324 P. G. JARVIS 



There are inherent difficulties in attempting to use data from experiments 

 of this type in field situations (Sands andRutter, 1959). The conditions of 

 root growth in a restricted exploited soil volume and the technique of 

 watering the whole soil mass back to saturation are essentially artificial. 



During germination, seedlings of oak produce a very long tap-root from 

 their acorn reserves. Hence, they are largely insulated from the effects of 

 drying-out of the upper part of the soil profile by shaUow-rooted her- 

 baceous plants. However, as a result of the intensive exploitation of the 

 rooting volume in an estabhshed wood, SMTs, which arise from the drying 

 action of tree roots, are usually uniformly distributed, at least in the upper 

 parts of the profile (Boggess, 1956; Jarvis, P.G., i960). Such conditions 

 are not unlike those in a container. 



In sessile oakwoods in the region west of Sheffield, SMTs develop only in 

 exceptionally dry summers, e.g. 1959. Reductions in growth as a result of 

 increases in SMT are likely to be of greater importance in less moist areas, 

 such as the New Forest. This experiment does, however, demonstrate the 

 probable normal occurrence of unsuitable conditions for root growth in 

 certain oakwood soils. 



ACKNOWLEDGMENTS 



This experiment was carried out as part of an investigation into the factors 

 influencing the growth and regeneration of sessile oak in the Sheffield 

 region. I should hke to thank Dr. CD. Pigott for his interest and super- 

 vision, and Professor A.R. Clapham, F.R.S., for the facihties of his depart- 

 ment during the course of this work. I am also grateful to the Department 

 of Scientific and Industrial Research for a Research Studentship which 

 made this work possible. 



REFERENCES 



Bergman, H.F. (1959) Oxygen deficiency as a cause of disease in plants. Bot. Rev. 25, 



417-485. 

 Birch, H.F. (1958) The effect of soil drying on humus decomposition and nitrogen 



availabiUty. Plant and Soil, 10, 9-31. 

 BoGGESS, W.R. (1956) Weekly diameter growth of shortleaf pine and white oak as 



related to soil moisture. Soc. Amer. Foresters Proc. 83-89. 

 BOYNTON, D. & Reuther, W. (1939) Seasonal variation of oxygen and carbon dioxide 



in three different orchard soils during 1938 and its possible significance. Amer. Soc. 



hort. Sci. Proc. 36, 1-6. 

 Fisher, R.A. (1921) Some remarks on the methods formulated in a recent article on 



'The Quantitative Analysis of Plant Growth'. Ann. appl. Biol. 7, 367. 



