Spring onset and seasonality patterns during the Lateglacial in the eastern Baltic region

Abstract. Spring onset is an important phenological observation that is sensitive to modern climate change and can be traced back in geological time. The Lateglacial (~14500–11700 cal yr BP) spring onset and growing season (growth-degree-days) dynamics in the eastern Baltic region were reconstructed using the micro-phenological approach based on the dwarf birch (Betula nana) subfossil leaf cuticles. The presented study sites, Lake Lielais Svetinu (eastern Latvia) and Lake Kosilase (central Estonia), are located ~200 km apart in the region affected by the south-eastern sector of the Scandinavian Ice Sheet. During the Lateglacial period the region and its biota were influenced by the retreating glacier and the different stages of the Baltic Ice Lake. The plant macrofossil data confirms that the study sites were located in different vegetation zones (arctic-to-boreal) during the Lateglacial period. The dynamics of the estimated length of the growing season and spring onset, combined with the regional collection of plant macrofossil records, suggest the importance of local settings to species migration. During the Lateglacial warming period (Bølling/Allerød), a notable spring warming and longer growing season was calculated based on micro-phenology, but the treeline did not extend beyond central Estonia. The comparison of pollen- and chironomid-inferred past temperature estimations with spring onset, growth-degree-days, and plant macrofossil data shows coherent patterns during the cooler Older Dryas and warmer Bølling/ Allerød periods while suggesting more complicated climate dynamics and possible warmer episodes during the Younger Dryas cold reversal.