All individuals of P. major and P. rugelii were mapped at approximately 3-wk intervals from May.-Oct. during 3 growing seasons in permanent 0.16 m2- quadrats near London, Canada. The quadrats were established on either fairly old pasture or recently disturbed pasture, where either 1 sp. was present originally or both. Many gains and losses were found for P. rugelii at Site 1 (12 yr old pasture), but only where seeds were added at the start of the study did the population increase significantly. At the end of 3 growing seasons 65% of the plants of P. rugelii at this site were 2 yr old or less, and the population size had been maintained during the study. Plants of P. major represented 1 age-class and were lost at a constant rate with very few additions to the population. Populations of both mature plants and seedlings of P. rugelii had longer half-lives than corresponding populations of P. major. The negative exponential model, with the assumption of constant mortality at all ages (on an annual scale) fitted the survivorship data of most cohorts of both species. Seedling mortality for both species was greatest during the 1st 6 wk. Also, seedlings were subject to a seasonally greater risk of mortality; it was greater in the spring and late summer than in the mid-summer and winter. These peaks in mortality occurred when the pasture was growing actively. The sizes of natural populations of both species were manipulated. The density was increased immediately following an addition of 100 seeds/quadrat; the increase was only temporary with seeds of P. major. The removal of spikes for 2 yr lead to a reduced density of P. rugelii. A greater proportion of plants of P. major flowered than of P rugelii. Also, P. major flowered earlier during its development. Both species were either monocarpic or polycarpic, the latter condition being allied with relatively weak competitive stress.