The altitudinal climatic effect on the stable isotope compositions of Agave and Opuntia in arid environments - A case study at the Big Bend National Park, Texas, USA

Several environmental factors potentially influence the stable isotope compositions of plants, depending on species and local environment. Although several studies have discussed these aspects in C-3 and C-4 plants, only a few have attempted to identify the major drivers of delta C-13, delta O-18, and delta N-15 in CAM plants. In this study, stable isotopic ratios of CAM plants collected along an altitudinal transect in the Chisos Mountains/USA during two growth seasons, are used to understand major factors governing isotopic variability. A strong and significant relationship between delta C-13 and altitude, positive for agave at <1700 m and negative for both agave and cacti at >1500 m, suggests variable carbon isotope fractionation at different altitudes, which may be unique to CAM plants. Our data suggest that temperature primarily governs delta C-13 of the studied plants. The significant negative correlations observed between the delta O-18 of agaves and the recorded air temperature (0.59 parts per thousand/degrees C) are similar to the delta O-18 of precipitation vs. temperature variability in global data. The delta N-15 variations suggest a similar N-source for both agaves and cacti, and reflect progressive changes in the dominant N-source across desert-grassland-woodland zones. We conclude that the stable carbon and oxygen isotope composition of agave reflects the climatic conditions along the elevation transect. (C) 2013 Elsevier Ltd. All rights reserved.