The topic of the paper is importance of dead and decaying wood to sustaining forest biodiversity. These days, a problem of dead wood is considered to be significant as many people do not understand an important value of dead wood for forests. Many people believe that dead trees are only trash and throw them away. However, it just seems that dead wood is transformed into unnecessary garbage. Even after death, it continues playing an important role in the forest ecosystem. In fact, the deceased tree is a whole universe inhabited by various organisms. In such a way, it is highly important to preserve the needed amount of dead and decaying wood in forests to sustain forest biodiversity.
Keywords: dead wood, decaying, tree, insects, species, important
In the natural forest, trees constantly die and in their place the new ones grow. Dead trees form a large amount of dead wood, which is different by breed, size, and the degree of decomposition. Standing deadwood and snags are home to many species of insects, fungi, mosses, and lichens. The process of construction of the house can take hundreds of years before the state of the dead wood will meet requirements of a specific bug or plant. The process of the emergence and expansion of dead wood in the forest is continuous. However, conduction of intensive forestry interrupts this process. Thus, there is a smaller variety of dead wood until it completely disappears from the forest. It can lead to the disappearance of species found in decaying wood. In such a way, it is rather important to save dead and decaying wood in the forest. The greatest attention should be paid to the preservation of such categories of dead wood, which are formed for hundreds of years. First of all, it includes large-sized snags, steady deadwood that can stand for many years, and high stumps of the natural origin. The purpose of the paper is to study the importance of dead and decaying wood to sustaining forest biodiversity.
The Process of Decomposition of Wood
There are many contradictions and disputes around the use of dead wood in forests. Some scientists believe that if to take dead wood from the forest, its biodiversity will decline by more than 30% (Agnoletti, 2006). In turn, there are biologists who consider that its presence is a source for the formation of foci of stem pests and fungal diseases of the growing forest (Bastmeijer, 2016). Despite these opposite opinions, there are still more scientists who state that dead and decaying wood is of the great importance for forests as dead wood in the forest is more alive than when it was a growing tree (Agnoletti, 2006). It is connected with the fact that it is full of living organisms. Dead wood is highly useful to the natural forest ecosystem and, thus, many experts believe that it is better to cut down a living tree than to take out dead wood from the forest.
There are several causes for the appearance of dead wood. Kees Bastmeijer (2016) states that “standing and fallen dead wood is created by tree mortality, which in natural forests is caused by fire, wind, snow breakage, drought, competition, insects, and pathogens” (p. 80). In such a way, dead wood originates from windfalls, logging, and fires. In addition, there is competition of trees in the forest. Such trees are more likely to die standing. They are weakened and have fewer nutrients as a substrate. Another cause is associated with natural death due to age. During life, dead areas are formed on the tree, which are inhabited by fungi and organisms that boost the process of decomposition of wood. In some woods, beavers become a real threat for trees. Long-term flooding and atmospheric phenomena are considered to be one more reason of the occurrence of dead wood.
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The process of wood decomposition is rather difficult. In the book Wildlife Habitat Management, it is noted that “dead and dying trees function differently in each stage of forest succession and the trees themselves progress though a succession of decay stages” (McComb, 2007, p. 158). Windfalls are the most common cause of the occurrence of dead wood. When the tree has fallen due to the wind, small twigs are broken off at first. Then, bigger branches and thick boughs are broken off. From this moment, the tree that has not yet been in contact with the ground and has been supported only by the branches and twigs falls lower and finally falls on the soil surface. Due to the increased humidity of wood tissues and enhanced penetration of soil organisms under the bark or in an increasingly soft wood, an accelerated process of decomposition begins (McComb, 2007). Insects mechanically pound wood, facilitating penetration of bacteria and fungi deep into the trunk. These organisms decompose wood to simple chemical compounds. In many latitudes, frost plays an essential role in expansion of wood (McComb, 2007). On the stages when decaying wood contains much water, freezing water breaks the surface tissue of wood, softens it, and changes the structure. At higher temperature changes and severe frosts, it can be a significant factor contributing to the facilitation of access of different organisms of flora and fauna to such wood.
Over time, a relatively smooth surface of the log is softened and undergoes fragmentation. There are recesses in it, in which spores of liverworts, mosses, ferns, and flowering plants seeds are delayed (Agnoletti, 2006). With time, other kinds of mosses grow in the trunk, which previously would have simply dried up due to the lack of water (McComb, 2007). They partially displace species that have grown there before them and later are superseded by other ferns and flowering plants. Thick carpets of mosses covering some trunks hold enough moisture. It contributes to a more rapid decomposition of wood and leads to the change of micro climate of the forest in the vicinity. Development of flowering plants is possible only when the thickness of the layer of decomposed rot reaches several centimeters (McComb, 2007). With every passing year, the trunk settles deeper into the soil, losing its initial form. In such a way, a process of decaying of wood is extremely complex and long.
