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Chapter 11.  Biomass and Utilization of Trees

Components of Tree Biomass                                               

Biomass Utilization                                                               

Volume of Standing Trees                                                     

      Individual Tree Volume                                                   

      Forest Inventory 



Chapter 11.  Biomass and Utilization of Trees

        Biomass is a measure of biological matter, customarily expressed in weight. The biomass of a forest is a complex topic that includes all organ­isms, trees, fungi, insects, and so forth, and is beyond the scope of this book. This chapter focuses on biomass of trees. Tree biomass may be that of a single individual or all individuals occupying a unit of area. Since trees have a substantial moisture content (Chapter 1), weights may be either with (i.e., green) or without (i.e., oven-dry) moisture. The remainder of this chapter is concerned with oven-dry biomass of individual trees and the rela­tion between biomass and utilization.

Components of Tree Biomass

        The biomass of trees is often subdivided into above- and below-ground components with further subdivisions of each. For example, above-ground biomass includes foliage, branches, stem, and bark. Various researchers may define components some­what differently. Table 11-1 presents equations for estimating oven-dry biomass components of many commercial tree species found in the Pacific North­west (Gholz et al. 1979). Figure 11-1 shows an example of biomass dis­tribution of a 16 inch (40.64 cm) dbh Douglas-fir tree as calculated from the equations in Table 11-1. About 83% of the biomass of this tree is above-ground and 17% is below-ground. The stem, in­cluding bark, is about 72% of the total biomass. Care must be exercised in interpreting biomass data. Some researchers report only the above-ground portions; the stem (with bark) of the 16 inch Douglas-fir represents 87% of its above-ground biomass. Some may consider the stump as part of the stem, while others include it with the roots. The stem biomass may be the entire stem to the tip of the tree or it may be measured to a minimum top diameter with the remainder considered part of the crown. The original researchers' report must be examined to be certain that definitions of compo­nents are clearly understood. The reference for Table 11-1 is a compilation from many sources and lists the original sources.

Biomass Utilization

        Harvesting systems are involved in removing portions of the above-ground biomass. Whole-tree harvesting converts most of the above-ground bio­mass into logs and chips. Whole-tree harvesting is relatively uncommon because markets for chips that contain bark and foliage are often weak. The more common harvesting practice converts just the stem into logs of specific sizes that are later con­verted to poles, lumber, veneer, and other products. Each of these processes has a minimum size of log that can be used. This generally means that the portion of the stem less than about 4 inches in diameter is not made into logs and is left in the forest along with the crown and below-ground biomass. In some trees, a portion of the stem may also be unusable because it is too crooked, rotten, or broken. Generally these losses, termed defect percent, are low in young-growth trees but can be as high as 70% or more in old-growth trees with advanced decay.

        Figure 11-2 presents a material balance for the 16 inch Douglas-fir tree with the following as­sumptions: (1) The minimum top diameter is 4 inches (10.16 cm). The weight of wood and bark in the stem above this point was estimated by sub­stituting this diameter into the stem wood and stem bark biomass equations. Thus 19.6 kg of stem wood and 3.8 kg of stem bark were left in the forest in the form of an unused top. (2) The defect percentage to account for other stem losses, perhaps a region of rot or crookedness, is 2%. (3) Five logs, 16 feet in length and having small end diameters (inside bark) from 4 to 12 inches, were obtained from the stem. The material balance in Figure 4-1 for processing logs into dimension lumber was applied to each log. The combined results are:

         40%    becomes surfaced-dry lumber

         43%    becomes chips

           7%    becomes sawdust

         10%    becomes planer shavings/dry trim

       100%    total


Table 11-1.  Equations for predicting tree biomass (in kilograms) of Pacific Northwest species.

Species

Y

X

Equation

 

Abies, species
True fir
(pooled)

Total foliage
Live branches
Stem wood
Stem bark

dbh
"
"
"

ln Y = - 3.4662 + 1.9278  ln X
ln Y = - 4.8287 + 2.5585  ln X
ln Y = - 3.7389 + 2.6825  ln X
ln Y = - 6.1918 + 2.8796  ln X

 

Abies amabilis
Pacific silver fir

Total foliage
Live branches
Stem wood
Stem bark

"
"
"
"

ln Y = - 4.5487 + 2.1926  ln X
ln Y = - 5.2370 + 2.6261  ln X
ln Y = - 3.5057 + 2.5744  ln X
ln Y = - 6.1166 + 2.8421  ln X

 

