Juvenile Tree Growth on 

Some Volcanic Ash Soils 

Disturbed by Prior Forest 

Harvest

J. Michael Geist, John W. Hazard, and Kenneth W. Seidel

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

Research Paper

PNW-RP-573

February 2008
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Authors

J. Michael Geist was supervisory research soil scientist (retired), Forestry and 

Range Sciences Laboratory, 1401 Gekeler Lane, now 701 Sunset Drive, La Grande, 

OR 97850; John W. Hazard (deceased) was Station statistician, Pacific Northwest 

Research Station, Portland, OR; Kenneth W. Seidel was principal research 

silviculturist (retired), Silviculture Laboratory (closed), now 2955 NE Alpen 

Glow Place, Bend, OR 97701.
Abstract

Geist, J. Michael; Hazard, John W.; Seidel, Kenneth W. 2008. Juvenile tree 

growth on some volcanic ash soils disturbed by prior forest harvest. Res. Pap. 

PNW-RP-573. Portland, OR: U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station. 22 p.

The effects of mechanical disturbance from traditional ground-based logging and 

site preparation on volcanic ash soil and associated tree growth were investigated 

by using two study approaches in a retrospective study. This research was 

conducted on volcanic ash soils within previously harvested units in the Blue 

Mountains of northeast Oregon and southwest Washington. We assessed soil and 

tree attributes and their association with higher and lower levels of soil disturbance. 

The two approaches were independent efforts that yielded very different results. 

In the first study approach, we used larger measurement plots linked to a portion of some randomly established monitoring transects in 10 harvest units. Sets of 

higher and lower disturbance plots were chosen and positioned in each unit based 

on predetermined soil bulk densities along the midlines of the plots. Crop trees in 

the plots were then measured. Although our two sets had significantly different 

bulk densities, we found no associated differences between sets of tree attributes. 

The second approach involved soil disturbance in five harvest units. Therein we 

mainly used visual cues to identify portions having higher and lower levels of soil 

disturbance. Smaller plots were purposely assigned and positioned within some of 

those conditions. Plot sampling of soils and trees followed, and the results reflected 

significant differences in soil disturbance, i.e., 24 percent greater bulk density (from 

compaction) and 38 percent less ash thickness (from displacement and compaction) 

between disturbance levels. Soil disturbance differences were associated with 17 to 

29 percent less juvenile tree size and growth. Longer term effects are unknown.

Keywords: Volcanic ash soils, soil disturbance, soil displacement, soil compaction, soil productivity, logging, harvest impacts, site preparation, juvenile forest 

growth, forest regeneration. 
Summary

Volcanic ash soils occur extensively in the Blue Mountains of northeast Oregon 

and southeast Washington and they support productive conifer forests. The ash 

layer originated from Mount Mazama in south-central Oregon, and it is readily 

displaced and compacted by mechanized equipment traditionally used in groundbased logging and site preparation. Whether these soils might be altered enough to 

reduce subsequent tree growth has been a common and sometimes controversial 

question among forest managers and the public. We sought to answer this question 

in a retrospective study of some harvested areas showing mechanical soil disturbance imposed by logging and site preparation. Attributes of the subsequent crop of 

juvenile trees and associated volcanic ash soils were compared between lower and 

higher levels of soil disturbance. 

Two study approaches were used. One approach used larger measurement plots 

(about 7 by 16 m) where soil disturbance level was assigned based on the bulk density of soil samples taken at about 3-m intervals along the midline of the plot. The 

plots were centered on a portion of several randomly oriented transects previously 

used in standardized soil monitoring of 10 forest harvest units. 

A subsequent independent approach involved a more restricted population of 

soil disturbance, smaller plots (about 5 by 8 m), more intensive soil assessments, 

and five harvest units. We used mainly visual characteristics of the soil surface and 

groundcover vegetation, plus occasional shovel probing, to find areas of higher and 

lower mechanical disturbance within each harvest unit. Measurement plots were 

then purposely located and positioned within a portion of those disturbance conditions. Assessments of tree and soil attributes followed and were more intensive for 

soils, as compared to larger plots. Measured soil attributes included a grid sampling 

of bulk density, ash thickness, and some chemical properties.

The larger plots mostly involved harvest units that contained a varied mix of 

tree species, whereas smaller plots involved units that were essentially monocultures of either lodgepole (Pinus contorta Dougl. var. latifolia Engelm.) or ponderosa 

(Pinus ponderosa Dougl. ex Laws.) pines. In both study approaches, measurement 

of all plot trees, 1.2 m or taller, followed after soil measurements. 

The results we obtained differed according to study approach. Our methods of 

choosing plots for their contrasting soil disturbance and using larger plots produced 

significantly different sets of bulk density between the disturbance classes, but there 

were no associated differences in tree attributes. The methods used with a more 

restricted population of soil disturbance and smaller plots revealed some differing 

sets of soil properties that did have associated differences in tree size and growth.
Our field observations with larger plots revealed a varied and complex array of 

machine-caused soil disturbance. That perspective was consistent with the variable nature of bulk densities along transects. We found this variation prevented 

us from constructing a higher average bulk density for higher disturbance plots to 

potentially contrast more with lower disturbance plots. We interpreted this as an 

indication that using only the midline bulk density may be problematic as a way to 

characterize soil disturbance conditions of whole plots. Unfortunately, we have no 

scientific means to test whether this speculation is correct. 

We found the scale of smaller plots fit well within the field conditions selected 

as higher and lower soil disturbances. Smaller plots appeared more homogeneous, 

and both soil bulk density and ash thickness results appeared to support the visual 

designations of higher and lower disturbance. Height, height growth, and radial 

growth ranged from 17 to 29 percent less at higher disturbance. Associated differences in soil properties included 38 percent less thickness of volcanic ash and 

24 percent greater bulk density at higher disturbance. We inferred that both soil 

displacement and compaction had occurred with mechanical disturbance. Because 

they occurred together and our study is retrospective, we could not separate and 

assess their individual effects. We do not know the larger scale or longer term 

effects of higher disturbance. We would, however, expect them to vary in relation to 

the specific conditions within a harvest unit. We cannot extend our results to stand 

levels on a per-acre basis, because our sampling focused on mechanical impacts 

within tree-producing areas and intentionally excluded intermediate disturbance 

conditions, burned slash piles, and haul roads. Further, we did not measure proportions or areas of lower or higher disturbance.

