Effects of Soil Disturbance on the 

Fundamental, Sustainable Productivity 

of Managed Forests1 

Robert F. Powers2 

Abstract 

Environmental policies in the United States and abroad are reducing timber harvests 

while wood demand is mounting. Reduced harvesting on public lands means that 

privately owned lands will be managed with greater intensity in the United States and 

that wood will be imported from other nations lacking strong environmental safeguards. It 

is imperative, therefore, that both public and private forest lands be managed to sustain their 

productivity, and many nations are seeking effective monitoring methods. Central to this is 

our ability to estimate a site’s fundamental capacity for growing vegetation, and to detect 

changes in this capacity caused by management. Because soil is the factor of a site modified 

most easily and profoundly by management, and because soil largely is independent of the 

current condition of vegetation, soil-based variables offer our most effective and practicable 

indices of sustainable productivity. The North American Long-Term Soil Productivity 

cooperative research program (LTSP) is the world’s most extensive coordinated effort to 

address questions of sustainable productivity in managed forests. Early findings from the 12 

LTSP sites in California illustrate the physical importance of organic soil cover in reducing 

soil erosion and maintaining favorable soil temperature and moisture relations during 

summer drought. Findings also show that the biological significance of soil compaction 

depends on soil texture. Moderate compaction degrades vegetative growth on fine-textured 

soils but can enhance growth on coarse-textured soils where drought is a factor. Impacts of 

soil compaction on tree growth often are masked by effects of competing vegetation. 

Measurements taken under operational conditions show that compaction associated with 

mechanized thinning can reduce soil rooting volume by as much as one-half. Subsoiling 

seems to mitigate the effect. Root damage caused by subsoiling did not adversely affect the 

growth of residual trees. Results are providing practicable field methods for monitoring 

management impacts on sustainable productivity. 

Forests offer many values and commodities beyond wood production. 

Concerned that forests have suffered from overemphasis on timber, 

environmentalists call for more conservative forest management practices that 

reduce wood harvest and preserve or restore other ecological values (Drengson 

and Taylor 1997). On many public lands of the western United States— 

historically a major source of domestic timber—harvesting continues, but at a 

rate less than one-third that of the last decade (USDA Committee of Scientists 

1999). Pressure to de-emphasize wood production here and abroad comes while 

the land area formerly available for production forestry is shrinking at an 

annual rate of 0.4 percent (FAO 1997). 

Paralleling the trend on public lands of the United States, a “green 

advocacy” has gained momentum and has spawned an expansive international 

industry to certify what is, and is not, “sustainable forestry” (Hammond and 

Hammond 1997, Journal of Forestry 1995). In general, leading forest scientists 

1 	An abbreviated version of this 

paper was presented at the 

Symposium on the Kings River 

Sustainable Forest Ecosystems 

Project: Progress and Current 

Status, January 26, 1998, Clovis, 

California. 

2 	 Senior Scientist, Leader of the 

Fundamental Factors of Site 

Productivity Science Team, and 

cofounder and Technical Chair 

of the North American LTSP 

Cooperative Research Program, 

Silviculture Laboratory, Pacific 

Southwest Research Station, 

USDA Forest Service, 2400 

Washington Ave., Redding, CA 

96001. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 63 
Powers Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

agree that timber harvesting, if conducted so as to preserve potential site 

productivity, need not compromise other ecosystem values (Attiwill 1994, 

Kimmins 1996). Many in North America’s private forestry sector are skeptical of 

third-party “green certification” where criteria may be based more on speculation 

than on science (Berg and Olszewski 1995). Yet, ignoring green certification 

could limit markets for industrial wood. 

Progress has been made toward developing more uniform and objective 

standards for green certification. The central international body is the Forest 

Stewardship Council with two affiliates in the United States—SmartWood and 

Scientific Certification Systems (Mater and others 1999). Green certification aims 

at protecting multiple forest values and long-term site productivity, but 

advocates often seem naïve and myopic. If all the world’s forests were managed 

under green certification standards of the Forest Stewardship Council, harvests 

would average 0.7 m3 ha-1 annually (Binkley 1997). Unfortunately, this average 

would require a one-third increase (1.1 billion ha) in global forest area just to 

meet the current wood demand of the world’s population. Kimmins (1996) 

points out that popular standards for green certification often are so stringent 

that harvested yields are lowered by more than if the sites had been severely 

degraded by exploitative management. Implications of mandating unnecessarily 

low harvests are serious. World population and demand for wood products are 

rising at similar rates (FAO 1997). This demand permeates all societies (FAO 

1997) and increases by 70 to 80 million m3 annually—a volume equivalent to 

British Columbia’s entire allowable cut in 1993 (Kimmins 1996). 

Reduced wood production from green-certified nations creates a strong 

incentive for other countries to accelerate forest harvesting beyond 

sustainable levels to reap the rewards of global demand (Kimmins 1996). A 

scarcity in domestic wood supply will raise wood prices, stimulating 

consumer preference for nonrenewable substitutes. Recent studies show that 

with each 1 percent rise in the price of softwood lumber, the use of cement 

rises by 0.15 percent, structural steel by 0.3 percent, and brick by 0.65 percent 

(Binkley 1997). Sustaining the productivity of United States forests, regardless 

of ownership, is in our national interest. 

Currently, a simple definition of “sustainable forestry” lacks international 

consensus (Nambiar and Brown 1997, Sullivan 1994). No one in good conscience 

can support management practices that degrade forest productivity. The problem 

is how to produce more wood from less area without impairing the land’s 

potential to provide other social benefits now or in the future. Clearly, the need 

for a closer linkage between management and research in the forest planning 

process has never been stronger (USDA Committee of Scientists 1999). 

This paper describes the genesis of the North American Long-Term Soil 

Productivity (LTSP) cooperative research program and summarizes results from 

the various component studies done to date in the Sierra Nevada and Cascades 

of California. 

Developing Indices of Sustainable Forestry 

An International Movement 

The United Nations Conference on Environment and Development of 1992 (“The 

Earth Summit”) led to a nonbinding agreement to establish principles for 

sustainable forest management (United Nations 1992). As a result, international 

committees have formed to develop criteria and indicators for the conservation 

and sustainable management of forests of the world. One such committee met 

informally in Montreal, Canada, in 1993. Deliberations of what has come to be 

called “The Montreal Process” culminated in a 1995 meeting in Santiago, Chile. 

There, in the “Santiago Declaration,” representatives from Argentina, Australia, 

64 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

Canada, Chile, China, Japan, Republic of Korea, Mexico, New Zealand, Russian 

Federation, Uruguay, and the United States agreed to develop, implement, and 

continually update nonbinding criteria and indicators for the sustainable 

management of temperate and boreal forests (Canadian Forest Service 1995). A 

first step is to find an unambiguous, effective, and objective way to monitor the 

land’s health that covers all levels of management intensity. 

