Agriculture Reference
In-Depth Information
long fire-free periods since seeds are generally released upon death of the
parent plant, regardless of fire.
The extraordinary importance of serotiny in South African and Australian
shrublands stands in stark contrast to the limited presence of serotiny and nearly
complete dependence on soil-stored seedbanks in other MTC ecosystems. Lamont
(Lamont
et al.
1991
; Lamont & Enright
2000
) points out that this high degree of
serotiny in these southern hemisphere systems is correlated with extremely nutri-
ent deficient soils and more predictable rainfall, patterns that will be explored
more fully in
Chapter 9
.
Delayed Postfire Seedling Recruitment from Resprouts
Many woody and herbaceous taxa recover from fire by resprouting with no
immediate postfire seedling recruitment and are termed obligate resprouters.
Subsequent seedling recruitment is largely a function of growth form. Resprouts
from herbaceous perennials, suffrutescents and subshrubs grow rapidly, flower
and disperse seeds in the first postfire season. These taxa have limited seed
dormancy and many recruit en masse in the second and subsequent years until
canopy closure (see obligate resprouting subshrubs in
Table 3.3
) (Keeley & Keeley
1984
; Denham & Auld
2002
). Similar patterns are evident with the synchronous
flowering after fire in some Australian grasstree
Xanthorrhoea
species (Gill &
Ingwerson
1976
; Taylor & van Staden
1998
). Subsequent seed dispersal largely
restricts seedling recruitment to the second and third postfire years in some species
(Lamont
et al.
2004b
). Although these are immediate postfire obligate resprouters,
they have reproduction that is fire dependent.
Delayed Postfire Seedling Recruitment from Parent Trees
Surface fire regimes have selected for a very different suite of adaptive traits in the
dominant overstory species that are largely concerned with ensuring survival of
parent seed trees (Keeley & Zedler
1998
). Such fires are considered the historical
norm for many northern hemisphere conifer forests (Agee
1993
; Allen
et al.
2002
),
and a common fire regime in certain Australian eucalypt forests (Bell
et al.
1989
;
Williams
2000
; Gill & Catling
2002
).
The strategy of trees in this fire regime is to ensure survivorship of parent seed
trees that can disperse seeds into gaps opened up by higher-intensity burning. Two
traits that have adaptive value are self-pruning of lower dead branches, which
creates a mid-story gap between surface fuels and canopy fuels, and thick bark
that reduces lethal effects of surface fires. Conclusions about the selective role of
fire in generating these traits requires close examination because they could have
evolved in response to other environmental factors. Bark, for example, insulates
against fire because of a series of dense cork layers, which could also insulate
