EFFECTS OF ELEVATED CO2 AND TEMPERATURE ON GROWTH AND ALLOMETRY OF 5 NATIVE FAST-GROWING ANNUAL SPECIES

Whereas much is known of the short-term growth response to elevated at mospheric CO2 concentrations, [CO2](elev), there is relatively little information on how the response of native species is modified by tempe rature, despite the fact that an increase in global mean temperature i s expected to accompany the rise in [CO2]. In this study, five functio nally related annual native species were exposed to different combinat ions 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 a ssessed 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 glasshous es, programmed to track outside ambient conditions and provide a repli cated experimental design. Treatments comprised (i) current ambient [C O2] and temperature, (ii) elevated [CO2] (ambient + 34 kPa), and ambie nt temperature (iii) ambient [CO2] and elevated temperature (ambient 3 degrees C) and (iv) elevated [CO2] and elevated temperature (T degr ees C-elev). All five species responded positively to [CO2](elev), alt hough the response was statistically significant for only one, Poa ann ua L. Averaged over all five species, [CO2](elev) increased total plan t biomass by 25 % (P = 0.005) at 56 d, reflecting a proportionally gre ater increase in leaf and stem mass relative to root weight. Elevated [CO2] had no effect on leaf area, either at the individual species lev el or overall. Elevated T degrees 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 degrees C were observed, with the CO2 response generally greater at elevated than am bient T degrees C. Both [CO2](elev) and T degrees C-elev resulted in a transient increase in relative growth rate, (RGR), during the first 1 4 d exposure and a 3 degrees C increase in temperature had no effect o n the duration of the response. CO2 stimulation of growth operated thr ough a sustained increase in net assimilation rate. (NAR), although th e 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 degrees C-elev was generally opposite o f that to [CO2](elev). For example, T degrees C-elev increased LAR thr ough an increase in SLA and this, rather than any effect on NAR, was t he major factor responsible for the stimulation of RGR. Allometric ana lysis of CO2 effects revealed that changes in allocation observed at i ndividual harvests were due solely to changes associated with plant si ze. Elevated T degrees C, by contrast, had a direct effect on allocati on patterns to leaves, with an increase in leaf area expansion relativ e to whole plant mass during the initial stages of growth and subseque nt increased allocation of biomass away from leaves to other regions o f the plant. No change in the allometric relation between roots and sh oots were observed at either elevated [CO2] or T degrees C. We conclud e, therefore, that allocation of biomass and morphological characteris tics such as SLA, are relatively insensitive to [CO2], at least when a nalysed at the whole-plant level, and where changes have been observed , these are the product of comparing plants of the same age but differ ent size.