Tree-ring analysis elucidating palaeo-environmental effects captured in an in situ fossil forest - The last 80 years within an early Permian ecosystem

The early Permian Chemnitz Fossil Lagerstatte (Leukersdorf Formation, Chemnitz Basin, SE Germany) represents a diverse T-0 assemblage of a fossil forest ecosystem around the Sakmarian-Artinskian transition (290.6 +/- 1.8 Ma), which was preserved by pyroclastic deposits of a multi-phased volcanic eruption. The multi aged plant community consists of predominantly hygrophilous elements, which grew on an alluvial plain mineral substrate under sub-humid conditions, representing a wet spot environment. Strong seasonality triggered the formation of annual tree rings in arborescent woody plants, such as pycnoxylic gymnosperms, medullosan seed ferns and calamitaleans. From several hundred fossil trees, the 53 best-preserved specimens were selected and investigated in detail by measuring 2,081 tree rings in individual sequences of up to 77 rings. Ring sequences were analysed by standard dendrochronological methods to determine both annual growth rates and mean sensitivity. Morphological and statistical analyses on single tree rings reveal different tree-ring types according to the different plant groups. Pycnoxylic gymnosperms have distinct and regular tree rings, whereas medullosan seed ferns and calamitaleans show indistinct and regular tree rings as well as so called event rings. Results reveal differences between plant groups regarding their physiological reactions or adaptations to seasonal fluctuations. In comparison to pycnoxylic gymnosperms, both medullosan seed ferns and calamitaleans exhibit reduced growth rates and more sensitive reaction to environmental perturbances as water deficiency pointing to comparably lower adaptation to seasonally dry palaeoclimate. In this context, event rings are in many cases traced back to plant physiological stress during particularly severe drought periods. Altogether, these fossil trees serve as sensitive environmental archives, which shed light on growth conditions several decades back in time from the entombing eruption.