Does the size-density relationship developed for bamboo species conform to the self-thinning rule?

Unlike most plants that increase size with age, species in bamboo subfamily have a unique growth pattern and their vegetative sprouts reach adult size during the first growing season. However, the adult size of bamboo varies greatly among species, and the range of sizes among different bamboo species may be an analog to the range of size of a non-bamboo tree species due to its growth with age. Therefore, we hypothesize that the self-thinning rule developed for individual plant species can be applied to describe the size-density relationship among different bamboo species. To test this hypothesis, we quantified the size-density relationship using data collected from 50 bamboo species in fully stocked stands and compared this relationship with the self-thinning rule. The 50 bamboo species included 10 dwarf bamboo species, 20 shrubby bamboo species, and 20 woody bamboo species, with their size covered almost the entire size range within the bamboo subfamily. In addition to biomass, ground diameter (GD) was also used as a measure of size in our study because of its close relationship with biomass. Our results indicate that the self-thinning rule described the size-density relationship very well, with a slope of −1.184 that is slightly lower than −3/2. We conclude that size differences among bamboo species, instead of the conventional size differences due to growth with age within a single species, can be used to develop the self-thinning rule for bamboo subfamily. Compared to the biomass, the ground diameter resulted in a better fit with density, suggesting that ground diameter should be used as the measure of size when studying size-density relationship for bamboos.