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A Total Forest Management Plan
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Specific gravity of wood is the ratio of the weight of oven-dry wood to the weight of a volume of water equivalent to the overall volume of the wood. It is numerically the same as density (Forbes 1955: Chapt. 21, p.8). It varies with many factors but there are clear signs of an interplay and tradeoffs over the eons in developing structure to collect and store energy and secondarily to resist losses from factors present in native sites (e.g., insects and disease).
The measure is correlated with the tendency of trees to form cavities potentially useful for wildlife and is well correlated with the energy in the heat value of woods. The one factor in determining the relative goodness of a forest stand for producing cavities is thus believed to be its average specific gravity. Decomposition rates as well are a function of specific gravity. Similarly, specific gravity can be used as a basis for marking and as an incentive in marketing various forest stands for increasing or decreasing select wildlife species, for firewood and energy forests, and for selecting trees for future forests. The high energy of resins suggests that separate analyses be made of conifers and hardwoods.
It can be easily hypothesized that firewood cutters will eventually be prone to go after the high energy-value woods. This is likely to influence understory development and to decrease dead and down woods which are cover for the insect and small mammal food base of the forest. Similarly by encouraging certain cuts based on the energy-quality of wood, small area cuts can be cost-effectively created for deer forage and specific songbird populations.
My hypothesis is that the average specific gravity will be demonstrated to correlate well with other variables in the forest ecosystem such as
The following list of specific gravities (Table 1) has been prepared from many sources, chiefly: Brown et al. 1949; Forbes 1955: Chapt. 14, p. 26-29; Phillips 1981; and Clark 1982. The numbers vary with many site factors. The central tendency is reported.
General rules for specific gravity of woods (Phillips 1981):
I have subsequently used the list of tree species in each Society of American Foresters (SAF) Forest Type as the basis for estimating an average specific gravity for each type. The relationship, summed over all species in a type, is:
Gj = 
Si PijS = specific gravity for a species (green wood)
P = proportion of the species in a forest type
i = each species
j = a SAF forest type
Based on the average specific gravity, an energy index Bj in BTU/cord of wood was calculated by
B
(dry wood) Bj = 28.907509 + 15.6816827 ln Gj
r2 = 0.94
a regression based on data in Forbes 1955, Chapt. 14, p. 74. The value of R2 is 0.92. A cord of wood is 80 cu. ft. (multiply by 0.2832 to get cubic meters).
Table 2 presents SAF numbers (column 2) with computed weighted average specific gravity and available heat per cord of wood for each. The differences are greater than expected and allow a fair amount of discrimination among types. Aggregations of trees, with little regard for their taxonomy, will likely be found well related to the major northerly and southerly aspects, elevation, proximity to groundwater or free water, and perhaps slope position (McCombs 1998).
| Table 1. Green-weight specific-gravity values for eastern US forest tree species. |
|