Monitoring Productivity Directly 

The fundamental indicator of a forest’s well-being is the rate at which 

atmospheric carbon is captured photosynthetically and accumulated as organic 

matter. This rate is termed “net primary productivity” (NPP). In turn, NPP is the 

common basis for most fundamental ecosystem processes that produce the 

characteristics of forests valued by society. Accordingly, degrading a site’s NPP 

potential also degrades its potential for producing flora, fauna, habitat, clean and 

abundant water, and recovery from disturbance. Monitoring the departures in 

NPP from baseline conditions would give us a sensitive measure of the health of 

a forest ecosystem and whether it is agrading, degrading, or stable. 

Unfortunately, NPP is extremely difficult to measure. Current rates are affected 

not only by site quality, but also by the present age, stocking, and structure of the 

forest. Therefore, they may not indicate the site’s true potential at full stocking, 

or “leaf area carrying capacity” (Grier and others 1989, Powers 1999b, Waring 

and Running 1998). And even at full stocking, it is almost impossible to measure 

NPP accurately in forests of irregular structure. An unbiased surrogate for NPP 

is needed that is independent of the current condition of the vegetation. 

The Soil Quality Approach 

Soil can be a strong and independent surrogate for measures of potential NPP. 

Together with climate and biotic potential of vegetation, soil forms the foundation 

for forest production. As recognized in the Montreal Process and Santiago 

Agreement, soil-based indicators of sustainable forestry must include measures 

of erosion, organic matter, compaction, nutrient cycling, and pollution 

(Ramakrishna and Davidson 1999). The USDA Forest Service recognized this 

requirement well in advance of the Montreal Process. The National Forest 

Management Act of 1976 mandates that the USDA Forest Service must manage 

public forest lands without impairing their permanent productivity. Accordingly, 

and in consultation with Forest Service Research, the Watershed and Air 

Management Staff of the USDA Forest Service adopted a program for monitoring 

the effects of management practices that is based on the following logic: 

management practices create soil disturbances; soil disturbances affect soil and 

site processes; and soil and site processes control forest productivity. 

Monitoring soil and site processes is not feasible at an operational scale. 

Therefore, USDA Forest Service monitoring strategy centers on measurable soil 

variables, which, if altered beyond a threshold, indicate that potential 

productivity has been degraded. These thresholds of soil quality are based partly 

on research, but largely on professional judgment. Threshold standards for the 

USDA Forest Service Regions of the United States have been summarized by 

Powers and others (1998). Current standards for the Pacific Southwest Region 

are shown in table 1. Although these standards represent a progressive step, they 

are not universally accepted. 

The LTSP Program 

Guidelines suggested by the Montreal Process are vague and too general to be 

useful in operational monitoring. The USDA Forest Service’s standards for 

monitoring soil quality (table 1) are much more specific but still based on 

conclusions drawn largely from scattered, anecdotal—and sometimes 

contradictory—research. This creates problems. Because the standards have not 

Powers 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 65 
Powers Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

Table 1—Current standards of soil quality adopted by the Pacific Southwest Region, USDA Forest Service (Powers and 

others 1999b). Standards indicate thresholds for significant soil degradation. 

Variable Quality standard 

Operational area Standards extend to all land capable of growing vegetation 

Erosion Not to exceed rate of soil formation, or about 2 Mg ha-1 yr-1

.


Soil cover Forest floor covers less than 50 percent of area.


Organic matter Litter and duff cover less than 50 percent of area. Fewer than


12 decomposing logs ha-1 at least 30 cm in diameter and 3 m 

in length. 

Infiltration	 Reduced to ratings of 6 or 8, as defined by Regional Erosion 

Hazard ratings. Extent depends on cumulative watershed 

effects analysis. 

Compaction Total soil porosity reduced more than 10 percent, depending 

on soil type, over an area large enough to reduce productive 

potential. 

Displacement	 Soil organic matter in upper 30 cm reduced more than 15 

percent from natural conditions. Affects enough area that 

productive potential is reduced. 

been calibrated against true measures of potential productivity, they may be too 

restrictive in some cases and too lenient in others. Without convincing evidence 

that such standards are accurate, forest managers in the private sector are not apt 

to take them seriously. 

In the late 1980s, I arranged a series of small group meetings among key 

USDA Forest Service scientists and leaders in National Forest System to 

explore prospects for a definitive study national in scope. We agreed that 

guidelines for detecting changes in fundamental productivity were 

cumbersome and inconclusive. After an exhaustive review of the world’s 

literature, our core group agreed on the following principles for guiding such 

a study. (1) Within the constraints set by climate and relief, the productive 

potential of a site depends on soil resources. (2) Management practices cause 

soil disturbances that affect soil properties and processes. In turn, these 

processes govern potential productivity. (3) The main soil processes 

controlling potential productivity involve physical, chemical, and biological 

interactions between soil porosity and site organic matter. 

The third principle provides a framework for research. Recognizing that it is 

unlikely that any simple response would apply to all climates, soils, and forest 

types, our core group agreed on a common experimental design that would be 

applied consistently to a spectrum of benchmark sites across the United States. 

Following discussion among international scientific peers, a study plan was 

drafted and reviewed nationally by silviculturists and soil scientists in both 

research and management arms of the USDA Forest Service. In 1989, the final 

plan for a Long-Term Soil Productivity (LTSP) cooperative study was approved 

by the Deputy Chiefs for Forest Service Research and National Forest System. 

Simultaneously, our rationale and proposal were presented at a major 

international conference, reviewed technically, and published in the proceedings 

(Powers and others 1990). Along with their counterparts from the National 

Forest System, principal investigators from USDA Forest Service Research began 

implementing the LTSP experiment in 1989. Funding for the installation phase 

came from the Washington Office through excess timber sale receipts 

(approximately $9 million between 1989 and 1998). 

66 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

The four main objectives of the LTSP program are to: (1) determine how site 

carrying capacity for NPP is affected by pulse changes in soil porosity and site 

organic matter; (2) develop a fundamental understanding of the controlling 

processes; (3) produce practicable, soil-based indicators for monitoring changes 

in site carrying capacity; and (4) develop generalized estimation models for site 

carrying capacity, as conditioned by soil and climatic variables. 

The effort is hypothesis driven and involves manipulation designed to 

stress the soil’s capacity for NPP. It focuses on major forest types of the 

United States within the component classified as commercial forest land. 

LTSP centers on closed-canopy, young-mature forests growing near the 

culmination of mean annual increment. It is chartered to run to at least the 

culmination of mean annual increment on each site (a rotation, or planning 

horizon). Details of this remarkably successful, North American-wide 

program are described by Powers (1999a). 

