3. Coarse Woody Debris – Nutrients and Essential Elements

77.  What makes a healthy tree or plant?  The availability in the proper proportions of the right "STEW" - Space, Temperature, Elements and Water.  And the energy of the sun will be used optimally making a tree into the most efficient system on earth.  Everything is recycled. 

78.  Forest managers need to know what actually happens in order to plan harvests that will protect essential element and nutrient cycles and streams from low pH precipitation (Hornbeck, 1992, page 151).

79.  Increasing demands for wood products, especially chips for fuel and pulp, coupled with new, highly mechanized logging equipment are resulting in more intensive harvesting of wood out of once fertile forests. When applied in the form of whole-tree clear- cutting, intensive harvesting is a severe disruption of forest nutrient cycles and essential elements. The first 5-10 years after harvest are especially critical in terms of nutrient and essential element transformations, movement, and loss from the ecosystem.  (Hornbeck et al., 1990, pg 55)

80.  In New England, intensive harvesting (wood removal) in the form of whole-tree clearcutting results in important losses of plant essential elements such as Ca, K, and N.  Shortages of plant-available essential elements might develop in regenerating stands, particularly in the years immediately after harvest when leaching losses and plant uptake are high.  Net losses in input-output budgets and preferential uptake by trees for essential elements such as Ca suggest that there also could be essential element limitations during future rotations. Until these concerns are researched more carefully, whole-tree clearcutting should be applied with caution (Hornbeck et al., 1990, page 63)

81.  Research in old-growth Douglas-fir forests, have shown about as much nitrogen accumulates in decaying, fallen trees as in the forest floor. Other essential elements, such as calcium and magnesium also accumulate in decomposing woody substrates.  Although here we are concerned with Douglas fir, neither decaying wood nor research data are unique to forests of the Pacific Northwest   (Maser and Trappe, 1984, pg16-par2).    

82.  Decomposition of fallen trees releases essential elements for microbial and plant growth (Maser, Tarrant, Trappe and Franklin, 1988, pg37-par1).  
Elements other than nitrogen such as calcium and magnesium, also accumulate in decomposing woody substrate.  (Maser and Trappe, 1984, pg 16-par2)
83.  A snag may accumulate moisture – carried essential elements and have a higher essential element capital when it falls than does a tree with symplast (Maser and Trappe, 1984, pg 19-par 2).    

84.  Woody duff, regardless of type or size, takes considerably longer to decompose than needle and leaf duff.  Needles, leaves, and small twigs decompose faster than larger woody material and essential elements are thereby recycled faster in the forest floor. About 140 years are needed for essential elements to cycle in large, fallen trees and more than 400 years for such trees to become incorporated into the forest floor; they therefore interact with the plants and animals of the forest floor and soil over a long period of forest and stand successional history (Maser, Tarrant, Trappe and Franklin, 1988, pg37-par2).   

85.  Although nitrogen fixation in wood is modest compared with that occurring in other substrates in forests, the persistence of decaying wood allows small increments of nitrogen to accrue over many decades (Maser and Trappe, 1984, pg 16-par3).   

86.  Further, decomposing wood undergoes changes in other chemical constituents and pH as well as physical structure.  Very old, decayed wood can even become somewhat humified and leave long lasting substrate resistant to further decay (Maser and Trappe, 1984, pg 16-par 4).  

87.  Decaying trees comprise considerable accumulations of mass, nutrients and elements in unmanaged, old growth forest.  Some of the largest accumulations occur in the unmanaged forest of the Pacific Northwest. Coarse woody debris can range from 130 to 276 tons per acre in stands from 100 to more than 1,000 years old. Although here we are concerned with Douglas fir, neither decaying wood nor research data are unique to forests of the Pacific Northwest.  McFee and Stone ( 1966) Observed that decaying wood persisted for more than 100 years in New York and others pointed out that substantial accumulations  in old-growth forest in Poland.  These observations evidence the long-term continuity of decaying trees as structural components in forest (Maser and Trappe, 1984, pg 16-par1).

