58 Weinberger et al.: A Review of Spatial Variation of Allergenic Tree Pollen Within Cities pollen varies spatially within cities. These findings were consistent across cities in different geographic regions and across different sampling method- ologies. In addition, several studies presented evi- dence that the distribution of urban trees is a driver of spatial variation in tree pollen within cities. SEARCH METHODOLOGY The study authors developed a search strategy to identify studies that characterize intra-urban spatial variation in tree pollen. The authors conducted a search in PubMed, an electronic citations database of medical literature, using the following search terms: pollen AND [spatial* OR spatio* OR geography* OR site* OR location* OR neighborhood* OR station*] The authors did not restrict the years cov- ered in the search. The authors screened the articles returned by the search first by title, then by abstract, and finally by the full text for inclu- sion in the review using the following criteria: 1) article is available in English and published online or in print in a peer-reviewed journal on or before February 1, 2014; and 2) tree pollen monitoring was conducted at a minimum of two stationary sites within a single metropoli- tan area. Due to the lack of previous reviews of this topic, the authors did not place limits on the length of pollen sampling, the species considered (e.g., allergenic versus non-allergenic), or the pollen sampling methodology and equipment. The PubMed search yielded 3,200 articles. Out of these 3,200 articles, the authors excluded 3,072 based on the title, 87 based on the abstract, and 34 based on the full text, leaving seven articles for inclusion in the review. An addi- tional four articles were identified for inclusion from the reference lists of the included articles from the PubMed search, for a total of 11 arti- Location cles. Finally, as not all studies of spatial variation in tree pollen may be included in the PubMed database, Web of Knowledge, a scientific cita- tions database, was used to identify articles citing these 11 papers. This resulted in the inclusion of an additional two articles, for a final total of 13. RESULTS Regional Distribution of Studies Out of the 13 studies identified for inclusion in this review, eight were conducted in European cities. Celenk et al. (2010) included sampling sites in both the European and the Asian parts of Istanbul, Turkey. Three studies were conducted in the United States, one in Japan, and one in Australia. The time of year at which tree pollen is present (i.e., the tree pollen season) is region- dependent, due in part to differences in climate. In temperate regions, the tree pollen season typi- cally begins in early spring and ends before the beginning of summer (Singh and Mathur 2012). Individual tree pollen taxa tend to be present for a more limited time window within the tree pollen season. For example, in the northeastern United States, the pollen of the cypress family (Cupressaceae) is present in March and April, while oak pollen (Quercus spp.) is present from late April through the end of May (Dvorin et al. 2001). Additionally, the taxa that contribute to the tree pollen season vary from region to region. Regional variations in the tree pollen season contribute to differences across included stud- ies in terms of the taxa studied and the timing and length of the sampling period, as detailed in Table 2. Most studies sampled pollen for a single tree pollen season or a single year. Studies that monitored pollen for a full year typically did so in order to capture non-arboreal pollen taxa. While the specific taxa examined depend on the region, Table 1. Selected tree-planting initiatives with unknown effects on health. Initiative MillionTreesNYC Million Trees LA Plant One Million Mile High Million Green Miami Tree Baltimore Million Tree Challenge Shanghai Roots & Shoots New York City, New York, U.S. Los Angeles, California, U.S. Pennsylvania, New Jersey, and Delaware, U.S. Denver, Colorado, U.S. Miami, Florida, U.S. Baltimore, Maryland, U.S. London, Ontario, Canada Shanghai, China Goal 1,000,000 trees 1,000,000 trees 1,000,000 trees 1,000,000 trees 30% tree canopy 40% tree canopy 1,000,000 trees 1,000,000 trees ©2015 International Society of Arboriculture
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