Within a climatic region and forest type, National Forests are solicited for 

sites that represent an array of the major soil types along a productivity gradient. 

Candidate sites are examined carefully for variation in soil and stocking. If 

satisfactory, and with concurrence of National Forest and Ranger District 

personnel, each forest stand is inventoried and at least 30 trees are felled and 

sampled to estimate the biomass and nutrient contents of their boles and crowns 

(Powers and Fiddler 1997). The understory, forest floor (all organic detritus 

above the mineral soil), and mineral soil to 100 cm in depth also are sampled for 

mass and nutrient content. Regression methods are used to expand sample data 

to an areal basis. This process characterizes the mass and chemical state of the 

forest immediately prior to treatment (table 2). 

Once sites have been characterized, all trees are felled and nine core 

treatments are assigned randomly to 0.4-ha plots. These treatments consist of 

three levels of organic matter removal/retention (commercial bole removed/ 

Table 2—Approximate biomass and nitrogen (N) content of ecosystem before and after removing various components 

at the Challenge LTSP site (Powers and Fiddler 1997) 

Powers 

Ecosystem Biomass in N content Cumulative Cumulative- Ecosystem N 

component component of ecosystem proportion of remaining 

component N content ecosystem N after removal 

Tree layer 

Boles 

Crowns 

Understory layer 

Shrubs 

Herbs 

Forest floor layer 

Woody debris 

Litter 

Mineral soil (1 m) 

Mg ha-1 kg ha-1 kg ha-1 percent percent 

425.3 440.3 440.3 5.1 94.9 

48.0 164.6 604.9 7.0 93.0 

0.4 3.1 608.0 7.0 93.0 

0.2 1.4 609.4 7.1 92.9 

9.6 29.7 639.1 7.5 92.5 

49.0 425.4 1,064.5 12.3 87.7 

— 7,630.0 8,694.5 100.0 00.0 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 67 
Powers 

Figure 1—Standardized field design 

of the factorial core treatments in 

the LTSP experiment. Each 

treatment cell measures 0.4 ha and is 

regenerated to the principal forest 

trees of the region. Regional 

vegetation is excluded on one half of 

each cell and allowed to develop on 

the other. 

Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

crown, understory and forest floor retained; all living vegetation removed/ 

forest floor retained; all vegetation and the forest floor removed/mineral soil 

exposed), crossed factorially with three levels of soil compaction (nil, 

intermediate, extreme) (fig. 1). Additional plots representing best management 

practices, as well as mitigation treatments (subsoiling, fertilization), are included 

if space is available. Plots are reforested with tree species native to the site. 

Following the reforestation phase, treatment plots are divided into two 0.2-

ha subplots. Native understory flora is allowed to develop naturally on one 

subplot. The other is kept only in trees by applied herbicides. These treatments of 

site organic matter and soil porosity produce a factorial, split-plot matrix 

encompassing the soil disturbances common to almost any operational 

harvesting practice. The vegetation control treatments permit complex and 

simple plant communities to develop side-by-side. With time, each plot and 

subplot provides a distinct measure of NPP as affected by pulse disturbance. 

Plots are sufficiently large that vegetative performance will not be confounded 

by edge effects from the surrounding forest. Measurement protocols have been 

standardized throughout the country (Powers and Fiddler 1997, Powers and 

others 1990). 

The first LTSP installation was completed on the Palustris Experimental 

Forest in Louisiana in 1990. The second was at Challenge Experimental Forest in 

California in 1991. By 1996, the LTSP concept had spread to the Canadian 

Provinces of British Columbia and Ontario. Soon the program expanded to 

complementary studies by collaborators in academia and the forest industry. 

Today, the network includes 62 core LTSP installations and 40 affiliated 

installations, making it the world’s largest and best-coordinated network of 

studies examining how soil disturbance impacts potential site productivity (fig. 

2). Of the 62 core installations, 12 are in the mixed-conifer forest type of the 

western Sierra Nevada of California (table 3). 

The oldest California installations have completed only twelve growing 

seasons, and the youngest only six. Intensive measurements are taken at 5-year 

intervals, partly because of restricted research funds, and partly because initial 

perturbations are not likely to indicate long-term trends. LTSP scientists agree 

that the first reliable indicators of long-term trends are not apt to appear until 

crowns have closed on all treatments at a site. All of the California installations 

have achieved their 5-year measurements, but none has reached crown closure 

on all treatments. 

68 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests Powers 

Figure 2—Distribution of core and 

affiliated experiments of the North 

American Long-Term Soil 

Productivity cooperative research 

program. Commercial forest area is 

shaded. 

Early findings have been presented at conferences as progress reports and 

are summarized in the following section, along with my interpretations. Most of 

the NPP data are still being analyzed, and growth data reported here reflect 

volume measures, not NPP, although trends between volume increment and 

NPP will track closely. Until all installations reach crown closure, these findings 

are simply a nest of case studies that should not (and generally cannot) be 

examined by inferential statistics. Early trends are not offered as long-term 

projections. Collectively, however, they show progress and provide important 

preliminary findings. 

Table 3—General characteristics of 12 LTSP sites in California (Powers and Fiddler 1997) 

___________________________________________________________________________

Place name	 Year Parent material Relative Site Forest Ranger 

establi- drought quality District or 

shed other ___________________________________________________________________________ 

Challenge 1991 Metabasalt Low High Plumas Feather River 

Wallace 1993 Volcanic ash Low Mod. Eldorado Georgetown 

Central Camp 1993 Granodiorite High Mod. Sierra Minarets 

Owl 1993 Granodiorite High Mod. Sierra Minarets 

Vista 1993 Granodiorite High Mod. Sierra Mariposa 

Blodgett 1994 Basalt Low High Blodgett U. of Calif. 

Brady City 1995 Basalt Low High Tahoe Downieville 

Lowell Hill 1995 Basalt Low High Tahoe Navada City 

Rogers 1996 Granodiorite Mod. High Plumas Feather River 

Aspen 1997 Volcanic ash Mod. Low Lassen Eagle Lake 

Bunchgrass 1997 Volcanic ash Mod. Low Lassen Hat Creek 

Cone 1997 Volcanic ash Mod. Low Lassen Eagle Lake 

___________________________________________________________________________


USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 69 
---------------------------------

Powers Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

Early Results from LTSP 

Infiltration and Erosion 

Several important findings—albeit preliminary—have emerged from the 

California LTSP study. Troncoso (1997), working with granitic soils from the 

three Sierra National Forest sites, found that severe compaction reduced water 

infiltation by 72 percent in surface soils. Lessened infiltration can lead to surface 

runoff during intensive rainfall and to reduced soil water recharge. Although 

soil water recharge may not be a problem if precipitation occurs as low intensity 

rain or snow, soil erosion is another matter. Surface erosion occurs when 

hydraulic forces exceed the binding strengths of soil aggregates, a relatively 

stable assemblage of organic material and mineral particles. Materials such as 

organic detritus on the soil surface will break the impact of raindrops. Therefore, 

erosion rates will be high if the soil surface lacks cover by organic matter and if 

the stability of soil aggregates is low. A simple way to measure soil erosion is to 

drive thin pins deeply into the soil, leaving a portion exposed above the soil line. 

