Increased C 3 productivity in Midwestern lawns since 1982 revealed by carbon isotopes in Amanita thiersii

How climate and rising carbon dioxide concentrations ( p CO 2 ) have influenced competition between C 3 and C 4 plants over the last 50 years is a critical uncertainty in climate change research. Here we used carbon isotope (δ 13 C) values of the saprotrophic lawn fungus Amanita thiersii to integrate the signal of C 3 and C 4 carbon in samples collected between 1982 and 2009 from the Midwestern USA. We then calculated 13 C fractionation (Δ) to assess the balance between C 3 and C 4 photosynthesis as influenced by mean annual temperature (MAT), mean annual precipitation over a 30 year period (MAP‐30), and p CO 2 . Sporocarp Δ correlated negatively with MAT (−1.74‰ °C −1 , 79% of variance) and positively with MAP (9.52‰ m −1 , 15% of variance), reflecting the relative productivity of C 3 and C 4 grasses in lawns. In addition, Δ values correlated positively with p CO 2 (0.072‰ ppm −1 , 5% of variance). Reduced photorespiration with rising p CO 2 accounted for 20% of this increased Δ, but the remaining 80% is consistent with increased assimilation of C 3 ‐derived carbon by Amanita thiersii resulting from increased productivity of C 3 grasses with rising p CO 2 . Between 1982 and 2009, p CO 2 rose by 46 ppm and the relative contribution of C 3 photosynthesis to Amanita thiersii carbon increased 18.5%. The δ 13 C value of Amanita thiersii may integrate both lawn maintenance practices and the physiological responses of turf grasses to rising CO 2 concentrations.