Reforestation of native Acacia confusa Merr. on landslide areas in Taiwan is important for agroforestry and soil conservation. To ensure high survival and growth vigor, A. confusa seedlings must develop a strong root system. Inoculating of acacia with symbiotic nitrogen-fixing bacteria (NFB) may ameliorate the problems associated with soil nutrient deficiency on landslide sites. In this study, under plastic house condition, a NFB was isolated from the root nodules of native A. confusa and identified as Bradyrhizobium elkanii, and its effects on growth, root system morphology and pullout resistance of acacia seedlings were investigated. Our results revealed that the growth of inoculated seedlings is significantly more vigor than that of the noninoculated controls. The enhancements in height, tap root length, shoot biomass and root biomass were 40, 100, 140 and 130%, respectively. Also, inoculated seedlings had significantly longer total root length (150%), larger external root surface area (130%), larger root volume (70%), and more root tip number (60%) than the controls. Moreover, the inoculated seedlings developed significantly stronger root functional traits, that is, root density (130%), root length density (60%) and specific root length (60%), than the controls. Consistently, the root pullout resistance of inoculated seedlings was significantly higher than that of the noninoculated ones. These results demonstrate that B. elkanii is an effective nitrogen-fixing bacterium capable of enhancing growth, root development and pullout resistance of A. confusa.

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.

Restoration of Alnus formosana (Burk.) Makino on landslide areas is important for agroforestry, forestry and soil erosion control in Taiwan. To ensure successful reforestation, A. formosana seedlings have to develop strong root system for nutrient and water acquisition as well as anchorage. Inoculating of A. formosana with symbiotic nitrogen-fixing actinobacteria Frankia may help mitigate drought and nutrient deficiencies on landslide sites. However, the effects of Frankia inoculation on growth, root architecture and mechanical properties of A. formosana seedlings are not well understood. In this research, a Frankia strain AF1 was isolated from actinorhizal nodules of local A. formosana and recognized as Frankia species, and its influences on growth performance and root mechanical properties of A. formosana seedlings were examined and analyzed. The results showed that the inoculated seedlings had significantly larger height and root biomass, longer root length, and more root tip number than that of the non-inoculated controls. Consistently, the inoculated seedlings had statistically significant higher uprooting resistance, root tensile resistance force and tensile strength than the controls. The results reveal that this native Frankia strain can promote growth performance, root system architecture, anchorage ability and root tensile strength of A. formosana.