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461.
462.
Jon E. Keeley 《The Botanical review》1991,57(2):81-116
Syndromes are life history responses that are correlated to environmental regimes and are shared by a group of species (Stebbins,
1974). In the California chaparral there are two syndromes contrasted by the timing of seedling recruitment relative to wildfires.
One syndrome, here called the fire-recruiter or refractory seed syndrome, includes species (both resprouting and non-resprouting)
which share the feature that the timing of seedling establishment is specialized to the first rainy season after fire. Included
are woody, suffrutescent and annual life forms but no geophytes have this syndrome. These species are linked by the characteristic
that their seeds have a dormancy which is readily broken by environmental stimuli such as intense heat shock or chemicals
leached from charred wood. Such seeds are referred to as “refractory” and dormancy, in some cases, is due to seed coat impermeability
(such seeds are commonly called hardseeded), but in other cases the mechanism is unknown. Seeds of some may require cold stratification
and/or light in addition to fire related stimuli. In the absence of fire related cues, a portion or all of a species’ seed
pool remains dormant. Most have locally dispersed seeds that persist in the soil seed bank until the site burns. Dispersal
of propagules is largely during spring and summer which facilitates the avoidance of flowering and fruiting during the summer
and fall drought. Within a life form (e.g., shrub, suffrutescent, etc.), the seeds of these species have less mass than those
of species with non-refractory seeds and this possibly reflects the environmental favorableness of the postfire environment
for seedling establishment. Regardless of when fire occurs, germination is normally delayed until late winter or early spring.
In the absence of fire, or other disturbance, opportunities for population expansion are largely lacking for species with
this syndrome.
The other syndrome, here called the fire-resister or non-refractory seed syndrome, includes species that are resilient to
frequent fires (mostly by vegetative resprouting), but require fire-free periods for recruiting new seedlings. Included are
shrubs, subshrubs, suffrutescents, lianas, geophytes and annuals. All are linked by the characteristic that their seeds germinate
in the absence of cues related to wildfires. In many cases no form of seed dormancy is present and the seeds germinate soon
after dispersal; consequently these species do not accumulate a persistent seed bank. Germination and seedling establishment
is independent of fire and thus opportunities for population expansion are also independent of fire. The demographic pattern
of seedling recruitment varies with the life form. For shrubs, seedling recruitment may be restricted to sites free of fire
for periods of a hundred years or more. Recruitment appears to require relatively mesic conditions and this may account for
the patchy distribution of these species within the matrix of relatively arid sites. Finding such sites has selected for propagules
specialized for wind or animal dispersal; the majority are bird dispersed. These shrub species all disperse fruits in fall
and winter and this may have been selected to take advantage of migratory birds as well as to time dispersal to the winter
rains typical of the mediterranean-climate. Germination typically occurs within several weeks of the first fall or winter
rains. Maturation of flowers and fruits during the summer and fall drought may account for the distribution of these species
on more mesic sites. Seed mass of these species is large and this may have been selected to provide an advantage to seedlings
establishing under the canopy of this dense shrub community. 相似文献