Soil salinity determines the relative abundance of C3/C4 species in Argentinean grasslands

Aim The C-4 and crassulacean acid metabolism (CAM) pathways are adaptations to compensate for high rates of photorespiration and water and carbon deficiency. This is the first attempt to compare the relative abundance of C-3 vs. C-4 + CAM species in temperate and subtropical grasslands across a latitudinal gradient in central Argentina. We predict that under the same rainfall regime, C-4 + CAM plants will have larger soil coverage in highly saline soils than in neighbouring non-saline ones. Location Data were taken from three phytogeographical provinces in the Santa Fe province of Argentina: Chaquenian, Pampean and Espinal. Methods The salinity of the soil was estimated through aqueous solution conductivity. The proportions of species belonging to C-3/C-4 + CAM photosynthetic pathways were compared among halophyte and non-halophyte communities with a chi(2) homogeneity test. The sum of cover percentages corresponding to the C-3 and C-4 + CAM photosynthetic pathways were calculated and compared using analysis of variance (ANOVA). Results The soil conductivity values were higher in the halophyte than in the non-halophyte communities for the same phytogeographical area. The C-4 + CAM plants had much higher soil coverage values in halophyte than in non-halophyte communities in the Pampean and Espinal phytogeographical provinces. The differences were not statistically significant in the Chaquenian province. Main conclusions Soil drought provoked by soil salinity results in a much higher soil cover by C-4 + CAM plants in regions with positive to neutral water balance (i.e. Pampean and Espinal). This differential abundance pattern in C-4 + CAM functional group is not observed in areas where a pronounced water deficit exists per se (Chaquenian region), and therefore C-4 + CAM plants predominate in all environments regardless of soil salinity. Our results suggest that one of the main environmental forces driving the upsurge of C-4 species in Argentinean grasslands might have been the strong local soil salinity gradient.