Agriculture Reference
In-Depth Information
In the northern hemisphere most of the dominant lineages originated in the
early Tertiary in small drought-prone pockets amidst a landscape of mesic forests
and woodlands. Seasonal drought necessary for fires was probably not annual and
fires could have been as infrequent as roughly once in the life span of these shrubs
and small trees and still be of selective importance. These conditions selected for
the obligate resprouting mode, suitable for persisting through periodic fires, but a
fire regime that did not favor postfire seedling recruitment. Factors responsible for
limited selection of postfire seeding would include: fire intensity insufficient to
produce widespread resprout mortality (i.e. high adult survivorship) and con-
straints imposed by drought avoidance strategies of deep root systems. The
island-like distribution of these fire-prone pockets selected for plant traits contrib-
uting to wider spatial dispersal, including feathery wind-dispersed achenes and
fleshy-covered vertebrate-dispersed fruits.
Through a combination of increasing length of drought, and coupling drought
with high temperatures, as well as the eroding effect of periodic fires on mesic
vegetation, the extent of fire-prone landscape expanded. As islands of fire-prone
vegetation expanded within a sea of less flammable woodlands, patch size alone
provided feedback by increasing the probability of being struck by lightning and
thus increased fire ignitions. In these closed-canopy shrublands and woodlands,
high-intensity fire created larger resprout-free gaps. This coupled with the marked
shift from closed-canopy resource-limited conditions to more open resource-
abundant burned sites increased opportunities for seedling recruitment to the
extent that species were selected to delay reproduction to a single postfire pulse.
Fire-dependent reproduction selected for niche construction traits such as
enhanced flammability created by retention of dead branches in the canopy and
other traits. As the predictability of gap formation increased some lineages took
the extra hazardous step of eliminating resprouting and becoming obligate
seeders. Postfire seeding, including obligate seeding, originated at different times
in different lineages and includes apparent origins in the Oligocene, Miocene and
Pliocene. Since the late Miocene through a combination of increased drought-
prone landscape and fire-favoring adaptations, many lineages have accelerated
speciation and contributed to making these MTC regions hotspots of plant
diversity.
Southern hemisphere MTC regions share some similarity in the early origins of
many contemporary taxa. However, in Western Australia and the Cape region of
South Africa, highly leached and infertile soils played a larger role than seasonal
drought in the early origins of many sclerophyll taxa. In southwest Australia the
early Tertiary sclerophyllous vegetation produced low-growing contiguous
expanses of heathlands and this potentially contributed to a much earlier import-
ance for fire. The greater expanse of flammable vegetation would have increased
the likelihood of capturing natural ignitions as well as enhancing fire spread and
this may have selected for postfire seeders earlier than in northern hemisphere
systems. Low-fertility soils in both Australia and South Africa changed the picture
in several ways. Fires could more easily drive the replacement of mesic woodlands
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