Changes in pollen season duration and their relationship with meteorological conditions in Lithuania

Climate change is a key factor determining changes in plant phenology. The start and end dates of the pollen season, as well as its duration, are closely linked to shifting meteorological conditions. Rising air temperatures have a particularly strong impact, but changes in other meteorological factors, such as precipitation, are also important. This study analyses two genera within the Betulaceae family, alder ( Alnus ) and birch ( Betula) pollen seasons from 2005 to 2023 in Lithuania, focusing on season dates, duration, pollen concentration, and their relationships with meteorological parameters. The dates and duration of the pollen season were evaluated using two definitions. The analysis is based on daily aerobiological observation data from Vilnius, Siauliai, and Klaipeda. The alder pollen season typically begins in March and lasts, on average, from 24 to 36 days in different study sites. Over the past two decades, a significant trend toward an earlier start of the season has been observed, with the beginning date moving up by 13–34 days, depending on the location and calculation method. The duration of the season varied slightly, and the end dates did not show statistically significant differences. The increase in air temperature during February and March was the primary factor driving the season's earlier start. The birch pollen season in Lithuania usually begins in mid-April and lasts about 30 days. Changes are statistically insignificant, despite minor shifts in the start and end dates. A weak but significant correlation exists between February–March temperatures and the beginning of the birch pollen season, while a weak negative correlation was observed between April–May temperatures and the season-end dates. • First 19-year analysis of alder and birch pollen monitoring in the Baltic region. • Alder pollen season start advanced by up to 34 days; birch shifts were less pronounced. • February–March warming strongly linked to earlier alder season; precipitation effects limited. • Results support allergy forecasting and urban planning for healthier cities.