**Abstract**

The causes, processes, features and conditional factors of wood decay and degradation due to fungi were reviewed. The degradation path of wood varies depending on diverse fungi and wood species, as fungi possess selectivity in the degradation of versatile wood components. The chemical treatments and preservatives are reviewed to understand their correlation with the decay mechanisms of wood. Environmentally benign wood preservatives are discussed, e.g. one based on chicken feather protein combined with copper and boron salts to replace the traditional wood preservatives together with several environmentally friendly preservatives based on wood extractives as a source of natural raw materials. Excellent functionalities of the protein-based wood preservative suggested that this eco-formulation could offer great potential to be used as an environmentally benign wood preservatives with a more competitive cost. This new system of wood preservatives provides a theoretical basis for further research and the reasonable utilization and scientific protection of wood products.

**Keywords:** wood degradation, mechanism, fungi, environmentally benign wood preservatives, chemical treatment

## **1. Introduction**

Wood, being a natural resource, exhibits its great value and importance, and can be used widely as a structural material and industrial raw material in many fields of the global economy. It is important to note that, despite wood is a remarkable material, it is susceptible to be biodegraded by the action of microorganisms, such as fungi and bacteria. Thus, wood is ready to be decomposed under proper or certain conditions and returns natural components to ecosystem cycling.

Wood is composed of three main components, namely cellulose, hemicellulose and lignin. The highest content of the three main components is cellulose, a long linear homopolymer, which is the main chemical component in wood cell walls, accounting for approx. 40–50% of the dry weight of wood substrates, composed of β-D-glucose molecules connected by [1–4] glycosidic linkages, and can be broken down into reducing sugars by cellulase. Hemicellulose is also a kind of polysaccharide molecules similar to cellulose, but a heterogeneous material which consists of various monosaccharides, and as such easier to be degraded by microorganisms than cellulose. Lignin, an aromatic heteropolymer, is the third major component of wood, accounting for approx. 25–30% of the wood dry weight [1, 2]. Unlike cellulose and hemicellulose, lignin is a polymer composed of condensed phenylpropane units (including benzene ring and aliphatic structure) with an extremely complex structure and chemical properties. The main function of lignin is to provide intensity and durability, as well as resist attacks from microorganisms and insects. Due to its complex structure, lignin is difficult to be degraded, which is often seen as a difficult obstacle in the production and utilization of wood, and only a few microorganisms, such as white rot fungi, possess the ability to completely degrade lignin [3].

In nature, the decay and degradation of wood products is a complex process that involves the combined action of various microorganisms, which can produce many types of enzymes and disintegrate woody materials as an organic substrate by the secreted enzymes [4]. By the loss of the constituents of wood cell walls, such as cellulose, hemicellulose and lignin, wood will lose its valuable strength and stability, and ultimately be bio-deteriorated by microorganisms. Among the microorganisms, white rot fungi, brown rot fungi, soft rot fungi, and bacteria are the most common causes that contribute to the decay of wood [2, 4]. White rot fungi are a few microorganisms that can completely degrade lignin. Brown rot fungi mainly decompose carbohydrates (cellulose and hemicellulose). Soft rot fungi are always ready to degrade polysaccharides. In addition, there are other microorganisms, such as bacteria, which can also degrade the components of wood [5].

Therefore, the relationship between the chemical components of wood and its degradation mechanism is strong. The degree of wood decay, or the degradation of wood chemical components, and the wood decay mechanisms vary depending on different environmental conditions [2]. For example, humid environments are more prone to cause wood degradation since microorganisms are more ready to colonize and reproduce in humid environments. In addition, different types of wood can also affect diverse types of degradation, leading to different levels of decay.

In order to extend wood service life, some treatments can be performed, such as heat treatment, chemical preservation treatment or other methods [6]. Meanwhile, the in-depth research on the wood degradation mechanism could also provide a theoretical basis and technical support for relieving and preventing wood decay and is also meaningful for both proper protection and reasonable utilization of wood.

## **2. Wood degradation by fungi**

#### **2.1 Wood decay fungi species**

Wood decay can mainly be caused by the infection of wood decay fungi. Basidiomycetes [4, 7], which are the most common wood decay fungi in wood and play predominant roles in terrestrial carbon recycling, are well known as members of Basidiomycota, one of two large divisions of *Eumycota*, together with the Ascomycota [8, 9]. Therefore, most wood decay fungi can be assigned to Basidiomycetes or Ascomycetes. In a terrestrial ecosystem, a large group of decomposers, wood rot fungi, has been found, with about 1500 species in Finland and about 2000 species reported in China [10].

*Wood Degradation by Fungi and Environmentally Benign Wood Preservatives DOI: http://dx.doi.org/10.5772/intechopen.112033*

In Division Basidiomycota, Gloeophyllales, an order of Class Agaricomycetes, is capable of producing brown rot of wood and contains several important species, among which *Gloeophyllum trabeum* and *Gloeophyllum sepiarium* are two important wood brown rot fungi species in common service above ground [9]. In particular, *G. trabeum* is well known as an important fungus for the test during the decay resistance trial, which has the tolerance of some kinds of organic wood preservatives.

