- The goal of the study was to collect data on the ecology and
reproduction of Sophora leachiana in order to help explain the rarity
and restricted range of this species. There are 13 known populations
of S. leachiana, occurring in a 29 by 6.4 km area in the Siskiyou
Mountains of southwestern Oregon. The species acts as a primary colonizer
in disturbed areas, especially following fires. When the tree canopy
becomes re-established, Sophora may persist vegetatively as rhizomes and
aerial shoots, but it ceases flowering.
Most populations occur on south or west slopes, in a region of low
summer rainfall. Adaptations of the plant include xeromorphic leaflet
anatomy, leaflet movements to avoid high insolation, and flowering that
depends on high levels of incident light.
The reproductive cycle of Sophora was analyzed, beginning with
meiosis and extending through pollen maturation, pollination, seed and
fruit development, seed anatomy, seed germination, and predators of
the plant. Two chromosome levels were found, tetraploid (n=18) and
hexaploid (n=27). Meiosis was more regular in the former than in the
latter, but the hexaploids showed a higher percentage of stainable
pollen. Species of Bombus are the principal pollinators, but low
levels of insect activity were seen in Sophora populations. The flowers
show a typical xenogamous syndrome, including protandry, high pollenovule
ratio, and low rate of spontaneous selfing. In controlled experiments,
only intrapopulation pollinations were successful.
In a vast majority of the flowering, the pistils show no development
past anthesis. Maturation of the fruit is limited to favorable
sites of high insolation and low plant density. Even in maturing
fruits, an average of 79 percent of the seeds abort prior to maturity.
Seed abortion appears to result from endosperm failure, due perhaps
to self-incompatibility. The mature seeds have the characteristic
anatomy of "hard" leguminous seeds and probably are adapted to survive
the heat of forest fires. In a series of tests, normal-appearing
Sophora seeds showed an average germination rate of only 58 percent.
Growth of the seedlings was variable, and some abnormalities were
observed. The inflorescences of plants in nature were often parasitized
by thrips, which caused premature dropping of the flowers. Larvae of a
species of moth were a common predator in some Sophora populations,
attacking both the vegetative parts and inflorescenses. The rust,
Uromyces hyalinus, was a plant pathogen of low incidence but sometimes
high density on individual shoots.
Sophora leachiana does have vigorous asexual reproduction by stout
rhizomes; but because the species eventually dies out as the forest
matures, it must depend on seeds in order to persist in an area. The
species is probably a Tertiary relic, depleted in biotypes and possessing
a restricted gene pool. Current seed production may be adequate
for continuance in the small area Sophora now occupies, but it does not allow for expansion of the species range. Forest clear-cuts are a
favorable habitat for the growth and flowering of S. leachiana. However,
forest management practices that would destroy the vegetative
clones and hence end seed reproduction in these sites would probably
lead to the extinction of the species.