Predicting future plant phenology and distribution: Does ecotype matter?

Background/Question/Methods

There is growing evidence that plants are flowering earlier in response to increasing temperatures and associated global change factors.   Rising levels of atmospheric CO₂ can also affect phenology by altering the carbon balance and other ecophysiological aspects of plant growth and reproduction.   Whether or not geographically distinct populations of a single, widespread species converge or differentiate in these phenological shifts is less understood.  Phenology determines a number of key processes such as mating, competitive ability, and size at reproduction, all of which influence reproductive success.  Thus, shifts in phenology can ultimately have important implications for future plant species distributions. In this study we tested for ecotypic variation in the reproductive and growth responses of the widespread allergenic weed, Ambrosia artemisiifolia (common ragweed) to elevated levels of carbon dioxide.  We collected wild-grown seed from three points along a latitudinal gradient in New England (Vermont, Massachusetts, and New York), sowed the seeds in pots, and grew the plants in a common garden experiment under experimental levels of 400 (ambient), 600, and 800 ppm CO­₂. 

Results/Conclusions

Ecotype and carbon dioxide both significantly influenced the timing of flowering.  Reproductive onset occurred earlier with increasing latitude, with concurrent increases in reproductive allocation. Elevated carbon dioxide also resulted in earlier reproductive onset in all ecotypes.  In the northern populations of Vermont, the reproductive onset was most impacted by elevated carbon dioxide. While elevated carbon dioxide did not impact length of flowering season, there was a positive correlation of flowering duration with latitude. Total life cycle length was negatively correlated with latitude, which resulted in more of the life cycle spent in the reproductive stage as latitude of source population increased.   We interpret our findings as evidence for ecotypic varation in ragweed with increased allocation to reproduction likely due to underlying genetic variation in CO2 response.  This increased investment in reproduction at northern latitudes could result in increased seed and pollen dispersal and possible enhanced range expansion compared with ecotypes in the mid and lower latitudes.  As a result the ecological and human health implications of common ragweed's response to global change are likely dependent upon provenance.