• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 64
  • 9
  • Tagged with
  • 74
  • 74
  • 74
  • 31
  • 27
  • 21
  • 21
  • 12
  • 11
  • 9
  • 9
  • 8
  • 7
  • 6
  • 5
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Alien plant invasion in relation to site characteristics and disturbance: Eragrostic lehmanniana on the Santa Rita Experimental Range, Arizona, 1937-1989

Anable, Michael Edward, 1965- January 1990 (has links)
Presence and composition of Eragrostis Lehmanniana was measured on 75 permanent transects every 3-8 years between 1958-1989 on the Santa Rita Experimental Range, Arizona. The number of transects occupied and average composition increased rapidly over this period. A repeated measures analysis of two soil groups with different water holding capacity and permeability revealed that the rate of increase of E. Lehmanniana was different between the groups. Repeated measures analysis of five grazing intensities revealed that the rate of E. Lehmanniana increase was different among intensities. It appears that the highest intensity had the highest rate of increase. The influence of grazing on E. Lehmanniana spread was also expressed by the lack of difference in relative composition measured in 1989 between ten livestock enclosures and adjacent grazed areas. This research suggests that E. Lehmanniana will spread and dominate with an average of 89 mm of summer precipitation in 40 days; a lower regime than estimated by other research.

The influence of invasive Lehmann lovegrass on two native grasses in the semi-desert grassland

Van Deren, Kurt Jon, 1964- January 1993 (has links)
This study evaluated the influence of Lehmann lovegrass (Eragrostis lehmanniana Nees) on the densities and spatial distributions relative to mesquite (Prosopis velutina Woot.) of two native grasses, Arizona cottontop (Digitaria californica (Benth.) Henr.) and Rothrock grama (Bouteloua rothrockii Vasey), in the semi-desert grassland. Also evaluated were the affects of independent variables range site, elevation, and proportion mesquite cover on these relationships. General Linear Model analyses showed no association between Lehmann lovegrass and either the densities or the distributions of the native species. Arizona cottontop showed an affinity for mesquite cover, while Rothrock grama and Lehmann lovegrass both showed an aversion to mesquite canopy. The relationship of Rothrock grama to mesquite cover was affected by range site. The relationship of Lehmann lovegrass to mesquite cover became less averse with increasing elevation and as its density increased. These results contradict suggestions that Lehmann lovegrass is associated with declines in native grasses.

Emergence and cool-season growth of Lehmann lovegrass and Arizona cottontop on different soils

Rogstad, Kristin Alix, 1972- January 1998 (has links)
Lehmann lovegrass (Eragrostis lehmanniana Nees.), a perennial bunchgrass, has established itself well since its introduction from South Africa. Arizona cottontop (Digitaria californica (Benth.) Henr.) is a native perennial bunchgrass that sometimes appears on the same sites and soils as Lehmann lovegrass. In a greenhouse, emergence was evaluated using line-source irrigation which simulated natural summer precipitation on two soil types collected from the Santa Rita Experimental Range (SRER). Density and cool-season growth of each species were monitored along transects located at the SRER on three different soils. I found that although Lehmann lovegrass and Arizona cottontop emerge similarly on two soils, Arizona cottontop un-emerged caryopses had a better percent survival rate. Also, Lehmann lovegrass plants had more green above-ground biomass from November to May than Arizona cottontop, and Lehmann lovegrass was able to greenup more rapidly following rain. This study showed these species have similar emergence characteristics, but that their cool-season greening differs.

Impacts of modernization on nomads and their use of rangeland resources in At-Taysiyah region, Saudi Arabia

Al-Haratani, Eisa Ramadan, 1958- January 1997 (has links)
Pastoral nomadism in arid and semiarid regions has evolved over many centuries as a rational response to uncertain climatic conditions and fragile ecosystems. Nomads are the main inhabitants of the bast area of marginal rangeland in Saudi Arabia. However, in the past few decades, modernization, in the form of improved transportation and access to water, combined with government subsidy of supplemental barley to the nomads of regions such as At-Taisyiah in the northern part of Saudi Arabia have provided the nomadic tribes with a buffer against uncertain climatic conditions and against the consequences of increasingly aggressive grazing of the range. In this study, an integrated approach is adopted to assess the impact of modernization and government intervention on the nomads and on the health of rangelands in At-Taisyiah. The approach consisted of interview with nomads in the region, a socio economic study of the region's nomadic lifestyle, analyses of several range capability assessment studies, and utilization of stochastic simulation models. Three major areas were identified in need of major improvements. These are the nomads' perception of range land resources, use of rangeland, and government policies towards nomads and rangeland. Improvement in these areas will play a major role in slowing the acceleration of desertification of marginal range lands in Saudi Arabia. comparison of Monte-Carlo simulations of four different grazing intensity scenarios and 12 different levels of government subsidies confirmed that lower grazing intensities provide the only means for sustainable utilization and for the combating of desertification of the rangelands in At-Taisyiah region.

