Agriculture Reference
In-Depth Information
after fire, which both limited survivorship of resprouting shrubs and increased
available resources for seedlings (
Fig. 9.5
). Increasing aridity and decreasing soil
fertility would increase gap size and also affect plant structure in ways that
increase flammability, which in turn would affect fire intensity and frequency,
both of which would increase gap size and predictability.
Delaying reproduction to postfire conditions requires the accumulation of a
seedbank during the inter-fire interval; two modes of seed storage include soil-
stored and canopy-stored seedbanks. Soil-storage is widespread throughout most
MTC crown-fire ecosystems whereas serotiny is largely of importance in South
Africa and Australia (see
Chapter 3
).
Soil-stored seedbanks
Heat shock, chemicals in smoke and charred wood provide signals that trigger
germination in seeds from fire-prone environments. These mechanisms enhance
fitness by cueing germination to postfire conditions when light, water and nutrient
resources are abundant. Thus traits enhancing postfire-seeding mechanisms are
viable candidates for true fire adaptations.
Heat breaks dormancy of hard-seeded species with water-impermeable seed
coats by several mechanisms: heat shock can trigger germination by rupturing
the seed coat layer, by shortening after-ripening, and by desiccation (Brits
et al.
1993
). Species have different heat dose optima for germination, and in some
cases there is a tendency for obligate seeders to have higher seed dormancy, higher
heat-stimulation requirements and lower heat-induced seed mortality than facul-
tative seeders (Paula & Pausas
2008
; F. Moreira & J.G. Pausas unpublished).
However, this is not the case in all floras (Keeley
1987
).
Heat shock is not a specific cue to fire as soil heating by sun may also trigger
germination on unburned open sites with bare mineral soil. Although soil tem-
peratures experienced on sun-exposed sites are much lower than those generated
by fires, the heat dose (i.e. temperature
time, e.g. Paula & Pausas
2008
) may be
similar due to the long exposition time on sun-exposed sites. However, the
relationship between seed germination and the different combinations of tempera-
ture and exposure time are complex and differ between species (Keeley
1991
).
Chemicals released by combustion and transferred to seeds by smoke or charred
wood (here referred to as “smoke”) appear to be a highly specific germination cue
for fire and indicative of a rather specific fire adaptation. Smoke-stimulated
germination in species from non-fire-prone ecosystems has been cited as contrary
evidence (Pierce
et al.
1995
), but this conclusion does not consider the fact
that smoke is a complex mixture of thousands of chemicals, many of which occur
in diverse ecological settings and stimulate many plant processes (Keeley &
Fotheringham
2000
). Hopper (
2003
) raises questions about whether smoke-
stimulated germination constitutes an adaptation because in the Australian flora
it appears to have arisen numerous times in taxa with an origin as far back as the
Cretaceous. A similar pattern is evident in the California flora as a number of
smoke-stimulated species appear basal in lineages with a long Tertiary history
