Agriculture Reference
In-Depth Information
Australia illustrates the problem. Aspects of the realized niche of rainforest are
confounded with attributes of fire refugia. Thus, apparent edaphic/climatic/
topographic correlates of rainforest distributions may also reflect boundaries
of reduced probability of fire imposed by the same suite of environmental
attributes. The potential edaphic/climatic niche of rainforest may be far broader
and overlap substantially with sclerophyll species.
Abandoning resource theory entirely for a fire-centric theory of vegetation
likewise misses the appropriate balance. Factors responsible for the displacement
of rainforest by sclerophyllous eucalypts in Australia spawned the “ecological
drift” theory of Jackson (
1968
). Novel elements of this theory included the influ-
ence of vegetation on probability of burning, multiple pathways of vegetation
change and the notion that particular fire regimes could eliminate species and
vegetation types. This theory postulated a central role for fire in the organization
of differing vegetation types. A major drawback of the theory in its original form,
however, is omission of varying soil types that control site productivity as a factor
that may influence growth rates of species and hence pathways of vegetation
dynamics (Bowman
2000
). Thus, a merging of elements of resource-based theory
with the principles of ecological drift offers the prospect of a more powerful
predictive basis for understanding vegetation distributions. Ultimately any future
theory of this kind has to account for observable correlations between edaphic and
vegetation patterns as well as the successional dynamics that emerge from the
interplay between functional types and fire regimes.
Equally problematical are attempts to explain the general lack of forest
growth in the Cape region of South Africa on nutrient availability, when
almost surely fire has had a heavy hand in determining fynbos/forest boundar-
ies (Bond
2010
). The South African MTC region differs from some other MTC
regions in lacking as intense a summer drought, and in the widespread exten-
sion of shrublands into high-rainfall areas that theoretically should support
forests. Extensive conifer and eucalypt plantations in the Cape are testimony
to the mismatch between the climate potential for forests and the actual
vegetation of extensive shrublands. Mesic Cape fynbos is not at equilibrium
with climate, as is the case with much of the global vegetation (Bond
et al.
2005
). Thus, the presence of shrublands over much of the region is not simply a
product of a winter-rainfall climate and extended summer drought. The scarcity
of forests in the Cape region MTC can be attributed to the interactive effects of
frequent fires and slow tree growth rates on nutrient-poor soils, and opportun-
ities for fire afforded by regular annual drought. Unlike Australia, which also
has nutrient-poor soils, the Cape region does not have eucalypts with their
remarkable fire tolerance and flammable properties, suggesting a potential
phylogenetic limitation to forests in the Cape region.
In northern hemisphere shrublands there has likewise been a tradition of
interpreting patterns solely in terms of climate, soils and mountain building.
Indeed, Axelrod (
1989
) believed that fire had played no significant role in the
evolution of chaparral traits. A huge body of paleoecology work in California