The mixed findings in our research and among findings of research elsewhere 

indicate broad generalizations and extrapolations of findings are inappropriate. 

Differences among studies include the kind and degree of disturbance impacts, 

harvesting equipment used, tree species and other vegetation present, climate, geology, soils, measuring tools, and experimental methodology. 

Only our smaller plot approach produced results that offer some credence to 

concerns about early tree growth loss with higher levels of mechanical disturbance 

of volcanic ash soils from logging and site preparation in the Blue Mountains. 

Long-term effects are unknown, but we believe it would be prudent for managers 

to apply strategies and practices that avoid or minimize higher levels of mechanical 

disturbance of volcanic ash soils. 

Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

Introduction

The disturbance effects of forest harvest practices on subsequent tree growth and 

long-term productivity of the soil have attracted considerable attention from land 

managers and private citizens. An expanding body of research indicates forest 

logging and site preparation can affect subsequent tree survival and growth. Rennie 

(1955) reported doubts about the sustainability of continuous forest growth in 

relation to harvest removal of nutrient reserves from the relatively infertile soils of 

Yorkshire, England. In his early concerns about nutrients, he has since been joined 

by other researchers who now share a broad array of concerns that constitute a 

worldwide focus on forest soil productivity and forest sustainability. Several symposia have addressed logging and site preparation effects on physical and chemical 

soil properties, soil biology, nutrient cycling, soil erosion, and their relations to 

productivity (Ballard and Gessel 1983, Harvey and Neuenschwander 1991, Perry 

et al. 1989, Slaughter and Gasbarrow 1988). 

In the Northwestern United States, conventional logging with ground-based 

machines has adversely impacted forest soils (Dyrness 1965, Geist et al. 1989, Lull 

1959, Sullivan 1988, Wert and Thomas 1981). Impacts differ both in kind and severity, but soil compaction has commonly been identified. In some cases, compaction 

was associated with reduced tree reproduction or growth (Cochran and Brock 1985, 

Froehlich 1979, Helms and Hipkin 1986, Lull 1959, Wert and Thomas 1981), but this 

is not always the case (Miller et al. 1996). The broad view of the literature shows 

mixed results, but the majority of studies indicate growth losses with compaction. 

In a review of 142 studies from 1970 to 1977, Greacen and Sands (1980) reported 82 

percent found yield reduced, 8 percent found yield increased, 6 percent found yield 

both increased and decreased, and 4 percent showed no effects related to compaction. 

The possibility of reduced productivity from harvest impacts, often narrowly 

focused on compaction, has been a special concern with volcanic ash soils that 

occur east of the Cascade Mountains in the Pacific Northwest. Origin of the ash is 

dominantly Mount Mazama in the Cascade Range. These soils support the more 

productive forests of the Blue Mountains in eastern Oregon and southeastern 

Washington. Our field and research experience indicates volcanic ash is easily 

compacted and displaced by mechanized equipment (Geist and Strickler 1978, 

Geist et al. 1989). Snider and Miller (1985) found no significant change in bulk 

density from tractor logging in eastern Oregon. In central Oregon, Cochran and 

Brock (1985) found compaction of volcanic ash originating from Mount Jefferson 

was associated with reduced tree growth, but they stated other unidentified factors 

were also involved. Page-Dumroese (1993) found Mazama ash in northern Idaho 
2

RESEARCH PAPER PNW-RP-573

was easily compacted by machine activity, and the compaction was greater with 

increased intensity of site preparation. In the Blue Mountains, volcanic ash soils 

can locally represent 50 percent or more of the commercial forest area, and much of 

that has already been influenced by one or more logging entries with ground-based 

equipment. Bulk densities of compacted volcanic ash after logging do not seem particularly high when compared to many other soils of different origins and textures. 

Compacted values of ash are usually less than 1.0 Mg m-3, which to some observers 

does not appear problematic. Yet, these elevated levels commonly represent bulk 

density increases of 15 to 50 percent above those of adjacent unlogged areas. Calculated in accordance with regional standards, the amount of area with detrimental 

soil conditions, reported dominantly as soil compaction, in logging areas ranges 

from 10 to 70 percent of the harvested unit, excluding the main haul roads (Geist 

et al. 1989, Sullivan 1988). There are potentially long-term considerations about 

production losses from compaction, because research indicates compaction below 

the surface few centimeters may require decades to recover naturally (Froehlich 

et al. 1985, Geist et al. 1989, Greacen and Sands 1980, Perry 1964, Wert and 

Thomas 1981).

Soil displacement during logging and site preparation affects surface organic 

layers and mineral soil. Displacement creates localized reductions in nutrient capital and water storage capacity, and subsequent growth reductions in nearby trees 

(Ballard 1978, Clayton et al. 1987, Nielson-Gerhardt 1986, Swindell et al. 1986, 

Tew et al. 1986). Displacement may also have long-term effects on growth. Because 

compaction, displacement, and other impacts often occur together or in proximity, 

their effects on production are often inseparable, thus experimentally confounded. 

In such situations, we must refer to tree growth reductions as associated with the 

aggregate of those disturbance impacts. 

Earlier research of volcanic ash soils in the Blue Mountains has not included 

growth relations of regenerated trees to soil conditions imposed by prior timber 

harvest. To begin addressing this deficiency, we initiated two retrospective studies 

that involved different approaches. Both approaches sought to answer the same 

question: are soil compaction and other machine impacts, i.e., mechanical soil 

disturbance, from ground-based logging and site preparation on volcanic ash soils 

great enough to cause growth differences in the subsequent tree crop? 

In this report, the terminology “harvest impacts” includes both logging and 

site preparation, i.e., skidding logs, loading logs, piling slash, and other incidental 

movement of mechanized equipment. Soil areas impacted by fire in slash reduction 

and areas devoted to the transportation system within harvest units were excluded 

from both study approaches.
Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

Materials and Methods

We sampled a total of 12 timber harvest units in our two approaches. Eleven 

were clearcut units and one was a seed-tree unit. The units were on the WallowaWhitman, Umatilla, and Malheur National Forests in the Blue Mountains of 

northeast Oregon and southeast Washington. The periods since harvest of the units 

ranged from 14 to 23 years. Units of these ages were chosen to provide a growth 

history of regeneration beyond the first few postharvest years. Skidding and slash 

piling were done with crawler tractors on 11 hand-felled units; 1 was whole-tree 

logged by feller-buncher in conjunction with rubber-tired skidders. Machine travel 

was unrestricted. These units provided a broad representation of ash soils and 

the effects of harvest practices typically used on extensive portions of the Blue 

Mountains. 

