MECHANISMS OF INTERACTION BETWEEN A LEGUMINOUS SHRUB AND ITS UNDERSTOREY IN A SEMIARID ENVIRONMENT

Retama sphaerocarpa shrubs in semi-arid environments often have a dens e understorey of annual and perennial herbs forming so-called ''island s of fertility.'' The effect of the canopy on soil fertility and micro climate and the combined effect of canopy and litter on species divers ity and productivity were assessed under Retama sphaerocarpa shrubs in a semi-arid environment in southeast Spain. Soil chemical properties differed significantly among three positions under the canopy, particu larly between inner and outer positions. The potential mineralization rate of organic matter was significantly higher in soils from an inter mediate position under the canopy than in soils from either the centre and the edge. Soil chemical fertility and estimated soil seed bank we re highest also in soil at an intermediate position and lowest in soil from the edge. The understorey flora was favoured by the lower temper ature and irradiation and increased soil fertility under the shrubs ca nopies. Species emerging from the soil seed bank separated clearly int o two groups which occupied inner and outer positions under the canopy . Different levels of addition of Retaina litter significantly decreas ed species richness and the number of emerged seedlings. In the field, pots placed near the centre, at an intermediate position and at the e dge of the canopy of Retama shrubs differed significantly in species r ichness and biomass production. Overall, micro-climatic conditions com bined with increased chemical fertility of the soil and inhibitory eff ects of litter to produce a large biomass of herbs at intermediate pos itions between the centre and the edge of the canopy. The high proport ion of litter from annual species in that position increases the miner alization rate and hence nutrient dynamics in a process which also cou ld benefit the shrub.