Carbon isotopic composition of Ambrosia and Artemisia pollen: assessment of a C₃-plant paleophysiological indicator.

There is limited evidence on how shifts in plant physiological performance influence vegetation variations in the paleorecord. To evaluate δ¹³C of pollen from C₃ plants as an indicator of community-level physiology, small quantities (10-30 grains) of untreated pollen and sporopollenin from herbarium specimens of Ambrosia (A. tomentosa and A. psilostachya) and Artemisia (A. frigida, A. ludoviciana and A. dracunculus), genera abundant in grassland pollen profiles, were isolated by micromanipulation. Their δ¹³C values were measured using a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer. Leaf δ¹³C was also measured. Carbon isotope discrimination (Δ) for untreated pollen, sporopollenin and leaves was compared with historic records of seasonal precipitation amount, vapor pressure deficit and the Palmer Drought Severity Index (PDSI). Each species showed positive correlations between Δ of untreated pollen and sporopollenin. Sporopollenin Δ was most strongly correlated with PDSI. Correlations among leaf Δ and moisture indicators were stronger for Ambrosia than Artemisia. These results suggest that sporopollenin Δ indicates the level of moisture stress in C₃ plants. Therefore, δ¹³C analysis of pollen promises to help address important paleoecological questions, such as how community-level physiology contributes to shifts in vegetation composition.