Plant communities associated with the harvest units are lodgepole pine/

pinegrass/grouse huckleberry (Pinus contorta Dougl./Calamagrostis Adans./

Vaccininum scoparium Leiberg); lodgepole/grouse huckleberry; grand fir/twinflower/forb (Abies grandis Dougl./Linnaea L.); and grand fir/grouse huckleberry 

(Hall 1973). Elevations range from 1460 to 1800 m; precipitation ranges from 600 

to 1500 mm, coming mostly in winter as snow; July and August are dry; slopes 

range from 3 to 35 percent.

The undisturbed soils associated with these plant communities typically have 

35 to 70 cm of silt loam or very fine sandy loam volcanic ash that commonly 

overlies a buried soil of loam to clay loam texture. The volcanic ash originated 

from the eruption of Mount Mazama about 6,600 years ago (Harward and 

Youngberg 1969). These soils include members of the Tolo series (medial over 

loamy, mixed, frigid Typic Vitrandepts), Helter series (medial over loamy, mixed 

Entic Cryandepts), Olot series (loamy skeletal, mixed Mollic Eutroboralfs) and 

closely-related Andic intergrades with shallower depths. We sought a broad array 

of ash soils, so no attempt was made to correlate specific series or taxonomic 

entities with the harvested study units. The volcanic ash surface layer has a 

naturally weak platy structure. Throughout the ash overburden there is little 

cohesiveness, few coarse fragments, and a high water-holding capacity. Natural 

bulk densities of the ash average 0.67 Mg m-3 and are comparatively uniform. 

Organic matter content is comparatively high in the surface and declines rapidly 

down the soil profile. The buried soils were residuum and colluvium, dominantly 

from basalts, andesites, and sedimentary rocks. At the buried soil boundary, the 

bulk density increases abruptly, as does clay content and coarse fragment content, 

but water-holding capacity is less (Geist and Strickler 1978). 
RESEARCH PAPER PNW-RP-573

We measured soil and tree-growth attributes to assess whether differences 

existed between higher and lower degrees of soil disturbance found within the harvest units. We hypothesized that if no growth reductions were detected at the higher 

disturbance conditions, then none should occur with intermediate conditions. 

It is noteworthy that the retrospective nature of our study precluded our ability 

to control the kind or degree of disturbance assessed. We used what was available to us. This also precluded any possibility of separating compaction from 

displacement or other impacts. Thus, the context of our study paralleled that of land 

managers who must prescribe practices in light of existing resource conditions. We 

believed this strengthened the practical relevance of study findings. 

We used two, very different sampling approaches to make our assessments. 

Because the first approach was completed before the second began, our efforts 

could be viewed as two separate studies. Owing to their commonality of objective, 

we chose to report this work as one study with two parts. Size of measurement plot 

(larger and smaller) was one of the approach differences and became a simple way 

to label discussion of the two approaches. The two study efforts were independent 

except for commonality of geographic area and some commonality in harvest units 

sampled. We designated at least three measurement plots in each of the two disturbance conditions (higher and lower) within harvest units studied by each approach. 

The procedures used to locate measurement plots and assign the two levels/conditions of soil disturbance were markedly different between the two approaches, 

and they are described below. No buffer strip was specified around either larger or 

smaller measurement plots. 

Larger Measurement Plots

We took advantage of soil monitoring transects established earlier by cooperating national forests in joint (research-management) efforts to assess soil impacts 

of logging (Geist et al. 1989). Larger measurement plots were linked to some of 

the transects in the 10 harvest units monitored. A randomly positioned grid of 

starting points had been established in each harvest unit for 15 randomly oriented 

transects, each about 30 m (100 ft) long. The larger plots were 7 by 16 m (20 by 

50 ft) and straddled a portion of a monitoring transect. We used soil bulk density 

assessments obtained by core sampling at intervals along the whole length of all 

line transects of a unit to determine the location of potential plots and to assign 

each plot a disturbance condition. Ten soil core samples (each about 25 mm long 

and 50 mm in diameter) spaced about 3 m (10 ft) apart were extracted from the 

10- to 15-cm (4- to 6-in) soil depth to measure bulk density. Sufficient thickness 

of volcanic ash was required, so only volcanic ash was included in the cores. The 
Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

higher and lower disturbance conditions and plot locations were determined by 

using sets of five consecutive core samples that provided the highest or lowest sets 

of bulk density values among the transects of each unit. Thus, we sought to achieve 

as much contrast in disturbance between plot conditions as possible, as accorded by 

the sets of bulk densities. The five consecutive core samples defined the midline of 

the long dimension of each large plot. At least three of the set of five bulk densities 

of a higher disturbance plot had to exceed the reference level of bulk density for the 

unit by 15 percent. The reference level was the average bulk density of core samples 

from three other transects (30 core samples total) that were randomly situated on 

unharvested area bordering the unit. All the bulk densities of lower disturbance plots 

had to be less than 15 percent above the reference level. Slash piles, fire-discolored 

soil areas, and the road system were excluded from soil sampling.

Soil cores were oven dried and and sieved to <2 mm, and both fractions were 

weighed. Bulk density was determined for the soil fraction by adjusting for the 

weight and volume of coarse fragments (mineral matter >2 mm) by using a particle 

density of 2.65 Mg m-3 (Geist and Strickler 1978). 

After designation of plots and disturbance conditions, all species of trees that 

were at least 1.2 m (4 ft) in height were hand felled and measured. Tree measurements included total height, prior-3-years height and growth, and diameter outside 

bark at breast height (d.b.h.). Measurements at 30 cm height included age, diameter 

outside bark, radial growth for the most recent 5 years (RG 1–5) and for the next 

prior 5 years (RG 6–10). Where tree stems were not round, the recorded values for 

diameter and radial growth were the average of two measurements, one taken along 

the long axis and the second at right angles thereto. A mixture of tree species usually 

occurred within the larger measurement plots and included grand fir, Douglas-fir 

(Pseudotsuga menziesii var. glauca (Beissn.) Franco), lodgepole pine, and ponderosa 

pine. Data were not separated by species because we sought overall growth comparisons. Further, separation would presume an unlikely similar exposure to within-plot 

soil disturbances by similar numbers and positioning of each species. One of the 

units had been precommercially thinned the year prior to our sampling. 