The Importance of Dead and Decaying Wood
A proper functioning of the forest ecosystem is in balance between the processes of growth, withering of trees, and wood decomposition of dead trees. Dead wood of forests is inhabited by one-third of forest biodiversity. The latter is highly important for the proper functioning of the forest. In the book The Analytic Hierarchy Process in Natural Resource and Environmental Decision Making, it is written that “biodiversity is presented as a decision objective in the hierarchy” (Schmoldt, Kangas, Mendoza, & Pesonen, 2001, p. 158). One hectare of the forest may contain from 100 to 200 cubic meters of dead wood (Agnoletti, 2006). According to the World Wildlife Fund, dead wood is considered to be a key indicator of naturalness of forest ecosystems (Tomppo et al., 2011). It plays a key role in maintaining productivity of the forest and its ecological functions, resilience of forests, and fixation of carbon dioxide (Agnoletti, 2006). Dead wood prevents land erosion, retains moisture, and fertilizes soil. Old hollow trees and dead wood provide forest fauna with the shelter and representatives of flora, invertebrates, and bacteria with food (Agnoletti, 2006). Calculations performed in France have showed that the provision of new protected areas where felling is prohibited can store as much carbon dioxide as planting of new forests (Tomppo et al., 2011). Dead wood is so important for the life of the natural forest ecosystems that its disappearance is able to radically change the composition of living organisms that live in these ecosystems, lead to the disappearance of many of them, violate natural processes of regeneration, and in some cases eventually lead to the formation of wastelands and glades on the former site of the forest.
Since the death of the tree, there occur impetuous processes of biotope changes in its immediate environment. The amount of light and water reaching lower layers of the forest increases. Conditions of life of bacteria, fungi, and animals living in the soil change (Schmoldt et al., 2001). The abundance of food and light causes a rapid growth of plants, which have been forced to endure hardships under the crown of the tree. The young generation of trees finally gets a real chance to break into the top tiers. This mechanism of change of generations of trees and maintenance of high biological diversity in many types of forests is called gap dynamics (Schmoldt et al., 2001). Currently, it is the main mechanism of self-maintenance of natural tropical, subtropical, and boreal forests.
Dead wood is necessary for the types of organisms that live in it or for which it serves as a breeding ground. Wood of dead trees serves as the habitat for many species of fungi, insects, and other living organisms. In the book Designing and Conducting a Forest Inventory, it is stated that “dead wood is an important habitat for many specialized insect, polypore, moss, and liverwort species” (Tomppo et al., 2011, p. 56). For many of these species, not only the presence of dead wood is important, but also its size, species composition, and number (Schmoldt et al., 2001). In forests where the amount of dead wood is several times lower than in indigenous forest ecosystems, many of these species have become a real rarity these days.
However, dead wood is no less important for many other forest species. A good example is forest trees themselves, for which dead wood is the main substrate for renewal in many types of forest ecosystems (Tomppo et al., 2011). For instance, in indigenous dark conifer forests with the dense ground cover from large forest herbs, ferns, and even blueberries, seedlings of spruce or fir have virtually no chances to survive on the soil surface (Tomppo et al., 2011). It is associated with the fact that too little light penetrates to the surface through a dense cover of forest grass. The same thing happens in the woods with a powerful moss cover (Tomppo et al., 2011). Seedlings of trees do not have time to reach roots of the mineral soil during the first months of life. As a result, they die due to lack of mineral nutrition or drought. Under these conditions, wood of dead trees is virtually the only suitable substrate for the resumption of trees. Decaying wood provides saplings with necessary mineral nutrition and moisture accumulated in dust of rotten wood is usually enough for the whole summer season. Trunks of trees, especially the large ones elevated above the surface of the grass cover, provide proper illumination of seedlings and young trees (Tomppo et al., 2011). As a result, in many types of forest ecosystems a viable undergrowth of forest trees can only be found on decaying tree trunks and stumps.
Lying tree trunks, especially in deciduous forests that have abundant grass cover, offer particularly favorable conditions of living for reptiles and contribute to their thermoregulation (Rotherham, Jones, Smith, & Handley, 2011). Other reptiles such as glass-lizard, grass snake, and adder also like gaps formed in dead wood. In addition, trunks and stumps of trees provide a good shelter for animals from predators. Old decaying stumps are also suitable for wintering. Highly decomposed wood serves for amphibious species as a refuge and a food source. Toads find there different invertebrates and moor frogs and newts hibernate in decaying trunks and stumps (Rotherham et al., 2011). Lying trunks retain much moisture and significantly alter the micro climate in their environment, which is conducive to the creation of appropriate conditions for amphibians that are especially dependent on the degree of humidity.
