THE EFFECTS OF EXPOSURE ON GROWTH 

 AND DEVELOPMENT 



P. H. Whitehead 



Imperial College, London 



The importance of wind as a factor in plant growth has long been recog- 

 nized. In the field of agriculture and forestry many empirical studies have 

 been carried out mainly associated with the design and location of 'wind 

 breaks'. Far less attention has been focused on the experimental investiga- 

 tion of effects that exposure to wind during growth may have on the 

 development and physiology of plants. 



At the beginning of the investigations presented here it was not even 

 clear to what extent wind effects could determine survival or death of 

 plants or communities. As a preHminary to laboratory investigations field 

 studies were carried out on M. Maiella in the Central Apennines of Italy. 

 This mountain has a fairly flat altopiano near the snow line covered with 

 Hmestone rock detritus and with typical 'alpine desert' vegetation. It is 

 particularly exposed to strong winds and the various community types, 

 often in close juxtaposition, appeared to be associated with the degree of 

 shelter from wind afforded by the rock fragments (Whitehead, 195 1). The 

 aerodynamic parameter Zq was found to be the best measure of the degree 

 of shelter from wind afforded by the surface irregularities. In these alpine 

 communities where the vegetation is sparse and the main drag on the air 

 stream is caused by the rock fragments themselves this parameter is not 

 open to the objections which can be made against its use in relation to 

 most vegetation types. The average values of Zq were determined in the 

 field for four of the community types described in Whitehead (195 1). Four 

 of these communities, A, E, G, and H, representing a gradation of 

 exposure from most exposed to least exposed were studied in detail. It 

 was found that there was a clear relationship between Zq and average 

 plant height; average inflorescence height and annual yield (Fig. la and b). 

 The larger the value of Zq and thus the greater degree of shelter the taller 

 were the plants and the greater the annual yield. 



The degree of shelter, expressed as Zq also appeared to be related to 

 both the number of species and the diversity of growth forms in a com- 

 munity (Fig. ic). The smallest number of species and least diversity of 



