62 DISEASES OF DECIDUOUS FOREST TREES. 



DECAY OF STRUCTURAL TIMBER. 



After the wood from any of the broadleaf trees has been cut from 

 the tree and it is exposed to the air or soil, decay occurs sooner or 

 later. Different species show a varying power of resistance to decay 

 when thus exposed, and as a result of this difference these woods are 

 usually classed into long-lived and short-lived woods. The long- 

 lived woods possess natural inherent qualities which enable them to 

 resist the attack of decay for a comparatively long period. In com- 

 parison with many coniferous woods, the broadleaf species have a 

 very much smaller power to resist decay, and the number of them 

 which are long lived is rather few. Among those woods which may 

 be considered very long lived are the catalpa {Catalpa speciosa War- 

 der), black locust (Robinia pseudacacia L.), and Osage orange (Toxy- 

 lon pomiferum Raf.). Woods like the white oak (Quercus alba L.), 

 chestnut (Castanea dentata (Marsh.) Borkh.), and shagbark hickory 

 {Hicoria ovata (Mill.) Britton) come next in durability; while among 

 the very short lived woods, the willows (Salix spp.), cottonwood 

 (Populus deltoides Marsh.), beech (Fagus atropunicea (Marsh.) Sud- 

 worth), and tupelo gum (Nyssa aquatica L.) may serve as examples. 



CAUSES OF DECAY. 



The decay of wood after it has been cut from the living tree may 

 be due to a number of causes. It may crumble away because of 

 influences usually referred to as " weathering." Wood fiber when 

 exposed to the air and to frequent wetting by rain is worn away 

 mechanically, giving the surface a roughened appearance. The 

 wood of most broadleaf species after being exposed turns grayish 

 white in color on the outside, due to the separation of groups of wood 

 fibers which stand out from the surface and give it a more or less 

 hairy appearance. 



The principal cause of decay is the growth of certain saprophytic 

 fungi in the wood fibers, which as a result of their growth bring about 

 changes in the wood which alter its structure and tensile quality. 



FACTORS FAVORING DECAY. 



The factors which favor the decay of wood in general have been 

 repeatedly referred to (79, 81) as (1) a certain amount of water, (2) 

 a certain amount of heat, (3) a certain amount of air, and (4) a cer- 

 tain amount of food supply. Where one or more of these four factors 

 are absent, decay will not be possible, because the growth of the 

 wood-destroying fungi is prevented. 



