More info for the terms: density, forbs, fresh, litter, monoecious, root crown, shrub, top-kill
French broom spreads by prodigious seed production. French broom may also
sprout from the root crown [11] or upper stem [12] when
aboveground parts are removed by cutting, freezing, or fire.
Breeding system:
French broom is monoecious and does not show appreciable levels of selfing [53].
Pollination:
Both native and nonnative insects pollinate French broom. Parker and others [51,53]
demonstrated pollinator limitation in French broom, underscoring the potential
importance of pollinators to its fecundity and spread. Patterns of fecundity
were not, however, strongly predicted by differences in pollen limitation
between species (French and Scotch broom) or between sites [53]. Because nonnative honeybees are
often themost common pollinators of brooms [52,72], potential negative
impacts of beekeeping on broom management have been suggested [52].
Seed production:
French broom becomes reproductive at 2 to 3 years of age, or on reaching a height of about 1.5 to 2 feet (45-60 cm) [1,11]. A medium-sized shrub can produce over 8,000 seeds a year (Bossard, unpublished
data, cited by [11]). Adams and Simmons [1] found an average of 7,400
pods per bush, with an average of 4.8 seeds per pod, and 5% of the pods damaged
by insects in a dense infestation of French broom in dry sclerophyll vegetation in
Victoria, Australia.
Seed dispersal:
French broom pods burst open explosively, ejecting seeds up to 13 feet (4 m). Seeds are further
dispersed by ants, birds, and other animals and in river water, rain wash, and mud,
and by vehicles and machinery [1,11].
Seed banking:
Dormancy of French broom seed is enforced by a hard coat that prevents imbibition. A large
proportion (60%-82%) of French broom seed is dormant upon dispersal ([1,49], and
references therein). The high rate of seed production
coupled with high rates of seed dormancy result in a rapid build-up of
persistent, soil-stored seed [1]. French broom seeds are known to
survive at least 5 years in soil (Bossard, unpublished data cited in [11]).
A large number of dormant French broom seeds in the soil seed bank can lead
to high germination rates following soil and vegetation disturbance such as that
caused by fire [1]. Even though seed densities tend to decrease with distance
away from broom stands, there
may be sufficient numbers to support a population expansion after fire ([49] and
references therein).
A review by Bossard [11] reports French broom seed bank densities range
from 465 to 6,733 seeds/m². An average of
10,000 dormant broom seeds/m² occurred in the litter and soil under mature
French broom stands in Marin County (Parker and Kersnar 1989, cited by [49]). Average French
broom seed bank densities of 3,774 seeds/m²
and 2,563 seeds/m² were recorded at 2 sites
in Australia [1]. Seed bank densities seem to increase with soil depth with the
age of the broom stand [49].
French broom seed bank densities may be related to stand age, although no
clear relationship was found in French broom
stands of varying ages on a coastal grassland site in Marin County, California
[3]. Seed density of French and Scotch broom, native perennial
grasses, native forbs, nonnative annual grasses, nonnative perennial grasses,
and nonnative forbs were compared in areas where the grassland matrix under the broom stands
was dominated by nonnative annual grasses. Scotch
broom was present only in small numbers (<5% of total aboveground plants),
and no distinction was made between seeds of the 2 broom species. Broom stands
ranged from 5 to 15 years old, and broom seed bank density ranged between 900
and 10,582 seeds/m².
Younger and older stands tended to have fewer seeds than stands in the middle
age range, with a general trend of increasing seed numbers as stands age, a
stabilization of numbers in middle-aged stands, and a decrease in seed density
among the oldest stands [3]. A similar trend was observed in a 4-year study of
Scotch broom in Australia [65]. However, variation in seed bank
density among sites was large, and there was no clear relationship between stand
age and seed bank density. Density of all nonbroom seeds as a group decreased
with stand age, although seed densities of individual species were not
different between sites of different ages. There were no significant
relationships (p=0.734) between stand age and the number of dormant or dead broom seeds in
the seed bank. However, there was a trend toward an increasing percentage of deeply
dormant seeds with increasing stand age. The large variation in broom seed
numbers among sites may be due to differences in productivity and fecundity
brought about by differences in availability of soil resources and/or
pollinators; or by differences in seed dispersal or predation by granivorous
insects among sites [3].
Germination:
French broom is polymorphic in its germination behavior, with about 18%
[1] to 40% (Parker and Kersnar 1989, cited in [49]) of fresh seed germinating immediately on exposure to suitable conditions,
and the remainder dormant. Scotch broom seed buried below about 3 inches (8 cm) does not
germinate [8], and very few
broom seeds occur below 4 inches (10 cm) (Parker and Kersnar 1989, cited in [49]), [3,24].
French broom seed may be stimulated to germinate by rain, scarification during soil disturbance, or fire.
In field studies in Australia, Adams and Simmons [1] observed
a flush of French broom seedlings after autumn rains in April 1988. Average seedling counts
were 790 seedlings/m² under dense mature stands without disturbance.
Several researchers have observed a flush of French broom seedlings following
fire [12,13,20,49], and laboratory studies indicate that heat scarification
stimulates germination of French broom ([1,11,49] and references therein) and Scotch broom [8,60,67,74] seeds. Heat of
122 to 212 °F (50-100 °C) doubled French broom germination rates, and seed mortality ensued at about
257 °F (125 °C) (Parker and Kersnar 1989, cited in [49]). Cheng (in press)
reports that heat treating seeds with temperatures of 149 °F (65 °C) improved germination
of seed in some populations but not in others [11]. Seeds that
were heat treated with boiling water "to simulate the cracking of the
testa by fire" germinated rapidly in light at 68 °F (20 °C), with over 50% of viable seeds
germinating within 12 days, and 90% within 14 days [1].
Hand scarification also stimulates germination of French broom seed [49].
Seedling establishment/growth:
Very little information is available on French broom seedling establishment and
growth. There is a high potential for seedling recruitment following fire [1,12,13,20,49] (see Plant Response to Fire).
Regular recruitment can lead to rapid French broom population expansion.
At a site in Australia the invasion front moved over 10 feet (3 m) in 12 months [1].
Asexual regeneration:
French broom can sprout from the root crown after cutting. Once seedlings are taller than
approximately 8 inches (20 cm), their rate of sprouting after cutting can be
over 90%, particularly if cut in the rainy season (Bossard, unpublished data in
[11]). Boyd [12] also reports sprouting from the stem
following top-kill by fire.