Graduate Thesis Or Dissertation
 

Influence of nitrogen and phosphorus on interference between medusahead and squirreltail

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  • Restoration of invaded aridlands is required to reduce the exorbitant ecological and monetary losses related to noxious weeds. An understanding of how reduced and increased levels of soil N and P influence interference between medusahead and squirreltail is imperative to understanding how squirreltail may be used in restoration of medusahead infested rangelands. Medusahead (Taeniatherum caput-medusae ssp. asperum (Simk.) Melderis) is an invasive, nonnative, annual grass that is rapidly displacing desirable rangeland plants in western North America. Evidence suggests that the native, perennial bunchgrass squirreltail (Elymus elymoides (Raf.) Swezey) may be able to displace medusahead under certain conditions, but the role of soil nutrients in this process is not well understood. I performed interference and growth analysis studies in a greenhouse to determine if soil nitrogen (N) and phosphorus (P) alter interference between medusahead and squirreltail. In both studies, plants were grown in pots containing a one-to-one mixture of sand and field soil from a site containing medusahead and squirreltail. In the interference experiment, medusahead and squirreltail were Redacted for privacy planted in density combinations of 0, 1, 5, 25, and 125 seeds per species per pot at four levels of N and P (loNloP, loNhiP, hiNloP, hiNhiP). Results indicated that medusahead and squirreltail competed for N. Increased soil N reduced medusahead and squirreltail's relative competitive abilities largely due to reductions in the intensity of intraspecific interference. High N also reduced the effect of medusahead density on squirreltail biomass. Soil P levels had little influence on predicted species aboveground growth or relative competitive ability. In the growth analysis experiment, plant growth, growth rates, and relative growth rates for above- and belowground biomass, total biomass, leaf area, total root length, and depth of root penetration of isolated individuals were recorded for harvested plants at 2-week intervals over a 70-day period. Results indicated that medusahead produced more absolute belowground biomass, aboveground biomass, total biomass, leaf area, and root length and had higher growth rates for these parameters than squirreltail. Squirreltail allocated more of its acquired resources to belowground growth endowing it with greater root: shoot ratios. Medusahead relative growth rates decreased in belowground biomass, aboveground biomass, total biomass, leaf area, and root length over the course of the study period, while squirreltail's relative growth rate for leaf area remained constant; however, medusahead still maintained higher relative leaf area growth rates during the experiment. Results from the growth analysis study matched results from the interference study in that medusahead was found to be a superior competitor over squirreltail for environmental resources. In order to understand the full benefits of squirreltail's relatively higher allocation of biomass to belowground growth, a long-term study would be necessary. A long-term study would allow for the differences in perennial versus annual resource allocation patterns to manifest themselves to their fullest extent. Over time and with consistently low nutrient availability, squirreltail might increase in medusahead infested rangelands. The maintenance of continually low levels of nutrient availability combined with a reduction of medusahead competition are prerequisites for reclamation of medusahead infested rangelands. Maintaining soil N at very low levels, over the long term, may diminish medusahead seed banks to low enough levels that squirreltail can increase and slowly replace medusahead. Competition between medusahead and squirreltail seedlings will likely be won by medusahead, but seedling-to- mature plant competition may be won by squirreltail. It appears that management inputs, e.g., seed drilling, herbicides, and best management practices that maintain low N availability, will be required to allow native perennials to firmly establish in invasive annual weed infested rangelands.
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