Distribution of ornamental urban trees and their influence on airborne pollen in the SW of Iberian Peninsula

Ornamental trees in urban environment are a relevant cause of illness due to allergenic pollen airborne dispersed. The work presented here analyses the importance of ornamental tree distribution and investigates airborne pollen records using volumetric spore traps. Three localities in the southwest of Spain were studied: Don Benito (DB), Plasencia (PL), and Zafra (ZA), in the Extremadura region. Volumetric Hirst type spore traps recorded pollen levels continuously for three years. Ornamental street trees in the study area were identified, counted and mapped. Airborne pollen records were found to be related to the number of trees, the proximity of the trees to traps, the different pollination systems and wind direction. A total of 17635 trees, from all localities were counted, belonging to 67 species, 58% anemophilous, 22% entomophilous, and 20% both pollination systems. Five species represented more than 50% of the total number of ornamental trees: Platanus hispanica, Olea europaea, Ulmus minor, Acer negundo and Melia azedarach. The average total pollen concentration in g m−3 was 102 (DB), 93 (PL), and 99 (ZA)… Omitting the data for Quercus pollen, a statistically significant correlation between the number of trees at 0.5 km around the spore traps and the types of pollen recorded was found. Some measures to decrease the amount of potential allergenic airborne pollen in urban environments were identified, such as reducing trees with anemophilic pollination systems, eliminating male trees and employing suitable pruning practices. In any case, the closer the source of ornamental pollen, the greater the allergenic risks.