The effects of tree planting on allergenic pollen production in New York City

Planting low-pollen trees could reduce aeroallergen exposure in cities, but the effects of historical tree planting on contemporary airborne pollen exposure remains largely unexplored. Whether tree selection decisions can effectively reduce airborne pollen will depend in part on which taxa are the most important sources of allergenic pollen and the extent to which they are intentionally planted vs. unplanted (i.e., recruiting naturally). Here, we use airborne pollen measurements to examine which taxa are of allergenic importance in New York City (NYC), and then combine two tree surveys with allometric equations to determine pollen production for several taxa. Future changes in pollen production as a function of growth, mortality, and recruitment are then simulated using i-Tree Eco Forecast. The most important producers of allergenic pollen in NYC are Quercus, Platanus, Morus, and Betula; they comprise 71% of airborne pollen measured and 93% of estimated pollen production (107 quadrillion pollen grains; however, pollen production was not estimated for all taxa). Previous municipal planting decisions have resulted in the high abundance of Platanus x acerifolia; it accounts for 34% of total street tree basal area and has an estimated annual pollen production of 27.7 +/- 6.4 quadrillion grains. In contrast, Morus and Betula are generally unplanted, indicating that tree planting decisions will have smaller effects on their future pollen production rates in NYC. Quercus trees produced an estimated 68.2 +/- 12.3 quadrillion pollen grains and accounted for almost 25% of airborne pollen collected, but were very abundant in parks and were mostly unplanted, suggesting that planting has not and will not have a major effect on city-wide pollen production. Overall, this study demonstrates that tree planting decisions can have important and long-lasting consequences for city-wide pollen production but only for species that are both primarily planted and large producers of allergenic pollen.