Smaller Measurement Plots

We drew from our larger plot experiences in follow-on efforts to investigate the 

study question. Size of plot and numerous other study elements were revised. Less 

time and fewer resources resulted in only five harvest units sampled with the smaller 

plots. The geographic area involved was very similar, in fact, three of the five units 

were previously sampled with the larger plot approach, but the smaller plot approach 

was completely independent of larger plots and monitoring transects. 
RESEARCH PAPER PNW-RP-573

A more restricted population of ash soil disturbance was investigated with 

smaller plots. Harvest units were reconnaissance surveyed for visual evidence of 

soil disturbance, while tree and stand characteristics were ignored. The visual cues 

related to soil disturbance were surface organic layer displacement and exposure 

of mineral soil, mineral soil discolored by slash burning, uneven soil surface that 

could indicate soil displacement or traffic depressions, and ground cover vegetation 

that indicated disturbance. The surface soil was occasionally probed with a shovel 

for evidence of increased firmness. As in all larger plot units, operational conventions at the time of harvest resulted in considerable machine movement within and 

around the units. Skid trails were not designated, and burned slash piles made with 

heavy equipment were common over the units and by landings. Soil disturbance 

was generally higher near slash piles, in and around landings, and in other places 

where more machine influence occurred. It was in the more machine-influenced 

areas that measurement plots for higher disturbance were situated. In each of the 

five harvest units, we purposely established three small measurement plots in both 

lower and higher conditions of soil disturbance (i.e., six plots per unit) discovered 

during reconnaissance. Prior observations of units helped define the smaller plot 

dimensions we thought would usually fit within the various disturbance situations. 

Measurement plots were about 5 by 8 m (15 by 25 ft), and were oriented as much 

within the respective disturbance conditions as possible. Plots were also scattered as much as conditions in each unit permitted. Although the specific nature, 

extent, and juxtaposition of disturbance varied within and among harvest units, 

our disturbance criteria for assignment of plots were consistent throughout. No 

change in plot assignments to disturbance conditions occurred after their original 

field designations. Our goal in locating plots was objective consistency within and 

among units. Lower disturbance conditions were indicated by the following: a soil 

surface with most of the preharvest organic layers and groundcover vegetation 

intact, comparatively little exposed mineral soil, little or no uneven soil surface 

indicative of vehicle mechanical influences, little or no increased firmness in the 

surface soil compared to unharvested area adjacent to each unit. Higher disturbance conditions were indicated by the following: a soil surface with disturbed or 

removed preharvest organic layers so exposed mineral soil was relatively common, 

prior groundcover vegetation commonly disturbed, presence of disturbance-related 

species (native or invader) or introduced erosion control species, commonly tracked 

and uneven soil surface, commonly increased firmness in the surface soil. 

We did not attempt to locate, map, or measure all the areas of lower and higher 

disturbance conditions that could have potentially been sampled within each of the 

five harvest units. As with larger plots, slash piles, fire-discolored soil areas, and the 
Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

road system were excluded from sampling, because our study question related to 

mechanical soil disturbance within tree production areas. 

Our soil sampling was more intensive within this approach. Both tree and 

soil attributes were sampled only after establishing the measurement plots within 

their respective disturbance conditions. We hand-felled all trees within the plots 

that met the same size criteria and performed measurements as on the larger plots, 

except age and radial growth were taken at ground level. The trees in two of the 

five harvest units were planted ponderosa pine, and in the other three units, the 

trees were naturally seeded lodgepole pine. We again sought an overall growth 

response of trees, so we made no attempt to separate species for reasons similar 

to those noted above for larger plot sampling. Precommercial thinning by hand 

felling had occurred a year before we measured one of the lodgepole pine units and 

4 years before in the two units supporting planted ponderosa pine. Two units were 

unthinned. Other vegetation was not measured. 

Soil bulk density was sampled at nine points of a systematic grid within each 

small plot. Core samples were rejected and replaced where inadequate ash thickness 

remained to obtain ash-only cores at the 10- to 15-cm depth. This avoided misinterpretation of differing bulk densities related to parent material and textural differences. At the four corners of each small plot, a bucket auger was used to sample the 

0- to 15-cm soil depth and measure the thickness of the volcanic ash layer. The four 

auger samples were aggregated into a single sample per small plot, air dried, mixed, 

and sieved to <2 mm diameter for subsequent chemical analysis. The same soil 

coring equipment and core processing was employed as in large-plot sampling. 

Soil chemical analyses included total nitrogen (N) by Kjeldahl method, organic 

matter by the modified Walkley-Black method (Jackson 1958), and net mineralizable N. The latter was the difference between the gross amount of ammonium-N 

mineralized during anaerobic soil incubation for 7 days at 40 °C and the initial 

ammonium-N extracted by 2M potassium chloride (KCl) (Keeney and Bremner 

1966). 

It was not experimentally possible to restrict stocking among the units to some 

predetermined range, and we did not conduct overall stocking assessments on 

the units. Stocking variation was obvious among units, but we avoided potential 

confounding effects of stocking on growth parameters, because field observations 

indicated stocking to be similar for higher and lower disturbance plots within a 

given harvest unit. We relied upon local silviculturists to make tree measurements 

on larger plots. They did not find, and no concern was raised about, within-unit 

stocking disparities among or between higher and lower soil disturbance conditions. 

After establishment of smaller plots, tree counts were made on the plots, and those 
8

RESEARCH PAPER PNW-RP-573

counts offered no evidence that tree numbers would have an influence on tree 

growth comparisons between levels of soil disturbance. 

Statistical Analysis of the Attributes Measured 

Data from the two study approaches were analyzed separately by applying a 

randomized block design and using standard ANOVA methods. We tested for differences between two “treatments,” represented by lower and higher levels of soil 

disturbance, and blocks represented by harvest units. Thus, with larger measurement plots, the statistical analysis we applied for each variable (soil or tree attribute) 

was a 2 (treatments) by 10 (harvest units/blocks) randomized block design (n = 10). 

In smaller plot sampling we applied a 2 (treatments) by 5 (harvest units/blocks) 

randomized block design (n = 5). The larger and smaller measurement plots were 

treated experimentally as subplots in the statistical analyses. The alpha level we 

chose to assign statistically significant differences in attributes between lower and 

higher disturbance conditions was 0.05. However, we have reported the observed 

P-values of attribute tests.

