4.1. Comparison of Indices and Conceptual Remarks
The indices IUGZA and IISA used in our study can be used as a management tool in urban green planning. These indices offer a comparison of the allergenic potential of different parks or planting scenarios, therefore, they can contribute to the development of allergy-friendly parks. Since the presented indices account for the total area of the examined green space (see Equation (1)), the methods presented in our study are applicable for any park of any size. In fact, recommendations for planting trees can be applied with little effort, leading to positive effects for pollen allergy sufferers.
facilitates an easy way to assess and predict the allergenic potential of an urban green space by the use of only a few mathematical calculations. To assess the individual-specific allergenic potential of urban green spaces, we developed an index (IISA
) that includes individual-specific foliage volume by accurate measurements of crown heights and surface areas occupied by each plant. Since there exists an apparent link between foliage volume and number of flowers (and therefore intensity of pollen emission [18
]), it is sensible to use individual-specific measurements. Therefore, the IISA
has advantages such as the improvement in accuracy and the reduction of a high influence of the parameters height and surface area. On the other hand, there are constraints in practicability and in speed of the analysis.
In general, the IISA
resulted in a lower value compared to IUGZA
(0.018 vs. 0.173). This difference is not surprising because the data obtained for calculating IUGZA
are based on mean values of mature individuals (which is different to the individual-specific measurement). The actual growth rate of trees is dependent on different site-specific factors such as climate, competition or nutrient supply [36
]. Therefore, the calculation of IUGZA
based on literature data may lead to an overestimation of the actual crown volume and in turn to an underestimation of the intrinsic allergenic potential. This fact was also mentioned by [37
] who applied this index to three localities in Spain.
The purpose of both indices is to compare an actual green space with a hypothetical green space with maximum allergenicity. Generally, plant crowns might overlap resulting in a higher surface area occupied by plants compared to the geometrical surface area of the park. Thus, a conceptual weakness of both indices is the fact that values can also be higher than 1 (see Methods section). In reality, however, the indices are rather small for parks (see Results [17
]), but not for dense forests [38
]. Even smaller values would be obtained when the constant of the formula for IUGZA
is adjusted to 648 (by applying the prevailing maximum values for the parameters used: ap = 4, pe = 3, ppp = 3 and H = 18). The value for IISA
(0.018) was considerably smaller since the constant used in the equation (1188) included sources of main local allergens (ap = 4) and was adapted to the maximum measured crown height (H = 33 m, T. tomentosa
). Some species, e.g., C. avellana
, will never reach a comparable height. Therefore, this index might yield in more meaningful values when the height of trees does not differ very much among planted trees or shrubs.
Another modification was the reduction of the number of considered individuals due to the fact that only mature species were included for the calculation of IISA. Considering all species (both mature and immature), the index was quite similar (0.019, data not shown). This is probably attributable to the fact that immature individuals are smaller in size and therefore do not contribute much to the magnitude of the index.
Allergenic pollen is not only restricted to trees and shrubs. The high sensitisation rate in German adults of 37.9% linked to grass pollen [39
] shows the importance of lawns. In our studied park, grass is cut on a regular basis and only some small areas covering less than 5% of the park’s area are not cut to promote spontaneous vegetation for insects. Thus, this study does not include the allergenic potential of non-woody plants but only the allergenic potential based on trees and shrubs. The inclusion of lawn area which accounts for 55.9% of the park area (1.3 ha) would result in an increase of IUGZA
by 0.00254 when a medium height of 0.2 m is assumed. Including grass for IISA
is not sensible since this index is based on accurate measurements of plant heights (which varies in the course of the vegetation season in the case of grass) and only includes mature individuals. Most grasses, however, do not flower in our park due to frequent cutting. Nevertheless, in other urban green areas, the inclusion of non-woody plants may has to be considered as well.
The employment of pruning can be an effective management tool. In our park, accurately pruned individuals of B. sempervirens
expose a lower risk since flowers are kept to a minimum. Ideally, information on pruning practices should be included in assessing the allergenic potential of parks. The studies of [17
] accounted for the effects of grass and flower species. Here, we excluded hedges and lawns since they are very frequently cut.
An important factor not considered in our study is the pollen emission of areas outside the study area [8
]. In case of Hofgarten, an inflow especially from south-eastern (plantings and lawns), south-western (large green areas and riverside vegetation) and north-western directions (plant nursery) is possible, despite walls and buildings enclosing this park. Depending on species and wind conditions, pollen can be spread over large areas and great distances [40
] substantially influencing the allergenic potential of a given place. Pollen abundance is further affected by numerous surfaces for impaction and filtration [42
] such as walls and buildings that surround Hofgarten. In addition, needles of conifers or leaves of deciduous trees may influence the impaction of pollen. Early flowering anemophilous plants such as hazel and birch may be able to disperse their pollen more efficiently since their own leaves and most leaves of other plants are not unfolded yet. Except for some fountains, lakes or ponds are not present in Hofgarten, but they allow the deposition of pollen [38
]. Since the length of the pollination period (ppp) may vary due to different weather conditions from one year to another [43
], phenological observations and/or airborne pollen measurement might contribute to a more accurate index in further studies. In the study of [37
], pollen concentrations were monitored at the roof of high buildings, but not in the investigated parks. An exact assessment of the influence of pollen transport however, is only possible with on-site airborne pollen measurements.
