The effects of urbanization and land use on ragweed distribution

Quantifying the spatial distribution of allergenic plants within landscapes could help target management efforts and predict human exposure to allergens at a scale relevant to human activity. We analyzed the combined effects of urbanization (proxied by surrounding impervious surface area) and land use on the distribution of common ragweed (Ambrosia artemisiifolia, Asteraceae), a major contributor to hay fever and asthma through its production of allergenic pollen. Along urban-to-rural transects in three regions of the northeastern United States, we recorded the presence of ragweed in 308 plots with different land uses: agriculture, developed, and forest. We also analyzed the effects of plot-level soil moisture, soil carbon-to-nitrogen ratio, and canopy cover on ragweed occurrence. Ragweed varied among land uses, but the effect of land use depended on the level of urbanization. The probability of ragweed occurrence was highest in urban forests, rural developed areas, and agricultural areas. Plot-level abiotic variables were not related to ragweed occurrence. Our results suggest that ragweed distributions are influenced to a greater extent by landscape-scale processes, such as propagule pressure and disturbance, than by macro-environmental effects of urbanization. From a human health perspective, our results suggest that ragweed pollen levels vary within and between urban and non-urban landscapes, which current pollen monitoring and forecasting efforts do not capture.