Agriculture Reference
In-Depth Information
The response of communities in more productive habitats (i.e. wetter/deeper or
more fertile soils) is less sensitive to variation in length of the interval between fires
than in drier, shrub-dominated communities. Thus the composition of swamps
and wet heaths on the one hand, and open forests on the other may be more
sensitive to variance in inter-fire interval length. Reductions in diversity in low-
stature herbaceous and shrub growth forms occur under competitive dominance
by obligate seeder shrubs in heaths and shrublands (Keith & Bradstock
1994
;
Bond & Ladd
2001
; Tozer & Bradstock
2003
; Keith
et al.
2007
). Diversity may be
dependent on high levels of variation in between-fire interval rather than the mean
length of the interval (
Fig. 8.6
, Tozer & Bradstock
2003
; see also Jacobsen
et al.
2004
for California). Effects may be more acute in moist compared with dry
habitats, where greater productivity and cover dominated by resprouters limit
understory diversity (Bond & Ladd
2001
; Clarke & Dorji
2008
).
Pekin
et al.
(
2009
) found a positive correlation between diversity measures and
fire frequency in MTC jarrah forests, reflecting positive responses of herbaceous
functional types to reductions in understory shrub cover. By contrast, Burrows &
Wardell-Johnson (
2003
) found that both frequent and infrequent fire in experi-
mental jarrah forest plots caused declines in abundance of woody taxa in the
understory, in accordance with the model in
Fig. 8.6
. Penman
et al.
(
2008a
) found
that differing frequencies of experimental burning had little impact on richness in
a southeastern, dry sclerophyll forest, though a long-term decline in richness was
detected in all treatments. This decline may be due to a failure of seedbanks to be
triggered through the absence of occasional moderate or high-intensity fire
(Penman & Towerton
2008
; Penman
et al.
2008a
,
2008b
).
Varied outcomes of studies of this kind may reflect inherent limitations in
measurement of fire regimes and plant responses. Fire history data covering
natural experiments is often limited to several decades at most. Manipulations
in formal experiments and the range of contrasting fire treatments are similarly
limited. Species richness and cover measures are inherently insensitive indicators
of composition and may mask major demographic changes within species and
functional types (Bradstock
et al.
1997
; Keith
et al.
2007
). Thus, the model may
respond differently to different functional types and perhaps even to different
fire regimes.
In conclusion, a variety of evidence supports the prediction that fire functions as
a non-equilibrium determinant of diversity in MTV communities through the
disruption of competition. Relatively frequent fire (
<
5-10 yr interval) may deplete
diversity due to low rates of recovery, though the nature of this effect may vary
with productivity. Infrequent fire (
30 yr interval) may also deplete diversity,
through inhibition of recruitment and replacement by dominant species capable of
recruiting in the absence of fire. Habitat variations affect these mechanisms via
productive potential and the availability of space created by fire as predicted by
Huston (
2003
). MTV spans a wide range of possibilities in this regard. In the least
productive communities such as dry, sandy or rocky habitat, competitive effects of
shrubs may be reduced in importance due to the ready availability of open space.
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