66 DEVELOPMENT OF FOREST TREES. 



only in small quantities or in exceptionally favourable years) 

 when they have been cultivated beyond the limits of their 

 natural home, as, for instance, sweet chestnut and elm in the 

 north of England, Other species are very accommodating; in 

 this respect, such as birch and Scotch pine. 



(3.) Liiiht. — Increased assimilation requires an increased 

 supply of light to the crown ; hence trees growing in the 

 open or on the edges of woods produce more seed than those 

 hemmed in. Suppressed trees rarely produce seed. 



(4.) Species. — The power of reproduction from seed dift'ers con- 

 siderably in the case of different species ; it depends on the total 

 quantity of good seed produced during a certain period, or on the 

 annual average. The quantityof seed is governed by two things : — 



(rt.) The average yield of each seed year. 



{h.) The frequency of seed years. 



As regards the average yield per seed year, the species can 

 be arranged into two sections as follows * : — 



Heavy crops: Beech, oak, spruce, Scotch pine, birch, horn- 

 beam, elm, alder, aspen, willow. 



Lighter crops : Ash, maple, sycamore, silver fir, larch. 



On the other hand, the species may, as regards frequency 

 of seed years, be arranged as follows : — 



Beech seeds only after intervals of six, eight, ten, and even 

 more j^ears. 



Oak, spruce, Scotch pine, and alder seed, on an average, 

 every three to five years. 



The remaining species seed after shorter intervals, and 

 some do so every year. 



Taking both factors into consideration, the species may 1)6 

 arranged into the following scale : — 



Best: Birch, aspen, willow. 



Next: Scotch pine, spruce, wych elm, hornbeam. 



Next: Norway maple, sycamore, silver fir, larch, lime, oak, 

 alder, ash. 



Last: Beech. 



* According to fJiiycr. 



