Airborne pollen trends in Tétouan (NW of Morocco)

Abstract

Trends of the airborne annual pollen integral (APIn) and pollen season of principal woody and herbaceous plants in Tetouan were analysed over a 10-year monitoring period (2008–2017). Pollen was continuously sampled by means of a 7-day recording volumetric pollen trap by Burkard. Pollen trends were analysed by using Mann–Kendall tests and Sen’s slope. Aerobiological data were correlated with temperature and rainfall. A significant decreasing trend in annual minimum temperature was revealed together with significant decreasing trends in the APIn observed for Cupressaceae, Cannabis, Parietaria, Pinus and Quercus, this being highly significant for Cupressaceae and Pinus. On the contrary, the seasonal intensity of Mercurialis, Morus and Olea showed nonsignificant trends. Besides this, 77% of the studied pollen types showed a tendency to decreasing the peaks value, these trends being significant for Cupressaceae (−204.67 pollen/ m3 per year) and Pinus (−14.33 pollen/ m3 per year). The end of the Quercus pollen season showed a marked tendency to occur earlier across the years (−4.5 days/year) and the start day of Cannabis, Cupressaceae, Pinus and Poaceae to occur later (+ 7.13, 2.33, 1.67 and 2.5 day/year, respectively), shortening the duration of the respective pollen seasons but not with a significant trend. Regarding the association between the pollen season intensity and meteorological parameters, six pollen types showed at least one statistically significant coefficient correlation. The decreasing and significant trend in the intensity of the APIn diminishes also the exposure to airborne pollen for allergic sufferers, having implications in the field of public health. Decreasing trends in annual minimum temperature and the general lack of significant trends and correlation coefficients between the parameters of the pollen season of different pollen types and monthly mean temperatures and rainfall suggest that interannual variability in the data is due to human interventions, deforestation, fires and the opposite response of some species to warming in Fall/Winter and Spring, and this could be the reasons for the observed behaviour in the pollen season.