Agriculture Reference
In-Depth Information
determining this competitive balance (Bond
2010
) and this will be discussed more
fully in the next subsection. Hill (
1994
) hypothesized that sclerophylly originated
in response to nutrient-deficient soils and this preadapted those taxa to xeric
conditions. One test of this hypothesis has been comparison of evolutionary
transitions that include xeromorphic traits. One such trait in Proteaceae is
the stomatal crypt, where stomata are deeply sunken inside surface indentations
of the leaf. In Australian Proteaceae stomatal crypts have evolved at least 11 times,
always associated with lineages in arid environments (Jordan
et al.
2008
).
Although sunken stomata are associated with a number of selective factors
(Gibson
1983
), in Proteaceae their origin has been interpreted as an indicator
of aridity and evidence that early origins of sclerophylly in response to nutrient-
poor soils pre-adapted these taxa to aridity. This idea has been supported
by phylogenetic reconstructions of lineages that have evolved stomatal crypts
(Mast & Givnish
2002
).
In southern Australia Hill (
1994
; Paull & Hill
2010
) suggests that Casuarinaceae
taxa such as
Casuarina
and
Allocasuarina
were present in the early Tertiary
rainforests but not found in the fossil record because they were restricted to dry
microsites such as ridgetops, and they expanded rapidly when the climate dried
later in the Tertiary. Other mid-Tertiary radiations include the rainforest progeni-
tors of the Elaeocarpaceae that radiated widely during the Oligocene and
Miocene, producing a diversity of arid-adapted shrubs (Crayn
et al.
2006
). In
the late Oligocene to early Miocene there was a shift away from
Nothofagus
-
dominated forests toward forests dominated by sclerophyllous-leaved Myrtaceae
and Casuarinaceae, more characteristic of the contemporary MTV found
throughout southern Australia (Hill
et al.
1999
).
Eucalyptus
(Myrtaceae) is not only the most diverse but also the most dominant
component of Australian landscapes. These sclerophylls thrive in nutrient-poor
arid sites and although fossils date the group to at least the Eocene, usually in
association with mesophyllous rainforest, they likely occupied more arid or
nutrient-poor sites within those communities (Holmes
et al.
1982
). As with the
northern hemisphere
Pinus
, Miocene was the time for
Eucalyptus
expansion, and
given its exceptional fire-surviving and even fire-promoting adaptations, it prob-
ably evolved in environments with a high fire frequency. Grasslands also
developed during this period, likely promoted by fire (Bowman
2000
; Bond
et al.
2003
; Keeley & Rundel
2005
) and it is interesting to speculate what sort of
coevolutionary relationship they might have had with the eucalypts.
Origins of South African MTC fynbos and related shrublands are poorly
recorded in the macrofossil record but some palynological records exist.
One characteristic that appears similar to Australia is the widespread presence
of sclerophyllous-leaved Proteaceae since the Eocene (Coetzee
et al.
1983
).
Sclerophyllous taxa originated in arid pockets during the Oligocene (Goldblatt
1978
) and many fynbos elements were present in small patches of “skeletal
soils” in the mountains, where soils were too shallow to support forest (Cowling
et al.
2009
).
