Increases in CO2 from past low to future high levels result in “slower” strategies on the leaf economic spectrum.

Depending on resource availability plants exhibit a specific suite of traits. At the interspecific level these traits follow the leaf economic spectrum (LES), traits related to slow turnover when resources are poor and fast turnover when resources are plentiful. Limited data shows that within species, CO 2 availability, low in the recent geologic past, high in the near future, has led to plants shifting their trait levels on the LES towards faster traits. We asked whether adjustments of physiological traits could underpin faster growth from low to high CO 2 and how these responses varied among plant functional types. We analysed the trait response of seedlings of up to 28 C3 plant species grown at low (160 ppm), near-ambient (450 ppm), and high (750 ppm) CO 2 . We measured growth, specific leaf area (SLA), leaf gas exchange, chemical composition and stomatal traits. On average photosynthesis was reduced by 59% at low CO 2 and increased by 14% at high CO 2 compared to ambient CO 2 . Respiration decreased by 21% at low CO 2 and increased by 39% at high CO 2 . Nitrogen content (N) per mass increased by 50% at low CO 2 and decreased by 9% at high CO 2 . Plants drastically increased SLA at low CO 2 so that despite lower carbon gain per area, carbon gain per unit mass was not reduced as much. Contrary to the responses to other resources, plant traits along the LES are adjusted towards the “fast” end of the spectrum (higher SLA, higher N) at low CO 2 and towards the “slow” end (lower SLA, lower N) with increasing CO 2 . For a limited number of species photosynthesis per unit mass showed the same, increase at low CO 2 . From a resource economics perspective plants thus adjust the cost for growth towards the availability of carbon and the rate of assimilation: at lower CO 2 the carbon costs decrease due to decreased respiration and lower leaf mass per area (higher SLA thinner leaves). At higher CO 2 the carbon costs increase due to increased respiration and higher leaf mass per area (lower SLA thicker leaves). This suggests that CO 2 increases from the past to the future are allowing plant species globally to combine faster growth with more robust, resource conservative leaves. [ABSTRACT FROM AUTHOR]