Agriculture Reference
In-Depth Information
Several Cape genera experienced rapid speciation toward the end of the
Tertiary. These include shrubs such as
Phylica
(Rhamnaceae) and herbaceous
perennials such as
Moraea
(Iridaceae). Such speciation is attributed to mountain
building (Cowling
et al.
2009
), although the increased spread of MTV due to
increasingly severe summer drought, coupled with increasing fire activity (
Fig.
10.9
) would also explain these patterns.
One interesting difference between the northern hemisphere MTC regions and
the southern hemisphere MTC regions in Australia and South Africa is the much
more limited presence of obligate resprouters with fire-independent seedling
recruitment and mostly fleshy-fruited vertebrate-dispersed seeds. This functional
type is poorly represented in the two southern MTC ecosystems of fynbos and
heath. South African Cape fynbos does have occasional
Rhus
,
Olea
,
Diospyros
and
Heeria
, but this functional type is largely absent from southwestern Australian
shrublands. In an earlier subsection it was hypothesized that, in the northern
hemisphere in the early Tertiary, this functional type would have been adaptive
in an environment where sclerophyllous shrubs were restricted to islands of
suitable stressful sites within a matrix of woodland. Under those conditions
long-distance vertebrate dispersal would be of immense selective value and
fire-independent recruitment an advantage on pockets of vegetation with a less
predictable fire regime.
A long history of extensive, highly weathered infertile substrates in southwest
Australia (Hopper
2009
) would have created a highly fire-prone vegetation and
the continuity of landscape fuels increased fire predictability. These habitats
would have selected for postfire seeders with localized dispersal and not provided
an impetus for vertebrate dispersal to target isolated islands of sclerophyll
shrublands, as hypothesized for northern hemisphere shrublands. An explanation
for the limited representation of obligate resprouters in these two southern hemi-
sphere MTC regions could be tied to nutrient-deficient soils. In the northern
hemisphere MTC ecosystems it is hypothesized that this mode is adaptive on
mesic fertile sites where vigorous resprouts limit gaps for seedling recruitment.
Infertile substrates in southern hemisphere MTC landscapes may limit the size of
postfire resprouts so that there is always a sizeable gap resource for seedling
recruitment after fires, thus providing less incentive for recruitment between fires.
Central Chile is particularly interesting in that today it seems to have little if any
natural source of ignitions due to blockage of summer thunderstorms by the
Andes (
Fig. 10.4
). The sclerophyllous matorral shrublands are highly resilient to
anthropogenic fires as almost all species resprout, and many do so from pre-
formed buds in lignotubers, a structure largely associated with fire-prone vegeta-
tion. Also a few hardseeded species in the Fabaceae and Rhamnaceae do recruit
some seedlings after fire from weakly dormant seedbanks. These characteristics
suggest an early association with fire that perhaps is on the decline since comple-
tion of the late Miocene Andean uplift eliminated sources of ignition only a few
million years ago. Examples of earlier matorral vegetation may be represented by
the fire response of shrubs and herbs in southern Chile and just over the border in