Abies procera
Noble fir

Total foliage
Live branches
Stem wood
Stem bark

"
"
"
"

ln Y = - 4.8728 + 2.1683  ln X
ln Y = - 4.1817 + 2.3324  ln X
ln Y = - 3.7158 + 2.7592  ln X
ln Y = - 6.1000 + 2.8943  ln X

 

Acer macrophyllum
Bigleaf maple

Total foliage
Live branches
Dead branches
Stem wood
Stem bark

"
"
"
"
"

ln Y = - 3.765 + 1.617  ln X
ln Y = - 4.236 + 2.430  ln X
ln Y = - 2.116 + 1.092  ln X
ln Y = - 3.493 + 2.723  ln X
ln Y = - 4.574 + 2.574  ln X

 

Alnus rubra
Red alder

Total foliage
Total wood and bark
  above ground
Stem wood plus bark
Root

dbh2 + H/100

"
"
"

Y = - 0.5124 + 0.1298X

Y =  0.02 + 2.09 X - 0.0015X2
Y =  0.02 + 1.60 X - 0.0005X2
Y =  0.1 + 0.48 X - 0.0005X2

 

Castanopsis
   chrysophylla

Golden chinquapin

Foliage
Live branches
Dead branches
Stem wood
Stem bark

dbh
"
"
"
"

ln Y = - 3.123 + 1.693  ln X
ln Y = - 4.579 + 2.576  ln X
ln Y = - 7.124 + 2.883  ln X
ln Y = - 3.708 + 2.658  ln X
ln Y = - 5.923 + 2.989  ln X

 

Chamaecyparis
   nootkatensis
+ Thuja plicata
Cedar (pooled)

Foliage
Live branches
Stem wood
Stem bark

"
"
"
"

ln Y = - 2.617 + 1.7824  ln X
ln Y = - 3.2661 + 2.0877  ln X
ln Y = - 2.0927 + 2.1863  ln X
ln Y = - 4.1934 + 2.1101  ln X

 

Pines (pooled)

Foliage
Live branches
Dead branches
Stem wood
Stem bark

"
"
"
"
"

ln Y = - 3.9739 + 2.0039  ln X
ln Y = - 5.2900 + 2.6524  ln X
ln Y = - 3.7969 + 1.7426  ln X
ln Y = - 4.2847 + 2.7180  ln X
ln Y = - 4.2062 + 2.2475  ln X

 

Pinus contorta
Lodgepole pine

Foliage
Live branches
Stem wood plus bark

"
"
"

ln Y = - 3.6187 + 1.8362  ln X
ln Y = - 4.6004 + 2.3533  ln X
ln Y = - 2.9849 + 2.4287  ln X

 

Pinus lambertiana
Sugar pine

Foliage
Live branches
Stem wood
Stem bark

"
"
"
"

ln Y = - 4.0230 +  2.0327  ln X
ln Y = - 7.637   +  3.3648  ln X
ln Y = - 3.984   +  2.6667  ln X
ln Y = - 5.295   +  2.6184  ln X

 

Pinus ponderosa
Ponderosa pine

Foliage
Live branches
Dead branches
Stem wood
Stem bark

"
"
"
"
"

ln Y = - 4.2612  + 2.0967  ln X
ln Y = - 5.3855  +  2.7185  ln X
ln Y = - 2.5766  +  1.444  ln X
ln Y = - 4.4907  +  2.7587  ln X
ln Y = - 4.2063  +  2.2312  ln X

 

Pseudotsuga menziesii
Douglas-fir

Foliage
Live branches
Dead branches
Stem wood
Stem bark
Roots

"
"
"
"
"
"

ln Y = - 2.8462  +  1.7009  ln X
ln Y = - 3.6941  +  2.1382  ln X
ln Y = - 3.529    +  1.7503  ln X
ln Y = - 3.0396  +  2.5951  ln X
ln Y = - 4.3103  +  2.4300  ln X
ln Y = - 4.6961  +  2.6929  ln X

 

Tsuga heterophylla
Western hemlock

Foliage
Live branches
Dead branches
Stem wood
Stem bark

"
"
"
"
"

ln Y = - 4.130  +  2.128  ln X
ln Y = - 5.149  +  2.778  ln X
ln Y = - 2.409  +  1.312  ln X
ln Y = - 2.172  +  2.257  ln X
ln Y = - 4.373  +  2.258  ln X

 

Tsuga mertensiana
Mountain hemlock

Foliage
Live branches
Dead branches
Stem wood
Stem bark

"
"
"
"
"

ln Y = - 3.8169  +  1.9756  ln X
ln Y = - 5.2581  +  2.6045  ln X
ln Y = - 9.9449  +  3.2845  ln X
ln Y = - 4.8164  +  2.9308  ln X
ln Y = - 5.5868  +  2.7654  ln X

 
 

Source: Gholz et al. (1979).                                         
dbh  =  diameter at breast height (4.5 feet  or 1.3 m), in centimeters.               
H  =  total height in meters.


