Variation in the vital rates of an allergenic plant, common ragweed (Ambrosia artemisiifolia L.), across an urban to rural and temperature gradient

Background/Question/Methods

Ten to twenty percent of Americans suffer from seasonal allergies caused by the pollen of common ragweed (Ambrosia artemisiifolia L.). In a previous study, the growth and flowering of A. artemisiifolia sown from seeds of a single population across an urban to rural gradient responded positively to higher urban temperatures and concentrations of ozone and carbon dioxide. Here, 24 study populations were surveyed along a land-use and temperature transect traversing Massachusetts with eight populations each in three groups (i.e., urban/warm, suburban/medium temperature, rural/cool) to better understand the regional variation in the species response to land-use and climate. At each population, 3-5 1m × 1m plots containing 50+ plants were established. The number of flowering plants per plot were recorded weekly and the survival, size, and number of flowers of a subset of 15 tagged individuals were noted biweekly. At the end of the growing season plants from each population were harvested to assess male inflorescence masses and seed set. Phenology data were analyzed with a split plot repeated measures analysis of variance (ANOVA) test, and individual plant data were analyzed with nested ANOVAs to determine effects of land-use/temperature, population, and plot-level variation.

Results/Conclusions

Plants in the rural, cool and suburban, medium temperature sites began flowering earlier than those in the urban, warm sites, suggesting that the allergy season varies in urban and rural areas. Individual survival was not significantly different across the study populations. Individuals in rural, cool populations had smaller crown heights at the beginning and end  of the growing season than plants at urban, warm sites. Plants at rural, cool sites produced a greater number of male inflorescences and had greater seed set than those at warm, urban sites. However, plants at rural, cool sites had lower pollen-producing male inflorescence masses than those at urban, warm sites. These findings suggest that people living in urban Massachusetts may be exposed to more Ambrosia pollen than people living in the rural part of the state.