Self-thinning exponents for partial organs in overcrowded mangrove Bruguiera gymnorrhiza stands on Okinawa Island, Japan

According to the Weller’s allometric model, the slope of the self-thinning exponent αx of a partial organ was calculated from the allometric constants θx and δx obtained from the allometric relationships of mean tree height H¯(∝w¯xθx) and of organ mass density d¯(∝w¯xδx) to mean organ mass w¯x. The self-thinning exponent, αx(=1/{1 − (δx + θx)}), was estimated to be 1.519 for stem, 1.515 for branch, 1.095 for leaf, and 1.482 for aboveground. The αx-value obtained from the estimates θx and δx cannot be used for its significance test because these two estimators are dependent of each other. The allometric relationship of the mean occupied area per tree s¯(=1/ρ∝w¯xϕx) to w¯x gives the self-thinning exponent αxk, calculated as 1/ϕx, which is mathematically and statistically (by ordinary least squares) equivalent to 1/{1 − (δx + θx)}. The ϕx-value was 0.6582 for stem, 0.6601 for branch, 0.9129 for leaf, and 0.6748 for aboveground mass. The ϕx-value was not significantly different from 2/3, but it was significantly different from 3/4 for stem, branch, and aboveground. This means that self-thinning exponents for woody parts were not significantly different from 3/2. This result suggests that the self-thinning exponent was close to 3/2 rather than 4/3. On the other hand, the ϕL -value for leaf was significantly different from both 2/3 and 3/4, but it was not significantly different from 1.0. This means that stand leaf biomass was constant regardless of population density. The self-thinning exponent for leaf had a negligible effect on that for aboveground because of the small amount of leaf mass compared with the combined mass of the woody organs. Therefore, the present results based on the allometric model of partial organs roughly support the 3/2 power law for aboveground self-thinning. For overcrowded Bruguiera gymnorrhiza stands, self-thinning could be explained by a simple geometric model rather than a metabolic model.

► This study was carried out in overcrowded Bruguiera gymnorrhiza stands.
► Weller’s model was used to examine the self-thinning exponents of partial organs.
► The self-thinning exponents of woody parts were insignificantly different from 3/2.
► The self-thinning exponent of leaf was insignificantly different from 1.0.
► The self-thinning of aboveground parts was explained by a simple geometric model.