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A Guide to Soil Quality Monitoring for Long Term Ecosystem Sustainability on Northern Region National Forests
ORGANIC MATTER FUNCTION IN THE WESTERN-MONTANE FOREST SOIL SYSTEM
ABSTRACT
Soil organic horizons are critical components of forest soil productivity. Understanding their unique roles in moisture retention and nutrient cycling before and after timber harvesting is key to managing postharvest productivity for future stands. Diverse habitat types, variable volcanic ash deposition, and temperature/moisture extremes make organic horizons especially crucial to productivity and management of western-montane forests.
INTRODUCTION
Productivity of western-montane forest soils is tightly bound to the organic matter component. Additions, alterations, and reductions of forest litter, humus, and wood residues influence both biotic and abiotic properties of any given site (Harvey and others 1987). Organic matter is especially important for soil water retention, cation exchange, nutrient cycling, and erosion control. Recent trends toward whole tree harvesting, increased woody debris removal (including slash burning and yarding unmerchantable material), and shorter rotations (McColl and Powers 1984) have increased awareness of effects of forest operations on soil processes and overall site productivity (Jurgensen and others 1990).
FOREST SOILS
The western-montane area encompasses the area from the eastern slope of the Cascade mountain range in Washington and Oregon south to the Sierra crest in California. It extends east to the Continental Divide in Montana and Wyoming. Most forest soils in the montane-west are Inceptisols (some Andosols, under the new taxonomy), developed from volcanic ash deposits (see Meurisse and others and Hironaka and others, these proceedings). The major ash-fall affecting the area is from Mount Mazama (now known as Crater Lake, OR) ash, which was deposited during an eruption 6,700 years B.P. The western-montane region was blanketed with a highly variable, sometimes extensive ash-fall up to 60 cm deep. Lesser eruptions from Mount St. Helens and Glacier Peak left thin deposits of ash.
Ash cap soils support large tracts of forested land in the Inland Northwest and have relatively high productivity and water-holding capacity compared to non-andic soils (Geist and others 1989). Below the ash cap in northern Washington, Idaho, and Montana is glacial outwash material characterized by a high percentage of rock fragments and low moisture and nutrient-holding capacity. Andic soils, once the organic mantle has been removed, are very susceptible to damage, particularly erosion, during harvest and site preparation.
The litter layer, also designated as Oi, consists of freshly fallen needles, twigs, and other debris that have undergone only slight decomposition. The fermentation layer (duff or Oe), is the organic material beneath the litter layer. Decomposition in this layer is very active, and the duff is usually permeated with fungal mycelia and root mats. Although this horizon is undergoing decomposition, plant parts are still distinguishable. Humus (Oa), is unrecognizable, dark brown or black, amorphous organic material that has undergone complete decomposition.
Typical horizon development includes some form of an organic horizon (O) underlain by an A and a Bs horizon (Fosberg and others 1979). Relatively undisturbed surface organic horizons typically consist of approximately 2-10 cm of litter, 0-5 cm of duff or humus (fig. 1) (collectively termed the forest floor), and varying amounts of decayed soil wood (the brown, crumbly mass left from decaying wood) (Harvey and others 1987). Surface organic horizon depth is highly variable, depending on climate, moisture, and topography. Northern Idaho forests, which are cool and moist, generally have substantial organic deposits, except in the driest habitat types. Central Idaho and northwestern Montana have similar organic horizon depths. However, soils influenced by lodgepole pine (Pinus contorta Dougl. ex Loud.) on warm, dry sites in central Idaho have almost nonexistent organic horizons and reflect a lower productivity potential (Steele and others 1981).
Figure 1—Average depth of the forest floor in selected habitat types in northern (Cooper and others 1987) and central Idaho (Steele and others 1981), and northwestern Montana (Pfister and others 1977) forests. [Text description of this figure]
Woody residue is a valuable component of montane-west ecosystems and has an important role in carbon cycling, nutrient storage, stream dynamics, erosion control, and animal activity (Harmon and others 1986; Jurgensen and others 1990; Maser and Trappe 1984). When the woody residue becomes incorporated into the forest floor, it is then termed soil wood. Organic horizons in combination with woody residues and soil wood comprise most forest soil organic matter (table 1), and in many cases the woody residue component may equal or surpass that of other soil components.
