Meta‐analysis reveals profound responses of plant traits to glacial CO 2 levels

A general understanding of the links between atmospheric CO 2 concentration and the functioning of the terrestrial biosphere requires not only an understanding of plant trait responses to the ongoing transition to higher CO 2 but also the legacy effects of past low CO 2 . An interesting question is whether the transition from current to higher CO 2 can be thought of as a continuation of the past trajectory of low to current CO 2 levels. Determining this trajectory requires quantifying the effect sizes of plant response to low CO 2 . We performed a meta‐analysis of low CO 2 growth experiments on 34 studies with 54 species. We quantified how plant traits vary at reduced CO 2 levels and whether C 3 versus C 4 and woody versus herbaceous plant species respond differently. At low CO 2 , plant functioning changed drastically: on average across all species, a 50% reduction in current atmospheric CO 2 reduced net photosynthesis by 38%; increased stomatal conductance by 60% and decreased intrinsic water use efficiency by 48%. Total plant dry biomass decreased by 47%, while specific leaf area increased by 17%. Plant types responded similarly: the only significant differences being no increase in SLA for C 4 species and a 16% smaller decrease in biomass for woody C 3 species at glacial CO 2 . Quantitative comparison of low CO 2 effect sizes to those from high CO 2 studies showed that the magnitude of response of stomatal conductance, water use efficiency and SLA to increased CO 2 can be thought of as continued shifts along the same line. However, net photosynthesis and dry weight responses to low CO 2 were greater in magnitude than to high CO 2 . Understanding the causes for this discrepancy can lead to a general understanding of the links between atmospheric CO 2 and plant responses with relevance for both the past and the future.