Figure 11-1.  Biomass distribution of a 16 inch dbh Douglas-fir tree.

Crown
Oven-dry weight
Percent
foliage
32 kg
2.8
barky live branches
62 kg
6.0
barky dead branches
19 kg
1.7
Total
120 kg
10.5 
Stem or Bole
 
wood
719 kg
62.8
bark
109 kg
9.5
Total
828 kg
72.3
Total Above Ground
948 kg
82.8
Below Ground
 
roots and stump
197 kg
17.2
TOTAL TREE:
1,145 kg
100.00

Figure 11-2.  Biomass distribution of a 16 inch Douglas-fir tree when processed into dimension lumber.

                                                                                                                                                                       


        It is assumed that the material balance values, developed for cubic log volume, can also be applied to log weight. The material balance shows that 33% of the total tree remains in the forest, serving useful ecosystem and soil stability functions. About 48% of the tree was converted to lumber and chips for paper, and 19% was converted by industrial boilers into energy for manufacturing the products. In many cases, the logs from the stem may be allocated to different processes; the first 17 feet may be allo­cated to plywood and the remainder to one or more sawmills. The material balance method can be easily expanded to show this allocation and the multiproduct conversions.

Volume of Standing Trees

Individual Tree Volume

        Standing trees are usually measured in terms of volume rather than biomass components. Volume may be estimated in cubic feet, cubic meters, or one of the board foot log scaling systems discussed in Chapter 2. Volume may represent the entire stem or the merchantable stem. Merchantable volume may be the volume between an assumed stump height and a minimum top diameter or the total volume of a series of fixed-length logs that must exceed a minimum diameter. There are several methods for developing volume estimates, and results are commonly presented in volume tables. The reader is referred to Avery (1975) for elaboration on methods and to Bell and Dilworth (1990) for volume tables for a number of Pacific Northwest species. Although the methods for obtaining tree volumes have some features in common with scaling actual logs after felling and bucking, some discrep­ancy can be expected between the estimated volume in the standing tree and the actual volume of logs obtained. Conversion factors between log scaling systems presented in Chapter 2 should not be assumed to be applicable to standing timber. Forest inventory reports of the U.S. Forest Service should be consulted for explanation of how tree measure­ments are taken, how volumes are estimated, and appropriate conversion factors between different volumes of standing trees. Conversion between tree volume and biomass, while seemingly straightforward, requires knowledge of average wood specific gravity of standing trees of the species in question. Specific gravity varies internally in a tree, with its age and genetics, and geographically, hence obtaining an accurate local specific gravity value is not easy. Conversions between volume and biomass may also be complicated by lack of consistency in the measurements and definitions used by volume table developers and biomass researchers.

Forest Inventory

        Inventories of standing timber are generally given in terms of volume stratified by species and stem diameter, which is taken outside bark at a height of 4.5 feet. This stem diameter is called diameter at breast height (dbh). Volume may be gross or, more commonly, net volume after taking into account unusable portions such as rot and poor form (sweep, crook, forks, etc.). The U.S. Forest Service conducts and publishes periodic forest inventories as "resource bulletins" for all states and subregions thereof, and provides information for various landowner categories. These bulletins also provide full descriptions of terminology, meas­urement standards, and conversion factors used.

      Volume may be expressed in cubic feet or in board feet, according to regionally preferred board foot log rules. Often this means Scribner in the West and the International 1/4 inch rule in much of the rest of the United States. Chapter 2 explains these log rules. It should be pointed out that the actual volume realized, when a tree is harvested and logs are measured, is likely to differ from the volume estimated by inventory methods. Volumes are commonly segregated according to

Growing stock:  Live trees of commercial species meeting certain standards of quality and vigor. When growing stock volume is reported, only growing stock trees 5.0 inches dbh and larger are included.

Sawtimber:  Live trees of commercial species that contain at least one 12 foot sawlog or two noncontiguous 8 foot logs that meet regional specifications for freedom from defect. Softwood sawtimber trees must be at least 9.0 inches dbh, while hardwood sawtimber trees must be at least 11 inches dbh.