1. AMERICAN ELM 0.46 2. ATLANTIC WHITE-CEDAR 0.31 3. BALDCYPRESS 0.42 4. BALSAM FIR 0.34 5. BALSAM POPLAR 0.31 6. BASSWOOD 0.32 7. BEAR OAK 0.56 8. BEECH 0.56 9. BIGTOOTH ASPEN 0.36 10. BITTERNUT HICKORY 0.60 11. BLACK ASH 0.45 12. BLACK CHERRY 0.31 13. BLACK GUM 0.46 14. BLACKJACK OAK 0.56 15. BLACK LOCUST 0.66 16. BLACK OAK 0.56 17. BLACK SPRUCE 0.38 1$. BLACK TUPELO 0.46 19. BLACK WALNUT 0.51 20. BLACK WILLOW 0.36 21. BLUE ASH 0.53 22. BLUEJACK OAK 0.56 23. BOXELDER 0.46 24. BUCKWHEAT TREE ? 25. BURR 0AK O.58 26. BUTTERNUT 0.36 27. CAROLINA ASH 0.45 28. CEDAR ELM 0.57 29. CHERRYBARK OAK 0.61 30. CHESTNUT OAK 0.57 31. CHINKAPIN 0.40 32. CHINQUAPIN OAK 0.59 33. COTTONWOOD 0.37 34. CUCU~BER TREE 0.44 35. DAHOON 0.50 36. DELTA POST OAK 0.60 37. DOGWOOD 0.61 38. DWARF CHINQUAPIN OAK 0.59 39. EASTERN REDCEDAR 0.44 40. FRASER FIR 0.34 41. GRAY BIRCH 0.47 42. GREEN ASH 0.53 43. HACKBERRY 0.49 44. HAWTHORN ? 45. HEMLOCK 0.38 46. HONEYLOCUST 0.60 47. HOP HORNBEAM 1.09 48. H0RNBEAM 0.58 49. JACK PINE 0.40 50. LAUREL OAK 0.56 51. LIVE OAK 0.80 52. LOBLOLLY BAY ? 53. LOBLOLLY PINE 0.47 54. LONGLEAF PINE 0.54 55. MAPLE 0.49 56. MOCKERNUT HICKORY 0.64 57. MOUNTAIN AStH ? 58. MYRTLE OAK 0.56 59. NORTHERN PIN OAK 0.58 60. NORTHERN RED OAK 0.56 61. NORTHERN WHITE-CEDAR 0.29 62. NUTMEG HICKORY 0.56 63. NUTTAL OAK 0.56 64. OVERCUP OAK 0.57 65. PAPER BIRCH 0.48 66. PAWPAW ? 67. PEACHLEAF WILLOW 0.36 68. PECAN 0.60 69. PERSIMMON 0.64 70. PIGNUT HICKORY 0.66 71. PIN CHERRY 0.47 72. PIN OAK 0.58 73. PITCH PINE 0.47 74. POND CYPRESS 0.42 75. POND PINE 0.51 76. POST OAK 0.60 77. PUMPKIN ASH 0.45 78. QUAKING ASPEN 0.35 79. REDBAY 0.42 80. RED MAPLE 0.49 81. RED OAK 0.56 82. RED PINE 0.41 83. RED SPRUCE 0.38 84. RIVER BIRCH 0.57 85. ROCK ELM 0.57 86. SANDBAR WILLOW 0.36 87. SAND LIVE OAK 0.80 88. SAND PINE 0.46 89. SAND POST OAK 0.60 90. SASSAFRAS 0.42 91. SCARLET OAK 0.60 92. SEPTEMBER ELM 0.57 93. SHAGBARK HICKORY 0.64 94. SHELLBARK HICKORY 0.62 95. SHINGLE OAK 0.56 96. SHORTLEAF PINE 0.47 97. SHUMARD OAK 0.56 98. SILVER MAPLE 0.44 99. SLASH PINE 0.54 100. SLIPPERY ELM 0.48 101. SOUTHERN MAGNOLIA 0.46 102. SOUTHERN RED OAK 0.52 103. SOURWOOD 0.50 104. SPRUCE PINE 0.41 105. SUGARBERRY 0.49 106. SUGAR MAPLE 0.56 107. SWAMP CHESTNUT OAK 0.60 108. SWAMP COTTONWOOD 0.32 109. SWAMP CYRILLA ? 110. SWAMP HICKORY 0.60 111. SWAMP TUPELO 0.46 112. SWAMP WHITE OAK 0.64 113. SWEETBAY 0.52 114. SWEET BIRCH 0.60 115. SWEETGUM 0.46 116. SYCAMORE 0.46 117. TABLE-MOUNTAIN PINE 0.45 118. TAMARACK 0.49 119. TURKEY OAK 0.56 120. VlRGINIA PINE 0.45 121. WATER HICKORY 0.61 122. WATER LOCUST 0.60 123. WATER OAK 0.56 124. WATER TUPELO 0.46 125. WHITE ASH 0.55 126. WHITE OAK 0.60 127. WHITE PINE 0.34 128. WHITE SPRUCE 0.37 129. WILLOW 0.36 130. WILLOW OAK 0.56 131. WINGED ELM 0.57 132. YAUPON 0.50 133. YELLOW BIRCH 0.55 134. YELLOW BUCKEYE 0.33 135. YELLOW-POPLAR 0.40 136. HARDWOOD (average) ? 137. RED ELM 0.48 138. STRIPED MAPLE 0.46 139. HICKORIES 0.61 140. MOUNTAIN MAPLE 0.46 141. SCRUB OAKS 0.57 142. OAK 0.58 143. TUPELO 0.46 144. WATER ELM ? 145. AMERICAN.HOLLY 0.50 146. APPLE ? 147. DWARF LIVE OAK 0.80 148. CHAPMAN OAK 059 |
| Table 2. Average green-weight specific-gravity for forest cover types typically found in Virginia | |||
| Trevey/George Washington National Forest Type Number |
SAF Type Number | Specific Gravity |
Available heat (million Btu) per cord of green wood (80 cu ft.) |
| 3 | 21 | 0.40 | 12.71 |
| 4 | 22 | 0.50 | 15.98 |
| 5 | 23 | 0.40 | 12.60 |
| 6 | 0 | 0.46 | 14.59 |
| 7 | 0 | 0.40 | 12.65 |
| 8 | 51 | 0.50 | 16.01 |
| 17 | 30 | 0.43 | 13.87 |
| 21 | 70 | 0.53 | 16.75 |
| 31 | 81 | 0.58 | 18.15 |
| 32 | 75 | 0.49 | 15.49 |
| 33 | 79 | 0.48 | 15.21 |
| 35 | 46 | 0.46 | 14.57 |
| 36 | 98 | 0.46 | 14.56 |
| 38 | 45 | 51 | 16.08 |
| 39 | 0 | 0.47 | 14.89 |
| 41 | 20 | 0.50 | 15.91 |
| 42 | 0 | 0.52 | 16.42 |
| 44 | 76 | 0.53 | 16.78 |
| 45 | 78 | 0.50 | 15.99 |
| 46 | 82 | 0.51 | 16.09 |
| 48 | 0 | 0.53 | 16.79 |
| 50 | 57 | 0.43 | 13.57 |
| 51 | 40 | 0.58 | 18.11 |
| 52 | 44 | 0.54 | 17.13 |
