Life cycle and age structure of ramets in an expanding population of Salix exigua (sandbar willow)
The sandbar willow (Salix exigua Nuttall, ssp. interior (Rowlee) Cronquist) is a dominant woody pioneer and stabilizer of riverbanks and sand dunes over much of North America. The growth rates, life-history phases, sex ratio, longevity, and age structure of a large population growing on a point bar in the Assiniboine River, Manitoba were assessed. Average annual height increments were 29.7 cm, and average annual girth increments were 2.6 mm/year. There were no significant differences between sexes. Significant linear correlations existed between basal stem diameter and age and between stem height and age. Flowering occurred in a few 2- and 3-year-old stems, regardless of sex. Flowering frequency increased to 93% at the age of 9 years. Stem mortality was highest in 3-, 4-, and 6-year-old ramets, and senescence was first observed at about 12 years. Though few stems at this site were older than 12 years, the oldest stem encountered was 31 years old and had sprouted soon after the point bar had started to form. The sex ratio of 1.7:1.0 significantly favoured males. Key words: willow, Salix, population dynamics, age structure, life cycle, sex ratio, clonal, ramets.