Total soil organic matter contents generally mirror site productivity. The most productive stands in our region have the deepest organic matter deposits and are usually in the cedar/hemlock (Thuja plicata Donn ex D. Don and Tsuga heterophylla [Raf.] Sarg.) types. The least productive stands, with the shallowest organic matter deposits, are ponderosa pine (Pinus ponderosa Dougl. ex Laws.) stands. The exception to this rule is subalpine fir (Abies lasiocarpa [Hook.] Nutt.) types; in these stands low temperatures limit organic matter turnover rates, leading to deep organic matter deposits, but limited tree growth.
Organic Matter and Nutrient Budgets
A primary portion of the nutrient capital, particularly nitrogen (N), in the forest ecosystem is contained in the Oi, Oe, Oa, and woody residue. Soils with the greatest N content usually have the largest organic horizon accumulations (tables 1 and 2). Generally, as N in the organic horizons increases, stand productivity increases. The exception to this is the warm, moist cedar/hemlock stands in Idaho where there is a rapid turnover of forest floor (Jurgensen and others 1990). In southeastern Wyoming, lodgepole pine (Pinus contorta ssp. latifolia [Engelm. ex Wats.] Critchfield) stands average 31 Mg/ha forest floor (Oe and Oi) volume and have an average N content of 33 kg/ha (Fahey and others 1985). On these stands, woody residue contributed 13 kg/ha N. Since lodgepole pine stands are usually N limited (Fahey and others 1985), inputs from decaying wood and forest floor can be very important for productivity.
| Location | Residue Mg/ha |
Forest floor Mg/ha |
Soil wood Mg/ha |
Mineral soil1 Mg/ha |
Yield capacity m3/ha/yr |
|---|---|---|---|---|---|
| 1Sampled to a depth of 30 cm. | |||||
| Montana | |||||
| Cedar/hemlock | 84 | 50 | 51 | 145 | 7.7 |
| Subalpine fir | 146 | 36 | 36 | 153 | 7.7 |
| Douglas-fir | 45 | 26 | 26 | 133 | 4.9 |
| Ponderosa pine | <20 | 7 | 2 | 160 | 2.9 |
| Idaho | |||||
| Cedar/hemlock | 154 | 23 | 48 | 201 | 9.5 |
| Location | Residue kg/ha |
Forest floor kg/ha |
Soil wood kg/ha |
Mineral soil1 kg/ha |
Proportion in mineral soil Percent |
|---|---|---|---|---|---|
| ns = not sampled. 1Sampled to a depth of 30 cm. 2From Jurgensen and others 1990. 3From Clayton and Kennedy 1985. 4From Fahey and others 1985. |
|||||
| Montana1 | |||||
| Cedar/hemlock | 125 | 787 | 341 | 1,729 | 58 |
| Subalpine fir | 219 | 570 | 344 | 1,686 | 60 |
| Douglas-fir | 68 | 438 | 419 | 2,183 | 70 |
| Ponderosa pine | <30 | 128 | 33 | 3,433 | 94 |
| Idaho | |||||
| Cedar/hemlock | 231 | 179 | 297 | 3,045 | 81 |
| Douglas-fir3 | ns | 248 | ns | 3,160 | |
| Wyoming | |||||
| Lodgepole pine4 | ns | 400 | 86 | 5,270 | |
Besides N, nutrients like calcium (Ca), magnesium (Mg), potassium (K), and phosphorus (P) are also found in abundance within organic horizons (table 3). The availability of all these nutrients is strongly influenced by the rate of organic matter decomposition. Again, nutrient concentrations vary depending on overstory species and stand locations, but O horizons provide a large proportion of nutrients critical for seedling establishment and growth.