In addition, volumes may be presented according to more than one merchantability standard, such as cubic feet for the whole tree, cubic feet from the stump to a 4 inch diameter top, and so on.

     Summary statistics for the entire United States are periodically prepared from the individual state reports giving both cubic foot and board foot (International 1/4 inch rule) volumes (Waddell et al. 1989). 

     Table 11-2 presents regional conversion factors based on these national statistics (Waddell et al. 1989). Ratios are presented separately for hard­woods and softwoods because (1) the average sizes of hardwood and softwood trees are normally different and (2) the lower limit for including a hardwood tree in the sawtimber category is 11 inches dbh rather than the 9 inch dbh for softwoods.  

     Columns 1 and 2 of Table 11-2 are ratios of cubic feet of growing stock and board feet of sawtimber. From the preceding definitions, note that all sawtimber size trees are included in the growing stock volumes. The percentage of total growing stock that is also sawtimber is given in column 3. The ratio of growing stock and sawtimber is a general index of tree size. It is also a rough index of quality since larger trees tend to be of better quality. Looking at column 1, as the ratio of growing stock volume (cubic feet) to sawtimber volume (board feet) decreases, more standing timber is in larger, higher quality sawtimber. This is also easily seen in column 3.

     Column 4 applies only to the sawtimber category and represents the conversion from 1,000 BF International 1/4 inch scale to cubic feet. Column 5 is the reciprocal of column 4, added by the author, and is the number of International 1/4 inch board feet per cubic foot. Since these ratios are averages for whole trees based on standardized forest inventory procedures, they are likely to be different from counterpart ratios obtainable when logs are actually measured and scaled.

Table 11-2.  Growing stock/sawtimber inventory ratios in the United States, by softwoods and hardwoods, region and subregion, 1987.




Region and Subregion

(1)
Cubic feet growing stock per board foot sawtimber

(2)
Board feet saw­timber per cubic foot growing stock

(3)

Percent of growing stock in sawtimber

(4)
Cubic feet per 1,000 board feet sawtimber

(5)
Board feet sawtimber per cubic foot

SOFTWOODS
North
    Northeast
0.3901
2.563
64.72
252.5
3.96
    North Central
0.3538
2.827
54.75
193.7
5.16

       Total
 0.3720

2.695

 59.73

 223.1

4.48
South
    Southeast

0.3004

3.329

69.78

209.6

4.77
    South Central
0.2511
3.982
76.82
192.9
5.18

        Total

0.2758

3.656

73.30

 201.3

4.97
Rockies
    Great Plains

0.2808

3.561

77.00

216.2

4.63
    Rocky Mountains
0.2543
3.932
78.70
200.2
5.00

        Total

 0.2676

3.746

 77.85

 208.2

4.80
Pacific Coast
    Pacific Southwest

0.1601

6.244

95.02

152.2

6.57
    Pacific Northwest
0.1725
5.798
88.56
152.7
6.55
        Pacific Northwest-West
0.1714
5.835
93.06
159.5
6.27
        Pacific Northwest-East
0.1736
5.762
84.07
145.9
6.85
    Alaska
0.2201
4.543
92.20
202.9
4.93

        Total

0.1842

5.529

91.93

169.3

5.91

United States

0.2749

3.953

75.91

198.4

5.04

HARDWOODS

North
    Northeast

0.4529

2.208

54.19

245.4

4.07
    North Central
0.3825
2.614
53.59
205.0
4.89

       Total

0.4177

2.411

53.89

225.2

4.44
South
    Southeast

0.3454

2.896

65.26

225.4

4.44
    South Central
0.3661
2.731
59.59
218.2
4.58

       Total

0.3557

2.813

62.43

 221.8

4.51
Rockies
    Great Plains

0.3000

3.333

65.86

197.6

5.06
    Rocky Mountains
0.5059
1.977
36.74
185.8
5.38

       Total

0.4029

2.655

51.30

191.7

5.22
Pacific Coast
    Pacific Southwest

0.3231

3.095

71.95

232.5

4.30
    Pacific Northwest
0.3081
3.247
63.69
196.1
5.10
        Pacific Northwest-West
0.3032
3.299
65.42
198.3
5.04
        Pacific Northwest-East
0.3130
3.195
61.95
193.9
5.16
    Alaska
0.5377
1.860
43.74
235.2
4.25

        Total

0.3896

2.734

59.79

 221.3

4.52

United States
0.3915
2.653
56.85
215.0
4.65

Source:  Waddell et al. (1989).  Column 5 added by the author.

 
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