Graduate Thesis Or Dissertation
 

The Role of Plant-soil Feedback in the Invasion of Brachypodium sylvaticum in Douglas-fir Forests

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  • Invasive plants have the capacity to transform landscapes and alter ecosystem function, causing significant economic and ecological damage. These effects include displacement and reduction of native flora and fauna, altered fire regimes, modification of biotic and abiotic soil properties, as well as local, regional, and global economic impacts. With such large impacts it is important that we better understand invasion dynamics to help with prevention, control and mitigation of invasive species. One process that has been associated with plant invasion is plant-soil feedback (PSF). A PSF occurs when plants alter biotic and abiotic soil properties through a variety of root exudates and litter decomposition such that subsequent plant growth is either positively or negatively affected. Positive conspecific and negative heterospecific responses have been theorized to be invasive species traits that promote invasion. Once an invasive species is removed from a system, there is a chance that PSFs generated by that species will persist in the soil, which is often referred to as 'plant legacies' or 'legacy effects' and may negatively influence restoration efforts. In the U.S. Pacific Northwest (PNW), Brachypodium sylvaticum (slender false brome), a perennial bunch grass native to Eurasia, is listed as a quarantined invasive species in California, Oregon, and Washington. Currently, B. sylvaticum is in the midst of rapid population growth and range expansion with populations in New York, Virginia, and Ontario, Canada. With a quickly expanding range research is critical for successful efforts to reduce the spread of B. sylvaticum. We developed two experiments to determine if PSF is a contributing factor to B. sylvaticum invasion in PNW forests. We hypothesized that 1) B. sylvaticum has positive conspecific and negative heterospecific PSF, 2) native species PSF has no effect on B. sylvaticum, and 3) PSF generated by B. sylvaticum will persist in the soil once removed, but over time, response of native species, soil nutrients and bacterial community composition will change from the invaded conditioned. To test our first two hypotheses, B. sylvaticum and five common native plants from the Oregon Coastal range, including the economically important tree, Pseudotsuga menziesii (Douglas-fir), were grown in a greenhouse on wild forest soils that had either been sterilized or kept live to condition the soil biotic community to the invader and the native species. Brachypodium sylvaticum was then grown on soil conditioned by itself and soil conditioned by natives; each of the five native species was grown on soil conditioned by B. sylvaticum and on their own conditioned soils. Plant biomass along with species specific measurements (number of leaves, stems, tillers, stem diameter and height) were recorded and a relative response (RR) index was used to determine the direction of PSF for the invader and native species. To test our third hypothesis, in March, 2015, ten plots were established in the McDonald-Dunn Research Forest located in Corvallis, OR where B. sylvaticum had at least 75% cover. Herbicide was applied to half of each plot to make two soil treatments: soil with B. sylvaticum and soil without B. sylvaticum. Over a nine-month period three soil collections took place where soil was collected from all plots and treatments. Plant response was evaluated by growing four native species and B. sylvaticum on both soil treatments and evaluating total biomass with a RR index; plant response (via growth), soil nutrients and bacterial communities were measured for each collection period. Bacterial communities were measured with phospholipid fatty acid (PLFA) analysis and high throughput 16s rRNA amplicon sequencing. Contrary to our hypotheses, the RR to PSF generated by B. sylvaticum was negative for the invader and P. menziesii and neutral for all other natives. Soils conditioned by Bromus vulgaris inhibited B. sylvaticum growth whereas soils conditioned by Prunella vulgaris and P. menziesii promoted B. sylvaticum growth. When testing for legacy effects, the RR of P. menziesii was negative when grown on soils where B. sylvaticum had been removed for six-months (six-month soils) but when grown on soils where B. sylvaticum had been removed for nine-months (nine-month soil) the RR of P. menziesii was neutral. The RR of P. vulgaris to six-month soils was positive while its RR to nine-month soils was negative. Nutrient and bacterial communities did not change in response to B. sylvaticum removal suggesting that the biotic and abiotic legacy requires longer than nine-months to be observed or B. sylvaticum does not affect the response variables measured. Overall, our data suggest that PSF generated by B. sylvaticum does not facilitate the invasion process but does differentially affect native species growth over time. PSF generated by native grasses may be a useful restoration tool to help prevent B. sylvaticum invasion and we suggest planting native species at least nine-months after B. sylvaticum removal.
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