Agriculture Reference
In-Depth Information
Van Wilgen
et al.
(
1990b
) have compared fuel properties of fynbos and forest. Fuel
mass in fire-prone fynbos was less than half that in the closed forest. However
there was a pronounced separation of litter and canopy fuels in the forest and the
latter were discontinuous and patchy relative to fynbos. Fuel moisture contents of
the forest fuels were 50-100% higher than fynbos, and dried out less in the dry
summer months, but heat yields and fat content of forest and fynbos fuels were
similar. Thus, the physical features of fynbos fuels promote flammability whereas
forests are much less flammable.
Although forests do not burn easily, forest margins do burn. Two species of
Virgilia
, hard-seeded legumes with long-lived seedbanks, are fire-stimulated obli-
gate seeders that occur only on forest margins abutting fynbos. No other forest
trees have been reported to have fire-stimulated recruitment. Rare fires may
influence forest composition for decades by selecting for species capable of
resprouting. Thus, forest margins are often dominated by multistemmed trees
(
Rapanea
,
Cassine
,
Pterocelastrus
) indicating past disturbance (Kruger
et al.
1997
). Tall single-stemmed individuals of these species, and of fire-intolerant
non-resprouting species (e.g.
Podocarpus latifolius
,
P. falcatus
,
Olea capensis
ssp.
macrocarpa
,
Curtisia dentata
), occur deeper in the forest.
Forest has been observed colonizing fynbos in several localities where fire has
been excluded for several decades (Masson & Moll
1987
; Manders
1990
; Manders
et al.
1992
; Luger &Moll
1993
; Cowling
et al.
1997b
). Many colonizing tree species
have bird-dispersed fruits and establish under perch sites on taller shrubs (Man-
ders
et al.
1992
; Cowling
et al.
1997b
). Forest saplings may take many years to
become fire proof: that is, with the capacity to resprout after burning. Manders
(
1990
) noted that all saplings of
Cunonia capensis
, a forest species colonizing
fynbos unburned for
30 yrs, died after being burned regardless of plant size.
Manders
et al.
(
1992
) concluded that forest could potentially occur in most fynbos
sites with > 650 mm precipitation but were prevented from doing so by the
combined effects of frequent fynbos fires and slow sapling growth rates.
∼
Determinants of Fire Regimes
It is interesting to contrast fynbos fire regimes with those of other MTC ecosys-
tems and with the other major fire-prone biomes in South Africa, which include
grasslands and savannas dominated by C
4
grasses. Fires burn more frequently in
fynbos than in California chaparral (see
Chapter 5
) but less frequently than
kwongan shrublands of Western Australia (see
Chapter 8
). Fynbos burns much
less frequently than mesic (> 750 mm annual precipitation) C
4
grassy ecosystems,
much of which burns annually or biennially (Balfour & Howison
2002
). Fires in
semi-arid savannas are strongly limited by the continuity of grass fuels, which
depends on preceding rainfall (van Wilgen
et al.
2004
). Though mesic fynbos and
savannas are both fire-prone and fire-dependent ecosystems, they have strikingly
different woody plant life histories reflecting their very different fire regimes
