Arboriculture & Urban Forestry 41(2): March 2015 taxa individually, so it is not clear if certain pollen taxa contribute to this discordance more than others. Gonzalo-Garjo et al. (2006) examined pollen concentrations at four sites in Badajoz, Spain, for a total of 27 days at intervals from March 7, 2003, to February 26, 2004. Sampling was carried out at six meters above ground at one suburban site and at ground level at three urban sites. Four major tree taxa were identified from the samplers: oak, olive (Olea spp.), the cypress family, and syca- more. No differences between any of the four traps were detected for the cypress family. For oak, the urban traps were not significantly different from each other, but they were significantly different from the suburban trap. The change in sampling height limit one’s ability to interpret whether dif- ferences in pollen concentrations between the urban and suburban traps were truly due to spatial variability, although the three urban sites them- selves are comparable. For olive and sycamore, the urban traps were significantly different not only from the suburban trap but also from each other. Alcazar et al. (2004) examined sycamore con- centrations at four sites in Cordoba, Spain (north- ern, southern, center, and western) for 26 days at intervals between February and April of 2000, and for 12 days at intervals between March and April of 2001. Samples at the western site were collected with a Hirst-type volumetric spore trap, while samples at the other three sites were col- lected with portable Lanzoni VPPS samplers. The Hirst-type trap has a wind vane so that the intake is always facing into the wind, while the three Lan- zoni samplers did not have this feature, limiting the comparability of the western site to the others. However, there were differences in the total amount of sycamore pollen collected over the sampling periods among the three sites that used the Lanzoni samplers. The total concentration ranged from 241 grains/m3 48,957 grains/m3 at the northern site to at the southern site in 2000 (a greater than 200-fold difference), and from 664 grains/m3 at the northern site to 16,335 grains/m3 at the center site in 2001 (a greater than 20-fold difference). The authors report that sycamore trees are less common in the northern part of the city, suggesting an influence of local vegetation. The four remaining studies employing volumet- ric samplers also revealed spatial variation in tree 61 pollen distribution but the evidence is less com- pelling. These studies all used two sites each, and in each case the sites were not at the same height. Myszkowska et al. (2012) measured pollen concen- trations at two sites in Cracow, Poland, between April and October of 2011. These sites were sepa- rated by only 1.5 km, one of the shortest reported separation distances among the studies included in this review. The total amount of tree pollen col- lected over the season was about 7% higher at site L than at site B (12,414 grains/m3 grains/m3 versus 11,631 ). More pronounced differences in total pollen collected over the season were observed for some tree taxa, including several known allergens. For example, total birch pollen was 34% higher at site B than at site L (2,130 grains/m3 grains/m3 versus 13 grains/m3 versus 1,584 times higher at site L than at site B (207 grains/ m3 ), and maple (Acer spp.) was almost 16 ). However, two issues limit our ability to infer spatial variation in pol- len in this study. First, different samplers were used at the two sites: a Lanzoni VPPS 2000 at site L and a Burkard volumetric spore trap at site B. Second, the Lanzoni trap was located 15 m high, while the Burkard trap was located 20 m high. Nowak et al. (2012) measured sycamore pollen at two sites at heights of 22 and 33 m in Poznan, Poland, from 2005 to 2009. Differences in annual total sycamore concentrations ranged from 10- to 20-fold. Rodriguez-Rajo et al. (2010) report pol- len measurements for six taxa (three tree taxa and three herbaceous taxa) from the same two sites as Nowak et al. (2012), but from a more limited set of years (2005–2007). The authors found that the total amount of each pollen type collected over the three-year period, as well as the number of days with pollen concentrations higher than 50 grains/ m3 m high site, although the differences were larger for the herbaceous taxa than the tree pollen taxa. Finally, Velasco-Jiménez et al. (2012) examined pollen concentrations at two sites in Cordoba, Spain, from 2006 to 2010. One trap was located in the southwest of the city at 15 m, and the other was located in the northwest at 22 m. The authors looked at differences in the daily (rather that annual total) pollen concentrations and found that daily concentrations of most pollen taxa were not significantly different at the two sites, with some ©2015 International Society of Arboriculture , was higher at the 22 m high site than at the 33
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