REFORESTATION IN NORTHERN IDAHO 47 



This, we believe, is an extreme case, but it illustrates possi- 

 bility of tree growth. The junior writer has shown 2 that although 

 the yellow pine stage in succession is usually preceded by shrubs, 

 especially Opulaster pauciUorus, Prunus emarginata, Spiraea 

 corymbosus, or species of Ceanothus, it is still able to invade the 

 prairie even without the protection of an intervening shrubby 

 stage (figs. 8 and 9). 



In a region such as the Pacific Northwest where summer 

 rains are rare or altogether wanting, a new growth of coniferous 

 trees depends upon soil moisture conservation. Soil moisture 

 in sufficient amount to tide seedlings over a four months' period of 

 drought and sunshine depends upon the presence of sufficient 

 humus to retain the moisture and at the same time support a 

 forest cover-growth of early maturing plants which will afford 

 some protection to the tree seedlings of all but the most xero- 

 phytic species such as the yellow pine and Douglas fir. That 

 the growth of herbaceous plants and shrubs may be so dense 

 as to preclude the development of the young conifers not in- 

 frequently happens. That the preclusion of tree seedlings is 

 usually due to light is a question to be determined by experi- 

 ment and in the field. Repeated observations and measurements 

 of the soil moisture content and the evaporating power of the air 

 together with light values show conclusively that, although the 

 soil moisture may be far more than sufficient for tree growth and 

 the humidity of the air relatively high, yet when the light values 

 drop to yi of normal sunshine our hardiest pioneer, the yellow 

 pine, is excluded. These observed facts invite the inference that 

 light is here the factor of prime importance. Of soil moisture 

 and humus there may be plenty, but if light beyond a certain 

 degree is shut out these seedlings will not be found. 