Results 

The marked differences in sampling approaches were accompanied by marked differences in the results obtained. Tables 1 and 2 show the overall means for, and the 

standard deviations of, attributes among the harvest units within lower and higher 

levels of soil disturbance as designated within each sampling approach. We could 

not test attribute differences between approaches, because of methodology differences, and we did not design our sampling with that intent.

Larger Measurement Plots

The results show that with this process we were able to construct two data sets 

for bulk density that were significantly different. But, all other attributes sampled 

within larger plots were not significantly different between our disturbance conditions, constructed in this way, at or near our chosen 0.05 level of significance (table 

1). The lower disturbance mean of bulk density, 0.64 Mg m-3, was very close to 

the 0.67 Mg m-3 mean of relatively undisturbed ash soils in the Blue Mountains 

reported by Geist and Strickler (1978). The mean bulk density for higher disturbance, 0.83 Mg m-3, was about midrange for harvest-affected ash soils reported by 

Geist et al. (1989) and was 0.19 Mg m-3 or 29 percent greater than the lower disturbance mean. At both levels of disturbance, the standard deviations of bulk density 

among units were low. We viewed these low variability values as some assurance 

of consistency in our methodology.
Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

Table 1—Volcanic ash soil and tree attributes sampled in 10 cutting units with larger plots to assess 

differences between lower and higher levels of mechanical soil disturbance from prior logging and site 

preparation

Lower disturbance Higher disturbance

 Standard Standard Mean Observed

Attribute Units Mean deviation Mean deviation difference P-value

Percent

Bulk density Mg/m3 0.64 0.05 0.83 0.07 +29* <0.001

Tree height cm 333 142 312 124 -6 .622

3-year height growth cm 97 30 99 30 +2 .697

D.b.h. cm 6 4 6 4 -5 .816

Diameter at 30 cm cm 7 4 7 4 -7 .638

Radial growth 1–5 mm 12 5 12 5 0 .314

Radial growth 6–10 mm 13 5 14 6 +8 .468

Age at 30 cm years 12 4 11 3 -8 .208

Mean difference (percent) for a given attribute is calculated as the higher disturbance mean minus the lower disturbance mean, divided by 

the lower disturbance mean, times 100. D.b.h. = diameter at breast height.

* indicates a statistically significant difference at alpha = 0.05. 

P-value is the statistical probability of observing a test statistic as extreme or more extreme than that observed assuming the null hypothesis is true.

In the array of large-plot averages, some tree growth or size attributes were 

slightly less at higher disturbance, whereas others were slightly more or were the 

same. Standard deviations for tree growth and size attributes were all much higher 

than for bulk density, especially in relation to their respective means.

Table 2—Volcanic ash soil and tree attributes sampled in five cutting units with smaller plots to assess 

differences between lower and higher levels of mechanical soil disturbance from prior logging and site 

preparation

Lower disturbance Higher disturbance

 Standard Standard Mean Observed

Attribute Units Mean deviation Mean deviation difference P-value

Percent

pH — 5.1 0.4 5.0 0.3 -2 0.267

Total soil N g/kg 1.2 0.4 1.0 0.2 -17 .364

Organic matter g/kg 48 19 35 11 -26 .057

Net mineralizable N mg/kg 25 7 19 6 -24 .143

Extractable NH4-N mg/kg 8 2 6 1 -25 .089

Bulk density Mg/m3 0.74 0.06 0.92 0.07 +24* <.001

Ask thickness cm 48 13 30 8 -38* .042

Tree height cm 399 150 284 157 -29* <.001

3-yr height growth cm 142 30 107 41 -25* .005

Diameter at 30 cm cm 11 6 8 6 -24* <.001

Radial growth 1–5 mm 23 10 18 11 -22* .004

Radial growth 6–10 mm 18 11 15 11 -17* <.001

Age at ground years 12 2 11 2 -8* .014

Mean difference (percent) for a given attribute is calculated as the higher disturbance mean minus the lower disturbance mean, divided by the 

lower disturbance mean, times 100.

* indicates a statistically significant difference at alpha = 0.05.

P-value is the statistical probability of observing a test statistic as extreme or more extreme than that observed assuming the null hypothesis is true. 
10

RESEARCH PAPER PNW-RP-573

We experienced sampling design concerns with the larger plot approach soon 

after going to the field and then again when we began to designate measurement 

plots. In the former instance, marked variability in soil disturbance was visually 

observed within potential measurement plots along transects. In the latter instance, 

our beginning criteria for designating higher disturbance plots required immediate modification so we could consistently designate three higher disturbance plots 

somewhere in every unit. Lower disturbance criteria did not require modification. 

We had initially planned for higher disturbance plots to have five consecutive bulk 

densities that exceeded a 20-percent increase above the reference value for each 

unit. That proved impossible with the variability in bulk density encountered. 

Instead, we had to settle on at least three of five consecutive bulk densities exceeding 15 percent of the reference value, as we reported above in “Materials and 

Methods.”

Smaller Measurement Plots

As with the larger plots, there were sampling design challenges with the smaller 

plot approach. In some units it was more difficult to identify potential plot locations 

with contrasting disturbance conditions than others, thus greater time was required 

for reconnaissance surveys than first anticipated. Disturbance variability among 

units and among timber sales was very evident when using this or the larger plot 

approach.

Test results from sampling with smaller measurement plots showed the means 

of 8 of the 13 attributes were significantly different (alpha equal to 0.05) between 

higher and lower disturbance conditions (table 2). The eight attributes included soil 

bulk density, ash thickness, and all tree growth and size parameters. Soil chemical 

attributes appeared to reflect less favorable conditions for plant growth at higher 

disturbance, but none tested significantly different at the 0.05 level of significance. 

Observed P-values associated with differences between higher and lower disturbance conditions for organic matter and extractable ammonium were fairly close 

to our chosen significance level. 