| Horizon | Ca kg/ha/yr |
Mg kg/ha/yr |
K kg/ha/yr |
P kg/ha/yr |
|---|---|---|---|---|
| 1From Clayton and Kennedy 1985. 2From Entry and others 1987. |
||||
| Ponderosa pine/Douglas-fir mixed forest–Silver Creek, ID1 | ||||
| Litter (Oi) | 347 | 340 | 340 | 190 |
| Mineral (0–10 cm) | 319 | 111 | 184 | 175 |
| Lodgepole pine–Lolo Pass, MT2 | ||||
| Forest floor (Oi,Oe) | 349 | 48 | 120 | 100 |
| Mineral (0–10 cm) | 278 | 40 | 177 | 100 |
Organic Matter and Moisture-Holding Capacity
As we have seen, forest floor material and decayed logs are a reservoir for nutrients. They also act as a storehouse for moisture. Fallen, decaying logs can contain especially large amounts of moisture (table 4). Amaranthus and others (1989) noted, in southwestern Oregon, that during the winter months decayed wood acts like a sponge to absorb water and retains much of that water throughout the following growing season. This water supply can be particularly important for seedling establishment, especially where available soil water would otherwise be insufficient for surviving summer drought or for maximizing growth in highly competitive situations.
| Location | Woody residue Percent dry weight |
Mineral soil Percent dry weight |
|---|---|---|
| 1From Amaranthus and others 1989. 2From Harvey and others 1979. |
||
| Southwestern Oregon1 | ||
| Ponderosa pine | 157 | 6 |
| Western Montana2 | ||
| Douglas-fir | 98 | 17 |
| Subalpine fir | 163 | 34 |
| Hemlock | 161 | 27 |
Comparisons of moisture contents on a dry weight basis do not provide a ready measure of how much is available for plant uptake. However, field capacity and permanent wilting point moisture data for a Douglas-fir stand in northern Idaho show soil wood has 5.5 times more available moisture than mineral soil per gram of substrate. On a weight/weight basis soil wood has an average available moisture of 84.5 percent, litter 18.7 percent, and mineral soil 15.4 percent (Page-Dumroese 1990). Although soil moisture levels fluctuate seasonally, decayed wood maintains higher water contents throughout the growing season (table 5) than the forest floor or underlying mineral soil. This makes decayed wood of particular importance to drier ecosystems where moisture is limited throughout the year. The forest floor, by acting as a mulch, may also be helpful for maintaining moisture levels in the mineral soil throughout the growing season.
Organic Matter and Cation Exchange Capacity
Organic matter, because of its many negatively charged sites, is a major source of a soil's cation exchange capacity (CEC) (Tate 1987). In a northern hardwood forest, Brooks (1987) found that in uncut stands the forest floor had six times greater CEC than surface mineral soil. After harvesting, an eightfold difference occurred in CEC between the forest floor and the mineral soil.
In northern Idaho, site preparation treatments that mound the soil organic matter and mineral top soil together (Page-Dumroese and others 1986, 1989) had significantly greater CEC's than a scalp treatment that removed the forest floor (table 6). The undisturbed treatment, with the forest floor left relatively intact, had a similar CEC to the mounded treatment. While knowledge about a soil's CEC is important, very little work has been done to link the effects of timber harvesting/site preparation to changes in CEC and resulting site productivity.
| Site treatment | Low elevation1 | High elevation2 | ||||
|---|---|---|---|---|---|---|
| O.M. Percent |
CEC cmol/kg |
O.M. Percent |
CEC cmol/kg |
|||
1Abies grandis/Symphoricarpos albus h.t., elevation 715 m. |
||||||
| Mounded | 15 | 15 | 28 | 18 | ||
| Scalped | 9 | 8 | 15 | 11 | ||
| Undisturbed | 14 | 11 | 29 | 20 | ||
Organic Matter and Disturbance
Stand disturbances, either natural or artificial, have a dramatic impact on the depth of organic horizons (table 7). Recent wildfires and intense, long-duration prescribed burns seem particularly devastating to organic matter depth (Harvey and others 1986). Destruction of soil organic horizons by repeated wildfires over the past 75 years may be a contributing factor to the development of aggressive shrubfields in northern Idaho (Harvey and others 1987).
