SPATIAL PATTERNS IN A 2-TIERED SEMIARID SHRUBLAND IN SOUTHEASTERN SPAIN

Single species and bivariate distribution patterns in a semi-arid shru bland in southeastern Spain, dominated by the tall leguminous shrub Re tama sphaerocarpa, were investigated by second-order spatial analysis based on Ripley's K-function. Shrubs were significantly clumped becaus e of a strong association of dwarf shrubs, mostly Artemisia barrelieri , under the canopy of Retama. Retama shrubs were randomly distributed, but when different size-classes were analysed separately, the pattern changed from significantly clumped to random and then to regular with increasing canopy diameter, suggesting increasing intraspecific compe tition with shrub size. Artemisia was significantly clumped at all sca les because of aggregation under the canopy of large Retama shrubs. Th e association between the species became stronger with increasing cano py diameter of Retama shrubs, suggesting that facilitation prevailed o ver interspecific competition because of niche separation in different tiers, both above and below ground. Retama shrub size thus determined both the type of pattern for its own size class and tier, and the sca le and intensity of the association with its understorey shrubs.