Agriculture Reference
In-Depth Information
woodland and forest development. The size of these islands of suitable habitat
likely varied along north-south gradients in temperature as well as along gradients
from coast to the interior, and varied in relationship to orogenic uplift. Seasonal
conditions conducive to fires were possibly not annual and the probability of
lightning-ignited fires would have been a function of habitat island size. These
conditions selected for the obligate resprouting mode, suitable for persisting
through periodic fires, but not modifying their reproductive biology to concen-
trate reproduction to postfire conditions. This strategy is tied to fire regime
characteristics, as well as to a life history syndrome that includes strategies for
avoiding drought stress with deep roots and dispersal strategies for finding other
suitable island habitats amidst a sea of more mesic woodlands. Deep roots
promote rapid resprouting, which on moister sites limits the window of opportun-
ity for seedling recruitment and has been an additional factor selecting against
postfire seeding in many of these taxa. With increasing aridity postfire gaps
became larger and more predictable and were critical to the evolution of postfire
seeding. 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 differ-
ent lineages and includes apparent origins in the Oligocene, Miocene and Pliocene.
Southern hemisphere MTC regions of South Africa and southern Australia are
distinct in the expanse of highly leached and infertile soils and in some cases these
have persisted since the Cretaceous. Sclerophyllous MTV developed early in the
Tertiary and evidence of fires extends back to the Eocene, with increasing fire
activity into the Miocene. The greater expanse of flammable vegetation would
have increased the predictability of fire and this may have selected for postfire
seeders earlier than in northern hemisphere systems. This would also have selected
against the obligate resprouter mode with vertebrate seed dispersal since the early
evolution of sclerophylls may not have been restricted to small islands of habitat.
Additionally, low-fertility soils would have reduced the capacity for resprouters
to dominate and thus increase gaps for postfire seedling recruitment, which would
have selected against fire-independent recruitment as in obligate resprouters.
Higher-fertility soils on Chilean landscapes represent a very different southern
hemisphere MTC story. Although lacking much of a fossil record we surmise that
early Tertiary evolution of sclerophylls was similar to the history described for the
northern hemisphere. As in these northern systems, fire-adaptive traits such as
resprouting, lignotubers and postfire seeding would have evolved on marginal
sites under various climatic scenarios of seasonality. However, during the late
Miocene, the Andean uplift almost totally blocked summer storms from bringing
lightning ignitions into central and northern Chile. This barrier has persisted to
this day and explains the presence of certain fire type traits on a landscape with
few natural sources of ignition. Chilean matorral represents an ecosystem in
transition from fire-prone to a non-fire-type system, and such transitions have
likely occurred in other biomes throughout evolutionary history.
