Hiroshi Nawata

Remote sensing methods for the evaluation of the mesquite tree (Prosopis juliflora) environmental adaptation to semi-arid Africa

In this study a remote sensing approach for the mesquite tree (Prosopis juliflora) control is proposed. The mesquite tree is well known for its high adaptability to arid and semi-arid conditions and characterized by very high water use efficiency. Introduction of the mesquite has caused several environmental problems in Sudan. In this study, to monitor mesquite water use efficiency the concept of a Normalized Difference Infrared Index (NDII), which is defined as the ratio of actual to foliar water content, have been applied and compared with the ground measurements of stomatal conductance (mmol. m-2. s-1), field spectral, volumetric soil water content. As results, on the base of the PALSAR L-band microwave polarimetric backscatter coefficient, the soil moisture (in bare soil area) and surface roughness (in dense forest covered area) could be estimated with a good accuracy for bare-soil surfaces.

Diurnal fluctuation of groundwater levels caused by the invasive alien mesquite plant.

Invasive alien plants such as mesquite (Prosopis juliflora) alter patterns of groundwater uptake. We measured the groundwater level beneath a mesquite stand in an arid area in Sudan. The changes in groundwater level closely followed plant water uptake. The groundwater level started to decline a few hours before sunrise, recovered around noon, and then continued to decline until a few hours after sunset, before recovering again during the night. Thus, groundwater level showed two peaks: just before sunrise and around midday. The midday recovery was due to the depression of photosynthesis by heat and light.

Evaluation of the Effect of a Rain Pulse on the Initial Growth of Prosopis Seedlings

Prosopis juliflora is a useful “multi-purpose” tree, but it invades rapidly in arid and semi-arid environments, causing livelihood degradation of local communities. A rain pulse under drought conditions has been reported to be an important factor to promote plant invasion. The purpose of this research was to evaluate the effect of a rain pulse on the invading process of Prosopis juliflora and to propose a feasible plan to control the expansion of this species in Sudan. First, seed germination was examined under eight irrigation conditions, corresponding to 4–32 mm of rainfall. Most seeds imbibed in all the irrigation conditions, about half or more of the imbibed seeds germinated in 12 mm or more of irrigation. Twenty mm or more of irrigation induced seed emergence. Second, initial growth of germinated seeds was examined under ten irrigation conditions (1–32 mm rainfall) over 54 hours or 138 hours of cultivation. Radicle elongated most vigorously in 24 mm irrigation, and extended over 20 cm in length. Allocation to radicle was dominant, and its elongation was promoted in higher soil moisture condition. Based on these results, we propose that seedlings of P. juliflora should be eradicated within a few weeks after a single large rainfall, which might be convenient and effective to control additional expansion of this species in Sudan.

Intraspecific variation in vocal repertoire among dugong populations

Previous studies have demonstrated that vocal signals facilitate acoustic communication of dugongs. We recorded wild dugong calls from around Talibong Island, Thailand (n = 586) and in Moreton Bay, Australia (n = 331). We also recorded vocalizations of a newborn calf (n = 315) kept at Phuket Marine Biological Center, Thailand, a 19 year old female (n = 73) at Toba Aquarium, Japan, and a 7 year old female (n = 203) at Underwater World, Singapore. Dominant frequency, duration and coefficient of frequency modulation were compared across populations and age. Statistical differences were found for almost all pairwise comparisons (p < 0.05) except between the captive dugongs kept in Japan and also between wild dugongs in Thailand and in Australia. A negative correlation was found between variance of the dominant frequency and dugong age, and a positive correlation was found between variance of the duration and age. The average dominant frequency of wild dugong calls collected in Thailand and in Australia were 5...