Monitoring changes in soil elevation on the pins over time provides an index of 

the rate soil erosion. 

Sandy Inceptisols with low organic matter content, such as the Dome soil 

series on the three Sierra National Forest sites, lack much profile 

development, and soil aggregate stability is very low. Therefore, soil particles 

are easily detached by raindrop impact and surface water flow if infiltration 

rate is too low. Although erosion pins were installed at the Sierra LTSP sites, 

funding limitations have precluded any measurements. Casual observations, 

however, indicate that erosion rates there are relatively high where soil 

surfaces were bared and severely compacted. 

Only at the Challenge site, the first California installation, have we taken 

measurements on erosion pins. There, soil elevations were measured after 3 

years on all plots. Using changes on the Control plot as a standard, relative 

erosion rates increased as much as sevenfold (table 4). Slope variation among 

plots precluded a smooth response surface across treatments, but some points do 

seem clear. First, soil compaction per se had no obvious influence on relative 

erosion rate. In fact, in three of four cases where soil was protected with slash or 

forest floor, relative erosion rates on compacted plots were lower than for the 

control plot. Second, the soil cover effect was profound. Regardless of compaction 

level, erosion always was much greater where the surface organic layer was 

gone. Soil aggregation is much stronger on the clay-textured soil at Challenge 

than on the sandy textures of the Sierra National Forest sites. Presumably, if 

rainfall intensity were as great at the Sierra sites as at Challenge, soil erosion 

rates would be far greater than noted. A solid conclusion is that surface organic 

cover comprises the first line of defense against the erosion of surface soil. 

Table 4—Relative rate of soil loss over 3 years at the Challenge LTSP site (control = 100, 

Powers and Fiddler 1997) 

Compaction level Relative rate of erosion when residual ground cover was: 

Logging slash Forest floor Bare soil 

percent --------------------------------

None 100 187 416 

Moderate 66 270 812 

Severe 84 43 330 

70 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests Powers 

Table 5—Effects of presence or absence of forest floor on mean monthly temperature and moisture at 20-cm depth of 

noncompacted soil, Challenge LTSP site (Powers 1999b) 

Month Soil temperature (°C) Percent soil moisture 

Present Absent Difference Present Absent Difference 

April 12.8 13.3 0.5 34 34 0 

May 17.8 21.5 3.7 26 14 -12 

June 19.0 22.8 3.8 21 13 -8 

July 20.2 24.6 4.4 25 18 -7 

August 18.5 23.7 5.2 20 13 -7 

September 17.5 21.0 3.5 15 12 -3 

Significance of Surface Organic Residues 

Organic residues that protect the soil surface from raindrop impact and runoff 

also insulate against high summer temperatures and evaporative moisture loss. 

From June through August at Challenge, bare soils averaged 23.7°C at 15 cm in 

depth—4.5°C warmer than soils covered by forest floor (table 5). Also, bare soils 

were essentially dry by May, whereas presence of a forest floor kept soils 

relatively moist into August. On the sandy soils of the Sierra National Forest 

sites, Troncoso (1997) found that surface soils were depleted of plant-available 

moisture by August, but moisture was available throughout the dry summer 

where logging slash had been retained (fig. 3). Studies a year later on these same 

soils (Swearingen 1999) showed that the mulching effect extends deep into the 

soil profile. Where logging slash had been retained, soil moisture at a depth of 75 cm 

remained well above the wilting coefficient (about 8 percent moisture content) 

throughout the summer, but soils were dry by mid-July where surfaces were bare. 

Obviously, a condition that extends the period of plant-available moisture favors 

plant growth under the conditions of a Mediterranean climate. Surface organic 

residues do that by acting as mulch against solar heat and evaporative losses. 

As soils dry, strength mounts between soil particles, regardless of the level 

of compaction. Increased soil strength means increased resistance to root 

elongation. Root stress increases greatly above 2 MPa (1 MPa = 10 atmospheres 

of pressure, or 10.3 kg cm-2) and growth essentially ceases at 3 MPa (Sands and 

others 1979). At Challenge, soil strength measurements taken in July of the 

third year showed that strengths averaged 2 MPa or less throughout the upper 

40 cm where a forest floor was present but a full MPa greater where it was 

absent (Powers and Fiddler 1997). Thus, surface organic residues affect root 

behavior in dimensions beyond simple water availability. 

A progressive view of the worth of surface residues is that their direct effect 

on soil moisture is important but ephemeral. The mulch value of a forest floor 

should dissipate as canopies close and transpiration dominates evaporation. 

However, organic residues are a major component of the carbon cycle that 

supplies an energy substrate to soil organisms that dominate soil processes. In 

turn, these processes control the storage and biological availability of soil water 

and nutrients. The nutritional significance of the forest floor is far greater than 

indicated by its biomass. For example, the mass of the forest floor before 

harvest at the Challenge LTSP site was only 11 percent of the total biomass 

above ground (table 2), but the forest floor contained 43 percent of aboveground nitrogen—nearly three times that in the tree crowns that either are 

exported or retained as logging slash. Therefore, loss of the forest floor may 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 71 
Powers 

Figure 3—Soil moisture trends at a 

depth of 15 cm as affected by 

presence or absence of surface 

organic residues. Sierra National 

Forest LTSP sites, third year. Dashed 

lines indicate moisture content at 

field capacity (upper) and at wilting 

coefficient (lower) (Troncoso 1997). 

Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

eventually have a profound effect on soil fertility on sites less fertile than at 

Challenge. Its importance probably exceeds that of the crown material in 

logging slash. 

The physical value of surface residues is conditioned by climate. At higher 

latitudes or at elevations where soil temperatures approach the cryic 

temperature regime, anything reducing soil temperature reduces primary 

productivity. In the frigid and cryic temperature zones, surface residues 

accumulate and insulate the soil, lowering soil temperature and diurnal flux 

(Fleming and others 1994). Moist sites remain wet and aeration may be 

impaired. On better-drained sites, water stress can develop because water 

viscosity increases rapidly as temperatures approach freezing (viscosity is 16 

percent greater at 5°C than at 10°C). The same insulating properties of surface 

residues that reduce primary productivity in cold forests become beneficial in 

warm, dry regions such as found in the mixed-conifer and pine forests of 

California (table 5, fig. 3). 

