<p> Russian olive is an exotic actinorhizal tree intentionally introduced to the U.S. in the early 1900’s. It has become a dominant component of riparian ecosystems throughout the western U.S. Unlike most other riparian trees in the semi-arid west, Russian olive germinates and grows both in the open and in the understory of mature cottonwood stands. As an actinorhizal species, it forms an endosymbiosis with soil actinobacteria in the genus <i>Frankia</i> that allows for atmospheric N<sub>2</sub>-fixation. This leads to higher soil N concentrations and mineralization rates underneath the tree’s canopy than outside. Russian olive’s high abundance and impact on soil N suggest it may alter plant communities, but these impacts have not been previously demonstrated. I investigated the impacts of Russian olive on shading, soil N availability, and plant communities and documented how those impacts varied across a semi-arid riparian ecosystem along the South Fork of the Republican River in eastern Colorado. Of the suite of environmental variables I measured, presence or absence of cottonwood canopy had the largest effect on Russian olive impacts. Russian olive increased shading, soil N availability, and proportion exotic plant and forb cover more in the open than underneath a cottonwood overstory.</p><p> Actinorhizal endosymbioses provides an important N source in terrestrial ecosystems, but N<sub>2</sub>-fixation rates decrease due to high exogenous N and low photosynthetically active radiation (PAR). The amount that these environmental variables reduce N<sub>2</sub>-fixation in host-<i>Frankia </i> symbiosis types dictates the strength and duration of those symbioses’ impacts on ecosystems. To understand how the two main types of endosymbioses (<i>Alnus-</i> and <i>Elaeagnus-Frankia</i>) differ in their response to environmental variability, I conducted a greenhouse experiment comparing growth and nodulation between two genera of actinorhizal species, <i> Elaeagnus</i> and <i>Alnus,</i> across exogenous N and PAR levels. Overall, <i>Elaeagnus</i> species had higher nodulation rates and tissue % N than <i>Alnus</i> species. Nodulation rate and growth response to nodulation were both lower at low PAR than high PAR for both genera. The reduction in the growth response to nodulation at high exogenous N was lower in <i>Elaeagnus-Frankia</i> symbiosis than <i> Alnus-Frankia</i> symbiosis. These results suggest that <i>Elaeagnus </i> species are more likely to cause a greater and longer-lasting increase in soil N than <i>Alnus</i> species.</p><p> A main objective of exotic species management is to increase native plant cover. However, few studies monitor plant community response to exotic species management, and the few that have suggest secondary invasion is likely, particularly when effects of the target invasive persists and management efforts cause disturbance. To measure the role of these two factors in plant community response to Russian olive removal, I monitored soil N availability and plant communities along the South Fork of the Republican River two years before and three years after the tree’s removal. Russian olive’s impact on soil N availability persisted, with levels staying high around removed Russian olive stems three years after removal. The plant community around removed Russian olive also had no increase in native plant cover but a dramatic increase in kochia (<i> Bassia scoparia</i>) cover following removal. My research demonstrates that Russian olive increases exotic plant cover in areas it invades and simply removing the tree does not promote native species recovery.</p><p>
| Identifer | oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:10604654 |
| Date | 14 November 2017 |
| Creators | Tuttle, Graham |
| Publisher | Colorado State University |
| Source Sets | ProQuest.com |
| Language | English |
| Detected Language | English |
| Type | thesis |
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