Climatic controls on the survival and loss of ancient types of barley on North Atlantic Islands

For ancient types of barley at sites in the Scottish Isles, Faroes, and Iceland, we calculated minimum temperature requirements for grain production (grain production threshold, GPT) as accumulated degree days over the cropping season. Site suitability for barley
from AD 1200 to 2000 was investigated by comparing these thresholds with reconstructions of annual cropping season degree days (CSDD) using temperature and tree-ring data.
In Iceland, between AD 1200 and 1500, reconstructed CSDD were more favorable in the
southwest (Reykjavik), with fewer years below the GPT, than in the North, East and West,
but there were two periods (1340–1389 and 1426–1475) with low average CSDD and several years below the GPT which possibly infuenced the abandonment of barley cultivation
around this time. Reconstructed CSDD for the Faroes (Tórshavn) had only one year below
the GPT, but 15 periods of four or more consecutive years with low CSDD which would
have challenged barley cultivation, especially in the thirteenth century. Reconstructed
CSDD were highest for the Scottish Isles, allowing a more prominent role of barley in the
farming system and economy. Nevertheless, years with poor harvests or famines were common and about half were associated with low CSDD, resulting in a signifcant temperature
link but also demonstrating the important contribution of other factors. Despite frequent
unfavorable years in both the Faroes and Scottish Isles, resilient production systems, welladapted barley strains and socio-economic factors allowed barley cultivation to continue,
and some ancient types to survive to the present day