Human impact, soil erosion, and vegetation response lags to climate change: challenges for the mid-Scandinavian pollen-based transfer-function temperature reconstructions

Precise and accurate reconstructions of past environmental parameters from high-quality palaeoenvironmental studies are critical for realistic testing of climate models. To ascertain the reliability of the reconstructions of the past, cross-validation from a variety of proxies and methods is essential. Mid-Scandinavia, showing a variety of palaeoecological studies, is a suitable region for comparing and validating environmental reconstructions. Here, pollen-based transfer-function reconstructions show inconsistent late-glacial temperature patterns. They also show that the Holocene Thermal Maximum (HTM) occurs at ca. 7.5–4.5 cal. ka b.p. However, thermal indicators (pollen, megafossils, plant macrofossils) place the HTM at no later than ca. 10–7.5 cal. ka b.p. It is argued that after the onset of the early Holocene warming equilibrium between vegetation and climate was established over a prolonged period; i.e. ca. 1,500 and 4,000 years in the mountains and lowlands, respectively. In the mountains, soil drought, wind and winter stress were important factors causing the lag, whereas inter-specific competition and soil development delayed the succession within the species-diverse lowland forests. These lags when vegetation was not filling its thermal potential result in a distortion of the temperature signal as derived by transfer functions which assume that vegetation is essentially in equilibrium with climate. Due to widespread human impact and erosion today, many modern training set samples are unsuitable as reference material for past environmental conditions. Various recommendations are suggested towards making improvements in the pollen-transfer function approach to climate reconstructions. To overcome the difficulties resulting from vegetation lags in the early Holocene, proxies that have a faster response time to climate, such as chironomids and aquatic plants including algae, may replace terrestrial pollen.