Annual degree day sums above 0°C calculated from daily averages obtained from hourly logging of soil surface temperature at every location. Freeze-thaw events calculated from hourly data obtained from temperature sensors at the soil surface. % indicate the percentage difference between chambers and control plots. Soil thaw date (day of the year) at which soil surface temperatures reach above freezing temperatures in and outside passive warming chambers for at least three consecutive days in spring. Note that soils at the Falkland Islands did not freeze for prolonged periods during winter.