Significance of Compaction 

Effects of soil compaction on soil moisture go beyond simple infiltration, but 

certain principles must be understood to interpret LTSP findings that, at first 

glance, may seem contradictory. Compaction alters soil pore size and volume— 

variables that influence the availability of soil water to plants. Soil pores are 

divided by convention into two size classes, based on water-retention properties 

(Childs and others 1989, Taylor and Ashcroft 1972). “Micropores” are voids so 

small that they remain filled with capillary water when gravitational water has 

drained and the soil is at field capacity. Much of this micropore water is held at 

tensions between 0.01 and 3.1 MPa (the lowest tension of hygroscopic water), 

meaning that not all of the water in micropores (that held at tensions > 1.5 MPa) 

is available to plant roots. “Macropores” are soil voids sufficiently large (> 14 

μm radius) that capillary water will not bridge their diameters following 

gravitational drainage. Loosely held water (0.01 to 1.5 MPa tension) is retained 

as a cohesive film on the surfaces of particles bordering macropores, and the 

remaining void is filled with air. This low-tension film is the principal source of 

water for plant uptake and accounts for roughly half of the total water-holding 

capacity of an uncompacted soil (a higher proportion for sands, a lower 

proportion for clays). As low-tension water is depleted from macropores by 

transpiration or evaporation, films become thinner. As films thin, a tension 

gradient develops and macropores are recharged partially by water held in 

micropores. Eventually, water films in all soil pores thin to such high tension (by 

convention, >1.5 MPa ) that the soil is said to have dried to “wilting coefficient.” 

When soil pore radii are less that 0.1 μm, the affinity between water molecules 

72 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests Powers 

Table 6—Effects of soil compaction (NC = not compacted, C = compacted) and understory vegetation on plant and soil 

properties for soils of contrasting textures (severe compaction on the clay, moderate compaction on the sand) (Powers 

1999b) 

Variable Understory vegetation absent Understory vegetation present 

Clay Sand Clay Sand 

NC C NC C NC C NC C 

Relative growth (pct) 

Abies concolor 100 56 100 167 22 33 67 100 

Pinus ponderosa 100 60 100 169 33 47 94 125 

Vegetative cover (pct) Trace Trace Trace Trace 91 56 55 68 

Soil bulk density (Mg m-3) 0.88 1.13 1.06 1.14 0.88 1.13 1.06 1.14 

Total soil porosity (pct) 67 57 60 57 67 57 60 57 

Change in AWC (pct) 0 -24 0 +65 0 -24 0 +65 

Predawnψp (MPa) 

Abies concolor -0.54 -0.63 -1.13 -0.93 -1.74 -1.15 -2.37 -3.47 

Pinus ponderosa -0.60 -0.66 -1.05 -1.14 -0.88 -0.87 -1.61 -2.05 

and pore surfaces is so great that water is said to be unavailable to plants. 

Soil macropores also are essential to infiltration of precipitation and 

exchange of oxygen and respiratory gases between the atmosphere and the 

root. A macropore volume of at least 10 percent of total soil volume is needed 

for proper root respiration by terrestrial plants (Grable and Siemer 1968, 

Vomocil and Flocker 1961). Soils with too few macropores may become 

anaerobic; those with too many may be excessively drained and droughty. 

Unfortunately, the distribution of soil pore sizes is difficult to measure and 

essentially impossible to measure in the field. 

Working with sandy soils on the Sierra National Forest LTSP sites, 

Swearingen (1999) showed that, at a depth of 15 cm, compacted soils retained 

plant-available water through late June, but noncompacted soils were depleted 

by mid-May. At a depth of 75 cm, the wilting coefficient was reached by late 

June on noncompacted soils and mid-July on compacted soils. Thus, a certain 

amount of compaction seemed to favor the storage of low-tension soil water. 

But is this effect universal? And how does it affect vegetative growth? 

To study these questions, the Challenge and Vista LTSP sites were chosen 

for more intensive study because they represent extremes in soil texture. Work 

centered on plots with complete organic matter removal and either no or severe 

compaction. As might be expected for a fine-textured soil, both tree growth and 

vegetative cover were suppressed (about 40 percent) by compacting the clayey 

soil at Challenge (table 6). Soil bulk density increased by 28 percent, equating to 

a reduction in total porosity of 15 percent. Such a reduction seems small, except 

that it comes at the expense of aeration porosity (macropores are the first to be 

deformed by compaction). Clay micelles (flat particles less than 0.002 mm in 

size) compact into a hard, platy structure. Although particle surface area is 

high, macroporosity is relatively low in clays. Measurements made in the 

laboratory with intact soil columns from treated plots at Challenge show a loss 

in plant-available water (AWC) of 24 percent. Therefore, compaction reduced 

both the soil’s aeration and its capacity to store available water. Lowered soil 

AWC measured in the laboratory is not simply academic. It is reflected by field 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 73 
Powers Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

measurements of predawn xylem water potentials (ψ

p) taken in August at the 

peak of drought. Predawn water potentials were 17 percent lower in white fir 

and 10 percent lower in ponderosa pine following compaction (table 6), meaning 

that trees on compacted plots were not rehydrating fully at night when stomata 

were closed. Lowered AWC translates to plant drought under Mediterranean 

climatic conditions, helping explain why both tree growth and ground cover 

were suppressed by compaction on the clayey soil. 

Different results were found on the sandy soil at Vista, where both trees 

and understory vegetation grew appreciably better on moderately compacted 

plots (table 6). Soil bulk density also was increased by moderate compaction, 

but only by 8 percent. Because sand grains are large, angular particles (0.05 to 

2.0 mm), they have less surface area per unit volume than clay particles. Total 

soil porosity generally is lower in sands than in clays and is weighted heavily to 

macropores. Consequently, sandy soils drain quickly and, because their 

microporosity and specific surface area are low, retain little water. Compacting 

the sandy soil moderately at Vista increased AWC by 65 percent (table 6). 

Apparently, this difference reflects a rearrangement of sand grains and a 

reduction of pore sizes. As a consequence, white fir ψ

p in August improved an 

average of 18 percent. Xylem water potentials were lower (trees were drier) 

after compaction in ponderosa pine—possibly because the pines were about 

five times larger than the firs. Larger trees mean greater crown surface area and 

greater transpiration rates. Presumably, larger trees on compacted soil depleted 

soil water more rapidly throughout the summer. From this I conclude that 

compaction effects on plant growth depend upon soil texture, the degree of 

compaction, and whether or not AWC is a limiting factor. 

