The Water Relations of Certain Heath Plants with Reference to their Ecological Amplitude: II. Field Studies

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Abstract
The annual and daily course of the water relations of three ericaceous heath plants, Calluna vulgaris, Erica tetralix and E. cinerea were followed in the field. Soils supporting Erica cinerea were the driest and those with Erica tetralix the wettest, while those soils with Calluna vulgaris were of intermediate moisture status. Relative turgidities were usually lowest in Calluna and highest in Erica tetralix. The daily course of transpiration showed no direct correlation with evaporating conditions, the lower transpiration of the afternoon indicating some stomatal control. In Erica tetralix transpiration was often depressed in the middle of the day and higher in the morning and evening. The relative turgidities in winter were generally lower than in summer. This did not appear to be correlated with environmental consitions but rather with the plants'' incapacity to make up water deficits when evaporation and soil moisture stresses were removed. In the presence of atmospheric moisture the plants could make up a considerable part of their deficits by the absorbtion of water through their aerial portions. The winter damage of Calluna vulgaris appears to be unconnected with frost but merely an aspect of the low winter turgidities. The wide ecological amplitude of Calluna is perhaps partly due to its tolerance of a wide range of water deficits, while Erica tetralix may be restricted to the wetter areas on its apparently greater sensitivity to water stress. Erica cinerea seems to depend on a higher level of root activity which may limit it to the better drained areas.