Beyond global change: lessons from 25 years of CO2 research

Over the past 25 years, countless experiments have been conducted on the impact of increased atmospheric CO2 concentration on various plants and ecosystems. While this research was motivated to better understand and predict how rising CO2 will affect the structure and function of ecosystems in the future, it also shed light on some general, CO2-research independent, aspects in ecological research. Interestingly, it is these general aspects that continue to create confusion and lead to misinterpretation. Here, we focus on seven interrelated key issues including (1) the confusion between fluxes and pools, (2) the stoichiometric aspects of growth and biomass production, (3) resource allocation within organisms, (4) data scaling and the choice of a reference metric, (5) the consideration of time and timing (experimental duration, ontogenetic shifts), (6) confounding and second-order (indirect or feedback) effects, and (7) the key role of biodiversity. The principles deriving from addressing these issues relate strongly to each other. Their concurrent consideration requires experimenters and modellers to likewise maintain a broad, holistic perspective. In this synthesis, we attempt to show how appropriate consideration of these principles can greatly enhance the assessment of the validity, plausibility and generality of experimental and modelling results. We conclude that neglecting to adequately address these key issues in ecological research may lead to overestimations of measured responses and/or simplistic interpretations. Our examples mostly originate from research on plant responses to elevated atmospheric CO2, but are also applicable to other areas of ecological research. We provide a checklist for the planning of ecological field experiments and the interpretation of their results that may help in avoiding common pitfalls.