Improved growth from compaction also occurred for aspen on droughty 

sands in Michigan (Stone and others 1999) and for ponderosa pine on the sandy 

soil at Rogers (Gomez and others 2002). Recent measurements at Central Camp 

and Owl LTSP sites on the Sierra National Forest show that growth was 

reduced by more severe compaction. Differing responses to compaction among 

sandy-textured soils on sites classified as the same soil series probably trace to 

subtle differences in distributions of pore size and/or particle size, which is 

fertile ground for further investigation. A contract was completed recently for 

detailed classification and textural analyses of the soils at all LTSP installations. 

Other recent LTSP measurements of ponderosa pine growth illustrate how 

this concept might extend to a variety of soil textures. Severely compacting a 

clay-textured soil (Challenge) can lower tree growth by nearly half (fig. 4). The 

effect was much less on a loam (Blodgett) or sandy loam (volcanic ash at 

Wallace) and even promoted growth on a moderately compacted sand (Vista). 

Work underway by University of California graduate students is quantifying 

how compaction affects pore size distribution, water availability, and tree 

growth on these and other LTSP soils. 

Confounding from Competing Vegetation 

Compacting the clayey soil at Challenge clearly reduced AWC and conifer 

growth rates (table 6). It also increased soil strength by about 1.5 MPa throughout 

the upper 40 cm (Powers and Fiddler 1997). In all, the soil’s productive potential 

was reduced substantially by compaction. Yet, this effect was apparent only 

when trees were free of competition from understory vegetation. Where 

vegetation was not controlled, trees were appreciably larger and their ψp equal or 

greater on compacted plots (table 6), although their absolute growth was 40 to 67 

percent less than on noncompacted plots where vegetation had been controlled 

(Powers and Fiddler 1997). This is because the density of understory vegetation 

was nearly twice as great on noncompacted plots (91 percent cover vs. 56 percent 

cover). Understory seedlings germinating from seed could recolonize 

noncompacted, clayey soil much more easily than compacted soil. Less 

74 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests Powers 

Figure 4—Effects of soil 

compaction on relative volume 

growth of ponderosa pine at four 

LTSP sites of differing soil texture. 

Symbols: not compacted (NC), 

compacted (C). 

understory competition for water and nutrients meant that trees planted in holes 

augered into compacted soil could perform better than trees under greater 

competition in noncompacted soil, at least in the short term. 

Where compaction improved soil water availability (the sandy soil at Vista), 

both trees and understory species grew faster (table 6). For white fir (which were 

smaller than ponderosa pine, and have weak stomatal control), predawn ψ was p

greater on compacted soil if understory vegetation was absent. Where understory 

was present, however, the higher vegetative cover on compacted soil overcame 

increased soil water availability and created greater competition for soil water, 

leading to lower ψp in both fir and pine (table 6). Furthermore, the low specific 

area of sand grains compared with clay micelles (Taylor and Ashcroft 1972), 

coupled with low precipitation at the Vista site means that the soil’s storage of 

water there is lower than at Challenge. Therefore—and regardless of improved 

soil water holding capacity from compaction—soil water would be depleted 

very quickly if both trees and understory vegetation are present. 

These findings suggest that, under certain conditions, the true effect of soil 

compaction can be masked completely—and seemingly reversed—by the 

confounding effect of competing vegetation. On sandy soils, available waterholding capacity may be improved somewhat by compaction. Because absolute 

soil water storage still is low, however, effects of competing vegetation will be 

particularly severe. Without accounting for the significance of understory 

vegetation, the true effect of soil compaction is not always recognized. 

Extending LTSP Findings to Operational Practices 

Soil Compaction 

Stanislaus Thinning Study 

Field monitoring methods developed from LTSP were applied to an operationalscale study established in 1994 on the Mi-Wuk Ranger District of the Stanislaus 

National Forest. The study was designed to evaluate the soil and stand impacts 

of alternative methods of mechanized thinning. The Wrights Creek Burn site had 

been salvage-logged in the mid-1950s following destruction of a mixed-conifer 

forest by wildfire. Slash and brush were piled into windrows by crawler tractor, 

thereby ensuring a legacy of soil disturbance that included soil compaction. In 

1959 the site was planted with ponderosa pine, and the ensuing stand was 

thinned by using ground-based equipment during the spring and summer of 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 75 
Powers 

3 Mention of trade names or 

products is for information only 

and does not imply endorsement 

by the U.S. Department of 

Agriculture. 

Figure 5—Soil-strength profiles 

for varying degrees of soil 

compaction following mechanized 

thinning at the Wrights Creek 

plantation. (A) Strengths were high 

beneath harvester/forwarder tracks 

following cut-to-length thinning, but 

not for the remainder of the unit. 

(B) Feller-buncher operations had a 

greater impact throughout the unit. 

Tillage (broken line) reduced soil 

strength in skid trails (Powers and 

others 1999). 

Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

1994, while soils were fairly moist. Harvesting was done both by whole-tree 

removal using a Timbco feller-buncher3 with shear and grapple skidder (FB), and 

by the cut-to-length (CTL) system using an FMG Timberjack harvester and 

forwarder that removed boles and retained slash. Unthinned areas were set aside 

as controls. Each system operated on plots of 2.5 ha and larger, with two replicates 

of each treatment. 

Because the FB had to drive to each harvested tree to shear it, traffic covered 

much of the plot. Traffic was funneled onto well-defined trails, however, 

transporting whole trees to the landing and leaving minimal slash on the plots. 

In contrast, CTL operations proceeded linearly through the woods with the 

felling head reaching about 10 m laterally. Limbs and foliage were stripped by 

the processing head in front of the harvester, and the lengths of cut timber were 

passed back to the forwarder for transport to the landing. Consequently, both 

harvester and forwarder drove atop a bed of freshly cut slash. 

After harvesting, the FB plots were split. Using a winged subsoiler drawn by 

crawler tractor, skid trails were tilled to a depth of about 0.5 m on half of the plots 

in the spring of 1996. The other half remained untilled. Preliminary findings 

from transects taken in each unit revealed that both types of mechanized thinning 

produced similar amounts of surface disturbance in traffic lanes, but the FB 

treatment had higher soil bulk densities beneath traffic lanes and twice as much 

general disturbance in the remaining area (Powers and others 1999). 

Soil strengths measured in April, when soils were near field capacity and 

strengths were lowest, were high beneath both FB skid trails and CTL traffic 

lanes (fig. 5) and exceeded 2 MPa in the 10-20 cm depth zone. Despite the 

similarity of soil strengths beneath traffic lanes in both treatments, however, 

strengths on the CTL plots were appreciably lower in all other disturbance 

categories (fig. 5A). For the minor disturbance class, 2 MPa strengths were 

reached at 13 cm in the FB treatment, but not until 25 cm in the CTL treatment. 

