Generalities in the growth, allocation and leaf quality responses to elevated CO₂ in eight woody species

Abstract

This paper reports general patterns of relative growth rate and related traits in response to elevated atmospheric CO₂ in eight woody species ranging widely in life form, leaf habit, taxonomy and ecology. Young plants of these species, all of comparable ontogenetic phases, were grown simultaneously in large containers with favourable nutrient and water availability in transparent outdoor chambers at 350 and 700 μl l⁻¹ CO₂ for one growing season. We found the following consistent responses. (1) All species grew faster at elevated CO₂, whereas the following leaf and allocation traits were consistently lower in CO₂-enriched environments: specific leaf area (quotient of leaf area and leaf weight), leaf area ratio (quotient of total leaf area and plant weight), weight-based foliar N concentration and, to a smaller extent, leaf weight fraction (quotient of leaf weight and plant weight). (2) There was important interspecific variation in the magnitude of the response of relative growth rate to CO₂. Specific leaf area at ambient CO₂ explained 88% of the variation in relative growth rate response to CO₂ among the eight species. At ambient CO₂, relative growth rate itself, was significantly correlated with the relative growth rate response to CO₂ only if the leafless species Ulex gallii was excluded from analysis. (3) The four deciduous species had a significantly stronger relative growth rate response to CO₂ than the four evergreens. This corresponded with their generally higher specific leaf area. (4) Specific leaf area and leaf habit might be useful for scaling up exercises, as easy-to-measure substitutes for growth responses of (woody) vegetation to elevated CO₂. However, the usefulness of such traits in this context needs to be tested in realistic, longer-term manipulative experiments in real ecosystems.