Temperature and Stature: A Study of Temperatures in Montane Vegetation

Abstract

Temperatures and aerodynamic resistances of the apical meristems of Pin us sylvestris L. and the dwarf shrubs Arctostaphylos uva-ursi (L.) Spreng. and Loiseleuria procumbens (L.) Desv. were measured simultaneously at four stations along an altitudinal transect in the Cairngorm Mountains of Scotland. Net radiation, air temperature and water vapour pressure were also recorded. Dwarf shrubs, at high altitude stations, experienced tissue temperatures that were no lower than those of trees growing at low altitudes. This resulted from the larger temperature differences between surface and atmosphere experienced by the dwarf life forms. On sunny days with very low windspeeds, the meristems in the short vegetation could be as much as 150C warmer than those of the atmosphere above, whereas in the tall vegetation the meristems of trees were never more than 7 0C warmer than the atmosphere. On windy or dull days the differences were smaller. Lapse rates of mean air temperature were negative over the altitudinal range 450-850m (-90C km-1) whereas the lapse rates of tissue temperature were practically zero because of the greater tissue-to-air temperature differentials in the shorter vegetation at higher altitudes. The high tissue-to-air temperature differentials are attributed to the higher aerodynamic resistances measured for the dwarf shrubs, which form an aerodynamically smoother canopy than the trees. The role of the stature/ surface temperature relationship in the distribution and evolution of montane life-forms is briefly considered. Key-words: Pinus sylvestris, temperature, microclimate, treeline, aerodynamic resistance