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• We computed a 1-ha resolution map of tree abundance at the national scale. • The selected allergenic tree species included alder, birch and hazel. • The combination of forest inventory with citizen science data was successful. • Maps show fine-scale variations across the urban-rural landscape of Belgium. • It offers many applications in the field of urban health and forest management. Mapping the distribution of allergenic plants in urbanized landscapes is of high importance to evaluate its impact on human health. However, data is not always available for the allergy-relevant species such as alder, birch, hazel, especially within cities where systematic inventories are often missing or not readily available. This research presents an approach to produce high-resolution abundance maps of allergenic tree species using existing forest inventories and opportunistic open-access citizen science data. Following a two-step approach, we first built species distribution models (SDMs) to predict species habitat suitability, using environmental characteristics as predictors. Second, we used statistical regressions to model the relationships between abundance, the habitat suitability predicted by the SDMs, and additional vegetation cover covariates. The combination of forest inventory data with citizen science data improves the accuracy of abundance distribution models of allergenic tree species. This produces a continuous, 1-hectare resolution map of alder, birch, and hazel showing spatial variations of abundance distributions both within the urban fabric and along the urban–rural gradient. Species abundance modelling can offer a better understanding of the existing and potential future allergy risk posed by green spaces and pave the way for a wide variety of applications at fine-scale, which is indispensable for evidence-based urban green space policy and planning in support of public health. [ABSTRACT FROM AUTHOR]

• We studied allergy risks (AR) of 18 green spaces in Brussels, Belgium. • We examined present and possible future AR across contrasting scenarios. • Increased tree allergenic potential resulted in 11–27% higher AR. • Increased pollen season duration resulted in 44% higher AR. • AR increased by 99% in combined scenarios despite preventive tree removal. Urban green spaces may improve human health and well-being. However, green spaces may also emit allergenic pollen and these may trigger asthma, allergic disease, and respiratory infections. How allergy risks in green spaces may be modified by environmental change is still not widely understood. This study analyzed tree inventory data of 18 urban green spaces (5940 trees; 278 taxa; 93 ha) in the Brussels Capital Region, Belgium. We investigated present tree pollen allergy risk (AR) and changes in AR driven by changes in tree species composition, allergenic potential and pollen season duration. AR was estimated by calculating the allergenicity index I UGZA (range 0–1, 1 worst) for the present situation and 13 scenarios. The average AR was 0.08 (SD 0.05; range 0.002–0.17). The AR increased by 11–27% in increased allergenic potential scenarios, and by 44% in the increased pollen season duration scenario. Preventive removal of birch, hazel and alder reduced the AR by 13%. The AR increased by 99–111% in combined scenarios with and without preventive removal of the main allergenic taxa. These findings indicate that tree pollen allergy risks could considerably rise when ongoing environmental changes lead to a combination of longer pollen seasons, increased pollen allergen potency, and increased sensitization for one or more species. The preventive removal of the main allergenic tree species cannot sufficiently counter allergy risks caused by other species and that are amplified by environmental change, highlighting the importance of careful tree species selection in urban green space policy and planning. [ABSTRACT FROM AUTHOR]