12 Biological Decomposition of Solid Wood T. K E N T KIRK U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI 53705
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E L L I S B. C O W L I N G School of Forest Resources, North Carolina State University, Raleigh, N C 27605
Decomposition of wood is an important part of the carbon cycle of nature. Decomposition is caused by fungi, insects, and marine borers that use the wood as food or shelter, or both. Lignin in wood provides a phys ical barrier to enzymatic decomposition of cellulose and hemicelluloses. This barrier is breached mechanically by insects and marine borers, biochemically by whiteand soft-rot fungi, and possibly by small nonenzyme cata lysts in the case of brown-rot fungi. Cellulose is degraded by e n d o - and exo-glucanases and β - g l u c o sidases, hemicelluloses by endo-glycanases and g l y c o s i dases, lignin by nonspecific enzymes, and perhaps by nonenzymatic, oxidative agents. Rapid strength loss oc curs with all decay fungi, but especially with brown-rot fungi. Strength loss due to insect attack is roughly pro portional to the amount of wood removed. Fungal de composition of wood can be prevented by keeping it below its fiber-saturation moisture content (approxi mately 27% of its dry weight) and by using the h e a r t - w o o d of naturally durable woods (species) or preser vative-treated wood. Useful application of wood-decom posing fungi is limited currently to production of edible mushrooms. Potential applications include biological pulping, pretreatment for enzymatic conversion of wood to sugars, and waste treatment. Many aspects of wood biodecomposition have not been researched adequately. Millions of tons of wood are produced every year in the forests of the world. Observation, however, tells us that the sum-total of wood upon the surface of the earth re mains fairly constant from year to year and from century This chapter not subject to U.S. copyright. Published 1984, American Chemical Society Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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to century. We must, therefore, conclude that there are destructive agencies at work by which millions of tons of wood are destroyed annually. Regarded in this light the problem of what these destructive agencies are, and how they act, becomes of general scientific and economic in terest.
Downloaded by CORNELL UNIV on September 23, 2016 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch012
A. H . R . B u l l e r , preeminent mycologist, 1 9 0 6 (J)
13ULLER
R E C O G N I Z E D T H A T W O O D IS T H E MOST A B U N D A N T O R G A N I C M A T E
RIAL o n e a r t h . H e a l s o r e c o g n i z e d t h a t t h e t r e e s t h a t f o r m w o o d through photosynthetic processes, a n d the fungi and other "destruc tive agencies" that destroy w o o d t h r o u g h respiratory processes are engaged i n a never-ending cycle of biosynthetic and biodecompositional forces. T h e s e relationships are s h o w n b y the f o l l o w i n g s i m p l i fied r e a c t i o n f o r t h e p r e d o m i n a n t p a r t o f t h e c a r b o n c y c l e o f t h e e a r t h : Photosynthesis b y trees a n d other plants 6nC0
2
+ 5 n H 0 + 6 7 7 , 0 0 0 η c a l o r i e s