Harvesting and different site preparation methods and their effect on stand nutrient balances can be seen in table 8. Clearcut and burn operations maintain more total N, P, and cations in the organic horizons than does a mechanical residue (bulldozer piling) removal system. Acceleration of nutrient loss and increased erosion occur after removing the protective organic mantle (Megahan and Kidd 1972). Soil organic matter promotes the formation of water-stable aggregates as long as substantial levels are maintained. Once the forest floor is destroyed, these aggregates break down and erosion increases. Clayton and Kennedy (1985) indicated it may take more than 50 years to restore a heavily disturbed ecosystem to its former nutrient status and perhaps centuries to restore soil lost through erosion.
| Harvest method | Ca kg/ha |
Mg kg/ha |
K kg/ha |
P kg/ha |
N kg/ha |
|---|---|---|---|---|---|
| 1Clearcut. | |||||
| CC1/residue left | 331 | 46.1 | 79.7 | 145 | 634 |
| CC/residue removed | 188 | 26,8 | 40.8 | 60 | 392 |
| CC/residue burned | 215 | 27.3 | 75.4 | 10 | 476 |
Organic Matter and Regeneration
Postharvest natural and artificial regeneration success depends, in many cases, on soil organic matter content. Increases after harvesting and site preparation in organic matter percentage in the surface mineral soil are most likely the result of forest floor and a considerable amount of logging slash being mixed into the surface mineral soil. This increase is usually short-lived (Cromack and others 1979) and decreases with new stand development (Kraemer and Hermann 1979). Therefore, planted seedlings, with reduced access to soil organics, may have, or will likely soon experience, growth declines (Graham and others 1989).
Organic horizon depth can directly influence seedling biomass production (fig. 2). Seedling weight of naturally regenerated ponderosa pine in a Douglas-fir (Pseudotsuga menziesii Beissn. [Franco]) habitat type is positively correlated with depth of the organic horizon. The correlation shown in figure 2 is particularly striking because organic matter depth did not exceed 1 cm. Although this study had a relatively small sample size, organic matter depth explained 51 percent of the variation in weight of these seedlings (Harvey and others 1988).
Figure 2—Ponderosa pine seedling weight response to increasing forest floor depth in the montane-west. [Text description of this figure]
In the past, mineral seedbeds for natural regeneration have been the "norm" (Haig and others 1941). However, soil organic components can also act as valuable seedbeds for natural regeneration (table 9). Organic substrates in the Canadian Rockies occupy a large portion of the stand and are used extensively as a seedbed.
| Seedbed | Area Percent |
Seedling distribution Percent |
|---|---|---|
| 1Data for area not available. | ||
| Muck | 1 — | 9 |
| Litter | 11.8 | 9 |
| Moss | 6.3 | 24 |
| Decayed wood | 16.5 | 24 |
| Humus | 44.9 | 12 |
| Mineral | 20.4 | 22 |
Harvey and others (1987) noted that, in terms of a competitive advantage, conifers seem to be the only species using woody debris as a substratum for regeneration. There is also species differentiation in the use of organic horizons for regeneration (Day and Duffy 1963). Lodgepole pine favors a mineral seedbed, but Engelmann spruce (Picea engelmannii Parry ex Engelm.) and Douglas-fir prefer organic seedbeds. Organic horizons and the upper 30 cm of mineral soil then become the primary rooting substrate as seedlings mature (Harvey and others 1986; Kimmins and Hawkes 1978).
Growth of planted seedlings after intensive site preparation on two soil types in northern Idaho was influenced by soil organic matter content (table 10). Western white pine (Pinus monticola Dougl. ex D. Don) and Douglas-fir growth was greater after 3 years in treatments with high organic matter content compared to scalped treatments. This may be due to several interacting factors including: (1) organic matter on the surface of the mounded treatments may have acted as a mulch to enhance water retention, (2) organic matter incorporated into the mounded treatments significantly lowered soil bulk density, and (3) organic matter left on the surface or incorporated into the mounded treatments improved the nutrient status of the soil (Page-Dumroese and others 1986, 1990). Scalping, which is commonly used in the montane-west, can in some instances, benefit seedling establishment and survival by reducing competition (Sloan and Ryker 1986). However, removal of the surface organic and mineral horizons can also severely limit growth and impair long-term survival.
MANAGEMENT IMPLICATIONS
Soil organic matter affects the cation exchange capacity, water-holding capacity, bulk density, nutrient budgets, and erosion potential. Removal of organic horizons during harvesting and site preparation may seriously reduce overall site productivity, stability, and regeneration potential.
