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This paper will report the results of a study commenced in 1935, at the Desert Experimental Range, Pine Valley, Millard County, Utah, to determine the relative effects of various grazing treatments upon the plant members of the shadscale [Atriplex confertifolia (Torr. And Frem.) S. Wats.] - winterfat [Eurotia lanata (Pursh) Moq.] plant association. Results of this study indicate that distinct vegetational changes have occurred within that plant association during twenty-three years of controlled grazing by sheep.

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Bibliography: p. 191-200. / 200 p. : ill. (chiefly col.), map (fold.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Persistence mechanisms of Erodiophyllum elderi are investigated in terms of life history strategies and patchy population theory. / Thesis (Ph.D.)--University of Adelaide, Dept. of Botany, 1999

Bibliography: p. 191-200. Persistence mechanisms of Erodiophyllum elderi are investigated in terms of life history strategies and patchy population theory.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

Individuals of Eriogonum abertianum Torr. (Polygonaceae) flower in spring, or following onset of summer rains, or both. Within populations flowering time is mainly environmentally determined: there is little genetic variance for flowering time, and experimental moisture limitation significantly delays flowering. In the field a Sonoran Desert population experienced significantly more mortality during the foresummer droughts, and had a significantly greater proportion of spring-flowering plants, than a Chihuahuan Desert population. Greenhouse experiments suggest a genetic basis for differences in size and time of flowering between these populations. Fossil and biogeographic evidence support an adaptive interpretation of earlier flowering in the Sonoran Desert. A model of selection comparing spring-plus-summer flowering with spring-only flowering suggests that expected summer fecundity may not offset the risk of foresummer mortality in the Sonoran population. Rather than switching to a spring-only habit as predicted by the model, the species' range ends where summer rainfall declines abruptly. The invariance of the spring-plus-summer habit is not explained by the demographic, historical, or genetic data. Plants which live for more than a year in the wild have offspring which, in the greenhouse, live longer than the offspring of the general population. This suggests a genetic basis for the occasional observed perennation. Analysis of a quantitative genetic model suggests that when adult survivorship is low, selection will generally reduce perennation. The annual habit is thus likely to persist even in the presence of genetic variation for perennation. Optimal control models of plant carbon allocation are extended to include within-season mortality and allometric growth constraints. When parameters are varied in numerical experiments, resulting predictions for easily measurable characters (e.g., time to first flower) often vary only slightly; most differences are in fitness, suggesting that satisfactory empirical tests may be difficult to conduct. Arbitrary mortality functions can optimally lead to multiple flowering episodes, and this can depend sensitively on parameter values. Optimal trajectories with allometric constraints are divided into a period of vegetative growth and another period of mixed growth.

Removal experiments were conducted to determine particular spatial and temporal conditions that can influence competitive interactions in several desert winter annual species. During the 1987-88 season, variation in the magnitude of competition at three habitats along a topographic gradient was demonstrated in two co-occurring species of winter annuals, Plantago patagonica and Pectocarya recurvata. Density effects on the survival and reproductive success of either species were weakest at the slope. However, the habitats where the two species experienced the most intense competition differed. Plantago was most affected by competition at the wash while Pectocarya was most affected at the base of the hill. The most striking pattern observed was that, for both species, the habitat with the highest reproductive success for plants that were not experiencing competition tended to be the worst habitat for plants in competition. A comparison of results from two experiments performed on Plantago patagonica during two growing seasons showed that competition occurred despite large seasonal differences in weather and plant performance. When wet and dry conditions of different year types were simulated by artificial irrigation during a dry season, competition was still detected in both rainfed and irrigated plants regardless of the marked differences in plant size as a result of the irrigation treatment. A neighborhood density roughly equivalent to 8 plants/dm² appeared sufficient to create competitive conditions for Plantago. Effects of competition were consistently manifested in reduced plant growth and fecundity. There was no evidence for density-dependent seedling mortality (self-thinning) even with seedling densities as high as 48 plants/dm². In two pairs of species tested, Plantago patagonica-Schismus barbatus and Plantago patagonica-Pectorcarya recurvata, there was no statistically detectable effect of neighbor species identity on target plants of Plantago and Pectocarya suggesting the possible equivalence of competitive effects in these species of desert winter annuals.

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