| 53 | 52 | 0.56 | 17.55 |
| 54 | 53 | 0.59 | 18.22 |
| 55 | 55 | 0.53 | 16.70 |
| 56 | 59 | 0.49 | 15.55 |
| 57 | 72 | 0.63 | 19.21 |
| 58 | 87 | 0.46 | 14.66 |
| 59 | 0 | 0.56 | 17.64 |
| 61 | 91 | 0.58 | 17.95 |
| 62 | 92 | 0.53 | 16.63 |
| 64 | 88 | 0.56 | 17.45 |
| 66 | 97 | 0.33 | 10.08 |
| 67 | 102 | 0.44 | 14.17 |
| 72 | 61 | 0.51 | 16.10 |
| 73 | 63 | 0.37 | 11.73 |
| 74 | 95 | 0.52 | 16.46 |
| 75 | 94 | 0.47 | 14.87 |
| 81 | 25 | 0.54 | 17.07 |
Notes
Species 13, Black gum has sp g. of 0.555 in understory, 0.475 in overstory (Phillips 1981; Clark 1982)
Species 30, Chestnut oak has reported sp g of 0.632
Species 37, Dogwood has sp g of 0.621
Species 60, reported 0.574
Species 70, reported 0.633 (mean of 0.568)
Species 80, reported 0.501
Species 91, reported 0.595
Species 102, reported 0.587
Species 115, reported 0.474
Species 123, reported 0.566
Species 125, reported 0.572
Species 126, reported 0.627
Species 135, reported 0.361 in understory, 0.412 in overstory
Davidson (1979) reported bigtooth aspen having 0.39, eastern white pine having 0.35, and northern red oak having specific gravities of 0.63.
Dickson et al (1995) showed data suggesting a strong negative correlation between percentage of a tree species standing after herbicide use and specific gravity of remaining trees as snags of value to wildlife.
Moisture content (green-weight basis) = ((A - B)/ A) x 100
Specific gravity = dry weight/ green volume = (B x F) / (C - E)
Specific gravity = dry weight / dry volume = (B x F) / (D - E)
Specific gravity = green weight / green volume = A / (C-E)
where A = green weight; B = dry weight; C = immersed green weight; E = weight of chip basket; and F = specific gravity of liquid (water=1 and kerosene = 0.80)
An equation for relating oven dry and air dry weight/volume at 12 percent moisture g/cm3 for tropical woods, all regions, (Reyes et al. 1992) is:
y = 0.0134 + 0.800 X (r2)
where y = wood density at overdry weight/green volume; g/cm3
X = wood density at air-dry weight/volume at 12-percent moisture; g/cm3
Robert H. Giles, .Jr., June, 1983 with revisions in May, 2001
Literature Cited and References
Brown, H. P., A. J. Panshin, and C. C. Forsaith. 1949. Textbook of wood technology. Vol. 1, McGraw-Hill Book Col, Inc. xxvii + 652 pp.
Clark, A. C., III. 1982. Predicted weights and volumes of firewood in hardwood trees in the southeast. USFS Southeastern Forest Exp. Station Research Paper SE-266, Asheville, NC. 28 pp.
Davidson, W.H. 1979. Hybrid poplar pulpwood and lumber from a reclaimed strip-mine, USDA Forest Serv, Northeastern For Exp. Sta, Research Note NE-282, Broomall, Pa 2pp.
Dickson, J.G. 1995. Longevity and bird use of hardwood snags created by herbicides. Proc. Southeastern Assoc Fish and Game Commissioners Annual Meeting, (Abstract)
Forbes, R.D. 1955. Forestry handbook. Ronald Press Co. N.Y. variously paged. 1130pp.
Gibson, M.D. , C.W. McMillian, and E. Shoulders. 1986. Moisture content and specific gravity of four major southern pines under the same age and site conditions. Wood and Fiber Science 18(3): 429-434
McCombs, J. W. 1998. Geographic information system topographic factor maps for wildlife management. M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA. 141 pp.
Phillips, D. R. 1981. Predicted total-tree biomass of understory hardwoods. USFS, Southeastern Forest Exp. Sta. Research Paper SE-223, Asheville, NC 22 pp.
Reyes, G., S. Brown, J. Chapman, and A Lugo 1992. Wood densities of tropical tree species, USDA Forest Serv., Southern For. Exp. Sta., New Orleans, La Gen Tech Rpt. SO-88, 15pp.
Saucier, J.R. 1972. Wood specific gravity of eleven species of pine. Forest Products J. 22(3):33.
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Last revision January 17, 2000.