Relationships between carbon and hydrogen isotope ratios and nitrogen levels in leaves of Clusia species and two other Clusiaceae genera at various sites and different altitudes in Venezuela

Samples of the Clusiaceae genera Clusia, Oedematopus and Dystovomita were collected at various sites and different altitudes in northern and south-western Venezuela. Analyses of stable isotopes of carbon and hydrogen and of leaf-nitrogen levels were performed on the dried samples. Correlations among these variables, i.e. carbon isotope discrimination (A), hydrogen isotope ratio (δD) and N-levels, and with altitude were assessed. In the samples, where values of Δ above 15‰ indicate predominant performance of C₃ photosynthesis, there were slight tendencies of increasing Δ, δD and N-levels with increasing altitude and of increasing Δ with increasing N. Although these correlations taken separately were not statistically significant, they support each other and indicate increasing transpiration and increased leaf-nutrient supply at increasing altitude. Performance of crassulacean acid metabolism (CAM) in species of Clusia appears to be restricted to altitudes below 1500 m a.s.l. There was a significant negative correlation of Δ with altitude in the samples, where values of Δ below 10‰ indicated predominant performance of CAM. This suggests that phases II and IV of CAM are progressively suppressed towards the upper altitudinal limit of CAM in Clusia in northern Venezuela. It is concluded that among the large number of environmental factors and combinations thereof, which determine the expression of CAM in Clusia and trigger C₃-CAM transitions in C₃/CAM intermediate species, low availability of water is the most important.