ORGANIC MATTER FUNCTION IN THE WESTERN-MONTANE FOREST SOIL SYSTEM

Deborah Page-Dumroese
Alan Harvey
Martin Jurgensen
Russell Graham

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]

Graph showing forest floor depth for selected habitat types in three regions.

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.

TABLE 1 
Volume of organic horizons and mineral soil in old-growth stands in the montane-west (from Jurgensen and others 1990)
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


TABLE 2 
Nitrogen content of organic horizons and mineral soil in several old-growth stands in the montane-west
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.

TABLE 3 
Soil nutrient budgets of organic horizons and mineral soil from selected undisturbed stands in the montane-west
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.

TABLE 4 
Moisture content of woody residue and mineral soil in the montane-west
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.

TABLE 5 
Seasonal moisture content fluctuations in soil substrates from an Abies lasiocarpa/Clintonia uniflora habitat type in western Montana (from Harvey and others 1978)
Month Humus
Percent dry weight
Woody residue
Percent dry weight
Mineral soil
Percent dry weight
May 130 204 37
July 74 118 27
September 141 244 40

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.

TABLE 6 
Cation exchange capacity (cmol/kg) and soil organic matter content (percent) as affected by site preparation technique in two northern Idaho stands
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.
2Tsuga heterophylla/Clintonia uniflora h.t., elevation 1,456 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).

TABLE 7 
Disturbance effects on soil forest floor (Oi, Oe, and Oa) depth in northwestern Montana (from Harvey and others 1986)
Site and disturbance Stand age
Years
Forest floor depth
cm
Subalpine fir
  Undisturbed >250 3.5
  Wildfire 80 1.8
  Clearcut and burn 15–20 1.5
  Wildfire 15 .5
Douglas-fir
  Undisturbed >250 2.3
  Partial cut/underburn 80–100 .6
  Selective cut 60–120 1.2
  Wildfire 50 1.9

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.

TABLE 8 
Moisture content of woody residue and mineral soil in the montane-west
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]

Diagram showing Ponderosa pine seedling weight response to increasing forest floor depth.

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.

TABLE 9 
Seedbed composition and natural seedling distribution in logged-over stands in the Canadian Rockies (from Day and Duffy 1963)
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.

TABLE 10 
Soil organic matter content and 3-year-old western white pine and Douglas-fir seedling biomass after three site preparation techniques in northern Idaho (from Page-Dumroese and others 1986; Graham and others 1989)
Treatment Organic matter
Percent
WWP biomass
Grams
DF Biomass
Grams
Mounded 27 19 16
Scalped 14 8 7
Undisturbed 23 8 9

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

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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.