Dead wood also plays an important role in the formation of specific micro-relief surface of the soil that is typical for many types of indigenous forests. Along with the so-called windfall-soil complexes, including bumps and depressions formed during the fall of large trees with roots, decaying tree trunks form a complex system of pits and mounds on the soil surface, which prevent surface runoff of water after heavy rains and especially during snowmelt (Schmoldt et al., 2001). As a result, most moisture is retained in the soil and is gradually absorbed by the soil. It also contributes to a higher water-holding capacity and water protection of forests (Schmoldt et al., 2001). Dead trees falling into streams – temporary or permanent – reduce the rate of flow of water, soil erosion, and removal of sand and clay in rivers.
Finally, one must not forget about the crucial role of dead wood in the carbon balance of forest ecosystems. Since the amount of oxygen released into the atmosphere of the forest is directly related to the amount of organic matter accumulated in the forest ecosystem, the role of dead wood in the overall carbon balance of the ecosystem is highly significant (Tomppo et al., 2011). The longer the trunk of a dead tree is stored in the forest or a large part of wood goes into relatively stable organic compounds of soil, the greater amount of bound carbon is stored in the forest ecosystem and the longer the appropriate amount of pure oxygen remains in the atmosphere (Tomppo et al., 2011). Therefore, it can be seen that dead and decaying wood is highly significant for the forest. Despite all the importance, these materials are in danger. Dead wood in the forests is considered to be the natural habitat, which is currently under the greatest threat of extinction (Rotherham et al., 2011). Old hollow trees and deadwood frequently fall under protection in many countries.
As a rule, there is no dead wood in parks and green areas. All decaying trees, twigs, and branches are removed or burned as they are considered to be unsightly or can threaten safety of people. In parks, living old hollow trees are quite common. Hollows with rot are a unique biotope in which many rare species of invertebrates evolve (Rotherham et al., 2011). Thus, trees with hollows and decaying wood should be not only in forests, but also in green areas and city parks.
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Environmental Damage from Clearing the Forest of Dead Wood
Firewood, stumps, and fallen trunks of trees are not a hotbed of harmful insects and fungi. It is the most important habitat of biodiversity. The presence of piles of firewood in forests is important for a successful nesting of a number of birds. Falling into rivers, dead trees provide a substrate for algae and insects. They are also a good place for spawning of fish. For example, in the United States fallen trees have created 50% of areas for the salmon breeding in streams (Stokland, Siitonen, & Jonsson, 2012). Decaying fallen tree trunks are the foci of the resumption of indigenous forest species in the burnt forest. In Sweden, the well-being of 805 species listed in the Red Data Book is dependent on dead wood (Stokland et al., 2012). According to the World Wildlife Fund, one third of the species of flora and fauna living in the forests inhabit dead wood (Tomppo et al., 2011). One dead tree can be inhabited from a few dozens to several hundred species of fungi, plants, and animals (Stokland et al., 2012). Dead wood plays an important role in maintaining the ecological balance, forest reproduction, moisture fixation, and protection of biodiversity.
Measures on liquidation of dead wood in forests lead to the destruction of habitats of many species of animals and plants. As shown by studies of American scientists, dead wood left after windfalls contributes to the rapid recovery of the forest (Stokland et al., 2012). In this regard, many ecologists demand to prohibit the elimination of dead and decaying wood in forests of the natural reserve fund. A planet is one large system where everything is connected. If to pick up dead trees from the forest, a habitant for many species of living organisms will disappear. Some people think that small bacteria are not important. Perhaps, these invisible bacteria seem to be useless, but people cannot know what will be necessary for the survival of the humanity in the future. For example, a yew was rather common in the Middle Ages. This strong and resilient wood was used to make bows. Then, fashion for such weapons passed and the type began to disappear. Nonetheless, in the 90-ies of the XXth century it turned out that the composition of the wood of the yew contains chemicals that are among the strongest anti-cancer drugs (Stokland et al., 2012). Thus, the smallest bacteria can be extremely useful for people. In such a way, despite the fact that many people think that dead wood is just trash, it is especially significant to preserve dead and decaying wood in forests.
A forest includes not only magnificent spruces and pines, birches and aspens. Not only deer, moose, and tigers live there, but also a great number of inconspicuous and secretive animals inhabit it. A forest is a complex system where along with areas covered by trees, there are also meadows and empty spaces. It is a system, which is constantly changing. Any forest represents a continuous process of life and death. Despite recent advances in forest sciences, there is still a misconception that wood of dead trees is a center of pest and disease breeding. However, it should be remembered that for forest nature dead wood is one of the most complicated stages of the constant flow of life. Decaying tree trunks, dry branches and twigs, as well as dead roots are highly important for the forest ecosystem and its inhabitants.