88.  Decayed logs on the floor of a once fertile forest are a reservoir for nutrients as well as essential elements.  They also act as a storehouse  for moisture providing moisture for plants and animals during dry times such as summer – so called - drought (Page-Dumroese, Harvey, Jurgensen and Graham, 1991).  Note: Trees absorb  of essential elements that are water-soluble and dissolved in water.  For the elements to enter the tree, moisture is required.

89.  Note:  I say “so called” because many trees and animals would have moisture during dryer times, i.e., if coarse woody debris were in place and functioning over time as designed (A unique survival feature of a forest).  Thus, with the removal of CWD come several depletions, which include, but are not limited to, the depletion of water, essential elements and nutrients for plants and animals above as well as below ground.  “The Demons Of D” at work.  So, drought is what we call the trigger puller, not the primary agent causing lack of water during dryer times.

90.  During decomposition, logs and other forms of coarse woody debris (defined as wood pieces more than ten centimeters in diameter and more than one meter in length) reduce erosion and affect soil development, store nutrients and water, provide a source of energy and essential element flow, serve as seedbeds, and provide habitat for decomposers and heterotrophs (Harmon and Hua, 1991).

91.  An important feature of woody debris is that nutrients are released at slower rates than from fine duff. This slow release allows nutrients to be retained within the ecosystem until tree production recovers. Timber harvest and salvage after disturbance reduces this pool of stable nutrients and essential elements (Harmon and Hua, 1991).

92.  Few studies have examined processes, other than nitrogen fixation, that are responsible for net changes in nutrient content of coarse woody debris. It is tempting to assume that the processes are the same as in fine duff, but recent research being conducted at Andrews indicates some differences. For example, during the early stages of log decomposition, fungal sporocarps transfer essential elements to the forest floor. Thus, in fine duff, fungi immobilize nitrogen, but in coarse woody debris they actively transfer it to the soil. Another important consideration in understanding nutrient release from coarse woody debris is that tree boles are composed of several distinct substrates. While wood may be slowly releasing nutrients, other parts such as the inner bark (phloem) decompose and release nutrients at rates similar to those of leaf duff. Hence an overall pattern of release from symplastless  trees may be a rapid loss of 10-20% of the nutrients followed by an extended slower release of nutrients. Finally, the role of fragmentation in transferring nutrients to fine duff in the later stages of woody debris decomposition is not revealed by patterns of net accumulation. The omission of transfers via fragmentation from previous calculations suggests (Harmon and Hua, 1991).  (NOTE:  it may be specifically unclear whether the paper is referring to  essential elements or a true nutrient.  Both exist, and are essential for system health.)

93.  During decomposition, logs and other forms of coarse woody debris (CWD) reduce erosion, affect soil development, store nutrients and water, are a potentially large source of energy (nutrients) and essential elements, serve as a seed bed for plants, and form an important habitat for fungi and arthropods. Despite growing recognition that symplastless trees play major roles in ecosystem function, many aspects of the specific processes involved are poorly understood. Consider, for example, the importance  in forest essential element cycles. Aside from nitrogen fixation, few studies have directly examined the processes responsible for the net changes in essential element content of decaying wood. The actual proportion of tree nutrition that is derived from CWD is not known  (Kropp, 1982).

94.  Symplastless trees are structural components of great importance for forest dynamics and forest biodiversity.  The decomposition of trees provides an important link in cycling on nutrients and essential elements in ecosystems.  In addition, many species of plants, fungi, and animals are dependent on symplastless trees for nutrients, essential elements, habitat or substrate and nesting (Kruys and Jonsson, 1999).

95.  Soil, nutrients and essential elements deposited along the up slope side of fallen trees reduce loss of nutrients from the site. Such spots are excellent for the establishment and growth of vegetation, including tree seedlings.  Vegetation becomes established on and helps stabilize this "new soil", and as invertebrates and small vertebrates begin to burrow into the new soil, they not only nutritionally enrich it with their feces and urine but also constantly mix it by their burrowing activities (Maser and Trappe, 1984 pg 4-par1&2).

96.  As a log decomposes, many organisms such as plant roots, mites, collembolans, amphibians, and small mammals, must await the creations of the inner space before they can enter.  The flow of plant and animal populations, air, water, and nutrients as well as essential elements between fallen tree and its surrounding increases as long as aging process continues (Maser and Trappe, 1984, pg 12-par1).

