Photosynthesis 139 



melt, is an important factor dictating the extent of canopy development 

 and therefore the uptake of carbon dioxide. 



Spatially, the uptake of carbon dioxide is under a similar control. 

 The productivities of the vegetation types are largely a function of the 

 plant densities or foliage area indices, which are determined by the mois- 

 ture and nutrient (mainly phosphorus) gradients. Wetter and more fertile 

 areas are more productive because the plants composing the vegetation 

 types in these areas allocate more carbon for photosynthetic tissues. The 

 photosynthetic rates on a leaf basis of a given growth form remain com- 

 parable across a wide range of microtopographic units. 



