Temporal trends in airborne pollen seasonality: evidence from the Italian POLLnet network data

Airborne pollen reflects local vegetative composition and is a proxy for flowering phase. Long-term pollen data might reflect changes in biodiversity and phenology, attributable also to the effect of climate change. The present study, based on pollen data collected within the Italian aerobiological network POLLnet, aimed to verify whether there is any evidence of temporal changes in pollen season timing and its relation with meteorological variables. To this purpose, nine stations located in North and Central Italy were selected, and twelve pollen taxa, both arboreal and herbaceous, were considered. For each taxon and station, 11–17-year datasets of airborne pollen concentration within the period 2000–2016 were analysed. Four different pollen season descriptors were elaborated (start, end and peak date, season length) and analysed their temporal trend, also in relation to temperature and precipitation. Overall, the results showed a negative temporal trend in pollen season starting date, which indicates a tendency towards an earlier flowering for Corylus, Quercus, Gramineae and Urticaceae in all stations (even if statistically significant in six out of 36 cases). The effect of meteorological parameters was evidenced by negative correlations between pollen season starting date and temperature. With the exception of Olea, Ambrosia and Artemisia, all the remaining pollen taxa showed significant (negative) correlations between pollen season start date and average temperature of the previous months in at least half of the stations. As for precipitation, no relevant correlations were detected with pollen season parameters. The results are also interpreted considering the different biogeographic areas in which the nine stations are located. Long-term pollen dataset is useful in phenological studies and for the detection of climate change effects