What have we learned from 15 years of free‐air CO2 enrichment (FACE)? A meta‐analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2

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Abstract
Summary: Free‐air CO2 enrichment (FACE) experiments allow study of the effects of elevated [CO2] on plants and ecosystems grown under natural conditions without enclosure. Data from 120 primary, peer‐reviewed articles describing physiology and production in the 12 large‐scale FACE experiments (475–600 ppm) were collected and summarized using meta‐analytic techniques. The results confirm some results from previous chamber experiments: light‐saturated carbon uptake, diurnal C assimilation, growth and above‐ground production increased, while specific leaf area and stomatal conductance decreased in elevated [CO2]. There were differences in FACE. Trees were more responsive than herbaceous species to elevated [CO2]. Grain crop yields increased far less than anticipated from prior enclosure studies. The broad direction of change in photosynthesis and production in elevated [CO2] may be similar in FACE and enclosure studies, but there are major quantitative differences: trees were more responsive than other functional types; C4 species showed little response; and the reduction in plant nitrogen was small and largely accounted for by decreased Rubisco. The results from this review may provide the most plausible estimates of how plants in their native environments and field‐grown crops will respond to rising atmospheric [CO2]; but even with FACE there are limitations, which are also discussed. Contents Summary 351 I. What is FACE? 352 II. Materials and methods 352 III. Photosynthetic carbon uptake 353 IV. Acclimation of photosynthesis 356 V. Growth, above‐ground production and yield 358 VI. So, what have we learned? 360 Acknowledgements 361 References 361 Appendix 1. References included in the database for meta‐analyses 364 Appendix 2. Results of the meta‐analysis of FACE effects 368

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