Agriculture Reference
In-Depth Information
overwhelmingly governed by soil depth and moisture availability. Thus, the
potential role of fire in organizing communities appears to be oblique, via effects
on soil and geomorphological processes (McIntosh
et al.
2005
; Shakesby
et al.
2007
) and reinforcement of habitat and community distributions and boundaries.
As a consequence, fire has been characterized as a recurrent perturbation with
minor selective effects on community composition (Florence
1996
; Specht &
Specht
1999
).
The notion that resources and disturbance in MTC ecosystems may have
coupled influences on diversity was recognized by Huston (
2003
) who proposed
models of diversity based on discrete and interactive effects of disturbance
frequency and productivity, and a classification of ecosystem types. In addition,
Huston (
2003
) linked disturbance frequency in fire-prone systems to flammability,
as an outcome of ecosystem productivity (i.e. an endogenous or “bottom-up”
model of flammability). The two mechanisms that drive these models are intensity
of competition driven by productivity, and disruption of competition by disturb-
ance. Species diversity is hypothesized to exhibit unimodal responses to variation
in each of these influences. Huston (
2003
) characterized the bulk of MTV as being
of low productivity and subject to low disturbance frequency. A high level of
sensitivity to variations in disturbance frequency and productivity were predicted
as a consequence.
This provides a basis for evaluating plant diversity responses to fire regimes at
differing levels or scales (i.e. within and between communities), including conse-
quences of habitat variations and spatial variability in fire regimes. Huston (
2003
)
postulated that plant recovery rates from disturbance and therefore competitive
pressures will tend to be low due to relatively low productivity. His model predicts
that high levels of patch-scale heterogeneity of fire regimes should be due to low
rates of regrowth, which implies discontinuity of fuels, and due to effects of fine-
scale habitat variability, which implies flammability variations. Both processes are
hypothesized to promote high plant richness and diversity at a variety of spatial
scales. Does the empirical evidence conform to these predicted plant diversity
patterns and responses to fire regimes?
Fire Regimes and Diversity
Patterns of within-community diversity (measured as species richness, see
Chapter 11
) in Australian MTV are a function of individual species' responses
to fire regime components as determined by life history markers and species'
vegetative and reproductive characteristics, which include: (1) persistence, (2) seed
supply constraints; (3) inhibition of establishment, and (4) risk of senescence.
These life history attributes will vary in response to habitat characteristics, thereby
influencing rates of recovery from fire and the intensity of competition. Models of
community response to fire regime variations can be inferred from an overview
of these responses and their patterns of variation. These topics are discussed in
order below.
