Population dynamics and trace metal biogeochemistry of the saguaro cactus (Carnegiea gigantea) in the Sonoran desert

Abstract

:The purpose of this study was to determine if smelter-derived air pollution in the form of trace metal toxicity could be responsible for either the apparent population decline or the epidermal browning of the saguaro cactus (Carnegiea gigantea) within the Saguaro National Monument (SNM). Population density and distribution measurements, estimates of epidermal browning, and tissue major ion and trace metal analyses were conducted in nine sites across the entire climatic range of Carnegiea as comparisons to patterns observed in SNM. Carnegiea from industrialized sites compared to pristine sites did not have statistically greater browning. DTPA-extractable copper (Cu) was significantly elevated in soils from two sites near copper smelters. However, tissue Cu concentrations were not at levels considered phytotoxic, and neither soil nor tissue Cu was significantly related to browning indices or population density measurements. No element showed any significant correlation to epidermal browning. The fact that tissue chemical indicators of photosynthetic enzyme and photosynthetic activity (N, Mg, and Fe in chlorenchyma) were not significantly related to browning suggests that epidermal browning may begin as a cuticular damage event, rather than from a tissue toxicity event that would damage the photosynthetic process. Densities of Carnegiea were greatest at both districts of SNM, with population densities at other regional sites being significantly lower. Our population analysis suggests that the Carnegiea population in SNM at the time of the establishment of the Monument was unusually dense; no present population had similar densities. Results suggest that trace metal pollution may not be causing premature Carnegiea death in SNM, and that browning may be a natural process that is unrelated to the population decline process.