Date of Award:

5-1-1993

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Biology/Ecology

Committee Chair(s)

Edward W. Evans

Committee

Edward W. Evans

Committee

Vincent J. Tepedino

Committee

Mary E. Barkworth

Abstract

Astragalus montii and A. humillimus are rare species that are isolated geographically as the result of growing on mountain tops or mesa tops. The breeding systems and pollinators of these two species were examined, as well as the effects of seasonality and water additions on fruit set of A. humillimus. The breeding system of A. montii was described and compared to that of two widespread sympatric congeners, A. miser and A. kentrophyta. Percent open-pollinated fruit set for A. montii was about as high as one congener and twice as high as the other. Only A. montii was autogamous. There was no significant difference in fruit set between the autogamous treatment and hand-pollinations; all were significantly lower than open-pollinated treatment. Bees were apparently more effective at pollen transfer than humans. Autogamously produced fruits had significantly fewer and lighter seeds. The pollinators of A. montii were compared with those of its congeners. More than 37 species of bees visited the three species, including Osmia (16 species) and/or Bombus (5 species), which were the primary pollinators. Most Osmia species visited the species indiscriminately, but Bombus species preferred A. miser and ignored A. montii. Astragalus montii was midway between its two congeners in its attractiveness to bees. The specific identity of bee species was unpredictable from site to site and from year to year. Open-pollinated fruit set of A. humillimus was 66%. Astragalus humillimus had significantly higher fruit set with hand-pollination or open-pollination than with autogamy. There were no significant differences in fruit quality components between treatments. Over 30 bee species visited A. humillimus, but there was doubt as to which served as pollinators. The 13 Osmia species almost certainly included important pollinators. Two sites less than 1 km apart differed in pollinator composition. Two flushes of plants bloomed in 1990. The early group had more flowers, which had better chances of setting fruit than those on late plants. Only date of flowering, and not water additions, affected fruiting ratios for early plants. Instead of increasing the fruiting ratio in late plants, water additions appeared to lower it. Both A. montii and A. humillimus had high fruit set compared to other perennials. They both set fruit autogamously, agreeing with the trend set by other rare Astragalus species. Much pollinator diversity was found.

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