Phenology and growth in four annual species grown in ambient and elevated CO2

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Abstract
The objectives of this study were (i) to test the hypothesis that changes in phenology with CO2 are a function of the effect of CO2 upon growth and (ii) to determine if CO2-induced changes in phenology can influence competitive outcome. We examined the effect of 350, 525, and 700 μL∙L−1 CO2 on Guara brachycarpa, Gailardia pulchella, Oenothera laciniata, and Lupinus texensis. Plants were grown as individuals in 150-, 500-, or 1000-mL pots and in competition in 1000-mL pots. Growth and development were monitored at twice-weekly intervals by recording the number of leaves and noting the presence or absence of stem elongation, branching, flower buds, and open flowers. Elevated CO2 affected both growth and phenology, but the direction and magnitude of effects varied with species and soil volume. Elevated CO2 did not appear to affect development through its effect on growth. Those treatments in which there were significant effects of CO2 on growth were generally different from those treatments in which CO2 affected phenology. Rather than affecting phenology by changing plant size, CO2 appeared to affect phenology by modifying the size at which plants switched from one stage to the next. The level of CO2 changed competitive outcome; the importance of Lupinus increased whereas that of Oenothera decreased with increased CO2. These changes were more closely related to the effect of CO2 on growth than its effect on phenology. Key words: time of flowering, size at flowering, competition, photoperiod, rate of development.

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