Carex humilis - a caespitose clonal plant: ramet demography, ring formation, and community interactions

Carex humilis forms rings of densely aggregated ramets in dry grassland vegetation in Central Europe. In the thesis I conclude that ramet demography is important both for analyzing fitness in clonal plants and for understanding how C. humilis rings are formed. I studied the ramet demography of C. humilis from 1993-1998 at the Hexenberg mountain, 300 m.a.s., 60 km east of Vienna, Austria. In total, the size of the ramet population increased during the study period but there was considerable variation in ramet population growth rate between years. Flowering ramets of C. humilis did not produce any offspring ramets, and at the study location, the seeds were either infected by smut of infested by insect larvae. The size of vegetative parent ramets positively correlated to both number and size of offspring ramets, i.e. larger parent ramets produced both more and larger offspring ramets.

The ramet population growth rate was higher in the periphery of rings than in the interior, mainly due to the ramets being larger in the periphery. Because of this, of the ramet population size in the periphery of rings increased, whereas the ramet population size in the interior decreased. However, the ramet density in the interior was lower than would be predicted from the population growth rate alone, and I conclude that the difference is due to the centrifugal dispersal of ramets. A simulation model showed that ramet populations with lower maximum population growth rate, or with more peripheral dispersal, formed rings, whereas populations with higher maximum growth rates or less peripheral dispersal formed filled circles. A size structured matrix model showed that although there was variance in the production of all size classes, both between years as well as between periphery and interior, it was only the variance in the production of larger ramets that was important for the variance in the asymptotic population growth rate.

Compared to the surrounding vegetation, the cover of most other plants was reduced inside C. humilis rings. In addition, the soil under C. humilis rings contained more moisture, nitrogen and phosphorus than the surrounding soil. Hence, the reason behind the lower population growth rate in the interior of rings was neither competition from other plants, nor depletion of any of the soil resources measured.