The difference between bulk density means of disturbance levels was 0.18 

Mg m-3, virtually the same as in the larger plot sampling. In contrast, the means 

themselves, 0.74 Mg m-3 and 0.92 Mg m-3, were notably higher in smaller plots, 

apparently indicating a different range covered. Standard deviations of bulk density 

among units were relatively low at both disturbance levels. As with larger plots, 

these low variability values offered some assurance of consistency in our methodology. The mean bulk density for the lower disturbance condition may reflect some 

machine activity, because it was 10 percent higher than the mean of 0.67 Mg m-3
11

Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

reported by Geist and Strickler (1978) for relatively undisturbed ash soils. The 

coefficients of variation (CV) of their means for undisturbed ash in similar Blue 

Mountain localities were comparatively low and ranged from 9 to 11 percent. So 

the 10-percent difference provided some confirmation of our lower disturbance 

designations. The mean bulk density for higher disturbance was 0.27 Mg m-3 or 

37 percent above the undisturbed mean of Geist and Strickler (1978). The higher 

disturbance mean was, as with larger plots, well above the 15- and 20-percent 

increases in bulk density commonly used in the Pacific Northwest to define 

detrimentally compacted soil conditions (Meurisse 1988; Sullivan 1988; Geist et 

al. 1989, 1991). The higher disturbance mean was not unusually high but did fall 

within the upper ranges of ash bulk densities reported in harvested areas by Geist 

et al. (1989). All bulk densities in the aforementioned references were corrected for 

coarse fragment content. 

The overall mean of ash thickness for higher disturbance was 18 cm or 38 percent less than the corresponding mean for lower disturbance. Ours is a retrospective study, so we cannot guarantee that lower and higher disturbance plots within 

a given unit had the same ash thickness before harvest occurred. The variability 

of ash thickness (CV = 22 percent) found by Geist and Strickler (1978) was about 

twice that for bulk density on relatively undisturbed areas, so thickness data offer a 

much less consistent benchmark for change. Therefore, our study design with multiple measurement plots per unit and unit replication was important to the strength 

of statistical testing and interpretation of the difference in ash thickness. 

At higher soil disturbance, tree size and growth attributes averaged 17 to 29 

percent less than corresponding attributes at lower disturbance. Consistently more 

variability among units, particularly in relation to their respective means, was 

noted for all tree attributes at higher disturbance compared to lower disturbance. 

The statistically significant (alpha equal to 0.05) 1-year difference in average tree 

age may indicate higher disturbance caused a delay in the natural establishment 

of lodgepole pine (ponderosa pine was all planted the same year). A season less 

growing time may account for part of the smaller growth with higher disturbance. 

Whether thinning in three of the units affected the data is speculative. Thinning at 

this age typically has minor influences on height growth, but radial growth is more 

sensitive and commonly increases. Thinning operations typically favor retention 

of the better performing trees and eliminate part of the poorer performers. If this 

were consistently the case in our units, we might expect a narrowing of growth 

differences between the two disturbance categories. However, a thinning effect on 

radial growth, if present, appears to be just the opposite. Radial growth was less in 

the earlier (RG 6–10) than the later (RG 1–5) growth period, and the radial growth 
12

RESEARCH PAPER PNW-RP-573

differences between disturbance categories were less in the earlier than the later 

period. We observed no reason to speculate further about thinning influences, and 

we are confident thinning had little or no bearing on our results and conclusions. 

Discussion

We believe both outcomes from our two study approaches provide valid information within the context of disturbance conditions in the field and the methodologies 

we used. Both approaches sought to determine if there was credible evidence of 

tree growth differences between higher and lower levels of mechanical disturbance 

of volcanic ash soil. Disturbance contrasts with one categorization approach yielded 

no difference in tree growth, whereas followup contrasts with a different disturbance categorization were associated with significant differences in growth. Given 

the many differences involved, we feel both challenged and obliged to provide 

our interpretation of the two findings, based on our observations, tested measurements, and our related experience. Our interpretations include speculation about 

the outcomes, but there is no way to scientifically assess their relative merit. We 

cannot quantitatively compare our two sampling approaches. There is no basis to 

statistically test the different outcomes, and we did not design our study with that 

objective.

Our two outcomes and the mixed results obtained from related research elsewhere indicates broad generalizations of findings from one study are inappropriate. 

Many variables exist among studies including the kind and degree of disturbance 

impacts, types of mechanized equipment used in harvesting operations, tree species 

and other vegetation, topography, geology, climate, soils, measuring tools, and 

experimental methodology. Given such an array of variables, a mixture of results 

could be expected. The 1980 review by Greacen and Sands reflects this, although 

the great majority of 142 studies found reduced tree growth associated with forest 

soil compaction. 

We could speculate at great length about why our two sampling approaches 

yielded such different results. With the many differences in our methodologies, 

the fact that we had differences in outcomes might not be surprising. Smaller plots 

being about one-third the size of larger plots often allowed them to be positioned 

wholly within a given disturbance situation. The processes for disturbance 

categorization with smaller plots resulted in a more restricted population of higher 

and lower soil disturbance than in larger plots where only midline bulk densities 

were used. Variation in bulk densities along transects in larger plots negated our 

ability to construct a greater average bulk density for higher disturbance plots 

that might contrast more with lower disturbance plots. Although the average 
13

Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

differences in bulk density with both approaches were similar, being in a higher 

part of the range with smaller plots may have been an advantage to detecting tree 

growth differences. Whether there were similar associated differences in ash 

thickness is unknown, because ash thickness was not measured on larger plots. The 

near monocultures in smaller plots may have been an advantage over larger plots 

containing a varied mix of tree species that together may respond differently to soil 

disturbance. Fewer samples of bulk density in relation to plot size and fewer soil 

attributes were sampled in larger measurement plots compared to the smaller plots. 

Instead of simply using a midline sampling of bulk density, a grid of samples was 

used to improve characterization of compaction in smaller plots, and ash thickness 

was measured as an indication of soil displacement. Visual cues and probing coupled 

with more measures of soil condition provided a potentially stronger characterization 

of disturbance conditions associated with smaller plots. Taken in total, the potential 

to detect tree-growth differences between properly characterized disturbance 

conditions appears more favorable and consistent with our research objective when 

using the smaller plot approach. However, given our limited efforts, we do not know 

how applicable and usable elsewhere the smaller plot approach might be. 

The midline bulk densities used to position larger plots on the transects initially 

seemed to hold promise as a rational method of identifying and sampling any effects 

of higher and lower soil disturbance on associated tree growth. In retrospect, disturbance appeared to be so patchily distributed, that larger plots located on randomly 

oriented transects incorporated considerable heterogeneity that lessened the potential 

to detect associated differences in tree attributes where they may exist, especially 

if the differences are small. Field observations and measurements in both sampling 

efforts led us to infer that harvest activities imposed a complex and variable kind, 

degree, and pattern of soil and vegetation conditions. Much of the soil disturbance 

was neither uniform nor simply straight-line in nature. Given enough complexity, the use of only midline bulk density may be insufficient to detect and assign a 

disturbance level that truly reflects the overall character of a sample plot this large. 

