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

Abstract

The objectives of this study were (i) to test the hypothesis that changes in phenology with CO 2 are a function of the effect of CO 2 upon growth and (ii) to determine if CO 2 -induced changes in phenology can influence competitive outcome. We examined the effect of 350, 525, and 700 μL∙L −1 CO 2 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 CO 2 affected both growth and phenology, but the direction and magnitude of effects varied with species and soil volume. Elevated CO 2 did not appear to affect development through its effect on growth. Those treatments in which there were significant effects of CO 2 on growth were generally different from those treatments in which CO 2 affected phenology. Rather than affecting phenology by changing plant size, CO 2 appeared to affect phenology by modifying the size at which plants switched from one stage to the next. The level of CO 2 changed competitive outcome; the importance of Lupinus increased whereas that of Oenothera decreased with increased CO 2 . These changes were more closely related to the effect of CO 2 on growth than its effect on phenology. Key words: time of flowering, size at flowering, competition, photoperiod, rate of development.