Arboriculture & Urban Forestry 41(2): March 2015 far from its source and that local sources are im- portant drivers of local pollen concentrations. Other studies mentioned that local vegetation may help explain their results but did not include systematic investigations such as tree surveys. DISCUSSION Out of the 13 studies that met the inclusion cri- teria for this review, 12 found evidence that the amount of tree pollen measured over the sampling period (or in each year in the case of studies that monitored for multiple years) was different at sites within a single city. In some cases, the magnitude of the variability was quite large. For example, Alcazar et al. (2004) found that the total annual sycamore concentration varied by a factor of 200 between two sites. White et al. (2005) did not find statisti- cally significant differences in daily total pollen concentrations at two sites in Cincinnati, Ohio, but did not examine the annual total, nor did they look at whether there were differences by pollen type. Study designs varied widely. The magnitude of the variability observed is not comparable across cities due to differences in sampler height, taxa examined, and the distance between and charac- teristics of the sites. Some studies only reported results for the aggregate of all pollen taxa (or in some cases, all arboreal pollen taxa), while others reported results for individual pollen families or genera. The latter group of studies found the mag- nitude of variation is taxon-specific. Sycamore, the most commonly investigated individual genus, was consistently found to vary substantially on an intra- urban scale. While the primary focus of this report is spatial variation in tree pollen, it is worth noting that non-arboreal pollen taxa may vary within urban areas as well. For example, Emberlin and Norris-Hill (1991) also examined differences in grass pollen (Poaceae) across sites in North Lon- don and found that the largest percent difference between any two sampling sites was 77%. Raynor et al. (1975) found differences in ragweed pollen concentrations at a very small spatial scale (1 m). Studies that did not meet the criteria for this review because they did not measure tree pollen also reveal spatial differences in grass and ragweed pollen (Puc and Puc 2004). For example, Barnes et al. (2001) monitored ragweed pollen using volumetric sam- plers at four sites in Kansas City, Missouri, U.S., and 63 found that daily ragweed concentrations tended to be higher in suburban sites than in an urban site. The most common study limitation was dif- ferences in sampling heights. This was an issue in six studies (Frenz et al. 1997; White et al. 2005; Rodriguez-Rajo et al. 2010; Myszkowska et al. 2012; Nowak et al. 2012; Velasco-Jiménez et al. 2012), including the study from White et al. (2005), which revealed no differences in daily pollen measured at two sites. In addition, the length of sampling varied widely between studies. Ishibashi et al. (2008) used the largest number of sampling sites (78 sites in 2006), but these samplers were only deployed for a single 24-hour period. It is impossible to say whether the patterns revealed during those 24 hours would have been stable over the course of an entire year. If tree pollen does indeed vary spatially within cit- ies, an important next question is whether patterns in tree pollen are stable across years. Five studies measured pollen for more than one year; however, three of these conducted pollen monitoring at differ- ent heights. Ishibashi et al. (2008) found that pollen levels were higher in suburban and rural areas than in urban areas in two years, but the sampling period was limited to a single 24-hour period in each year. Alcazar et al. (2004) monitored sycamore at uni- form height, and found that the northern site had the lowest totals in both years of monitoring. How- ever, the southern site had the highest total in 2000, while the central site had the highest total in 2001. Nowak et al. (2012) found that total sycamore con- centrations were between 10 and 20 times lower at a site that was far from the closest sycamore trees than at a site that was close to a stand of sycamore for all years 2005 to 2009, but the two samplers were placed at different heights. Clearly, more information is needed in order to draw conclusions regarding the stability of spatial patterns over multiple years. RECOMMENDATIONS FOR FUTURE RESEARCH Taken together, the results of all 13 studies pro- vide evidence that there is spatial variation in the distribution of tree pollen within cities. However, additional research is needed to characterize the stability of patterns of pollen distribution across years as well as the extent to which local veg- etation and other land-use characteristics affect local pollen levels. Multi-year pollen monitoring, in ©2015 International Society of Arboriculture
March 2015
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