High elevation does not predispose alpine plants to be more productive under elevated CO 2

In a recent publication by Chen et al. (Ecology Letters), it has been suggested that photosynthetic responses to elevated CO2 (termed CO2 fertilisation effect, CFE) resulted in increased gross primary productivity (GPP) and have boosted above-ground biomass in an alpine sedge-dominated grassland at 4585 m elevation in Tibet. Here, we offer a critical assessment of the experiment’s outcome reported by these authors. We note that photosynthesis was not measured in the study. Calculated as GPP, the increased flux was entirely due to increased ecosystem respiration, inappropriately assessed by short-term darkening of vegetation during bright daylight hours. We conclude that the data presented do not permit making a case for CFE on biomass production in such alpine grasslands. By what is known, these natural ecosystems are not carbon limited.