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In many areas of the Western U.S., diffuse knapweed (Centaurea diffusa Lam.) has invaded into plant communities dominated by bluebunch wheatgrass (Pseudomegnaria spicata [Pursh] A. Love). The objectives of this study were to compare growth response of both species when grown under altered moisture and temperature regimes while in the seedling stage and to further elucidate the competitive ability of diffuse knapweed compared to bluebunch wheatgrass. Isolated individuals were grown in four different environmental chamber conditions (12 hr day length, 10 and 16C and -0.01 MPa and -0.03 MPa soil moisture). Diffuse knapweed penetrated quicker than bluebunch wheatgrass starting on day 20, regardless of temperature and moisture and knapweed penetration was greatest under warm and wet soil conditions. Bluebunch wheatgrass developed more root length initially under warmer and drier conditions, but those differences diminished after thirty days growth. Warmer and wetter soil conditions favored diffuse knapweed leaf area production in later stages of seedling growth. Diffuse knapweed had more rapid root penetration than bluebunch wheatgrass under the conditions studied. Diffuse knapweed maximized shoot production, indicative of a relatively fast growing species, while bluebunch wheatgrass maximized root production, characteristic of a relatively slow growing species. Diffuse knapweed seedlings grew best under warmer and wetter conditions. Comparing plant efficiency (indices of plant efficiency based on the measurement of a plant attribute divided by the total biomass of the plant), diffuse knapweed was apparently more efficient than bluebunch wheatgrass at producing the competitive attributes of root penetration, leaf area and root length. / Graduation date: 1998

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In recent years, global warming awareness has resulted in an increased demand for clean sources of energy such as hydropower. As a consequence, its impact on riparian vegetation must be studied. In this research, I aimed to assess how different functional riparian groups from northern Sweden respond to hydropeaking (i.e. short-term flow regime changes due to differences in the daily energy requirements). I selected seedlings of eight species natural from Swedish riparian ecosystems which can be grouped in three different guilds (forbs, graminoids and woody) according to their habitat and morphological traits.  A seven week greenhouse experiment in which the seedlings were subjected to two watering treatments that simulated prolonged and deep submergence and frequent and short shallow submergence conditions was developed. I measured the root, stem and leaf biomass, followed leaf changes and stem growth over the weeks and evaluated the health status. The study showed how some species and guilds responded differently to the treatments although survival rates were similar. Forbs was the most resilient group unlike the woody guild.  Graminoids grew longer and thinner roots in frequent submergence situations. Small seedlings appeared to be more sensitive to prolonged submergence. No significant differences were found for leaf variables. Collectively, these results suggest that hydropeaking could significantly modify the riparian vegetation. More and longer studies are needed in order to understand the capacity that hydropower has to modify the riparian vegetation and therefore the riverine ecosystems.