97.  Duff fall and throughfall are major pathways for the flow of essential elements and energy within forests,  they contribute essential elements, nutrients and water to so-called rotten wood. The larger a fallen tree, the more duff it accumulates on its surface and the more essential element - rich moisture it intercepts from the canopy.  The moisture gathers essential elements as it passes through the accumulated duff and soaks into the fallen tree (Maser and Trappe, 1984, pg 19-par 2).

98.  CWD, and the associated epiphytic bryophytes act as both essential element and moisture buffers for the ecosystems (FEMAT, 1993). This buffering allows the slow release of water and essential elements to surrounding plants. In mature and old growth coastal forests, a large proportion of western hemlock and Sitka spruce seedlings germinate and grow on CWD substrates (Harmon and Franklin 1989; G. Davis, pers. comm., 1994).

99.  The main chemical differences among substrates are: (1) nitrogen content; (2) mineral or ash content-phosphorus. Potassium, calcium, magnesium; (3) the carbon matrix-cellulose, lignin, pentosans and (4) the content of other organic compounds-waxes, pigments, carbohydrates,  fats,  resins,  phenolic compounds (Maser and Trappe, 1984 pg11-par2).  

100.  Plant - essential elements.  The succession of plants on fallen trees is mediated by changes in essential element availability and physical properties over time.  Three broad phases can be defined: initial, optimal, final. Early invaders prepare the tree for later colonization by altering its physical and chemical properties during the initial phase.  The  altered tree provides the best substrate for a wide array of organisms during the optimal phase. Ultimately, the depletion of essential elements and physical deterioration of the wood during the optimal phase diminish its value for many organisms, so fewer species inhabit the final phase (Maser and Trappe, 1984, pg 25-par 5). 

101.  Besides nitrogen, other essential elements such as Calcium , Magnesium , Potassium, and Phosphorus and other essential elements play key roles in soil, plant and tree health as well as the health of the other associated living organisms (Page-Dumroese, Harvey, Jurgensen and Graham, 1991).

102.  In addition, coarse woody debris has the potential to store a large amount of carbon in the ecosystem. The role of coarse woody debris in storing carbon is often overlooked, with only living plants or soil carbon being considered. Relatively little is known about the formation and rate of decay of coarse woody debris or the factors controlling these processes, despite the relevance of this information to the global carbon cycle (Harmon and Hua, 1991). 

103.  As fallen trees progress from decay class I to class II, the scavengers are replaced by competitors with the enzyme systems needed to decompose the more complex compounds in wood. The fungi involved in this activity are often mutually antagonistic, so that a given part of the tree may be occupied by only one fungus that excludes others by physical or chemical means (Maser and Trappe, 1984,pg27-par4).   (We call this altered area a niche)

104.  The decomposing wood of a fallen tree serves as a savings account of essential elements and organic material in the forest soil (Maser and Trappe, 1984, pg 16-par2).

105.  Fallen trees interact with essential element cycling processes in a forest through such mechanisms as duff  fall (freshly fallen or slightly decomposed plant material from the canopy),  throughfall (rain or dew that picks up elements as it falls through the canopy), nitrogen fixation, and essential element uptake by plants associated with the fallen trees (Maser and Trappe, 1984, pg19-par2). 

106.  Ground contact by fallen trees creates opportunities for various interactions with the biotic components of soil and duff. Fungi, for instance, translocate essential elements within the soil- system, as both decomposers and root symbionts. Fungi also immobilize translocated essential elements and thereby enrich the decomposing wood substrates they inhabit. In addition, the colonization of decomposing fallen trees by nitrogen-fixing bacteria permits additional nitrogen accretion within the decaying wood (Maser and Trappe, 1984, pg 19-par 3).        
107.  Western hemlocks colonize so-called rotten wood over many decades to insure long-term interactions by root zone processes. Decaying wood thus serves as a savings account of soil organic materials and essential elements in forest (Maser and Trappe, 1984, pg19-par4).   

