have been collected from the south coast of the Cape Floristic
Region (Deacon
1992
). There are abundant hearths in human occupation sites
starting from the Late Pleistocene (120 000 to 60 000 yrs BP) implying that, from
this time, humans could make fire at will. Yet, though humans learned the use of
fire in Africa, the archaeological record of early human use of fire is fragmentary
and very poorly known, especially relative to Australia and North America. It
has been speculated that “fire-stick” farming may have been used to manage
fynbos for food resources for
100 000 yrs (Deacon
1992
). Corms of geophytes
(
Watsonia
,
Gladiolus
,
Ixia
,
Moraea
) dominate the food debris of archaeological
sites throughout the fynbos region and these shrublands may have been burned
frequently in autumn to promote this food resource (Deacon
1992
; Le Maitre
1984
). However, the extent of hunter-gatherer impacts on fire regimes of the
region during the Stone Age period are probably slight and localized, with the
∼
