INVESTIGATIONS ON THE WATER RELATIONS 

 OF SAND-DUNE PLANTS UNDER NATURAL 



CONDITIONS 



A.J. Willis & R. L. Jefferies 



Department of Botany, University of Bristol 



INTRODUCTION 



Although many studies of the water relations of plants have been made, 

 only a few of them relate to the behaviour of plants under the normal 

 conditions of their habitat. The aim of the present investigation has been 

 to gain information concerning the water balance of plants in their natural 

 environment of sand dunes. In this habitat, conditions tend to be somewhat 

 extreme, especially on the dunes themselves where rates of evaporation are 

 often high, and the water supply limited during the summer. Furthermore, 

 conditions are strongly contrasting within the dune system, as although in 

 some parts insolation is considerable and the water content of the sand 

 very low, elsewhere the sand may be water-saturated and some shelter 

 afforded. These habitat differences are most marked during the summer, a 

 time when any physiological differences between plants with regard to 

 their water relations are likely to be clearest. The data reported here are of 

 studies made in mid-June (1952-60) on a range of plants of the dune 

 system of Braunton Burrows, North Devon. 



Previous investigations have stressed the importance of the water factor 

 in respect of plant distribution on the Burrows (Wilhs, Folkes, Hope- 

 Simpson and Yemm, 1959), many species being restricted to low-lying 

 'slacks' where their roots are always within reach of the capillary fringe of 

 the water table and others occurring characteristically on the drier dune 

 slopes. An attempt was made to estimate to what extent plants of these 

 various sites differed in respect of their water relations. Hygen (195 1, 1953) 

 has attempted an ecological characterisation of plants in terms of their 

 rates of stomatal and cuticular transpiration, and has shown the usefulness 

 of transpiration decline curves in this connection. Somewhat similar studies 

 have been made in the present investigation alongside assessments of 

 stomatal behaviour and water deficits. In particular, the diurnal course of 

 transpiration has been followed under natural conditions, measurements 

 being also made of physical factors of the environment such as temperature, 

 relative humidity and hght intensity, as well as the rate of evaporation, at 

 different times of the day and night. 



