126 COMPOSITION OF FOREST CROPS. 



nally attempted, in which case a pure forest from the outset would 

 obviously have been preferable. 



2. On the suitability of the climate and locality for the more or 

 less prosperous growth of more than one species. This is self-evident. 

 The expression "more or less prosperous" must be understood here 

 in the same sense as in the discussion of the preceding condition. 



3. On the ability of the mixed species jointly to maintain and im- 

 prove the fertility of the soil. If the mixed growth were such that 

 the soil deteriorated under it, then the proportion of some one 

 species, the hardiest and most tenacious of the mixture, would, in 

 the majority of cases, gradually increase to the detriment of the 

 rest, and in time the crop would consist wholly, or almost wholly, 

 of that species. 



4. On the requirements of the several species as regards light be- 

 ing different. All woody species require light in order even to 

 simply remain alive, but some can endure shade better others. The 

 ability of a plant to endure shade is usually proportionate to the 

 density and shape (spread and depth) of its crown, as well as to the 

 height of the latter above the ground. If all the associated species 

 possessed the same requirements as to light, the mixture would be 

 no better than a pure forest, and, unless the species in question 

 were extremely shade-enduring, the ultimate condition of the leaf- 

 canopy would become more or less unsatisfactory. 



5. On the relative hardihood of the several species. The hardi- 

 hood of the various species during their youth is vary various, and 

 depends in a great measure on the character of the locality and soil 

 in any given case, a fact that should never be forgotten by the 

 forester. In certain localities, and in certain soils, some species 

 cannot do without the protection against frost and drought aiforded 

 by shelter and cover, while others again, although able to grow up 

 in the open, nevertheless thrive better when sheltered. 



6. On the relative rapidity of their growth in height at various 

 ayes, especially during their youth. It is a matter of very great mo- 

 ment whether in a mixed crop the more shade-enduring species 

 grow up with greater, equal, or less rapidity than the rest, the im- 

 portance of this consideration being greatest during their youth. 

 To take only a single instance: if the crop consisted of individuals 

 of more or less the same age or height, it is obvious that the more 

 rapidly growing species, provided they were also the most shade- 

 enduring, would, as soon as they had joined crowns over the rest, 

 kill or at least permanently throw back these latter ; while, if the 



