Graduate Thesis Or Dissertation
 

Biosystematic studies of the Mimulus moschatus complex in the Pacific Northwest

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  • Populations of an unknown species of Mimulus, herein described as Mimulus hymenophyllus Meinke, were located and studied near Horse Creek, a tributary of the Snake River in eastern Wallowa County, Oregon. The new species is differentiated from its apparent closest relative, Mimulus jungermannioides Suksd., by an annual habit, long petioles, corollas that are three to four times the length of the calyx, rounded capsules, and seeds up to 0.85 mm long. Mimulus hymenophyllus and M. iungermannioides are members of the section Paradanthus, and are morphologically allied with Mimulus moschatus Dougl. in Lindl. and several related taxa. The Mimulus moschatus complex of species is distinguished within the genus by having firmly adherent placentae, glandular-sticky foliage and stems, prismatic calyces with equal to sub-equal teeth, elongate pedicels, and deciduous, funnelform corollas ranging in color from canary to lemon yellow. Mimulus hvmenophyllus is endemic to the steep canyons of Horse and Cow Creeks in Wallowa County, at altitudes ranging from 850 to 1300 m. The species is restricted to the damp crevices of vertical basalt cliffs, often growing in the shade under overhangs. The surrounding plant community is dominated by several coniferous species, primarily Pseudotsuga menziesii, pinus ponderosa, and Abies arandis. The habitat of M. hymenophyllus is isolated, and is presently not in danger from human disturbance. However, the species is currently maintained as a state and federal candidate for listing as threatened or endangered, based on limited abundance and narrow geographic range. Studies of the seed and seedling biology of Mimulus hymenophyllus and its Pacific Northwest relatives (i.e., M. moschatus, M. floribundus, M. jungermannioides, M. patulus, M. pulsiferae, M. washingtonensis, and M. breviflorus) were also conducted. Germination trials indicated that seeds of M. moschatus, M. jungermannioides, and M. floribundus are capable of germination immediately or soon after capsules dehisce, and do not become dormant upon later exposure to warm or cold temperatures. Seed lots of M. hymenophyllus and M. breviflorus germinate rapidly when first mature, but subsequently develop complete or partial dormancy after prolonged exposure to late summer temperatures. Dormant seeds of these species, as well as those of M. patulus, M. Dulsiferae, and M. washingtonensis which are dormant when capsules dehisce, will germinate readily after several weeks of cold-wet stratification. Four of the five species lacking innate seed dormancy develop inflated fruiting calyces that temporarily trap seeds as capsules dehisce. This prolongs the dispersal rate of seed crops, thereby minimizing the potential for catastrophic seedling mortality due to mass germination of cohorts during unfavorable environmental conditions. These species retain seed on the parent plant for significantly longer periods than species with unmodified calyces. Seed dispersal from parent plants is accomplished by wind and flowing water, except in M. hvmenophvllus, where negatively phototropic pedicels orient ripe capsules towards the darkened cliff substrate for dispersal directly into crevices. Two of the rarest species studied, M. hymenophyllus and M. patulus, were found to co-occur in nature with an indigenous and rather weedy, unrelated species of Mimulus, i.e., M. nasutus Greene. Experimental plantings showed that seedling lots of M. nasutus emerge sooner and over a shorter period than those of the two uncommon species, suggesting that M. nasutus may outcompete them and thereby contribute to their rarity. Seedlings of M. breviflorus and M. floribundus, which typically occur in habitats subject to sudden fluctuations in soil moisture, exhibit adaptations for accelerated sexual reproduction under experimental drought conditions. This trait, when coupled with inflated fruiting calyces, enables these species to exploit unpredictably arid environments despite the potential disadvantage of nondormant seeds. Mimulus jungermannioides and M. moschatus are not adapted for precocious sexual maturation, and offset their lack of seed dormancy by having a perennial life cycle and by occurring in more predictably mesic habitats. Significant differences between the annual and perennial species were observed in the timing and amount of seed production during droughtstress. The information provided in this thesis will be valuable to systematists and conservation biologists. Several of the species studied have been promoted for protection under endangered species laws. The formal naming of Mimulus hymenophyllus legitimizes efforts for the management of this unique species, while seed and seedling biology data will be important to germplasm studies, phylogenetic assessments, and possible reintroduction of species or populations should extirpation occur.
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