OXYGEN AND CARBON ISOTOPIC VARIATIONS IN COLUMNAR CACTI: ARE RESPONSES TO CLIMATE RECORDED IN SPINES?

The past and future behavior of the North American monsoon (NAM), particularly with respect to times of drought and its relationship to the Pacific-North American (PNA) teleconnection pattern and the El Nino/Southern Oscillation (ENSO) is of great interest to climatologists and water managers. The sparseness of long-term instrumental precipitation and tree ring records in the southwestern United States and northwestern Mexico at low elevations has hindered research on NAM variability at interannual timescales. The long-lived saguaro cacti (Carnegiea gigantea) and other columnar cacti in North and South America have the potential to record climate variability with high temporal and spatial resolution. The vertical sequence of spines on the saguaro’s exterior represents a high resolution (4 to 6 per year), and long (>150 year) record of environmental change.

We present a mechanistic model of isotopic variation that reflects the physiological responses of Saguaro to climate variation over seasonal to century long time-scales and the rationale for a new method to determine the growth rate of columnar cacti using the radiocarbon bomb spike. Our measurements reveal that oxygen and carbon isotopic variation among the sequentially produced and persistent spines covering the saguaro body record fluctuations in saguaro water balance and photosynthetic behavior. The model successfully predicts isotopic variation in spines and constrains controlling variables, yielding a powerful and high-resolution stable isotope index of water stress in the low desert.

Development of an isotopic model for saguaro will serve as the framework for models applied to other species of columnar cacti in North and South America. The tropics have been singled out as an important source of global climate variability at different time-scales. Despite this, their role in global climate change is poorly understood and precise chronologies of tropical climate change are needed to place empirical constraints on competing theories and models. In particular, the use of terrestrial records from columnar cacti in South America could identify ENSO periods in the last century and provide empirical constraints on the inputs of Atlantic (monsoonal) versus Pacific (winter) moisture to the Altiplano during ENSO and other important climatological events.