FACE facts hold for multiple generations; Evidence from natural CO₂ springs

Rising atmospheric CO₂ concentration is a key driver of enhanced global greening, thought to account for up to 70% of increased global vegetation in recent decades. CO₂ fertilization effects have further profound implications for ecosystems, food security and biosphere-atmosphere feedbacks. However, it is also possible that current trends will not continue, due to ecosystem level constraints and as plants acclimate to future CO₂ concentrations. Future predictions of plant response to rising [CO₂ ] are often validated using single-generation short-term FACE (Free Air CO₂ Enrichment) experiments but whether this accurately represents vegetation response over decades is unclear. The role of transgenerational plasticity and adaptation in the multigenerational response has yet to be elucidated. Here, we propose that naturally occurring high CO₂ springs provide a proxy to quantify the multigenerational and long-term impacts of rising [CO₂ ] in herbaceous and woody species respectively, such that plasticity, transgenerational effects and genetic adaptation can be quantified together in these systems. In this first meta-analysis of responses to elevated [CO₂ ] at natural CO₂ springs, we show that the magnitude and direction of change in eight of nine functional plant traits are consistent between spring and FACE experiments. We found increased photosynthesis (49.8% in spring experiments, comparable to 32.1% in FACE experiments) and leaf starch (58.6% spring, 84.3% FACE), decreased stomatal conductance (g_s , 27.2% spring, 21.1% FACE), leaf nitrogen content (6.3% spring, 13.3% FACE) and Specific Leaf Area (SLA, 9.7% spring, 6.0% FACE). These findings not only validate the use of these sites for studying multigenerational plant response to elevated [CO₂ ], but additionally suggest that long-term positive photosynthetic response to rising [CO₂ ] are likely to continue as predicted by single-generation exposure FACE experiments.