Variation of pollen season trends under mediterranean climate: a systematic review

The present review examined the perceptions concerning the impact of climate change on pollen production, as well as the timing of the start and end of pollen seasons across various taxa and countries in the Mediterranean region. We analyzed 36 major taxa, focusing on long-term trends. Most studies on the airborne pollen in this region primarily focused on Quercus (n = 9), followed by Poaceae and Pinus (n = 7), Cupressaceae (n = 6), and Amaranthaceae (n = 5). The studies ranged in duration from 10 to 31 years. The results indicate a general increase in annual temperatures across Spain. However, a decline in minimum, mean, and maximum temperatures was observed in Tétouan (Morocco), Jaén and Ourense (Spain) before months when species were expected to flower. Based on data extracted from peer-reviewed articles, pollen season intensity has generally decreased across Mediterranean countries, mainly due to deforestation, habitat fragmentation, land use changes, and intense human activity, all of which have diminished biodiversity in these areas, except in Spain. More than 25% of the data reviewed showed a delayed start to the pollen season, except in Turkey, where no data was found regarding the pollen season's start. Over 98% of studies conducted in Morocco and Spain indicated a delayed end to the pollen season. However, both delays and advances were observed in Greece and Turkey. These phenological changes are primarily linked to shifts in temperature, and could be connected to climate change and rising CO² levels. Based on our research, the timing of pollination, whether earlier or later than expected, differs from the results of other studies and is strongly influenced by the particular years examined. Another factor that could affect the results is the application of various methods or criteria to determine the pollen season.