Nutrient contents of three Atriplex species (Atriplex cancensces, atriplex linearis and Atriplex polycarpa) under different management practices and site conditions

Mehramiz, Mohammad Reza January 1998 (has links)
There are thousands hectares of abandoned farmlands in Arizona. Research on practical methods for establishing native plants and wildlife on abandoned farmlands is under way at the Desert Botanical Garden of Phoenix and the University of Arizona. This research is aimed at improving understanding of the primary variables affecting restoration of abandoned farmlands. These variables include: water treatments, planting dates, mulch and water catchment, seasonal planting and plant species composition. The objectives of the study is to investigate the effect of land restoration practices on nitrogen, protein, fiber (NDF and ADF), ash and dry matter contents of three Atriplex species under different management and site conditions. The amount of nutrient contents in Atriplex species growing on abandoned farmlands under irrigation and conservation practices is higher when compared to no irrigation or control in Casa Grande research site. Atriplex species growing under mulch and water catchment condition had significantly higher nutrient content compared to other treatments. Furthermore, seasonal planting in abandoned farmlands affected the nutrient contents of Atriplex cultivars at the University of Arizona farm. The Atriplex cultivar planted in the winter had higher nutrient contents when compared to those planted in the summer and irrigated with a sprinkler irrigation system for some cultivars. A better understanding of how these variables affect nutrient contents of Atriplex species used in this restoration study will play an important role in directing public interest towards the revegetation of abandoned farmlands in southern Arizona for food production.

Establishment of native Atriplex species evaluated under a line-source sprinkler irrigation system during the summer and winter

Watson, Mary Carolyn, 1949- January 1999 (has links)
Atriplex canescens (Pursh) Nutt., Atriplex polycarpa (Torr.) Wats., and Atriplex lentiformis (Torr). Wats. have been considered candidate perennial shrubs for revegetation of abandoned farmlands in southern Arizona. Objectives of the 1992-1994 field studies were to investigate the establishment characteristics of populations of these species and to estimate water requirements for establishing transplants and/or seedlings under a line-source sprinkler irrigation system. During the summer on a sandy loam soil, seedling establishment occurred at total water amounts greater than 200 mm but was absent at amounts less than 150 mm The poor stand establishment was attributed to high soil temperatures at 1 to 3-cm depth which were not optimum for seed germination. Transplant survival percentages were greater than 89% except for accessions of A. canescens var. linearis (S. Wats.) Munz, which were affected by rabbit herbivory. Transplants were successfully established by planting into a wet soil profile followed by cumulative precipitation amounts of 60 to 70-mm. Establishing Atriplex taxa during the summer using transplants was more promising than direct-seeding. On a silt loam soil during the 1993 winter, stand establishment was not increased under cumulative precipitation and irrigation amounts greater than 100 mm compared to 66 mm of precipitation. On a clay loam soil during the 1994 winter, supplemental irrigations increased the probability of seedling emergence and stand establishment was higher under total water amounts greater than 100 mm. Differences between years in response to the line-source irrigation gradient were attributed to the number of consecutive days when soil moisture at 1 to 3-m depth was high for optimal seedling emergence. During the winter, plant heights were not affected by total water amounts between 182 to 248-mm (1993) and between 119 to 150-mm (1994), whereas heights were reduced at total water amounts less than 100 nun in 1993, and less than 90 mm for all species in 1994 except A. polycarpa. In southern Arizona where precipitation is erratic in amount and timing, supplemental irrigations may increase the probability of successful seedling establishment of Atriplex shrubs.

