Exploring Holocene growing season variability in Central Europe: Evidence from Vegetation Proxies

The lengthening of the growing season in the Northern Hemisphere is a key response of terrestrial ecosystems to climate warming, yet long-term perspectives on Holocene growing season dynamics remain limited. Growing Degree Days (GDD), a widely used metric for assessing growing season thermal conditions, can be reconstructed using the micro-phenological method, a relatively recent proxy based on changes in leaf epidermal cell morphology. When combined with pollen-based reconstructions, this integrated approach provides robust estimates of past growing season thermal conditions.

In this study, we explore Holocene growing season variability at Linje peatland in northern Poland by combining Betula nana leaf micro-phenology with pollen-based GDD reconstructions derived from the same sediment sequence. Linje peatland represents a mid-latitude microrefugium where B. nana has persisted throughout the Holocene and where long-term peat accumulation, together with modern hydrometeorological monitoring, provides a unique opportunity for local proxy calibration. Building on an existing micro-phenological model developed for northern Finland, a site-specific inference model was established using annually collected modern leaves and applied to subfossil B. nana remains spanning approximately the last 11,350 years.

Preliminary results suggest broadly coherent long-term patterns in growing season thermal variability during the Late Holocene, while intervals of divergence between the two proxies are more pronounced during the Early Holocene. Interestingly, these differences may reflect contrasting proxy sensitivities or ecological response times. Overall, this study illustrates how combining micro-phenological and pollen proxies can be used to investigate past vegetation-climate interactions, growing season dynamics, and their relationship to prehistoric and historic human societies.

This research was supported by ESF project PRG1993, the Doctoral School of Tallinn University of Technology, the (Estonian) Ministry of Education and Research Centre of Excellence grant TK215, and the National Science Centre, Poland (grant nos. 2021/41/B/ST10/00060 and 2022/45/B/ST10/03423).