The objective of this study was to investigate the duration of flowering and fruiting and the growth of riparian species in the natural forest along Phra Prong River, Watthana Nakhon District, Sa Kaeo province. The occurrence of flowering and fruiting was recorded every month from February 2015 to January 2016 (12 months) along a 7km distance of the riversides. Sixty six species belonging to 36 families were seen flowering and fruiting in this study. They were observed during February – April. The peak of flowering and fruiting occurred in March (early summer). Hydnocarpus anthelminthicus, Uvaria rufa, and Combretum latifolium had a long lasting flowering and fruiting period. Oxystelma esculentum, Capparis micracantha, Connarus cochinchinensis, Derris scandens, and Ficus racemosa produced flowers and fruits more than once a year. Those species may play an important role in availability of food resources for animals in the riparian forest. The appropriate time to collect seeds in Phra Prong riparian forest starts from February and lasts until August. The germination test indicated that Xanthophyllum lanceatum and Crateva magna had a high germination rate, about 70%. Growth of some riparian species, including Cinnamomum iners, Dipterocarpus alatus, Hydnocarpus anthelminthicus and Hopea odorata was monitored every three months from July 2015 to June 2016. Diameter growth of D. alatus was significantly higher during the wet season (Jul-Sep) than other seasons. In contrast, the rest of the species showed no significant differences among monitoring periods. Height growth rate of C. iners was significantly highest late in the dry period (Jan-Mar).

Responses of riparian woody species, especially in the tropical forests of Thailand, under flooding condition remain unknown. The effects of flooding on growth of the native species, Hydnocarpus anthelminthicus and Xanthophyllum lanceatum, which dominate in the lowland tropical forests in the East of Thailand, were observed during 16 weeks. The growth and morphological responses were determined in one-year-old seedlings, which stems were submerged at a level of 3 cm above the soil surface (flooded). They were compared to the control (unflooded) at every 2, 4, 8, 12, and 16 weeks. Flooding did not suppress shoot elongation in H. anthelminthicus and X. lanceatum over the study period. In general, leaf, stem, and root biomass were not significantly different between flooded and unflooded seedlings in both species. Adventitious roots were found in flooding seedlings of both species, while hypertrophied lenticels were not formed during the submergences. In addition, senescence, necrosis, abscission, or mortality were not observed in the flooded seedlings in this study. Preliminarily, H. anthelminthicus and X. lanceatum could be considered as potential species for restoring the riparian forest, especially in the studied region.Citation: Moungsrimuangdee B, Waiboonya P, Yodsa-nga P, Larpkern P. Responses to flooding of two riparian tree species in the lowland tropical forests of Thailand.