The late-Holocene relationship between peatland water table depth and summer temperature in northern Poland

Abstract

Although the contribution of current summer warming to the widespread drying of European peatlands is undeniable, summer temperature fluctuations alone are unable to explain all the hydrological shifts of these ecosystems. We aim to determine whether the current coupling of high summer temperatures and low water tables of peatlands in northern Poland is recent or has been stable over long timescales, and to infer the climatic context driving past relationships between temperature and peatland hydrology. We compare the previously published 4000-year long paleohydrological record from Kusowo bog with a new chironomid-based summer temperature record covering the last 4.5 ka from nearby (~7 km) Lake Spore. Although several simultaneous shifts in our hydrological and temperature reconstructions are evident on multi-decadal to centennial timescales, for instance during the peak cooling of the Migration Period or Little Ice Age (~1.4 and 0.4 cal ka, respectively), the records reveal mostly independent fluctuations. However, the relatively high uncertainty of our quantitative estimates suggests caution is warranted when inferring the strength of the relationship between the reconstructed variables at this temporal resolution. However, at centennial to millennial timescales, the relationship between the records is surprisingly strong. Overall, warmer summers (mean July temperature = 18.7 °C) before ~2.3 cal. ka corresponded well with lower ground water tables at Kusowo bog (mean = 11.2 cm), whereas cooler summers in the following period (mean = 17.4 °C) were accompanied by higher water tables (mean = 7.9 cm). Particularly wet bog conditions during the last ~0.7 ka were also the coldest. The finding that on long timescales the drying of Kusowo bog coincided with warming suggests a persistent regional climate pattern in which drought conditions change along with summer temperatures. If this millennial-scale pattern holds for the projected warming, further peatland drying can be expected in northern Poland.