. Die kanadische Feld - naturforscher. 1999 Notizen 297 800 700 600 o 500 400 300 200 100. GOCOCdOOGOOOOOCOQOO} 03 d>d> Jahr Abbildung 1. Cirsium pitcheri Populationen, 1981-1997, Pukaskwa Nationalpark, Ontario. Deskriptive Statistiken wie z.b. Mittelwert und Standardabweichung der Ergebnisse angewendet wurden Trends in den Daten über die 17-jährige Frist zu offenbaren. Ergebnisse über 17 Jahre die Daten zeigt an, eine mittlere gesamte pop Uebergangsregelung von 401,7 Pflanzen entlang der beiden Strände innerhalb von pukaskwa Nationalpark (Abbildung 1). Es ist ein Stan-dard Abweichung von 182,9. Es gab eine Hohe von 760 Personen im Jahr 1985 und einem Tief von 1.

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. The Canadian field-naturalist. 1999 Notes 297 800 700 600 o 500 - 400 300 200 100. GOCOCdOOGOOOOOCOQOO}03d>d> Year Figure 1. Cirsium pitcheri populations, 1981-1997, Pukaskwa National Park, Ontario. Descriptive statistics such as mean and standard deviation have been applied to the results to reveal trends in the data over the 17-year period. Results Over 17 years the data indicates a mean total pop- ulation of 401.7 plants along the two beaches within Pukaskwa National Park (Figure 1). There is a stan- dard deviation of 182.9. There was a high of 760 individuals in 1985 and a low of 153 in 1991. In 1986, a beaver dam upstream from the thistle colony burst releasing a substantial volume of water (Sahanatien 1985*). This forced the re-routing of the creek near its mouth clearing out 63% of the thistle population. Between 1986-1991, the population remained low (< 300 total population) until 1991 with a population of 153 individuals. Since 1991 the population has rebounded to a high of 497 in 1996. Discussion The C. pitcheri population appears to have sur- vived the catastrophic impact to its habitat in 1986. As suggested by Mosquin (1990*), periodic distur- bance may actually assist the survival of the plant by reducing the encroachment of later successional species. Since 1986, however, the re-routing of the creek into Oiseau Bay has reduced the annual level of dis- turbance at the site (Mosquin 1990*). The population near Creek Beach (the larger of the two concentra- tions), is no longer influenced by deposition or peri- odic water fluctuations. With this lack of distur- bance, Mosquin suggested that the population would have difficulty competing with other encroaching vegetation (Mosquin 1990*). However, the total plant population has actually increased since 1991, initiated by a higher number of seedlings in 1992. Reside (1992*) suggests that the cool weather and moist conditions that were prevalent during the sum- mer months of 1992 may have contribute

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