Climate and the meristem temperatures of plant communities near the tree-line

Temperatures of terminal meristems of forest, krummholz and dwarf shrub vegetation were measured at altitudes of 450, 600, 650 and 850 m in the Cairngorm Mountains of Scotland. Simultaneously, the air temperature above the vegetation was recorded, so that it was possible to calculate the difference between meristem and air temperature, sometimes called the excess temperature. This temperature increased linearly with the net radiation absorbed at each station, and the slope was dependent on wind speed and the height of the vegetation. In the extreme cases the slopes were practically zero for forest and 0.028° C W^-1 m² for dwarf shrubs. The latter implies a temperature excess of about 15° C in bright sunshine and low wind speeds. A model is developed to calculate the excess temperature from a knowledge of vegetational height and climatological variables.