Abiotic site conditions affect photosynthesis rates by changing leaf functional traits

Photosynthesis is a main driver of plant performance and varies between and within species. This study investigates the effects of plant functional traits as well as abiotic site conditions on the intra- and interspecific variability of photosynthetic performance measured via maximum carboxylation capacity (Vcmax) in five widespread species (Campanula glomerata, Centaurea jacea, Plantago media, Salvia pratensis and Trifolium montanum) and on 18 dry calcareous grassland sites across Europe. In addition to that we assessed plant traits associated with plant performance like specific leaf area, leaf nitrogen and carbon status and stable nitrogen isotope content in parallel on each individual. Climate variables, site characteristics and soil nutrients were recorded to test whether abiotic conditions had a direct impact on photosynthesis rates, or whether that influence was mitigated by their impact on the leaf functional traits measured. Leaf functional traits and abiotic site conditions had an influence on Vcmax both, within and between species. However, the results differed between these scales with differences between species, where mainly T. montanum responded differently than the other species. Leaf nitrogen content had the strongest link of all parameters analysed to Vcmax and was positively related to it both, intra- and interspecifically. Slope, soil nitrogen, irradiation and temperature influenced Vcmax yet we found that mainly leaf traits had direct effects on Vcmax when we analysed all traits and site conditions together using structural equation models. However, the indirect effects of abiotic site conditions via changing leaf functional traits were strong. We thus conclude that abiotic site conditions change Vcmax mainly via affecting leaf functional traits, thus representing an indirect effect. This effect has to be considered when using abiotic site conditions and leaf functional traits for forecasting and measuring photosynthesis. © 2021 Gesellschaft für Ökologie