Limitations on the climatic and ecological signals provided by the δ~(13)C values of phytoliths from a C_4 North American prairie grass

摘要

Silica phytoliths, which are deposits of opal-A that precipitate in the intra- and intercellular spaces of plant tissues during transpiration, commonly contain small amounts of occluded organic matter. In this paper, we investigate whether the δ~(13)C values of phytoliths from a C_4 grass, Calamovilfa longifolia, vary in response to climatic variables that can affect the carbon-isotope composition of plant tissues. There is no significant correlation (r~2<0.3) between climate variables and the δ~(13)C values of C. longifolia tissues (average δ~(13)C_(tissue)=-13.1±0.6 ‰; n=70) across the North American prairies. However, plant tissue δ~(13)C values are lower for grasses collected in populated areas where the δ~(13)C value of atmospheric CO_2 is expected to be lower because of fossil fuel burning. Phytolith δ~(13)C values are more variable (δ~(13)C=-27.3 to -23.0‰; average=-25.1±1.3‰; n=34) and more sensitive to changes in aridity than whole tissue δ~(13)C values. The strongest correlations are obtained between the δ~(13)C values of stem or sheath phytoliths and humidity (r~2=0.3), latitude (r~2=0.4) and amount of precipitation (r~2=0.5). However, use of these relationships is limited by the wide spread in δ~(13)C values of phytoliths from different plant tissues at the same location. We have been unable to infer any relationship between δ~(13)C values of phytoliths and expected variations in the δ~(13)C values of atmospheric CO_2. The C. longifolia phytoliths are depleted of ~(13)C relative to tissue carbon by 10-14‰. This means that the phytoliths examined in this study have carbon isotopic compositions within the range reported previously for phytoliths from C_3 plants. This observation may further limit the usefulness of soil-phytolith assemblage δ~(13)C values for identifying shifts in grassland C_3:C_4 ratios.