Global environmental change has severe impacts on plants and ecosystems. Anthropogenic carbon emissions, for example, lead to elevated CO₂ (eCO₂) 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 CO₂ emissions are taken up and stored in the biosphere which slows down the growth of CO₂ in the atmosphere and dampens climate change. The stimulation of A_n and g_s by eCO₂ is thought to critically contribute to this carbon uptake. If eCO₂ 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 eCO₂ 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 eCO₂ 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 CO₂. New and innovative tools are therefore needed to understand how eCO₂ 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 eCO₂. 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.

Ansgar Kahmen

ORAL