Tillage reduced soil strength in FB skid trails to less than that for the 

undisturbed class (fig. 5B), suggesting that compaction that occurred during 

salvage and site preparation decades earlier may have been mitigated. Roots of 

trees bordering the trails were severed during tilling operations. Analysis of 

diameter growth of about 80 trees bordering tilled and nontilled skid trails, 

however, gives no indication that tillage of the skid trails had any effect on tree 

growth in the 3 years since tillage (Powers and others 1999). Net growth was 

neither depressed from root damage nor enhanced by reducing the high soil 

strengths in the compacted trails. Whether this trend persists remains to be seen. 

A B 

76 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

I conclude from this that techniques for measuring soil strength, developed 

from LTSP studies, can be used to distinguish physical changes in the soil that 

are caused by field operations. Therefore, soil strength should have a prominent 

place in the operational methods for monitoring soil quality by the USDA Forest 

Service’s Pacific Southwest Region (table 1). 

The Roseburg Study 

To further extend LTSP principles of physical soil mitigation to an operational 

setting, we began a second experiment in fall 1998 with cooperation from 

Roseburg Resources Co. in the Pondosa Burn north of Burney, California. The 

test forest is a 15-year-old plantation of ponderosa pine established after salvage 

logging in the aftermath of a 1977 wildfire. Although soils were compacted 

somewhat by salvage logging, tree survival was excellent. Canopies have closed, 

and the plantation is being thinned by whole-tree harvesting to maintain high 

rates of tree growth and to reduce fuel buildup. Four primary treatments have 

been established on 0.4-ha plots in a randomized design of four blocks: (1) 

unthinned control, (2) thin and remove every third row of trees, (3) thin and 

remove every third row, subsoil the traffic lanes, and (4) thin and remove every 

third row, fertilize and subsoil the traffic lanes. 

Thinning was done with a three-wheeled Morbark Wolverine shear and 

grapple skidder. Whole trees were skidded to a landing, where they were 

chipped. Traffic lanes within thinned strips were tilled to a depth of about 0.5 m 

along wheel tracks, using two passes of a winged subsoiler drawn by crawler 

tractor. Fertilization involved granular urea and ammonium triple phosphate 

applied at 224 kg N ha-1 and 336 kg P ha-1 in the tilled tracks. Tillage provided an 

opportunity to work N and P into the rooting zone of residual trees. Penetrometer 

readings taken shortly after treatment in fall 1998 showed that mechanized 

thinning led to soil compaction and reduced the friable rooting depth by half (fig. 

6A). In contrast, subsoiling the compacted traffic lanes lowered soil strength to 

prethinning levels (fig. 6B). This treatment should create a furrow effect that 

collects and retains soil moisture and extends tree growth into the summer. 

Early results from this LTSP-related study reinforce the significance of soil 

strength for detecting treatment differences on soil’s physical properties. They 

also indicate that: mechanical row thinning compacted the soil beneath the traffic 

lanes; compaction extended to at least 40 cm; and subsoiling seems to have 

mitigated the effect. How this translates to soil moisture availability, understory 

diversity, root growth, and tree growth will be determined in the coming years. 

Organic Residues 

Principles learned from LTSP indicate that replenishment of organic matter is 

critical to sustained productivity of forested ecosystems. Organic carbon in forest 

detritus is the substrate energizing most soil biotic processes that control nutrient 

Powers 

Figure 6—Soil strength MPa 

isolines at Pondosa in fall 1998 

following mechanized thinning of 

every third row (Powers and 

others 1999). (A) Thinned only. 

Friable rooting zone was reduced 

by half beneath traffic lanes. (B) 

Thinned and tilled along harvester 

tracks. Tilth returned to 

prethinning levels. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 77 
Powers Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

and water availability, aeration, and soil structure. Detritus can create a fuel bed 

that increases the risk of forest destruction by fire, however, particularly in our 

summer-dry climate. The problem facing management is how to reduce fire risk 

without depleting organic matter on and within the surface soil. 

Conventional methods of managing residues include low-intensity burns or 

mechanical removal of some of the fuel load through whole-tree harvesting. Both 

methods remove organic carbon from the site. While solving the immediate fuel 

problem, they offer nothing for long-term carbon storage or improvement of soil 

quality. On more mesic to xeric sites with less fertile soils, losses of surface 

residues likely will lead to deficiencies of N and P when the stand is at leaf-area 

carrying capacity, and nutrient uptake peaks (Powers 1999). Two experiments 

are underway and a third is planned to test effective alternatives to burning or 

removal of organic residues. Each involves retention of residues in chips. 

The Sierra Pacific Study 

The first experiment in the Sierra Pacific study began in 1993 with commercial 

harvesting of a 90-year-old mixed-conifer stand on what is now Sierra Pacific 

Industries land near Blodgett Forest in Eldorado County. Logging slash (about 

50 Mg ha-1 containing about 640 kg N ha-1) was treated in three ways: residues 

were scattered and broadcast burned; residues were piled by tractor into 

windrows and the windrows were burned; and residues were chipped on the 

site and returned to the ground as linear, 30-m rows of wood and foliage 

shredded to sizes averaging between 2 and 5 cm in length. The first two 

treatments are conventional, but the third is an innovative means of retaining 

organic matter while reducing its flammability and concentrating it to create a 

critical mass that retains water and perhaps creates an ideal medium for 

nonsymbiotic nitrogen fixation. Rows of chips resemble large, fallen, 

decomposing trees spaced about 20 m apart. Treatment plots, 0.2 ha each, were 

replicated four times and planted with mixed species of conifers in spring 1994. 

Concentrating chips into piles offers several benefits. First, it reduces the fuel 

profile throughout the unit to a compact, localized condition. Second, it avoids 

high temperature effects in broadcast chip applications that may heat-girdle 

some conifer seedlings. Third, chip piles dry from the surface inward, creating 

cool, moist conditions in their interiors and in the soil beneath them. Finally, 

once chip piles have decomposed, conditions may be ideal for free-living, 

nitrogen-fixing bacteria. Nitrogen fixation rates have been shown to be much 

higher in chip piles (because of the organic carbon source and anaerobic 

conditions) than in any other field medium (Jurgensen and others 1980). The 

trick is to reduce chips as quickly as possible from large, flat objects with low 

specific surface area to small, amorphous bodies with great surface area for 

retaining moisture. This reduction occurs in natural decomposition by the 

removal by fungi of linear molecular chains of cellulose to leave a residuum of 

lignin (Blanchette and Shaw 1978). Cooperating with scientists in the USDA 

Forest Service’s Rocky Mountain Research Station, we are attempting to speed 

this process by inoculating some chip piles with pure strains of Postia placenta— 

an aggressive brown rot fungus particularly adept at consuming cellulosic sugars. 

