Distribution of plant species in Israel in relation to spatial variation in rainfall

Relations between the distribution of herbaceous and woody species from the flora of Israel and the variation (25 -900 mm) in mean annual rainfall are described using a grid-based approach integrating multivariate techniques (cluster analysis and DCA-ordination) and GIS. The analysis was based on an extensive data base of vascular plant records in 10 km x 10 km grid cells representing the climatic zones of Israel. Cluster analysis revealed three geographically distinct clusters of grid cells which could be arranged alon g the main rainfall gradient in the study area. The main subdivision is bet ween cells from the Mediterranean region and cells from the more arid regio n. The second subdivision separated cells from the arid region into two dis tinct groups which differed significantly in rainfall. Clustering of herbac eous and woody species was similar, but boundaries separating adjacent clus ters based on woody species were consistently linked to more rainy areas. C ell scores on the first DCA-axis were significantly correlated with mean an nual rainfall, but this relationship was not linear. Log-transformation of the rainfall data resulted in a higher correlation between the DCA-scores a nd rainfall, indicating that variation in rainfall had a stronger effect on species composition in relatively dry than in more mesic regions. Slopes o f linear regression models relating cell scores on the first DCA-axis to me an annual rainfall, as well as the corresponding R-2-values, were highest i n the desert, intermediate in the dry Mediterranean, and lowest in the mesi c Mediterranean territory. These results indicate that both the per-unit ef fect of rainfall on plant species composition and its relative importance a s a determinant of compositional variation decreased from relatively dry to more mesic regions. The length of the first axis, as well as the slopes of regression models re lating cell scores on the first DCA-axis to mean annual rainfall were consi stently higher in the case of woody species which are characterized by a hi gher rate of compositional variation. Due to lack of comparable data, it is difficult to evaluate whether our results can be generalized to other regi ons. However, an analysis of data obtained from different parts of the worl d points to a considerable variability in the degree to which rainfall affe cts patterns of floristic variation. Our findings that compositional respon ses to rainfall may vary between different groups of species and may depend on the part of the rainfall range which is being considered may partially explain this inconsistency.