Shoot Growth and Mortality Patterns of Urtica dioica, a Clonal Forb

Abstract

The growth and mortality patterns of the clonal forb Urtica dioica were investigated at the level of the individual shoot in two growing seasons, 1991 and 1992, in a natural stand. Shoot height and diameter at ground level of each shoot tagged in spring were measured repeatedly five times during the growing season. Dry weights of these repeatedly measured shoots were estimated using an allometric relationship between dry weight, height and diameter of harvested shoots. A large decrease in shoot density occurred with stand development from the beginning of the growing season in both the years: (1) shoot survival rate was about 30% at the end of the growing season; (2) shoot mortality rate per 10 × 10 cm subplot between censuses was positively dependent on shoot density per subplot; (3) the mortality rate of individual shoots was negatively dependent on shoot size (height, diameter and weight) at each growing stage, suggesting one-sided competition between living and dying shoots; (4) shoot size (height, diameter and weight) variability in terms of the coefficient of variation and skewness decreased in accordance with shoot mortality. Symmetric competition between living shoots was detected by regression analysis based on a model for individual shoot growth considering the degree of competitive asymmetry. However, the competitive effect on individual shoot growth was very small (nearly absent). The mortality pattern of Urtica dioica indicates that shoot self-thinning occurred from the early growing stage as in non-clonal crowded monospecific stands, and contrasts with many clonal plants where shoot self-thinning rarely occurs or, if any, is confined only to a short period of the later growing stage. The pattern of growth and competition between living shoots of Urtica dioica contrasts with non-clonal crowded plants undergoing intense competition (usually asymmetric) between individuals, but is a common feature of many clonal plants where shoot competition is supposed to be reduced by 'physiological integration' between shoots. These form a new pattern not reported yet for clonal plants. It is pointed out that clonal plants show a wider spectrum of the growth, competition and mortality patterns of shoots than non-clonals. Some possible mechanisms for the pattern of Urtica dioica are discussed.