Uprooting resistance of two tropical tree species for sand dune stabilization

Coastal windbreak restoration is important in Taiwan for agroforestry and sand dune stabilization. Australian pine (Casuarina equisetifolia Forst.) is the main species in windbreaks. It often suffers from serious uprooting and waterlogging damages, whereas sea hibiscus (Hibiscus tiliaceus L.) is more resistant to wind and tolerant to waterlogging. It is suggested that sea hibiscus can be substituted for Australian pine in coastal windbreak restoration. However, the adaptive mechanism of its root system to wind is not well understood. In this study, a field experiment was conducted to investigate the anchorage capabilities and root morphology of 10-year-old Australian pine and sea hibiscus plants. The results showed that root system morphologies of Australian pine and sea hibiscus plants belonged to taproot system and heart system, respectively. Root systems of both species were distributed towards northeast and southwest, which coincided with the monsoon directions. Sea hibiscus plants had significantly larger root collar diameter, longer taproot length, larger root biomass and shoot biomass than that of Australian pine plants. Additionally, sea hibiscus plants had significantly larger root volume than Australian pine plants. Moreover, sea hibiscus developed significantly stronger root functional traits, that is, root density (245%), root tissue density (300%) and the root to shoot ratio (138%) than Australian pine plants. Consistently, the root maximum uprooting resistance of sea hibiscus plants was significantly higher than that of Australian pine plants. These results demonstrate that sea hibiscus plants have stronger anchorage capability and they are more suitable for windbreak restoration and sand dune stabilization.   Key words: Anchorage, pullout, root system morphology, uprooting resistance.