Author

David R. Duba

Date of Award:

1976

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Advisor/Chair:

Brien E. Norton

Abstract

Field and laboratory investigations were conducted relating to factors controlling distribution and abundance of annual species growing in a deisturbed portion of the salt desert shrub zone on fine-textured soils. Dominant species on the site in the two study years were the nonnative, Halophytic, late-summer maturing, Halogeton glomeratus (Bieb.) C.A. Mey., and Bassia hyssopifolia (Pall.) Kuntze. Contrasting amounts of precipitation were received in the two years; in 1974, spring conditions were so dry that two early-maturing, sub-dominant annuals, Descurainia pinnata (walt.) Britton, and Lepidium perfoliatum L. were essentially absent. They successfully reproduced under more favorable 1975 conditions.

Studies on species distribution centered around factors responsible for occurrence of large (20-30m across), adjacent, essentially pure stands of Bassia and Halogeton. It was determined that few seeds dispersed more than a couple of meters from parent plants; and that seedling establishment success of the species was significantly different in the two vegetation types, commensurate with soil surface physical and chemical differences. The less drought tolerant Bassia was confined to soils which had higher water potential in summer than the areas occupied by the succulent halogeton. Bassia responded with more rapid growth when water availability increased, and suppressed halogeton in both growth chamber competition studies. The adjacent pure communities are believed to be maintained by Bassia's inability to establish on soils dominated by Halogeton, and Bassia's competitive superiority when both species become established together.

Studies on plant abundance involved making quantitative estimates of numbers of seeds or plants per unit area in different life cycle phases during the study period. The objective of these studies was to determine the manner in which each year's seed production was utilized, in terms of dormancy of seeds in soil, germination, mortality, or reproduction. It was determined that residence time of seeds in soil was short, since seeds germinated freely after overwintering. Consequently the vegetation composition was closely related to the previous year's seed production. Tremendous mortality occurs during the seedling establishment phase, as only eight percent of seeds sown into marked quadrants survived until mid May. Mortality during summer was highly density-dependent. Plant plasticity served a homeostatic function on one site which suffered a seed crop failure in 1974. The few plants established there in 1975 became very large so that total seed production was not greatly different from areas which had a substantial amount of seed production in 1974.

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