Spatial patterns of plant isotope tracers in the Los Angeles urban region

Plant-environment interactions are difficult to study in urban areas, in part due to the confounding factors that affect physiology, including alterations to atmospheric composition and climate. We wished to determine whether the spatial distribution of urban plant isotope and chemical tracers could be used to distinguish among the many environmental factors that may influence plant physiological processes. We extensively sampled winter annual plants in the region in and surrounding Los Angeles, USA, and analyzed plant material for stable carbon, nitrogen and oxygen isotopes as well as carbon and nitrogen content; and radiocarbon composition. We then overlay maps of the spatial distribution of pollutant, climatic, geographic, and population variables to determine if there were significant relationships. Multiple regression analysis indicated that the radiocarbon content of winter annual plants was strongly related to ozone and carbon monoxide concentrations. Nitrogen isotopes and leaf nitrogen content were related to atmospheric NO₂ and ozone concentrations. Oxygen isotope ratios were correlated with atmospheric vapor pressure deficit and ozone concentrations. These relationships suggest that plant processes are influenced by anthropogenic N uptake and ozone damage in this region. For stable carbon isotopes, spatial variability was correlated with temperature and the distribution of pollutants and point sources, but the functional relationships were less clear. While further studies are needed to confirm the mechanisms, these results highlight the potential for mapping of plant isotopes as a tool for studying complex plant-environment interactions in urban landscapes.