108.  Internal succession is also influenced by temperature, moisture, and stage of decay. A class I fallen tree, for example, has many readily available essential elements that support opportunistic colonizers. As decay proceeds its moisture holding capacity increase but essential elements become less available because either they have been used or the remain locked in the more decay resistant compounds of the wood.  Ultimately, the rapidly growing opportunists are succeeded by organisms with more sophisticated enzyme systems, and decay continues (Maser and Trappe, 1984, pg37-par2).  

109.  External succession is related to the changes that take place in the plant community surrounding a fallen tree. A fallen tree is a connector between the successional stages of a community;  it provides continuity of habitat from the previous forest through subsequent successional stages. A large fallen tree therefore provides a physical link – an essential element savings account – through time and across successional stages.  Because of its persistence, a fallen tree provides a long- term, stable structure on which some animal (both invertebrate and vertebrate) populations appear to depend on for survival (Maser and Trappe, 1984, pg 38-par 1).    

110.  Machine entry on an area, which contains trees, reduces diversity because heavy equipment fragments and scatters class IV and V so called rotten wood.  Habitat diversity declines to a fraction of what had been available, probably fewer kinds of organisms can thrive.  Further, because woody substrates serve as long-term soil organic material and essential element reservoirs, increasingly intensive timber management, coupled with shorter rotations, could significantly alter the role of decaying wood in the essential element cycling processes (Maser and Trappe, 1984, pg 48-par 1).   

111.  Humus formation is important in regulating the incorporation of nitrogen into humic materials.  Because of its high cation exchange capacity and slow decomposition, so called rotten wood can retain available mineral nitrogen from throughfall and decomposition as well as organic nitrogen compounds mineralized within the wood chemical matrix.  Roots and mycorrhizae of plant species that colonize decaying wood use its available nitrogen (Maser, Tarrant, Trappe and Franklin, 1988, pg40-par2).    

112.  The long-term input by nitrogen fixation in decaying fallen trees and by canopy inhabiting lichens maintains a positive balance of nitrogen in the ecosystem  (Maser, Tarrant, Trappe and Franklin, 1988, pg40-par5).    

113.  Decaying wood has long-term potential for contributing nitrogen for tree growth as residual lignin and humus are decomposed (Maser, Tarrant, Trappe and Franklin, 1988, pg41-par1).   

114.  With respect to tree maturity, habitats, both external and internal, are influenced by tree size – maturity ( Internal Regulating System). An uninterrupted supply of new, immature wood in young forests decomposes and recycles essential elements and energy rapidly. Habitats provided by the death of the symplast of young trees are short-lived and rapidly changing.  (E.g., specifically speaking, species of young trees, which produce protection wood such as heartwood, would have not formed heartwood).  In contrast, the less frequent, more irregular mortality of the symplast of large trees in old forests is analogous to slow-release fertilization. The lasting quality of large fallen trees creates stable habitats in which large woody debris accumulates. Scattered accumulations of large woody debris are associated with openings in the forest canopy. Large fallen trees in such an area often contact each other physically, creating external habitats of intense biological activity (Maser, Tarrant, Trappe and Franklin, 1988, pg44-par2).   

115.  Decaying, fallen trees contribute to long-term accumulation of soil organic matter, partly because the carbon constituents of well-decayed wood are 80-90 percent residual lignin and humus. Decaying wood in the soil and establishment of conifer seedlings and mycorrhizal fungi on dry sites are positively correlated. Fallen trees also create and maintain diversity in forest communities. Soil properties of pits and mounds differ from those of surrounding soil; such chemical and topographic diversity in turn affects forest regeneration processes.  All this, especially large fallen trees that reside on the forest floor for long periods, adds to spatial, chemical, and biotic diversity of forest soils, and to the processes that maintain long-term forest productivity (Maser, Tarrant, Trappe and Franklin, 1988, pg44-par3).

116.  Mycorrhizal fungi can colonize logs presumably using them as sources of water and  essential elements.  (Franklin, Cromack, Kermit, et al. others, 1981). 

117.  Coarse woody debris is a significant factor in essential element cycling processes (Harmon et al. 1986; Caza 1993). Although the relative concentration of essential elements in wood and bark is low, much of the essential elements capital and carbon are stored here because of the large biomass involved (Harmon et al. 1986; Caza 1993) (Voller and Harrison, 1998).       

