Abstract. We studied the differentiation diversity (β-diversity or species turnover) patterns of the three main cactus growth forms (columnar, opuntioid and globose) in 318 (1° × 1°) squares covering Argentina. We analysed the degree of association between species turnover of each growth form with the spatial variation of a set of 15 environmental variables.
Species turnover was estimated in two ways: (1) by calculating species turnover along latitudinal and longitudinal gradients and (2) by evaluating the species turnover between each square and its eight surrounding neighbouring grid cells.
For the three growth forms, species turnover in latitudinal transects was mostly related to the mean within-transect values of certain environmental variables, while in longitudinal transects it was related to the variation of some environmental variables within the transect rather than to their mean values. For columnar species, transect species turnover was mainly associated with variation in temperature, confirming the temperature-sensivity of this growth form. For opuntioid species, turnover along transects was mainly related to topographic variables. In the case of globose cacti, transect turnover was associated with variation in temperature and rainfall.
For the three growth forms, areas of high turnover coincided with marked transitions between different biogeographic provinces, while the areas with lowest species turnover coincide with topographically and climatically uniform plains. Species turnover between individual squares was positively associated with the proportion of summer rainfall in globose cacti, the variation of mean annual temperature in columnar cacti and was negatively related to mean annual temperature in opuntioid cacti. Compared to the other growth forms, globose cacti presented a much larger proportion of squares with a high species turnover.
In general, differentiation diversity was lower for the opuntioid and the columnar species, two growth forms with higher dispersal ability and was highest for the globose cacti, which have the lowest dispersal capacity. Environmentally heterogeneous areas, where large-scale transitions between biomes occur, have exceptionally high species turnover, and are important target areas for the conservation of biodiversity.