Regardless of the difficulties, questions, and findings that arose in the larger plot 

study, they have no bearing on the original utility or validity of monitoring general 

soil conditions with the random transect method developed by Hazard and Geist 

(1984) and further elaborated by Geist et al. (1991).

We believe the second study’s results provide some credible evidence that 

higher degrees of ash soil disturbance from traditional ground-based harvest can 

be detrimental to the early growth of associated trees. In this case, the trees were 

ponderosa and lodgepole pines. Sampling refinements and population restriction 

in conjunction with smaller plots showed higher soil bulk density (interpreted as 
14

RESEARCH PAPER PNW-RP-573

soil compaction) and lesser ash thickness (interpreted as soil displacement and 

compaction) at higher disturbance were associated with significantly less tree 

growth and size. Association is not necessarily evidence of direct cause and effect. 

We saw no necessity to construct, and we doubt we achieved, worst-case 

comparisons of soil disturbances in our methods, yet we offer some comments in 

that regard. The lower and higher disturbance means for bulk density in both larger 

or smaller plot approaches were indicative that neither approach achieved worstcase comparisons of soil compaction. In both approaches, our ranges were notably 

less than the ranges of bulk densities for mechanically disturbed volcanic ash soils 

reported by Geist et al. (1989). With larger plots, the lower disturbance value was 

near the low end and the higher disturbance value was about midrange for bulk 

densities of mechanically disturbed volcanic ash soils reported by Geist et. al. 

(1989). With smaller plots, the lower disturbance value was above the low end but 

less than midrange, and the higher disturbance mean was in the upper range of 

mechanically disturbed volcanic ash soils. Our results indicate disturbance effects 

on ash thickness should also be considered in constructing or assessing a worst-case 

comparison, but reference data comparable to that of bulk density are lacking. 

Further, more knowledge would be needed about what combinations of bulk 

density, ash thickness, and perhaps other attributes exist to help characterize a 

worst-case comparison of disturbance. 

We are unable to use data from smaller plots to estimate what the overall 

effect of growth reductions from higher disturbance areas might be, or from either 

sampling, what an overall average growth would be for a stand, a harvest unit, or 

all units combined. Estimation of those values was beyond the scope of our study 

objective, and we did not sample with that intent. Overall effects require assessments spanning the range of soil disturbance, associated tree growth, and proportions of harvest units represented by each. In neither approach did we determine 

the amounts of higher and lower disturbance present in harvest units sampled. We 

purposely excluded assessment of intermediate mechanical disturbance and some 

other disturbances, as described earlier. 

Our visual surveys indicated higher mechanical disturbance represented less 

area than the combination of moderate and lower disturbance, but we made no 

area measurements to confirm that perspective. Our experience further indicates 

the kinds and relative proportions of various disturbances can vary greatly among 

harvest units. There would likely be little effect on overall growth from higher 

disturbance areas where they are proportionately small. But, we do not know what 

growth effects might exist within intermediate mechanical disturbances and other 

disturbances we excluded. We also do not know longer term effects. Regardless 
15

Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

of these unknowns, we expect that standard inventory processes conducted in 

the usual course of forest management will account for whatever soil disturbance 

effects may exist within the overall growth and yield of forest stands. Therefore, no 

special monitoring should be needed in this regard.

The overall percentage differences for most soil and tree parameters associated 

with our smaller plots showed considerable similarity in their magnitudes, although 

they varied among units. Our overall differences with smaller plots agreed well 

with a relationship constructed by Froehlich and McNabb (1984) who suggested 

that percentage difference in bulk density might relate better to the percentage 

growth loss associated with compaction than would absolute values of bulk density. 

Of course, the results from our larger plots showed no such agreement. Our overall 

percentage differences with smaller plots were also similar to those of Cochran and 

Brock (1985) who studied compaction relations to tree growth in central Oregon on 

deep soils with 50 to 60 cm of surface volcanic ash from Mount Jefferson (coarser 

textured than our Mazama ash). They used plots similar in size to our larger plots 

that were also linked to randomly oriented transects. Plot averages of total height or 

periodic height increment for 5-year-old seedlings of ponderosa pine were regressed 

against plot averages of soil bulk densities associated with the measured trees. They 

found both growth measures declined significantly with increased bulk density. 

Clayton et al. (1987) studied soil disturbances and young tree growth within three 

clearcuts in the northern Rocky Mountains of central Idaho; soils contained significant ash content in the upper 30 cm (generally much thinner ash than our study 

locations). The specifics of which soil assessments related to which tree growth 

assessments differed among locations and sampling tools. They did, however, find 

some instances where soil compaction and/or displacement were associated with 

less growth of trees aged 15 to 25 years. Bosworth and Studer (1991) studied early 

tree growth following clearcutting on volcanic ash soils of northern Idaho. They 

assessed juvenile tree performance in relation to three site preparation methods: 

broadcast burn, bulldozer pile, or no site preparation. Among the methods studied, 

they found greater height growth in 8 of 9 years with broadcast burning. Lower 

tree growth in bulldozer-piled areas was attributed to both soil compaction and the 

excessive displacement of surface litter and humus layers into slash piles. They 

speculated the removal of litter and humus resulted in decreased nutrient supply 

compared to broadcast burning.

Tree growth effects detected with smaller plot sampling appear large enough 

to warrant some management consideration. Our results coupled with findings by 

others lend support for harvesting methods that prevent and minimize excessive 

machine disturbance of volcanic ash soils in the Blue Mountains (Clayton et al. 
16

RESEARCH PAPER PNW-RP-573

1987, Dyrness 1965, Froehlich et al. 1981, Geist et al. 1989, Greacen and Sands 

1980, Sullivan 1988, Wert and Thomas 1981). Management concerns about adverse 

soil conditions from past timber harvests in the Blue Mountains were bolstered 

when 68 percent of the harvest units monitored on the Malheur National Forest by 

Sullivan (1988) averaged between 28 and 70 percent detrimental soil conditions. 

Detrimental compaction (defined in the Northwest Region as at least a 15 or 20 

percent increase in soil bulk density) dominated his findings in volcanic ash and 

other soils following ground-based logging and site preparation. Our findings cannot validate definitions of detrimental soil compaction or displacement for ash soils 

as applied in the Pacific Northwest Region by the U.S. Forest Service. Validation 

would have required a different study that involved the whole range of soil disturbances.

