GROWTH DYNAMICS AND SIZE STRUCTURE OF SHOOTS OF PHRAGMITES-AUSTRALIS, A CLONAL PLANT

1. Growth dynamics and size structure during one growing season were investigated at the level of the individual shoot of Phragmites australis, a clonal plant. These were based on the diffusion model for three shoot populations which are described as even-aged sparse (the least crowded), even-aged dense and uneven-aged dense (the most crowded). Irrespective of the difference in the degree of crowdedness, these three shoot populations converged to the same size structure in height and weight as they grew, suggesting a regulatory mechanism between shoots. There was no growth in shoot diameter, and hence size-structure dynamics of shoot height and weight were almost parallel. 2. From these size-structure dynamics and from direct estimation of the growth pattern of shoots for the sparse shoot population, two types of shoot growth pattern were inferred for the uneven-aged dense shoot population of Phragmites australis which consisted of young small replacement and old established shoots: type 1 has a linear G(t,x) function (mean of absolute growth rates of shoots of size x at time t in the diffusion model) with respect to x together with the size-independent D(t,x) function (variance of absolute growth rates of shoots of size x at time t in the diffusion model) whereas type 2 has a concave G(t,x) function with respect to x and/or positive growth rate for the smallest shoot in the stand (in this case, the functional effects of D(t,x) are generally small). 3. In both cases growth of small young shoots is guaranteed or supported but not suppressed by large old shoots either stochastically (type 1) or deterministically (type 2), thus leading to little variablility in shoot size, even in the uneven-aged crowded stand which can be regarded as an extreme situation of asymmetric competition. These growth patterns and size-structure dynamics are in striking contrast to those of non-clonal plants, which generally have a convex G(t,x) function with respect to x and a positively size-dependent D(t,x) function, and thus greater size variability under more crowded conditions, especially in uneven-aged stands consisting of small young and large old individuals. These differences suggest physiological integration between shoots in Phragmites australis. 4. The size-independent D(t,x) function is common to many clonal plant species including Phragmites australis, whilst the G(t,x) function differs between species. This suggests that effective physiological integration between shoots works at least stochastically in the growth dynamics of shoots in many clonal plant species (and in some clonal species, deterministically as well), at least in the form of a controlled allocation of remobilized resources from the rhizomes to the growing shoots especially at the early growing stage. It was also inferred that the stochasticity in growth of small shoots plays an important role in the establishment and persistence of clonal plants.