Relationships between soil moisture and growth of herbaceous plants in a natural vegetation mosaic in Niger

The 'tiger bush' is a particular vegetation pattern in which densely vegetated bands alternate with bare areas. In general, bare areas are working as impluvium for the downslope vegetated stripes. The intraseasonal regimes of soil moisture are quantified during a rainy season, and their influence on the growth of the annual vegetation is analysed. Data were collected along the line of maximum slope crossing two vegetated bands. Density and phenology of herbaceous layer are related to the changes of soil moisture and to the associated soil surface features. The herbaceous layer comprises only annual species, mainly composed of the tiny grass Microchloa indica and Cyanotis lanata (Commelinaceae). Both are indicators of long-term overgrazing. The distribution of Microchloa indica is well correlated with the sedimentary crusts located immediately upslope of the thicket. Cyanotis lanata is the only species recorded in the downslope part of the transect. Its distribution is well correlated with the biological crusts located in the core of the thicket, where infiltration first occurs and high infiltration rates are seen, as long as shade and litter are not limiting for seedling development. As the rainy season progresses, infiltration also increases upslope of the thicket which, until then, remains bare and where only Microchloa indica germinates provided the seed stock is not limiting. (C) 1997 Academic Press Limited.