Historical plant collections provide unique insight into the long-term and in-situ physiological responses of pants to global environmental change

Global environmental change has severe impacts on plants and ecosystems. Anthropogenic carbon emissions, for example, lead to elevated CO 2 (eCO 2 ) in the atmosphere which can stimulate photosynthesis (A n ) and stomatal conductance (g s ) with impacts for carbon, water and nutrient pools and fluxes in terrestrial ecosystems. In fact, approximately 25% of the annual anthropogenic CO 2 emissions are taken up and stored in the biosphere which slows down the growth of CO 2 in the atmosphere and dampens climate change. The stimulation of A n and g s by eCO 2 is thought to critically contribute to this carbon uptake. If eCO 2 will continue to stimulate A n and g s and ecosystem carbon uptake in the future is, however, unclear. This is because important questions regarding the effects of eCO 2 on A n and g s and how these effects are influenced by different plant species or different environmental agents such as nutrient and water availability are unresolved. Experiments are often too short-lived to resolve the complexity of interactions by which eCO 2 affects A n and g s in natural ecosystems. Also, monitoring programs are often not sufficiently long-term to capture in-situ responses of plants to rising atmospheric CO 2 . New and innovative tools are therefore needed to understand how eCO 2 and other global change drivers impact A n and g s in plants and to resolve with this a key uncertainty in the coupled carbon-climate system. The analysis of archived plant material, e.g. in herbarium collections, offers an exciting new opportunitiy to complement experiments and long-term monitoring programmes to reconstruct the long-term in-situ physiological responses of plants to environmental change. In my presentation, we will introduce a new approach that allows for the first time the quantitative reconstruction of A n and g s from the carbon isotope composition and nitrogen content per unit leaf area in archived plant material. I will show how we have applied this new approach to 3000 plant samples from the Herbaria Basel that have been collected across Switzerland from 1850 to today to infer for the long-term in-situ physiological responses of plants to global environmental change. Our data indicate a uniform 20% increase of A n between 1850 and today and a small, yet steady decline in g s . Most interestingly we found very little differences in these responses among different plant functional types or among plants originating from different habitats (wet - dry or nutrient poor - nutrient rich), suggesting a uniform in-situ physiological response of plants to eCO 2 . Our data contributes a new approach to assess the long-term physiological responses of plants to global environmental change and has important implications for modelling past and future carbon and water relations in terrestrial ecosystems.