Agriculture Reference
In-Depth Information
hemisphere it is a coniferous phenomenon, being limited to species of eastern
hemisphere
Cupressus
and western hemisphere
Hesperocyparis
(Cupressaceae),
Pinus
(Pinaceae),
Sequoiadendron
(Cupressaceae) and the monotypic genus
Tetraclinis
(Cupressaceae), which are distributed in a range of habitats that include
boreal forests and semi-arid shrublands (Lamont
et al.
1991
; Keeley & Zedler
1998
).
Although these are seemingly very different ecosystems, they share certain features
that contribute to the crown fire regime, including an abbreviated growing season
that maintains fuels near the crown, leading to high-intensity crown fires.
In the southern hemisphere, primarily in MTC shrublands of the South African
Cape region and Western and South Australia, serotiny is found in about a dozen
conifers in three genera of Cupressaceae (
Actinostrobus
,
Callitris and Widdringtonia
),
and is very widespread in angiosperm families, including the Asteraceae, Brunia-
ceae, Casuarinaceae, Ericaceae, Rosaceae, Myrtaceae and Proteaceae (Lamont
et al.
1991
; Oliver & Fellingham
1994
). In the last two families are several genera
with 40 or more serotinous species:
Banksia
,
Eucalyptus
,
Hakea
,
Leucadendron
and
Protea
. Serotiny is substantially more important in both diversity and abundance in
these southern hemisphere ecosystems than any other region of the world.
In all of these taxa, cones or fruits open en masse and disperse seeds within days
of being scorched by fire, resulting in a pulse of seedling recruitment that generates
stands of even-aged cohorts. Seeds are generally not dormant, are short lived and
fire plays little direct role in stimulating germination beyond inducing cone or fruit
opening (Lamont
et al.
1991
).
There are intraspecific patterns of varying degrees of serotiny and thus popula-
tions may be even aged or uneven aged, possibly tied to fire return intervals
(Keeley & Zedler
1998
; Ne'eman
et al.
2004
). Retention of seeds is also highly
variable across species, ranging from a few months (e.g.
Pinus muricata
cones
remain closed through the autumn fire season and disperse seeds in winter,
Keeley & Zedler
1998
) to many decades (Lamont
et al.
1991
). Delayed dispersal
in
Sequoiadendron giganteum
in the Sierra Nevada of California might also be
considered a form of serotiny. Although seed dispersal is not strictly tied to fire,
cones can remain closed many years after reaching maturity and then shed seeds in
mass numbers in the first few days following a surface fire with plumes of
convective heat transfer that dries the cones.
There is species-specific variation in retention of seeds in cones in the absence
of fire. When seeds are released in unburned stands seedlings typically have a
low probability of successful recruitment (Zammit & Westoby
1988
). Among
the biggest threats to long-term sustainability of serotinous populations are
Caption for Fig. 3.5. (cont.) Australia, (b) unopened
Hakea
fruits from Victoria, Australia,
(c) unopened
Erica sessiliflorus
infructescence from Cape region fynbos, South Africa,
(d) opened
Protea
infructescences with dispersed seeds and seedlings beneath following recent
fire; and serotinous cones of gymnosperm trees in northern hemisphere MTC ecosystems,
(e) unopened cone crop in
Pinus halepensis
in southern Spain, and (f)
Hesperocyparis
(
formerly
Cupressus) forbesii
after a recent fire with opened serotinous cones dispersing seeds
in southern California chaparral. (Photos by Jon Keeley, a-d, f, and Juli Pausas, e.)