Effects of fire on Agave palmeri

Johnson, Roxane Jeannette January 2001 (has links)
I investigated the effects of prescribed fire on Agave palmeri , an important seasonal food source of the federally Endangered bat, Leptonycteris curasoae yerbabuenae. Three different treatments were randomly assigned to plots containing agaves within a burn unit: plots were burned with extant fuel, plots were left unburned, and plots were burned with an augmentation of fuel. Agaves were surveyed before the fires, immediately after the fires, and one and two years after the fires. Mortality and survivorship with the fuel load, agave size and the type of clusters in which the agaves grew. Agaves near mesquite and acacia trees or dead, dried agaves experienced higher mortality than agaves growing elsewhere. Agaves in plots with added fine fuels also had higher rates of mortality. One year post-fire, mortality was low in all treatments and recruitment was higher on augmented and burned plots than on unburned plots. Two years post-fire, mortality of small Agave palmeri was associated more strongly with rainfall than with fire treatment, while mortality of larger height classes of agaves exhibited a delayed response to fires.

Restoring a prairie: Testing effectiveness of Chinese tallow tree (Sapium sebiferum) mulch to reduce seedling emergence

Donahue, Candice January 2004 (has links)
The invasive Chinese tallow tree (Sapium sebiferum) is difficult to control because of its large seed bank and ability to resprout from cut stumps. I performed laboratory and field experiments to evaluate the effectiveness of mulching live trees for restoring invaded prairies. Herbicide use was limited to manual application to cut stumps. I manipulated mulch depths and types in the field and measured soil temperatures beneath them. At depths of as little as 5 cm, Sapium mulch damped soil temperature fluctuations and reduced seedling emergence. Reduced seedling emergence was not the result of allelopathic compounds in Sapium mulch because other mulch types suppressed emergence similarly. Substantial regrowth of native vegetation occurred through the mulch. Independent manipulations of mulch depth and temperature fluctuations in a lab experiment confirmed that mulch suppressed seed germination indirectly via soil temperature effects. This prairie site can now be managed by mowing or burning.

Use of uprooted invasive buffelgrass (Pennisetum ciliare ) parent plants as thatch to reduce progeny seedling emergence

Jernigan, Marcus Brendon 14 February 2014 (has links)
<p> Buffelgrass (<i>Pennisetum ciliare</i>) is a perennial bunchgrass native to Africa that has invaded ecologically intact areas of the Sonoran Desert in southern Arizona. It threatens many native plant species by means of competitive exclusion as well as increased fire frequency and intensity. Since the 1990s, efforts have been underway in southern Arizona to control buffelgrass using manual removal. A problem with this method is that the resulting bare, disturbed soil provides a favorable environment for buffelgrass seed germination. This study examined whether thatch composed of uprooted buffelgrass parent plants spread over disturbed areas reduces the density of progeny seedlings. A secondary goal was to determine whether light attenuation and/or autoallelopathy were major factors involved in the effect of thatch on buffelgrass seedling density. The effect of light attenuation on seedling density was tested in containers in the field and in the greenhouse. The propensity of thatch to produce autoallelopathic chemicals was tested in the greenhouse. Field plots with thatch had 1.9 buffelgrass seedlings/m<sup>2</sup> which was significantly fewer (<i>p</i> = 0.03) than the 2.9 seedlings/m<sup> 2</sup> in plots without thatch. These results suggest that the placement of thatch over areas disturbed during manual treatment of dense stands of buffelgrass will increase the efficiency of follow-up control of buffelgrass progeny seedlings in these areas. Results of the field container study suggest that light attenuation does not play a significant role (<i>p</i> = 0.39) in the reduction of seedling density by thatch, whereas those of the greenhouse shade treatment study indicated that light attenuation is a significant factor (<i>p</i> = 0.004). However, because percent germination was very low in the field container study, those results may be of little value compared to the greenhouse shade treatment study results which indicate that light attenuation is a mechanism by which thatch reduces buffelgrass seedling emergence. Chemicals leached from decomposed buffelgrass thatch did not have a significant effect (<i>p</i> = 0.09) on buffelgrass seedling density. Only the combination of thatch and leached chemicals significantly reduced (<i>p</i> = 0.014) seedling density. Thatch may also increase the activity of other factors that could reduce seedling density such as pathogens, and predators of seeds and seedlings.</p>

The interaction of arvicoline rodents and sheep in Norwegian alpine rangeland /

Saetnan, Eli Rudinow. January 2008 (has links)
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 6531. Adviser: George O. Batzli. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Page generated in 0.111 seconds