Effects of elevated CO₂ and temperature on growth and allometry of five native fast-growing annual species

Whereas much is known of the short-term growth response to elevated atmospheric CO₂ concentrations, [CO₂ ]_elev , there is relatively little information on how the response of native species is modified by temperature, despite the fact that an increase in global mean temperature is expected to accompany the rise in [CO₂ ]. In this study, five functionally related annual native species were exposed to different combinations of ambient and elevated [CO₂ ] and temperatures in order to assess their response in terms of growth and allometry. Fast-growing annuals were selected for the study because their growth responses could be assessed over a major portion of the plant's life cycle and in as short a period as 8 wk. Plants were grown in eight hemi-spherical glasshouses, programmed to track outside ambient conditions and provide a replicated experimental design. Treatments comprised (i) current ambient [CO₂ ] and temperature, (ii) elevated [CO₂ ] (ambient+34 kPa), and ambient temperature (iii) ambient [CO₂ ] and elevated temperature (ambient+3°C) and (iv) elevated [CO₂ ] and elevated temperature (T°C_elev ). All five species responded positively to [CO₂ ]_elev , although the response was statistically significant for only one, Poa annua L. Averaged over all five species, [CO₂ ]_elev increased total plant biomass by 25% (P=0·005) at 56 d, reflecting a proportionally greater increase in leaf and stem mass relative to root weight. Elevated [CO₂ ] had no effect on leaf area, either at the individual species level or overall. Elevated T°C, by contrast, had little effect on shoot growth but increased root mass on average by 43% and leaf area by 22%. Few interactions between elevated [CO₂ ] and T°C were observed, with the CO₂ response generally greater at elevated than ambient T°C. Both [CO₂ ]_elev and T°C_elev resulted in a transient increase in relative growth rate, (rgr), during the first 14 d exposure and a 3°C increase in temperature had no effect on the duration of the response. CO₂ stimulation of growth operated through a sustained increase in net assimilation rate. (nar), although the potential benefit to rgr was offset by a concurrent decline in leaf area ratio (lar), as a result of a decrease in leaf area per unit leaf mass (sla). The response to T°C_elev was generally opposite of that to [CO₂ ]_elev . For example, T°C_elev increased lar through an increase in sla and this, rather than any effect on nar, was the major factor responsible for the stimulation of rgr. Allometric analysis of CO₂ effects revealed that changes in allocation observed at individual harvests were due solely to changes associated with plant size. Elevated T°C, by contrast, had a direct effect on allocation patterns to leaves, with an increase in leaf area expansion relative to whole plant mass during the initial stages of growth and subsequent increased allocation of biomass away from leaves to other regions of the plant. No change in the allometric relation between roots and shoots were observed at either elevated [CO₂ ] or T°C. We conclude, therefore, that allocation of biomass and morphological characteristics such as sla, are relatively insensitive to [CO₂ ], at least when analysed at the whole-plant level, and where changes have been observed, these are the product of comparing plants of the same age but different size.