Resources and Environmental Sciences

Centaurea maculosa (spotted knapweed), a perennial tap-rooted forb, was introduced to the Pacific Northwest in the late 1800s from Eurasia. It has no natural enemies in North America and has rapidly spread throughout native grassland systems in the western United States. Semiarid grasslands are characterized by low nutrient and water availability. Generally, competition for soil resources in low-nutrient environments is intense. Centaurea ’s success in semiarid grasslands may be attributed to its ability to preempt limiting resources, or alternatively, its ability to use these limiting resources more efficiently. We hypothesized that as a weedy species, Centaurea has a greater ability than dominant native grasses to: 1) take advantage of pulses of nitrate availability, 2) preempt N under low and moderately high N levels, and 3) use water more efficiently. Three studies were used to test these hypotheses. First, we conditioned Centaurea and two native grass species, Pseudoroegneria spicata and Pascopyrum smithii, to different durations (pulses) of nitrate availability. Plants were grown in mixed- and monoculture pots. 15N- nitrate was use to determine N uptake. Second, we grew Centaurea, Pseudoroegneria, and Pascopyrum in mixed- and monoculture, and supplied them with different rates of N. These N supply rates mimicked low and moderately hig