Our results arise from traditional ground-based harvest operations, but they 

potentially apply to newer ground-based mechanical harvest systems. The latter are 

gaining popularity in cultural operations such as thinning of overstocked timber 

stands to address concerns about forest health, growth, and catastrophic wildfire. 

We would encourage operational guidelines that minimize skid trails, landings, and 

other high-disturbance areas and reuse of such areas when possible and appropriate. Consideration of soil disturbance is important with every management entry 

because of the potential for cumulative negative effects on tree growth and possibly other ecosystem attributes. Mechanical treatment to reduce soil compaction 

remains problematic, and treating compaction does not resolve soil displacement 

and other disturbance concerns.

If values are corrected for coarse fragments (>2 mm), then the bulk density of 

Mount Mazama ash provides a comparatively sensitive attribute to use for detecting 

change from compacting influences. Natural bulk densities are reported to be quite 

uniform both spatially and throughout the deposited thickness, so the local variability is relatively low (Geist and Strickler 1978, Geist et al. 1989). Yet, cores must 

be extracted in the field with considerable care, and this is time-consuming, tedious 

work. Diligence and consistency in both field and laboratory processes is crucial to 

the integrity and validity of soil disturbance studies that use core sampling to measure bulk density for assessment of compaction. Unfortunately, the higher natural 

variability of ash thickness makes averages more problematic for assessments of 

displacement in disturbed areas. 

Managers would benefit from less time- and labor-demanding assessments 

of soil disturbance that have reasonably wide quantitative ranges reflective of 

tree growth. Clayton et al. (1987) found that higher classes of soil penetration 

resistance (higher soil strength) were associated with less growth in young trees. 
17

Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

They used shovel probing to assign trees to classes after they checked their ability 

to distinguish those classes with penetrometer measurements. Penetrometer readings between classes differed in the range of two- and threefold. Lull (1959) stated 

infiltration was commonly the soil characteristic most sensitive to compaction. 

Perry (1964) applied this concept and used a simple infiltration measurement to link 

compaction to reduced growth of juvenile southern pines. 

Conclusions 

Our study approaches yielded valuable insights to the complexity of soil disturbance associated with traditional ground-based harvest. We concluded that natural 

and harvest-imposed variability in ash soil conditions make possible tree growthdifferences associated with mechanical disturbance difficult to detect, especially if 

the differences are small. Simply stated, the results we obtained differed between 

the two study approaches. In our first study approach, we found no differences in 

tree attributes between lower and higher soil disturbance. In our second approach, 

with a more restricted population of disturbances and modified sampling, we 

did find some credible evidence of tree size and growth differences between soil 

disturbance levels. Because our sampling efforts with both approaches were limited 

in number, we cannot say how representative our findings may be overall in the 

Blue Mountains. 

With our second approach, it appeared we were able to discriminate between 

higher and lower levels of soil disturbance mainly by visual means supplemented 

by occasional shovel probing. We do not know how well our procedures would 

work elsewhere, but the significant differences between disturbance levels for soil 

bulk density and ash thickness offered a measured confirmation of visual assignments. We would hope visual indicators can be useful to forest managers in identifying problem soil conditions on the landscape. 

Findings from our second approach offer some credence to concerns raised 

by the general public and land managers about potential tree-growth reduction 

from ash soil disturbance. Those concerns about soil disturbance have commonly 

focused only on compaction. We believe concern is also needed about the frequent 

and confounded association of soil displacement with compaction. It appears 

displacement is not only a valid concern, it might affect juvenile tree growth as 

much as or perhaps more than compaction. But there is no simple remedy for either 

impact. We do not know the long-term effects on growth of trees and tree stands, 

but we expect they will differ among units according to the amount and specific 

nature of the disturbances therein. Regardless, we believe it would be prudent for 

managers to use strategies and practices that prevent and minimize higher levels of 
18

RESEARCH PAPER PNW-RP-573

mechanical disturbance of volcanic ash soils like those of the Blue Mountains. 

This strategy is relevant to current and future management entries of any kind.

We addressed a relatively small number of soil and tree attributes in our study 

of harvest-associated impacts within forest ecosystems. From our smaller plot 

approach we concluded some of these attributes were relevant to the question investigated. Yet, there remains an immense ecological complexity that extends beyond 

our narrow investigation. That complexity includes a host of other attributes we did 

not measure, such as other vegetation associated with the tree crop. Those attributes 

could be working in concert with or in opposition to the ones we found relevant 

to tree growth. So, the knowledge we gain in a narrowly focused study is just one 

of many steps toward a fuller understanding of the natural and human-caused 

complexity of forest ecosystems. Mindful of that complexity, we are less likely to 

oversimplify the issues and questions we address and the answers we find.

Acknowledgments 

The authors gratefully acknowledge the assistance and cooperation of the following: U.S. Forest Service, Pacific Northwest Region soil scientists Dr. Bob Meurisse 

(retired) and Steve Howes; regional silviculturist Walt Knapp (retired); forest soil 

scientists Dan Harkenrider, Tom High, Earle Rother, and Tim Sullivan; district 

silviculturists Lyle Kuchenbecker (retired), Gary Rollins, and Kenneth Rockwell; 

and Pacific Northwest Research Station research technician Michael Snider. We 

also gratefully acknowledge the valuable contributions of reviewer comments from 

Dr. Pat Cochran, retired research soil scientist; Dr. Bob Meurisse, retired regional 

soil scientist; Dr. Hersch McNeil, soil scientist Malheur National Forest; Dr. Richard Miller, emeritus research soil scientist, and Dr. Tim Max, Station Biometrician, 

Pacific Northwest Research Station. 

English Equivalents

When you know: Multiply by: To find:

Millimeters (mm) 0.0394 Inches (in)

Centimeters (cm) 0.394 Inches (in)

Meters (m) 3.28 Feet (ft)

Milligrams (mg) 3.5 × 10-5 Ounces (oz)

Grams (g) 0.035 Ounces (oz)

Kilograms (kg) 2.21 Pounds (lb)

Megagrams per cubic meter (Mg/m3) 62.4 Pounds per cubic foot (lb/ft3)

Degrees Celsius (C) 1.8 °C + 32 Degrees Fahrenheit
19

Juvenile Tree Growth on Some Volcanic Ash Soils Disturbed by Prior Forest Harvest

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