The ecology and reproduction of Sophora leachiana Peck (Fabaceae) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r494vp238

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  • 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.
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