ヒルギダマシ植林による砂漠沿岸緑化に関する研究 : ペルシャ湾沿岸におけるヒルギダマシ種子の生態

抄録

In the United Arab Emirates, the flowering season of the gray mangrove, Avicennia marina (Forsk.) Vierh, starts by mid May, and mature crypto-viviparous seeds are present from late August to mid October. Feeding of seeds on mother trees by leafhopper larvae was the main reason for mortality, which was observed from mid September. The number of injured seeds increased daily, and almost all seeds on trees were damaged by mid October. The heart-like green seed is covered with a thin waterproof pericarp for flotation in seawater. The mature seed breaks out of the pericarp within a few minutes after dropping in the water, then sinks and settles to the bottom. Most mature seeds sink around mother trees on the tideland. Immature seeds spread further and can reach the middle of the Persian Gulf. Germinating seeds start to draw seawater through their radicle and excrete the excess salt on the surface of their cotyledons. The salinity of the seawater is 42‰-43‰. After settling on the tideland, the germinating seeds develop as follows: 2 days, roots appear from the tip of the radicle; 4-6 days, the seedling has settled firmly; 14-16 days, it develops its epicotyl and leaves. In a survey on the tideland, 21% of seedlings survived from 515 germinating seeds after 45 days, but most then died during the next 10 months because they were covered by floating algae and plastic rubbish. In comparison, 2.3% of seedlings survived that settled around the community of mother trees, because young trees and pneumatophores of mother trees prevented the algae and rubbish from being deposited on them. These results indicate that seedlings can settle smoothly on an appropriate area of intertidal zone, but they need protection from both natural and artificial stresses to survive.