Agriculture Reference
In-Depth Information
(1) They imply that MTC shrublands are not at equilibrium with climate and that
features of forests and thickets, together with those of the shrublands, have to
be compared to identify which plant traits are common to MTCs
(2) Similarly for low nutrient soils: what traits, if any, are convergent in both fire-
prone and fire-resistant vegetation growing on the same nutrient-poor sub-
strates or differ depending on their disturbance regime?
(3) What traits are convergent because of shared responses to fire or mammal
herbivory (e.g. spinescence), rather than to climate and soils?
This chapter focuses on fynbos shrublands since these are by far the most studied.
However, we do discuss the other fire-prone and fire-resistant vegetation types
with a view to understanding the major vegetation patterns in the region. Within
the wider African context, the Cape region is a tiny enclave of shrubby vegetation
relative to the vast expanse of flammable C
4
grassland ecosystems. The fynbos and
renosterveld crown fire regimes are very different from the surface fires of grass-
land and savanna ecosystems and have selected for entirely different reproductive
and persistence traits. Indeed the most distinctive feature of MTCs in the Cape
may be that they select for crown fire regimes with return intervals much longer
than the frequent surface fires fueled by C
4
grasses.
Fynbos Fire Regimes
Fynbos has a crown fire regime (see
Chapter 2
). Fynbos fires come in a large range
of sizes from a maximum of 58 000 ha (a lightning fire in the Cedarberg in 1988) to
less than a hectare. A database of digitized fire maps for the Cape region has
recently become available derived from fire scars sketched onto 1:50 000 maps
since as early as the 1920s (Scientific Services, Capenature, Stellenbosch). The
database has more than 2000 fires, mostly from protected areas in mountain
fynbos across the biome, and is most consistent and reliable from the 1970s.
It has been used in several fire regime analyses and shows that large fires
(> 1000 ha) accounted for only 22% of the total number of fires but 85% of
the area burned (Forsyth & van Wilgen
2007
,
2008
; Seydack
et al.
2007
; Southey
2009
; Wilson
et al.
2010
).
Most fynbos fires occur at intervals of 10-30 yrs (Seydack
et al.
2007
; Forsyth &
van Wilgen
2008
; Southey
2009
; Wilson
et al.
2010
). Fires seldom burn in stands
younger than 7 yrs and few stands survive more than 40-50 yrs without burning.
Fires can burn in any season, including winter, though the largest number of fires,
and total area burned, is in summer (
Fig. 7.5
). There is some geographic variation
in fire season with winter burns being common in the humid coastal ranges in the
eastern mountains of the region (van Wilgen
1984
; Southey
2009
). These are
generally associated with long periods of warm dry foehn-like winds, called
bergwinds
(see
Box 1.3
). Under experimental conditions, fynbos fire intensities
range from 500 to
20 000 kW m
-1
with flame lengths of 2-7 m and rates of
∼
