Agriculture Reference
In-Depth Information
Canopy-stored seedbanks
Serotiny is a special case of bradyspory in which seeds are maintained in “cones”
(here we take botanical liberties by using this term for both true gymnosperm cones
and serotinous angiosperm fruits) on the parent plant and released suddenly in
response to an environmental trigger. It is a trait that appears frequently in gymno-
sperms (Pinaceae and Cupressaceae) in most MTC regions, but in the southern
hemisphere it is also found in about a dozen angiosperm families (see
Chapter 3
)
and is widely distributed in the Proteaceae (see
Box 7.1
). Serotiny involves delayed
seed release and often seeds may be stored within cones for decades and then rapidly
dispersed following fires in canopy-consuming crown fire regimes. Postfire seedling
establishment is very successful, in contrast to the occasional seeds dispersed during
the inter-fire interval, which have very low seedling survivorship (Bond
1984
).
Although delayed seed release occurs in some arid-land herbaceous species as a
mechanism for timing germination to rainfall events, including annuals (Ellner &
Shmida
1981
), whereas woody species that accumulate seeds from several annual
crops appear almost exclusively in fire-prone environments.
Thus, it would seem inescapable that fire was the selective factor responsible for
delaying reproduction to a single pulse of postfire recruitment. However, Axelrod
(
1980
) contended that in the case of serotinous pines, fire was a recent anthropo-
genic factor and that serotiny evolved in response to drought. Not only did he fail
to recognize the importance of fire throughout land plant evolution, but he also
did not recognize that serotiny, which times cone opening to postfire conditions,
would not be an effective means of avoiding drought; rather drought should select
for a bet-hedging strategy with multiple seed dispersal events throughout the
plant's lifetime. Despite the inescapable importance of fire as a selective force,
fire need not be the only selective factor and sometimes it may be acting in concert
with other stresses such as granivory (Lamont
et al.
1991
).
Northern hemisphere gymnosperms in crown fire regimes are mostly postfire
seeders with serotiny, and none have soil-stored seedbanks, suggesting phylogen-
etic constraints on adapting recruitment to postfire conditions in crown fire
regimes. This may be tied to the presence of cones in ancestral species that pre-
adapted some taxa to canopy storage, or to the lack of appropriate seed coat
layers that limited the possible evolution of seed dormancy in the soil. The latter
may be a consequence of the apparent origin of gymnosperm serotiny from
arboreal ancestors in surface fire regimes where long-term soil-stored seedbanks
have never been selected (Keeley & Zedler
1998
; Schwilk & Ackerly
2001
).
However, phylogeny and ecology may both be involved since fire return intervals
are commonly much longer in these northern hemisphere MTC regions and,
reflecting the arboreal ancestry, the serotinous conifers are all very long lived,
which would be critical for serotiny under such fire regimes. In the southern
hemisphere where crown fire regimes prevail, many angiosperm plant families
have numerous serotinous taxa. Such taxa are in lineages that also have taxa with
soil seed storage, suggesting phylogenetic constraint is an unlikely explanation for
canopy seed storage.
