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 v egetated bands alternate with bare areas. In general, bare areas are w orking as impluvium for the downslope vegetated stripes. The intraseas onal regimes of soil moisture are quantified during a rainy season, an d their influence on the growth of the annual vegetation is analysed. Data were collected along the line of maximum slope crossing two veget ated bands. Density and phenology of herbaceous layer are related to t he changes of soil moisture and to the associated soil surface feature s. The herbaceous layer comprises only annual species, mainly composed of the tiny grass Microchloa indica and Cyanotis lanata (Commelinacea e). Both are indicators of long-term overgrazing. The distribution of Microchloa indica is well correlated with the sedimentary crusts locat ed immediately upslope of the thicket. Cyanotis lanata is the only spe cies recorded in the downslope part of the transect. Its distribution is well correlated with the biological crusts located in the core of t he thicket, where infiltration first occurs and high infiltration rate s are seen, as long as shade and litter are not limiting for seedling development. As the rainy season progresses, infiltration also increas es upslope of the thicket which, until then, remains bare and where on ly Microchloa indica germinates provided the seed stock is not limitin g. (C) 1997 Academic Press Limited.