Pioneers
Weeds take over bare ground in waves.
"Weeds" are taking over this bare patch of earth in the same way they colonized bare patches of rocky soil left behind by the glaciers. The first, pioneer, plants to invade newly bare areas of ground like a lot of sun, dry soils and heat. The next wave of plants, adapted to less barren ground, thrive where pioneer plants protect them from sun and wind, and stabilize the soil.
Some plants are made to be pioneers.
Fireweed, willow and common silverweed are three of the pioneer plants in this plot. Fireweed and willow are common in burnt and disturbed areas of the Yukon. Their numerous wind-blown seeds make them perfect pioneers. When the ice sheets started melting about 18,000 years ago, sage, mountain avens and grasses were probably some of the first plants to put down roots behind the retreating glaciers.
Common silverweed, one of the pioneer plants growing in this plot. Photo:
Barb Hogan
Inset: Siberian aster, one of the pioneer plants growing in this plot. Photo:
Barb Hogan
Plants move north as the climate warms.
Many plants migrated north from ice-free southern areas when the glaciers retreated during warmer periods of the last glacial age. By studying fossilized pollen in the soil and present geographical distribution, we can tell that common silverweed has arrived since the glaciers left. The southern migrants crowded out many of the plant species that survived the last glacial age in Beringia.
Drummond's mountain avens, a pioneer plant during the last glacial age.
Photo: Bruce Bennett
Inset: Dwarf hawksbeard, a pioneer plant during the last glacial age. Photo:
Bruce Bennett
With our warming climate, will invasive weeds like sweet clover invade the Yukon and crowd out native plant species? Or is the northern spread of boreal forests more of a threat to their survival?