Niger tiger bush as a natural water harvesting system

Tiger bush in Niger has been often hypothesised to act as a natural water-harvesting system. The interband:band ratio (IBR) and the related efficiency of the system in terms of water concentration and woody biomass production appeared as crucial questions to be addressed and documented. This study focused on the role of annual rainfall upon the structure and the functioning of the tiger bush system. A regional transect, totalling approximately 200 km in length was established north/south across the rainfall gradient. It included 10 local transects each spanning five interbands and bands. Along the local transects, surface features were carefully recorded as well as woody biovolume. One of these local transects was completely harvested so that biovolume could be related to biomass. Annual woodcutting was estimated after a detailed regional study. In addition to field measurements performed in 1995, the IBR was measured on sequential aerial photographs (1950, 1955, 1962, 1975, 1992), using a 16× binocular magnifier combined with a stage micrometer with 0.1 mm division. A simple model of rainwater redistribution was designed based on surface conditions properties. The best correlation of IBR was obtained with annual rainfall averaged over the last 15 yr (Ra15). This ratio was dramatically increased where Ra15 declined below a threshold of approximately 350 mm. The ratio between the water shedding zone (RZ) and the infiltration zone (IZ), RZ:IZ, based on field crust type survey, was a better predictor for the water-harvesting efficiency of the system than was IBR. The water harvesting model could be used as a satisfactory predictor for woody biomass (R2=0.83). Best simulations were obtained when wood cutting during the last 8 yr was accounted for. The water harvesting and concentration enables wood production which equals that of the forest in much more humid southern zones. The thicket production in the banded patterned systems succeeds in over-compensating the barrenness of the interbands, even exceeding woody biomass of industrial plantations in the same region. Due to the gradual thickening of the infiltration zone, the water-harvesting system becomes no longer effective at the wetter southern end of the tiger bush domain (Ra15=685 mm). Conversely, the decreasing favourable conditions to infiltration should lead to a dry limit of (Ra15=155 mm). Therefore, woody biomass production of the tiger bush is controlled by two opposing trends linked to mean rainfall. The result is that the production of these structures reaches a maximum at 550 mm mean annual rainfall. The sustainable use of tiger bush in the longer term requires the careful maintenance of this natural water harvesting system, with no attempts of afforestation in the interbands, no agriculture, and only moderate wood harvesting in the vegetation bands. Rehabilitation strategies which mimic the natural tiger bush ecosystem are the most appropriate under these climatic circumstances.