Polyporales is another order of Class Agaricomycetes in Division Basidiomycota and includes many of the fungi species. In Polyporales, *Rhodonia placenta* is another most common species of brown rot fungi and a common test fungus applied in evaluating new wood preservatives. It is interesting that *Trametes versicolor* is another species in Polyporales, but a white rot fungus, which is also a test fungus commonly used for the assessment of the function of wood preservatives especially in hardwoods.

### **2.2 Inhabiting conditions of decay fungi**

Wood decay fungi have certain requirements in inhabiting conditions for growth and survival [11], several factors affecting fungal colonization and propagation are as follows:


In-depth comprehension of the fungal inhabiting or living conditions is extremely significant as this information can provide us with a better understanding of how these wood decay fungi survive and multiply, and furthermore how to efficiently protect wood and prevent serious degradation. For example, reducing the surrounding humidity or keeping wood dry below the fiber saturation point (25–30%) could efficiently eliminate the fungi growth. In addition, scientific treatments could be carried out to better protect wood resources, and a new wood preservative system could be developed. Besides, novel biotechnology can also be developed to pretreat the woody raw materials or waste wood even including some organic waste, which can promote the development and progress of environmental protection.

#### **2.3 Wood decay process and feature**

#### *2.3.1 Wood decay process*

As mentioned before, wood rot fungi are traditionally divided into white rot and brown rot fungi, and different fungi can decompose the different wood chemical

compositions due to their own selectivity and action mechanism [12, 13]. During the process of wood degradation, white rot fungi can mainly secrete a key group of extracellular oxidases (oxidative enzymes) to degrade lignin, i.e. lignin peroxidase, manganese (II)-dependent peroxidase, and laccase, which is the most typical and common oxidative enzymes possessing the relative strong degrading ability. Hydrolytic enzymes, such as cellulase, hemicellulase, amylase and pectinase can as well as be secreted by white rot fungi, in consequence, white-rot fungi can completely deconstruct the lignocellulose cell wall materials [4]. In addition, according to the degradation and removal of wood chemical components by white rot fungi, it can be classified into "selective" and "simultaneous" decay path [14]: (1) Selective rot initially degrade the hemicellulose and lignin, but retaining the cellulose [15]; (2) Simultaneous rot degraded cellulose, hemicellulose and lignin in a rather uniform depletion [16, 17]. It is noteworthy that the same white rot fungi can cause selective or simultaneous rot when it decayed different substrates, and even both types rot in the same substrates [18, 19].

Unlike the white-rot fungi, brown rot fungi can secrete a large amount of carbohydrate enzymes, such as cellulase and hemicellulase, pectinase including amylase. Brown rot fungi extensively depolymerize the carbohydrates (cellulose and hemicellulose), leaving the fragments of the degraded cellulose and hemicellulose but retaining the modified lignin which is not depolymerized seriously [20–22]. It has long been thought that these basidiomycetes do not decompose the lignin seriously, and their activities on lignin, the abundant aromatic biopolymer, are limited to minor oxidative modifications [23, 24].

As mentioned before, different enzymes display rather various effects on wood chemical components, especially from various fungi as well as different degradation stages, which could be attributed to the fungi's instinctive motivation and selectivity. Thus, different fungi biodegrade wood in their own selective path, and different biodegradation paths vary between different wood species (soft and hard wood).

#### *2.3.2 Decay features*

According to the shape, wood decay can be classified into white rot, brown rot, and soft rot [4]. Most wood decay fungi species are subordinate to Basidiomycota (Basidiomycetes), typically classified into two types, either white- or brown-rot fungi [7]. Brown rot fungi cause significant degradation of cellulose and hemicellulose but with little degradation of lignin, which can only be modified. The typical features of wood brown rot are shrinks and fragmentations, which easily to be decomposed into soft cubic shapes with brown discoloration, due to the lack of cellulose and hemicellulose, and the oxidation of lignin. Conversely, white rot fungi mainly degrade lignin, causing a whitish, needlelike texture or fibrous shape of the decayed wood [25]. Softrot fungi, broadly as "non-*Basidiomycete*" destroyers, resemble the brown-rot fungi which utilize exoglucanases, and endoglucanases to degrade cellulose, which was reported that the attack is limited to the amorphous cellulose zones in the microfibrils [26]. Generally, the attack of soft rot fungi is limited primarily to the carbohydrates in cell walls, and limited modification of lignin, such as demethoxylation.

#### **2.4 Variation of wood property**

Once wood is infected by fungi, wood degradation will be presented outside or/ and inside of wood, with the result of alterations in chemical compositions, physical and mechanical properties, and changes of microstructures. Some alterations of wood properties, such as mass losses, chemical components and microstructures are summarized as below:
