A COMPARISON BETWEEN THE WATER 

 RELATIONS OF SPECIES WITH CONTRASTING 

 TYPES OF GEOGRAPHICAL DISTRIBUTION 

 IN THE BRITISH ISLES 



Margaret S.Jarvis 

 Department of Botany, University of Sheffield* 



INTRODUCTION 



In the Sheffield region, where the experiments to be described were con- 

 ducted, many species of plants attain the north-western or south-eastern 

 limit of their range in Great Britain. The distribution of species such as 

 Prtinus padus L. and Saxifraga hypnoides L. corresponds with the predomi- 

 nantly upland part of Great Britain to the north-west; that of species such 

 as Thelycrania sangtiinea (L.) Fourr. (Cornus sanguinea (L.)) and Filipendula 

 vttlgaris Moench. with the more lowland part to the south-east. Hence there 

 are correlations between the patterns of distribution of the species and 

 various features of climate such as annual rainfall, amount of sunshine and 

 average maximum or minimum temperatures. Similarly, the distribution 

 of such species on slopes of various aspects in the Derbyshire dales can be 

 correlated with features of the microcHmate. Species at the south-eastern 

 Hmit of their range tend to exhibit preference for slopes of northerly 

 aspect, and, conversely, species at the north-western Hmit of their range 

 are restricted to, or most abundant on, the south-facing slopes. 



Measureinents made with plaster-of-Paris electrical resistance blocks in 

 the summer of 1959 confirmed that on such south-facing slopes the soil 

 dried out more rapidly and to higher soil moisture tensions than the soil on 

 north-facing slopes. For example, after a fortnight without rain in August, 

 1959, the soil moisture tension at 4 cm depth was 20 atm on a slope of 

 south aspect and from i-i to 2-9 atm on the opposite slope of northern 

 aspect. 



An investigation of the response of plants of these species to increases in 

 soil moisture tension and to low atmospheric humidity was made experi- 

 mentally in the course of an attempt to elucidate the factors which were 

 hiniting distribution. Two herbaceous species, S. hypnoides and F. vulgaris, 

 and two woody species. P. padus and T. sanguinea were used. 



* Now at The Institute of Physiological Botany of the University of Uppsala, 

 Uppsala, Sweden. 



