Self-thinning of stems, branches, and leaves in overcrowded mangrove Kandelia obovata stands on Okinawa Island, Japan

• Self-thinning exponents for leaves, branches and stems in Kandelia obovata stands were examined.
• Self-thinning exponents for woody organs were not different significantly from 3/2.
• The self-thinning of K. obovata could be explained by a simple geometric model.
• Stand leaf biomass was constant regardless of population density.
• Aboveground biomass density was constant regardless of stand structure.

The self-thinning exponent of stems, branches and leaves of the subtropical mangrove Kandelia obovata stands was investigated over seven years. According to Weller's allometric model, the slope of the self-thinning exponent αx of a partial organ “x” is calculated from the allometric constants θx and δx obtained from the allometric relationships between mean tree height H¯ (m) and organ mass density d¯ (kg m−3) with mean organ mass w¯x (kg). The self-thinning exponent, αx, was estimated to be 1.509 for stem, 1.647 for branch, 1.090 for leaf, and 1.507 for aboveground. The ϕx - value which is the allometric coefficient between s¯ (mean occupied area per tree) and w¯x (kg) was 0.6629 ± 0.0250 for stem, 0.6072 ± 0.0229 for branch, 0.9167 ± 0.0356 for leaf, and 0.6637 ± 0.0297 for aboveground. The value did not significantly differ from 2/3 but did significantly differ from 3/4 for stem, branch, and aboveground, indicating that the self-thinning exponents for woody parts did not significantly differ from 3/2. Our result suggests that the self-thinning exponent is closer to 3/2 than to 4/3. In contrast, the ϕL - value for leaf significantly differed from both 2/3 and 3/4 but did not significantly differ from 1.0, indicating that stand leaf biomass was constant regardless of population density. The self-thinning exponent for leaf had a negligible effect on that for aboveground mass due to the combined mass of all woody organs. Our data support the 3/2 power law for aboveground self-thinning. For overcrowded K. obovata stands, self-thinning could be explained by a simple geometric model rather than a metabolic model.