Precipitation effect on flowering and propagule setting in mangroves of the family Rhizophoraceae

Abstract

Fijian seashores are dominated by Rhizophora samoensis (Hochr.), Rhizophora stylosa Griff. and Bruguiera gymnorhiza (L.) Lam. These three species were studied at two locations—western (dry zone) and eastern (wet zone) parts of Viti Levu (the Main Island of Fiji)—over 2 years consisting of a normal-precipitation and a low-precipitation (drought) year. The research was conducted to understand the flowering pattern, propagule development and propagule size at maturity before dispersal from the mother plant. The flowering pattern and number of propagules at maturity were observed to be significantly different among the three species and within species between a normal-precipitation and very low-precipitation (drought) year. In the drought year (1998), the number of flowers produced per plant and propagule setting were significantly lower than in the normal-precipitation year (1997) at both locations. Only 1–2% of total flowers in each species became mature propagules in the normal-precipitation year. This percentage was significantly lower in the drought year for all three species. Propagule size (weight, length and girth) was found to be significantly different in the three species. R. stylosa produced the biggest propagules, followed by R. samoensis and B. gymnorhiza. Longer and heavier propagules were recorded in the normal-precipitation year than in the low-precipitation year. Flowering patterns, propagule setting and propagule size at maturity are species specific, while differences within species between two zones could be due to different climatic conditions (the amount of rainfall, relative humidity and temperature regimes) prevailing over the 2 years.