The Roseburg Study 

Along with the main-effect treatments with thinning and tillage described 

previously, four secondary treatments, each occupying 0.1-ha subplots, were 

added to each block of the Roseburg study at Pondosa as supplements to 

Treatment 3 described earlier. They involved retention of woody residues as 

chips to reduce fuel volumes while retaining site organic matter. They were: (3a) 

all thinnings chipped, returned to traffic lanes, subsoiled, and rototilled; (3b) 

traffic lanes subsoiled, thinning chips added to the surface; (3c) thinnings chipped 

and returned, fertilized with N and P, subsoiled, and rototilled; and (3d) traffic 

lanes subsoiled, thinning chips added to the surface, fertilized with N and P. 

78 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

Chips were returned to the site to see if residue retention would improve soil 

water storage capacity, soil fertility, and carbon sequestration. The purpose of 

chip fertilization was to lower the C:N ratio to favor microbial decomposition 

(chemical analyses are not available at this time). Chips either were retained on 

the surface to act as mulch or tilled into the soil to increase decomposition. 

The Kings River Study 

A third residue modification study is planned for the Kings River 

Administrative Unit of the Sierra National Forest. The study area is characterized 

as the drier end of the westside Sierra Nevada mixed-conifer and ponderosa pine 

forest types. Soil AWCs are moderate to low. Fire suppression has led to heavy 

fuel accumulations in the understory. The challenge is to manage the Unit for a 

variety of resources while lowering fire risk. The common strategy when creating 

fuel breaks or site preparation following timber harvest is to remove whole trees 

during the harvest and to pile and burn remaining residues. Unfortunately, such 

treatments deplete the site of organic materials, potentially affecting soil fertility, 

AWC, and erodibility. 

Recent mechanical innovations provide another choice. Preliminary tests of 

an innovative rotary mulcher in the southern pine region of the eastern United 

States show a high potential for reducing fuels, retaining site organic matter, and 

improving physical soil properties important to plant growth. Attached to a 

crawler tractor, the rotary mulcher grinds stumps, shrubs and logging slash into 

fine residues. These residues may be left on the soil surface to serve as an 

evaporative mulch or mixed into the surface soil in a single operation. Grinding 

residues into a fine particle size not only reduces the fuel profile, but it also 

increases the specific surface area of the biomass. Higher specific area spells 

greater rates of microbial decomposition. Incorporating these fine materials into 

mineral soil offers a huge bonus of increasing AWC, soil aggregate stability, 

nutrient retention, and carbon sequestration. A small grant was obtained for 

testing this technique in the fall of 1999. Work will center on shaded fuel breaks 

and on group selection openings. Rotary mulching will be compared against 

conventional best-management practices in its effect on seedling survival, 

growth, and vigor, and on soil AWC, nutrient storage, and aggregate stability. 

Emerging Indicators of Sustainability 

A major objective of the LTSP effort is to develop effective and practicable 

methods for monitoring changes in a site’s carrying capacity for NPP. Although 

LTSP is in its infancy, soil strength as measured with a recording cone 

penetrometer (figs. 5, and 6) has emerged as a premier method for assessing soil 

physical properties. The sensitivity of penetrometer readings to soil moisture 

and hardness shows its capacity for integrating several soil physical changes that 

affect root behavior. Management practices that increase soil strength above 2 

MPa during the potential growing season indicate that the site’s capacity for 

plant growth has been diminished. Accordingly, and based solely on early 

findings from LTSP, soil strength has been proposed as the single most useful 

index of soil physical condition in operational monitoring for sustainable forest 

management (Powers and others 1998). Other recommendations include 

anaerobically mineralizable nitrogen as a single, integrative measure of nutrient 

supply, and the presence or absence of biopores and fecal aggregates as an index 

of soil invertebrate activity (Powers and others 1998). Declines in either of these 

properties with time suggest declines in soil biotic function and, most likely, a 

declining productive potential. The foundation for these biotic indices of 

sustainable forestry practices was developed in part by regional and national 

findings from LTSP. 

Although the LTSP effort is young, findings are emerging rapidly. Already 

they are modifying standards for monitoring soil quality for National Forests of 

Powers 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 79 
Powers Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 

the United States, such as standards shown in table 1. As the concept of soil 

quality evolves, so will the effectiveness of soil-based standards. To be 

practicable, monitoring methods must focus on the simplest possible indices of 

key physical, nutritional, and biological properties and processes of the soil. 

Standards will be subject to continual refinement. When strong calibrations 

emerge between soil variables and NPP, as conditioned by climate and soil type, 

such indices will be universally accepted. Findings from LTSP research promise 

to be the primary means for achieving this acceptance. 

Acknowledgments 

While LTSP installation costs were covered by the National Forest System, 

research funds have come almost entirely through the Kings River sustainable 

Forest Ecosystems Project competitive grant. Besides operational research, these 

funds and field sites have helped support M.S. and Ph.D. research of students at 

four major universities in three states. California phases of the North American 

LTSP program would be impossible without the cooperation, faith, and 

generosity of many people. I am particularly grateful to Christine Nota, Mike 

Garcia, Brent Roath, Dave Smith, Mark Smith, Gary Thompson, and Dan Young 

of the Sierra National Forest; Gary Cabodi (retired) of the Eldorado National 

Forest; Martha Twarkins, Jerry Westfall, and Gary Cline of the Tahoe National 

Forest; Chuck Smay of the Plumas National Forest; John Fiske (retired) and Rob 

Griffith, formerly of the USDA Forest Service Regional Office; Ron Stewart of the 

Washington Office (who, as Regional Forester of the Pacific Southwest Region, 

USDA Forest Service, provided exceptionally strong support); Bob Heald of 

Blodgett Research Forest; and Terrie Alves, Kendell Kilborn, Cristina Siegel and 

Bert Spear of the USDA Forest Service’s Pacific Southwest Research Station. 

Thanks, too, to Alex Janicki of the Stanislaus National Forest, Jeff Webster of 

Roseburg Resources, and Henry Alden (formerly with Michigan-California 

Co.)—colleagues in extending LTSP results to an operational scale. Matt Busse 

and Rob Griffith provided several helpful suggestions for revising the draft 

manuscript. Special thanks go to Gary Fiddler of the Regional Office, Pacific 

Southwest Region, USDA Forest Service, who oversaw all phases of site 

characterization, contracting, and treatment installation. Without Gary, we 

would have made no progress. 

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