118.  Symplastless wood facilitates a slow release of essential elements, ameliorates leaching, and provides a growing substrate for bryophytes. These buffer water and essential element release from duff and aboveground processes, especially processes such as nitrogen fixation in aboveground plants such as hepatics (Harmon et al. 1986; FEMAT 1993; Samuelsson et al. 1994) (Voller and Harrison, 1998).     

119.  Free-living bacteria in woody residues and soil wood fix 30-60% of the nitrogen in the forest soil. In addition, 20% of soil nitrogen is stored in these components (Harvey et al. 1987). Harmon et al. (1986) reported that CWD accounted for as much as 45% of aboveground stores of organic matter.  Symplastless wood in terrestrial ecosystems is a primary location for fungal colonization and often acts as refugia for mycorrhizal fungi during ecosystem disturbance (Triska and Cromack 1979; Harmon et al. 1986; Caza 1993) (Voller and Harrison, 1998).    

120.  Colonization of symplastless wood by fungi and microbes may be one of the most important stages in essential element cycling (Caza 1993); however, these processes are still relatively poorly understood. Soil wood contains a disproportionate amount of the coniferous non-woody roots or ectomycorrhizae in forests (Harvey et al. 1987). As one of the dominant sources of organic matter, symplastless wood is an important determinant in soil formation and composition (Caza 1993) (Voller and Harrison, 1998)   

121.  Symplastless wood provides physical structure to the ecosystem and fills such roles as sediment storage (Wilford 1984), protecting the forest floor from mineral soil erosion and mechanical disturbance during harvesting activities. It ameliorates the affects of cold air drainage on plants, helps stabilize slopes, and minimizes soil erosion (Maser et al. 1988).  Symplastless wood provides elevated germination platforms with reduced duff fall accumulation and relatively consistent moisture regimes (Harmon et al. 1986; Maser et al. 1988; Caza 1993; D.F. Fraser, pers. comm., 1995). In stream ecosystems it protects stream banks from erosion and maintains channel stability (Triska and Cromack 1979; Sedell et al. 1988). Features that influence the ability  to fulfill these functions include size (length and diameter), whether roots are still attached, orientation, degree of burial, and proportion of the piece that remains submerged (Sedell et al. 1988)  (Voller and Harrison, 1998).      

122.  The substrate of poorest quality is the decay-resisting outer bark, which is low in moisture, carbohydrates, cellulose, and carbon to nitrogen (C:N) ratio but high in lignin, taxifolin, total extractives, and density. (Maser and Trappe, 1984 pg 1-par4 ). 

123.  In class IV element content of the fallen tree at this stage may exceed the original content because minerals have been added by duff fall from the canopy and by throughfall of rain, have been brought in by animals or have been translocated from underlying soil by fungi or roots.  Nitrogen may be added by similar means and by biological fixation.  These circumstances provide an excellent rooting medium for plants.  A great variety of fungi, both decomposers and symbionts, thrive in the complex of niches within the fallen tree (Maser and Trappe, 1984 pg 26-par 5, pg 27-par 1).  

124.  Conclusion:  What purpose and need is there, that the capacity and ability, of CWD, to function as a nutrient and essential element storehouse, go unobserved in this “Burn and Clearcut Project”?  Technical reports clearly point out that the long-term continuity of decaying trees, are structural components of forests.  CWD are reservoirs for nutrients as well as essential elements for long periods of time.  CWD provides a source of energy and essential element flow.  Timber harvest and salvage after disturbances reduces pool of stable nutrients and essential elements. Symplastless trees are structural components of great importance for forest dynamics and forest biodiversity.  Many species of plants, fungi and animals are dependent on symplastless trees for nutrients, essential elements, habitat or substrate and nesting. The benefits and their persistence, in the cycling of essential elements and providing nutrients is a function which contributes to system health and a obligatory function to operate at a high quality state, i.e., operating about the means in which is was designed.  Therefore the removal of such materials that would provide a physical link – an essential element savings account – through time and across successional stages is not indicative or technically published to be, a treatment, which would protect or increase forest health.  In all honestly, it will reduce protection thus forest health as well.

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John A. Keslick, Jr.


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