Postharvest treatments should be planned to limit damage to fragile organic horizons. There may be occasional instances of extreme competition or heavy fuel loading that warrant intensive site treatments and forest floor removal to achieve adequate regeneration. Although maintenance of the organic mantle may limit some initial site preparation options, in the long run productivity will be maintained or improved. Economic investments made to conserve organic matter or reduce bulk density in many stands in the montane-west can provide substantial returns in the form of improved long-term soil productivity.
REFERENCES
Amaranthus, M. P.; Parrish, D. S.; Perry, D. A. 1989. Decaying logs as moisture reservoirs after drought and wildfire. In: Alexander, E. B., ed. Proceedings of Watershed '89: a conference on the stewardship of soil, air, and water resources; 1989 March 21-23; Juneau, AK. Juneau, AK: U.S. Department of Agriculture, Forest Service, Alaska Region: 191-194.
Brooks, R. H. 1987. Effects of whole-tree harvest on organic matter, cation exchange capacity, and available water holding capacity. Houghton, MI: Michigan Technological University. 29 p. Thesis.
Clayton, J. L.; Kennedy, D. A. 1985. Nutrient losses from timber harvest in the Idaho batholith. Soil Science Society of America Journal. 49: 1041-1049.
Cooper, S. T.; Neiman, K. E.; Steele, R.; Roberts, D. W. 1987. Forest habitat types of northern Idaho: a second approximation. Gen. Tech. Rep. INT-236. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 135 p.
Cromak, K., Jr.; Swanson, F. J.; Grier, C. C. 1979. A comparison of harvesting methods and their impact on soils and environment in the Pacific Northwest. In: Youngberg, C. T., ed. Forest soils and land use: Proceedings, fifth North American forest soils conference; 1978 August; Fort Collins, CO. Fort Collins, CO: Colorado State University: 445-476.
Day, R. J.; Duffy, P. J. 1963. Regeneration after logging in the Crowsnest forest. Publ. 1007. Calgary, AB: Canadian Department of Forestry. 31 p.
Entry, J. A.; Stark, N. M.; Loewenstein, H. 1987. Effect of timber harvesting on extractable nutrients in a Northern Rocky Mountain forest soil. Canadian Journal of Forest Research. 17: 735-739.
Fahey, T. J.; Yavitt, J. B.; Pearson, J. A.; Knight, D. H. 1985. The nitrogen cycle of lodgepole pine forests, southeastern Wyoming. Biogeochemistry. 1: 257-275.
Fosberg, M. A.; Falen, A. L.; Jones, J. P.; Singh, B. B. 1979. Physical, chemical, and mineralogical characteristics of soils from volcanic ash in northern Idaho: I. Morphology and genesis. Soil Science Society of America Journal. 43: 541-547.
Geist, J. M.; Hazard, J. W.; Seidel, K. W. 1989. Assessing physical conditions of some Pacific Northwest volcanic ash soils after forest harvest. Soil Science Society of America Journal. 53: 946-950.
Graham, R. T.; Harvey, A. E.; Jurgensen, M. F. 1989. Effect of site preparation on survival and growth of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) seedlings. New Forests. 3: 89-98.
Haig, I. T.; Harris, D. P.; Weidman, R. H. 1941. Natural regeneration in the western white pine type. Tech. Bull. 767. Washington, DC: U.S. Department of Agriculture, Forest Service. 99 p.
Harmon, M. E.; Franklin, J. F.; Swanson, F. J.; Sollins, P.; Gregory, S. V.; Lattin, J. D.; Anderson, N. H.; Cline, S. P.; Aumen, N. G.; Sedell, J. R.; Lienkaemper, G. W.; Cromack, K., Jr.; Cummins, K. W. 1986. Ecology of coarse wood debris in temperate ecosystems. Advances in Ecological Research. 15: 133-302.
Harvey, A. E.; Jurgensen, M. F.; Graham, R. T. 1988. The role of woody residue in soils of the ponderosa pine forests. In: Baumgartner, D. M.; Lotan, J. E., eds. Ponderosa pine-the species and its management; 1987 September 29-October 1; Spokane, WA. Pullman, WA: Washington State University, Cooperative Extension: 141-147.
Harvey, A. E.; Jurgensen, M. F.; Larsen, M. J. 1978. Seasonal distribution of ectomycorrhizae in a mature Douglas-fir/larch forest soil in western Montana. Forest Science. 24: 203-208.