4.2. Planting Scenarios and Recommendations for Plantings in Urban Green Areas
The selection of 14 typical park trees in German cities [32
] according to their current proportional occurrence resulted in the highest allergenic potential (IUGZA
= 0.226) among all six planting scenarios. This finding suggests that the planting of common park trees in general is not very suitable for designing allergy-friendly green spaces. This may be attributed to some species whose pollen are known as main local allergens such as B. pendula
or F. excelsior
. Regarding the current state of the park, a comparison between these typical park trees (93 individuals) and the remaining trees and shrubs which are not very common or even exotic (138 individuals) shows that their proportional IUGZA
is almost equal (0.087 vs. 0.086; data not shown). This finding also suggests that typical park trees include species with negative effects for allergy sufferers. It was pointed out that a moderate planting of exotic species leads to an increasing floral diversification, but the overuse of these exotics should be avoided [14
]. Olea europaea
is an example of a species producing highly allergenic pollen that is mainly planted because of its exotic appearance [10
]. Frequently planted individuals of O. europaea
lead to an increase of airborne Oleaceae pollen and therefore to an increase of the sensitisation rate of German adults which is currently at 9.7% [39
]. This species might pose a risk since a high degree of family relationship with another more common species with allergenic pollen (Fraxinus
spp.) exists [44
]. The role of exotic species is controversially debated: Whereas [45
] attribute exotic species a preventive measure to reduce sensitisations, other authors noted that some species attracted negative attention. For southern Spain, [46
] mentioned allergy symptoms in autumn related to the genus Casuarina
that is native to Australia and Asia but extensively used as an ornamental tree, especially in coastal cities [47
The role of main local allergens for the allergenic potential of green spaces is evident: Regarding the current state of the park, 21 major local allergenic trees and shrubs are contributing with a value of 0.036 to the allergenic potential (data not shown). This means that only 9% of the trees and shrubs are responsible for 21.1% of the allergenic potential calculated with IUGZA
. For example, major local allergenic pollen of birch, hazel and grasses are triggering clinically relevant symptoms in over 90% of the German adults and on over 75% of Europeans [39
]. Thus, replacing main local allergenic plants (scenario 2) had a considerable impact on the park’s allergenic potential (IUGZA
= 0.147). The substitution was made by randomly selected individuals with a medium allergenic potential of ap = 2. Using solely non-allergenic plants would yield in even lower values of IUGZA
To assess the suitability of the current selection of trees, we incorporated three different scenarios. Although a uniform distribution of all planted species (scenario 3) was linked to a similar IUGZA
as the actual selection, an equal distribution of the ten most frequently occurring plants (scenario 4) resulted in a comparably high allergenic potential (IUGZA
= 0.197). This indicates that the actual preferential selection of trees is not very suitable with respect to their allergenic potential. An unequal distribution (scenario 5, IUGZA
= 0.156) reduced the high influence on IUGZA
of some species such as the main local allergenic trees Q. robur
and F. excelsior
as well as the high allergenic potential of T. tomentosa
because of its great foliage volume. As a result, the mass use of certain plants results in large quantities of monospecific pollen, probably affecting the frequency of new sensitisations and the aggravation of symptoms in people allergic to this pollen [14
The categorisation of climate-tolerant species by [34
] does not include any parameters related to the risk for allergy sufferers. The species that are able to perform well under changed climatic conditions (scenario 6) yielded similar values for IUGZA
compared to the current state. However, for these and for all other species, it has to be considered that plant-related factors such as the length of the pollination period or the pollen’s allergenicity might change with ongoing climate change [48
]. In the future, these changes might result in different assessments and have to be timely regarded in urban planning.
The results of our planting scenarios suggest that a greater diversity of trees, assessed using Shannon index, is not linked to a lower allergenic potential of the park. However, several studies claim that urban green planning should focus on biodiversity for ameliorating the allergenic potential of parks [14
]. Additional species are increasing species diversity but we suppose that the mass use of common pollen emitters leads to high pollen levels increasing the risk of new sensitisations and aggravating allergic reactions.
Allergy sufferers should have access to green spaces where they are not put at additional health risk. In Germany, pollen allergens can be found nearly year-round in the air [26
], which can strongly affect those concerned. The occurrence of highly allergenic and anemophilous plants should be limited or avoided, because of their high pollen emission [8
]. Furthermore, plants with a long pollination period and the increased planting of only male individuals of dioecious species pose a risk for allergy sufferers. It was suggested that the concept of a female park is suitable for allergy sufferers, because female individuals of dioecious species do not release any pollen [10
]. However, the sterility of such green spaces results in a minimised capability of pollination [8
]. In addition, female trees are often not favoured for planting due to higher amounts of litter or undesirable odour as observed in G. biloba