FOREST PATHOLOGY IN FOREST REGULATION. 31 



reaching to the heartwood admit air, and by evaporation change the 

 water content of the heartwood; by oxidation, changes in its chemistry 

 probably also take place. 



The change must necessarily be intensified, the more serious the 

 injury. Wounds exposing the heartwood heal over very slowly, and 

 the heartwood which receives all its water from the sapwood must be 

 modified very markedly in its chemistry and physics, particularly in 

 its water and oxygen content, by the long exposure to the air. More- 

 over, from the time the injury happened until the time the wound is 

 completely healed the heartwood is directly exposed to inoculation 

 from spores of wood-destroying fungi adapted to white fir. Although 

 the production of spores by a sporophore is enormous, by far the 

 greater number are carried by air currents to places where they can 

 not germinate for lack of moisture; many are intercepted by the 

 natural screen ("forest screen") formed by the foliage and trunks of 

 uninjured trees or by trees to which they are not adapted, and only a 

 very small number finally land in the cracks of exposed sapwood or 

 on exposed heartwood of white fir of the proper age. Of these, again, 

 a very small percentage find temperature and moisture favorable for 

 germination. This explains the fact that so many trees, although 

 badly wounded, are not infected. But it stands to reason that every 

 year during which the heartwood remains exposed adds to the danger 

 of the tree becoming infected. 



By far the deepest wounds are caused by fire. Although in white 

 fir the danger in repeated fires feeding on the pitch flow following a 

 first injury is comparatively slight, and although the lack of resin in 

 the wood does not favor the hollowing out of the interior of the tree 

 as it does in yellow pine, fire frequently causes very long wounds, which 

 reach into the heartwood. 



Spores can gain entrance to the heartwood through open frost 

 cracks. Low temperatures and sudden drops of temperature are 

 common throughout the range of white fir. Inside of the range they 

 are more or less confined to certain localities and zones. 



Lightning in white fir generally causes more or less superficial 

 wounds. The peculiar injuries to be traced to lightning in white fir 

 present many interesting features. Here we are only interested in 

 injury which might lead to the infection of the heartwood. As in 

 yellow pine, lightning sometimes tears long strips of bark off the tree 

 and leaves the cambium and sapwood unprotected. In such cases 

 both die and dry out, with the result that cracks in the sapwood lead 

 into the interior of the tree. Smaller superficial lightning wounds 

 locally kill tli'- sapwood, which is very commonly attacked by second- 

 ary fungi, which do not do very serious damage. However, logs with 

 lightning injury of this kind are liable to be thrown out as culls, 

 although fchey generally contain some sound heartwood. 



