Long-term physiological responses of herbaceous plants to global change from carbon and oxygen isotopes in herbarium specimen

The analysis of the isotopic composition in herbarium specimens gives insight into the physiological responses of plants to environmental change. Specifically, the carbon isotope composition of plants is linked to the time-integrated, leaf-level intrinsic water use efficiency (i.e., the ratio of net photosynthesis over stomatal conductance), while the oxygen isotope composition is linked to leaf stomatal conductance. Thus, by combining stable isotope analyses of carbon and oxygen, trends in integrated values for net photosynthesis and stomatal conductance can be determined. Here, we present results from analyses of carbon and oxygen isotope values from over 3000 sampled herbarium specimens, that have been collected across Switzerland over the past century covering more than 80 herbaceous plant species and a wide range of habitats. While plants across most taxa and habitats have increased their intrinsic water use efficiency over the last decades, the contribution of net photosynthesis or stomatal conductance to changes in intrinsic water use efficiency differs among different plant functional groups (herbs, legumes, grasses, and sedges). Applying a multi-model approach, our study demonstrates that the carbon and water relations of plants respond to long-term changes in the environment but that the physiological nature of these responses differs among plant functional groups and the plant’s ecological niche.