Plant distribution and colonization in extreme polar deserts, Ellesmere Island, Canada

grantor: University of TorontoPolar desert vegetation (with usually less than 5% vascular plant cover) represents 15.1% of the arctic tundra biome area, yet its composition and distribution are poorly known. The Canadian High Arctic contains 75.6% of the polar deserts of the world. They are characterized by very cold and extremely short growing seasons and minimal summer precipitation. This thesis investigates the vascular species richness and distribution based on the survey of 82 sites on central Ellesmere Island, Canada. In addition, it addresses fundamental questions regarding three major variables potentially limiting plant community development: the availability of 'safe sites'; the abundance of viable seeds; and the limited time for plant establishment since the site's deglaciation. The minimum sampling area of this sparse vegetation was measured to be 10-25 m$\sp2.$ Vascular species richness ranged from 2-22. Four key vascular species: Saxifraga oppositifolia, Papaver radicatum, Draba subcapitata and Draba corymbosa occurred on 60-100% of the sites. Some species were restricted to a specific substrate, either granite or carbonate. Bryophyte and lichen cover was frequently $<$0.1% and $<$5% respectively. Environmental variables measured explained a large portion of the vegetation variation (more than 67%). Elevation was the most important variable. Vegetation on low elevation sites was more abundant and diverse. Four vegetation groupings reflecting plant abundance, plant diversity and substrate type were distinguished with a two-way indicator species analysis (TWINSPAN). Plant distribution on a dolomitic plateau was contagious and plants tended to grow near protective boulders. Along two altitudinal gradients, flower buds were produced almost every year by Papaver radicatum, but flower and fruit production varied depending on site conditions and yearly summer climate. Germinable seed banks were found even in extremely poor sites. This study demonstrates the diversity of polar deserts in central Ellesmere Island. Reproduction by seeds on such sites is common although its success depends on local climate which varies greatly from year to year. Recruitment of new seedlings is rare although vacant 'safe sites' and seeds are available. Ground surface dessication may partly interfere with this process. The findings of this work suggest that recruitment is restricted to short periods when conditions are more favourable.Ph.D