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.