62 Weinberger et al.: A Review of Spatial Variation of Allergenic Tree Pollen Within Cities exceptions. Olive, sycamore, and the nettle family (Urticaceae) were higher in the southwest, while oak, cypress (Cupressus spp.), and poplar (Populus spp.) were higher in the northeast in most years. Evidence from Impactor Samplers Raynor et al. (1975) constructed sampling lines consisting of eight samplers each separated by a minimum distance of 1 m and a maximum dis- tance of 486 m in various locations in New York state. The authors looked at total tree pollen as well as ragweed pollen. Differences between pairs of samplers in a given sampling line increased as a function of distance in most tests. Even sam- ples taken only 1 m apart had an average 18% difference. Samples taken at the maximum dis- tance from each other (486 m) had an average 25% difference. These relationships were simi- lar across sampling locations and pollen types. The authors concluded that pollen near ground- level may not be well mixed, even in the absence of local sources, while noting that differences in the collection efficiency of the Rotoslide sam- plers could have contributed to the results seen. The remaining two studies provide less compel- ling evidence, again due to differences in sampling height. As part of a study on allergic sensitization to pollen, White et al. (2005) measured pollen at two sites in Cincinnati, Ohio, U.S. One site was in an urban area at 12.2 m high, while the other was a suburban site at ground level. In a paired t-test, daily total pollen concentrations were not significantly different between the sites (P = 0.24). No information on individual taxa was presented. Information on the annual total of tree pollen at each site was not presented. Frenz et al. (1997) report the results of moni- toring conducted at two sites 5.6 km apart in St. Paul, Minnesota, U.S. One site was located on a one-story building (Snelling) while the other was located on a two-story building (Wycliff). Total pollen over the sampling period was similar at the two sites (17,188 grains at Wycliff versus 17,341 grains at Snelling). However, differences appeared when results were examined by month, with higher counts at the Snelling site in April and May, and higher counts at the Wycliff site in June through October. The authors suggested that local vegeta- tion plays a role, as the Snelling site is close to streets ©2015 International Society of Arboriculture lined with trees that release pollen in the spring, while the Wycliff site was an industrial environ- ment more conducive to grass and weed growth. Evidence from Gravimetric Samplers The two studies that included the largest number of sampling sites both used gravimetric sam- pling methods. Emberlin and Norris-Hill (1991) sampled several pollen taxa at 12 sites in 1987 and at 14 sites in 1988 in North London, UK. They found substantial variation in annual pol- len influx across the sites. The largest difference between any two sites was 646% for sycamore pollen. The authors suggest that the presence of a great number of sycamore trees within the city contributed to the magnitude of the variability. Ishibashi et al. (2008) measured pollen in Tokyo, Japan, at 32 sites for one 24-hour period in March 2005 and at 78 sites for one 24-hour period in March 2006. Total pollen influx was approximately 10 times higher in 2005 than in 2006. However, in both years, influx was higher in suburban and rural locations than in the urban city center. No statistical testing was performed relating land-use characteristics to pollen counts, nor were any taxa-specific results presented. Effect of Local Vegetation Several studies presented evidence that local vegetation was a driver of spatial variation in tree pollen. For example, Gonzalo-Garjo et al. (2006) used information from a tree census in the study area and found that the site with the high- est sycamore pollen concentration was closer to major stands of sycamore trees than the two sites with lower concentrations, although the exact distances and densities of the stands were not reported. Katelaris et al. (2004) conducted vegeta- tion surveys in 2,000 m circular buffers of their three sampling sites and found differences in local vegetation that paralleled differences in total pollen concentrations. However, no taxa-specific information was reported. In the study by Nowak et al. (2012), one site was located 100 m from the closest sycamore trees, while the other was 6.5 km from the closest sycamore trees. Annual syca- more totals were 10 to 20 times lower at the site that was farthest from the sycamore trees, sug- gesting that most sycamore pollen does not travel
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