Airborne Pollen Trends during the 3 last decades in Spain

Airborne pollen significantly impacts air quality, human health, and ecosystems. Monitoring pollen trends is critical for understanding these effects, especially as pollen allergies affect millions of persons globally, reducing respiratory health and life quality. Pollen data provide relevant information about the impacts of climate change on vegetation, with shifts in flowering timing serving as indicators of ecological responses. These trends are also vital for forecasting agricultural yields, particularly in Mediterranean climates where water availability and temperature are increasingly erratic.

This study examines flowering timing and intensity trends for 12 anemophilous taxa across 12 locations in the Iberian Peninsula from 1994 to 2023. Using data from Hirst pollen monitoring method, we calculated annual trends in several flowering date indicators (e.g. peak pollen day, start of flowering, duration of pollen season) and other indicators for pollen production (e.g. peak daily pollen concentration or seasonal pollen integral). Statistical analyses assessed linear trends, comparing variations among woody and herbaceous taxa, as well as regional differences between Mediterranean and more temperate areas.

The date of maximum flowering advanced by an average of -0.054 days/year, with species-specific variations ranging from -4.2 (Amaranthaceae) to +7.3 days/year (Fraxinus). Woody taxa, especially those flowering in winter or early spring, exhibited varied responses. For instance, in some locations, Cupressus showed a slight delay with an average of -0.16 days/year, while Betula displayed a more marked delay, averaging -0.33 days/year. These delays are likely linked to insufficient chilling requirements in warm winters. Herbaceous taxa, such as Poaceae, advanced their flowering by an average of -0.13 days/year, equivalent to 1.3 days/decade, reflecting their sensitivity to rising temperatures and altered water availability. Notably, Rumex experienced a marked delay in flowering of -0.48 days/year, while Urticaceae advanced by +0.42 days/year.

Seasonal pollen integral displayed contrasting trends. Tree species such as Olea (average increase: +377.83 pollen grains/year, max: +1193.99 pollen grains/year in Jaén), Quercus (+180.81 pollen grains/year, max: +594.15 pollen grains/year in Córdoba), and Cupressus (+174.55 pollen grains/year, max: +853.45 pollen grains/year in Granada) showed significant increases. Conversely, certain herbaceous taxa, such as Poaceae (-11.89 pollen grains/year, min: -158.79 pollen grains/year in Badajoz), Amaranthaceae (-10.81 pollen grains/year, min: -30.07 pollen grains/year in Málaga) and Rumex (-6.67 pollen grains/year, min: -24.85 pollen grains/year in Badajoz), showed declining pollen loads, particularly in Mediterranean regions heavily affected by drought. In contrast, some taxa like Platanus displayed moderate increases averaging +101.11 pollen grains/year, demonstrating the complexity of pollen production responses to climatic variables.

Trends were not homogeneous across Spain. Southern regions exhibited more pronounced changes in flowering timing and intensity, aligning with greater climatic extremes. The observed trends in flowering timing and intensity underscore the complex ecological responses to climate change. Warming winters delay flowering in some taxa due to insufficient chilling, while rising temperatures and CO₂ levels could have driven increased pollen production in others, particularly in trees, but not so clear evidence in herbaceous. These findings highlight the need for continued pollen monitoring to mitigate the health impacts of allergenic pollen and to adapt agricultural practices to changing climatic conditions.