Abstract
Abstract. Blanket bog occupies approximately 6 % of the area of the UK today. The Holocene expansion of this hyperoceanic biome has previously been explained as a consequence of Neolithic forest clearance. However, the present distribution of blanket bog in Great Britain can be predicted accurately with a simple model (PeatStash) based on summer temperature and moisture index thresholds, and the same model correctly predicts the highly disjunct distribution of blanket bog worldwide. This finding suggests that climate, rather than land-use history, controls blanket-bog distribution in the UK and everywhere else. We set out to test this hypothesis for blanket bogs in the UK using bioclimate envelope modelling compared with a database of peat initiation age estimates. We used both pollen-based reconstructions and climate model simulations of climate changes between the mid-Holocene (6000 yr BP, 6 ka) and modern climate to drive PeatStash and predict areas of blanket bog. We compiled data on the timing of blanket-bog initiation, based on 228 age determinations at sites where peat directly overlies mineral soil. The model predicts that large areas of northern Britain would have had blanket bog by 6000 yr BP, and the area suitable for peat growth extended to the south after this time. A similar pattern is shown by the basal peat ages and new blanket bog appeared over a larger area during the late Holocene, the greatest expansion being in Ireland, Wales, and southwest England, as the model predicts. The expansion was driven by a summer cooling of about 2 °C, shown by both pollen-based reconstructions and climate models. The data show early Holocene (pre-Neolithic) blanket-bog initiation at over half of the sites in the core areas of Scotland and northern England. The temporal patterns and concurrence of the bioclimate model predictions and initiation data suggest that climate change provides a parsimonious explanation for the early Holocene distribution and later expansion of blanket bogs in the UK, and it is not necessary to invoke anthropogenic activity as a driver of this major landscape change.
Highlights
Blanket bog is a distinctive type of peatland confined to areas with cool and extremely wet climates
We use PeatStash to simulate the UK distribution of blanket bogs in the mid-Holocene (6000 years ago, 6 ka). We compare these simulations with a new compilation of blanket-bog initiation dates, in order to explore whether climate change could plausibly account for the expansion of blanket bogs during the later Holocene
PeatStash simulates the potential distribution of blanket bog (Gallego-Sala et al, 2010) based on mean annual temperature (MAT), mean temperature of the warmest month (MTWA) and a moisture index (MI) calculated from longterm monthly means of temperature, precipitation, and fractional sunshine hours
Summary
Blanket bog is a distinctive type of peatland confined to areas with cool and extremely wet climates. Blanket bogs occur where the mean annual temperature (MAT) > −1 ◦C, the mean temperature of the warmest month (MTWA) < 14.5 ◦C and the ratio of mean annual precipitation to equilibrium evapotranspiration (moisture index, MI) > 2.1 (Gallego-Sala and Prentice, 2013) These limits ensure that the site is outside the permafrost zone and not subject to cryoturbation, that summer temperatures are not too high for Sphagnum growth, and that there is sufficient moisture throughout the year to sustain peat growth on sloping ground. In addition to predicting accurately the present-day distribution of blanket bog in Great Britain, PeatStash correctly predicts the highly disjunct global distribution of blanket bogs (Gallego-Sala and Prentice, 2013), including its occurrence in places such as Newfoundland and Kamchatka that have experienced very different land-use histories from Great Britain and Ireland This finding strongly suggests that the present-day distribution, at least, of blanket bogs everywhere is controlled by climate. We compare these simulations with a new compilation of blanket-bog initiation dates, in order to explore whether climate change could plausibly account for the expansion of blanket bogs during the later Holocene
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