Variation in individial growth and the population structure of a woodland perennial herb, Paris tetraphylla
Hara, T. | Wakahara, M.
1. Year-to-year growth dynamics of a long-lived woodland perennial herb, Paris tetraphylla, were investigated based on the diffusion model. The past biomass growth of each harvested individual was traced back by measuring the rhizome volume between stem scars. The population density was 2.47 m-2. Interference between individuals was therefore expected to be almost absent. 2. Two-year field observations revealed that there was little mortality and only a few recruits by seed. Branching of rhizomes (vegetative reproduction or clonal growth) was also very rare. Mean absolute growth rate per year of individuals of biomass x at year t (defined as the G(t,x) function representing averaged species characteristics) was nearly 0 irrespective of x and t, whilst variance of absolute growth rates per year of individuals of biomass x at year t (defined as the D(t,x) function which is caused by environmental fluctuations, genetic variation, etc.) was proportional to Xb where b ranged between 1 and 2. 3. Theoretically estimated stationary size distributions of individual biomass based on the above results agreed well with the observed ones, suggesting that the Paris tetraphylla population was already long lived and that the size structure was at a stationary state. The variation factor D(t,x), but not the deterministic factor G(t,x), determined the stationary size structure of the population. 4. It was shown theoretically that fluctuations in mortality rate and the magnitude of the D(t,x) function around the values estimated from the field data affect the stationary size distribution only a little. Therefore, the Paris tetraphylla population studied here is regarded as a stable system. As mortality rate decreases and/or the magnitude of the D(t,x) function increases, stability of size structure increases. 5. The growth and size-structure dynamics of Paris tetraphylla are in striking contrast to those of most crowded annuals and trees, where one-sided or strongly asymmetric competition between individuals is the major determinant of size structure and brings about its stability through the G(t,x) function. Therefore, there are two types of stable plant communities: in one type, such as in most crowded annuals and trees, growth, size structure and stability are governed mainly by the G(t,x) function (i.e. effect of the D(t,x) function is relatively small), and in the other mainly by the D(t,x) function such as in P. tetraphylla studied here and the sparse populations. However, the inverse J-shaped size distribution of individual biomass or stem diameter is common to both types.Show more [+] Less [-]