Harvey, A. E.; Jurgensen, M. F.; Larsen, M. J.; Graham, R. T. 1987. Decaying organic materials and soil quality in the Inland Northwest: a management opportunity. Gen. Tech. Rep. INT-225. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 15 p.
Harvey, A. E.; Jurgensen, M. F.; Larsen, M. J.; Schlieter, J. A. 1986. Distribution of active ectomycorrhizal short roots in forest soils of the Inland Northwest: effect of site and distribution. Res. Pap. INT-374. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 8 p.
Harvey, A. E.; Larsen, M. J.; Jurgensen, M. F. 1979. Comparative distribution of ectomycorrhizae in soils of three western Montana forest habitat types. Forest Science. 25: 350-358.
Jurgensen, M. F.; Harvey, A. E.; Graham, R. T.; Larsen, M. J.; Tonn, J. R.; Page-Dumroese, D. S. 1990. Soil organic matter, timber harvesting and forest productivity in the Inland Northwest. In: Gessel, S. P.; Lacate, D. S.; Weetman, G. F.; Powers, R. F., eds. Sustained productivity of forest soils: Proceedings of the seventh North American forest soils conference; 1988 June 25-28; Vancouver, BC. Vancouver, BC: University of British Columbia: 392-415.
Kimmins, J. P.; Hawkes, B. C. 1978. Distribution and chemistry of fine roots in a white spruce-subalpine fir stand in British Columbia: implications for management. Canadian Journal of Forest Research. 8: 265-279.
Kraemer, J. F.; Hermann, R. K. 1979. Broadcast burning: 25-year effects on forest soils in the western flanks of the Cascade Mountains. Forest Science. 25: 427-439.
Maser, C.; Trappe, J. M. 1984. The seen and unseen world of the fallen tree. Gen. Tech. Rep. PNW-164. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 56 p.
McColl, J. G.; Powers, R. F. 1984. Consequences of forest management on soil tree relationships. In: Bowen, C. D.; Nambiar, E. K. S., eds. Nutrition of plantation forests. New York: Academic Press: 379-412.
Megahan, W. F.; Kidd, W. J. 1972. Effect of logging roads on sediment production rates in the Idaho batholith. Res. Pap. INT-123. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 14 p.
Page-Dumroese, D. S. 1990. Unpublished data on file at: U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Moscow, ID.
Page-Dumroese, D. S.; Jurgensen, M. F.; Graham, R. T.; Harvey, A. E. 1986. Soil physical properties of raised planting beds in a northern Idaho forest. Res. Pap. INT-360. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 6 p.
Page-Dumroese, D. S.; Jurgensen, M. F.; Graham, R. T.; Harvey, A. E. 1989. Soil chemical properties of raised planting beds in a northern Idaho forest. Res. Pap. INT-419. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 7 p.
Page-Dumroese, D. S.; Loewenstein, H.; Graham, R. T.; Harvey, A. E. 1990. Soil source, seed source, and organic-matter content effects on Douglas-fir seedling growth. Soil Science Society of America Journal. 54: 229-233.
Pfister, R. D.; Kovalchik, B. L.; Arno, S. F.; Presby, R. C. 1977. Forest habitat types of Montana. Gen. Tech. Rep. INT-34. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 174 p.
Sloan, J. P.; Ryker, R. A. 1986. Large scalps improve survival and growth of planted conifers in central Idaho. Res. Pap. INT-366. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 9 p.
Steele, R.; Pfister, R. D.; Ryker, R. A.; Kittams, J. A. 1981. Forest habitat types of central Idaho. Gen. Tech. Rep. INT-114. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 138 p.
Tate, R. L. 1987. Soil organic matter: biological and ecological effects. New York: John Wiley and Sons. 291 p.
Paper presented at the Symposium on Management and Productivity of Western-Montane Forest Soils, Boise, ID, April 10-12, 1990.
Deborah Page-Dumroese, Alan Harvey, and Russell Graham are Soil Scientist, Project Leader, and Research Forester, respectively, Intermountain Research Station, Forest Service, U.S. Department of Agriculture, Moscow, ID. Martin Jurgensen is Professor, Forest Soils, Michigan Technological University, Houghton, MI.