Soil water use and root system characteristics of C. maculosa and sympatric plants
Swan, Megan Cashman
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Centaurea maculosa Lam. (spotted knapweed) is an introduced perennial forb that invades disturbed and undisturbed semiarid grasslands in the western United States. It forms dense monocultures and reduces native and desirable vegetation. We hypothesized that C. maculosa may succeed through a superior ability to access soil water. This study had two objectives, to compare soil water uptake patterns under C. maculosa, perennial grass, and R. hirta monocultures in the field on semiarid rangeland, and to compare root characteristics of C. maculosa and R. hirta plants grown under two soil water regimes in the greenhouse. For the first objective, we used two field sites in western Montana. We created four blocks of adjacent monoculture plots of three species (perennial grasses, C. maculosa and R. hirta) arranged in a randomized complete block design. Soil water dynamics were monitored throughout the growing season at six hour intervals in the upper 0.3 m of the soil profile using TDR, and weekly to biweekly at greater depths using a neutron moisture meter. Centaurea maculosa used more water than resident grasses when averaged over the soil profile and had greater water use than grass at greater depths later in the growing season. R. hirta displayed similar soil water uptake to C. maculosa in the second year of our study. There was no "carry-over effect" of low soil water contents at deep soil depths from the end of the first growing season to the beginning of the next. To address our second objective, we grew 10 individuals of C. maculosa and R. hirta under either "dry" or "wet" soil water conditions for 3 months in the greenhouse, then extracted, cleaned, stained and scanned their roots to quantify several root characteristics. Centaurea maculosa had lower root mass, root length, specific length, root length density and greater average root diameter than R. hirta under both soil water regimes. Centaurea maculosa had higher root mass ratios than R. hirta, but this may have been due to phenological differences at time of harvest or differences in nitrogen utilization. Our results suggest that C. maculosa may invade and persist in western rangelands due to its ability to take up soil water unavailable to native grasses which allows it to continue growth and photosynthesis late in the growing season. However, similar water uptake patterns and greater total root system size and efficiency of the non-invasive R. hirta indicate that C. maculosa does not have unique characteristics indicating superior belowground competitive ability, but may in fact share traits common to other late season tap-rooted forb species.