The Role of Life History Processes in Primary Succession on an Alaskan Floodplain

Abstract

The pattern of primary succession on the floodplain of the Tanana River in interior Alaska resulted largely from interactions between stochastic events and life history traits of the dominant species. Seed rain by willow (Salix alaxensis), alder (Alnus tenuifolia), poplar (Populus balsamifera), and spruce (Picea glauca) varied substantially among years but was highest in the successional stage dominated by that species. Some seeds of each species arrived in all stages, and seedlings of all species were initially present in early successional sites. The copious, wind—dispersed seeds and rapid seedling growth rates of willow and poplar resulted in their abundant establishment on early successional vegetated—silt bars. Heavier alder and spruce seeds were less widely distributed, yet rapid growth rates of alder resulted in dense alder thickets within 20 yr of silt bar formation. We found no evidence of buried seed of the four study species. Sown seeds of willow, alder, and poplar established only in early successional vegetated—silt and willow sites. Spruce established in these same sites and in alder sites. Spruce was the only species that naturally colonized mid and late successional sites. Removal of the litter and forest floor enabled all species to germinate in all sites. Flooding resulted in substantial mortality of seedlings, in early successional sites. A combination of short life span, herbivory by hares, and shade intolerance eliminated willow from mid—successional alder—dominated sites. Thereafter differences in longevity explained successional change from alder to poplar to spruce. Facilitative interactions among species did not appear essential to explain changes in species composition in this primary successional sequence.