United States 

Department of 

Agriculture 

Forest Service 

Northeastern Forest 

Experiment Station 

Research 

Paper N E-509 

Measuring the Response 

of Conifer Seedlings 

to Soil Compaction Stress 

Howard G. Halverson 

Robert P. Zisa 
The Authors 

Howard G. Halverson is research forester and Project Leader 

of the Northeastern Forest Experiment Station's urban forestry research unit at University Park, Pennsylvania, He received a B.S. degree 

in forestry from Iowa State University and M.S. and Ph.D. degrees from 

the University of Arizona. He joined 

the USDA Forest Service in 1965 

and the Northeastern Station in 

1975. 

Robert P. Zisa is a forester with 

the Bureau of Indian Affairs, Hoquiam, Washington. During this study 

he was a member of the Northeastern Station's urban forestry unit at 

University Park. 

Manuscript received for 

publication 22 December 1981 

Abstract 

A test of seedling growth response to several levels of soil compaction showed that root penetration depth was best correlated with 

soil compaction. Shoot biomass, 

root biomass, root elongation, and 

seedling height were not well correlated with compaction. The results 

reveal that most measurements of 

growth do not give a good indication of seedling response to 

stresses induced by compaction. In 

fact, selecting a growth measurement to determine seedling response may require a separate test. 

There were good partial correlations 

between mean seedling height and 

all other measures of growth, but relationships must be developed for 

each growth response. When the relationships are known, mean seedling height can be used to predict 

the mean value of any of the other 

growth variables. 
Introduction 

Urban forests are often subjected to hostile environments that 

discourage tree growth. A number 

of stress factors were recently reviewed by Wimelick (1976), who 

found soil conditions to be important. Urban soils may be disturbed 

and compacted to densities well 

above normal and impose severe 

stresses on urban vegetation (Patterson 1976). Despite these 

stresses, tree seedlings can be established and grow on compact urban soils (Zisa et al. 1980). Although 

the seedlings grew, there were differences in growth, and little is 

known about growth pattern response to stresses induced by soil 

compaction. 

Measuring tree response to environmental factors, including soilinduced stress, is a necessary step 

in understanding growth patterns. 

Traditional measures of tree response have been establishment 

success and height growth, as used 

by Little and Trew (1979). 

Top growth and volume increment are important variables when 

maximum wood volume is the management objective. Wood volume is 

not a usual management objective 

in urban forests. Rather, other 

amenity values are primary, and 

values such as esthetics are subjective and not readily quantifiable. Because traditional measures of 

growth emphasize volume, they are 

not appropriate for urban seedlings. 

Other criteria must be used to 

evaluate tree vigor. 

Seedling response to different 

soil properties could be measured 

by a number 6f variables. The traditional measure is height growth. 

Forms of below-ground growth and 

biomass also are important measures of growth response. All of 

these variables have been used in 

the past, but none has been determined as the "best'heasure of response to stress. Pham and others 

(1978) showed that fertility and soil 

moisture interact and produce unusual growth responses, such as accelerated top growth and restricted 

root growth. This suggests that top 

growth may not be a good measure 

of seedling response to soil factors. 

The objective of this study was 

to determine the effects of soil 

compaction on seedling growth, and 

to identify the growth variable most 

responsive to soil compaction level. 

A secondary objective was to determine if the nondestructive measure 

of response, shoot height, was a 

good indicator of soil compaction 

levels. Also, we wanted to determine if height can be used to predict other growth variables. 

Methods 

Soil Type 

Two soils were used in the 

study. The soils were chosen because they are common in the urban areas and because a difference 

in clay and silt content suggested 

the soils might have different 

characteristics when compacted. 

One soil, a silt loam, was taken from 

an area classified as a poorly 

drained variant of the Nixon soil 

series. The other soil series, Lakewood, has a coarser texture and was 

classified as a sandy loam. The 

physical and chemical properties of 

the two soils were described by 

Zisa and others (1980). 

Soil Compaction Level 

The soils were sieved to remove coarse fragments and then 

transferred to 6.35-cm insidediameter cast acrylic transparent 

tubes. The soils were packed under 

static pressure into each tube in 1- 

cm increments until a soil column 

15 cm long was constructed. Soil 

was packed to a bulk density of 1.2, 

1.4, 1.6, or 1.8 glcmm3. A total of 48 

tubes of soil were constructed from 

each soil type. A perforated plate 

was attached to the bottom of each 

tube to allow for drainage and to retain the soil column. 

Except for screening, we did 

not alter the soil before it was compacted into the tubes. Neither fertilizer or lime was added, because 

earlier research on these two soils 

showed that heavy fertilization accelerated top growth and restricted 

root growth in maple (Pharn et at. 

1 978). 

Tree Species 

Several species of the genera 

Picea and Pinus are the most frequently planted urban conifers in 

the Northeast (Gerhold et at. 1975). 

Since pine and spruce are commonly planted, we used stratified seed 

of pitch pine (Pinus rigida Mill.), 

Austrian pine (Pinus nigra Arnold), 

and Norway spruce (Picea abies (L.) 

Karst) for the experiment. Seeds 

were soaked in water and treated 

with a fungicide to control disease. 

Twenty seeds of each species were 

sowed in each soil type and bulk 

density combination. Four replicates of each treatment combination were constructed. 

Growing Conditions 

The containers with sowed 

seed were transferred to a greenhouse bench; the study design was 

a randomized complete block. Soil 

water potential was maintained near 

- 113 bar for the duration of the 

study. We used water, not a nutrient 

solution, to moisten the soil to 

avoid the problem of zones of nutrient solution encouraging abnormal 

development. For example, Minore 

and others (1969) suggested that nutrient accumulation in a gap between a container wall and a soil 

column accelerated root development next to the container wall. To 

prevent microbial growth at the interior of the wall, we covered the 

outside of the tube with foil. Artificial lighting was used to maintain a 

14-hour photoperiod, Air temperatures during the period varied between 16" and 34°C; the relative humidity ranged from 26 to 98 percent. 

Thirty days after germination, 

the seedlings were thinned to one 

per tube. The selected residual 

seedling was centrally located on 

the soil columns. At this time, we 

also removed the foil covering to be 

sure the seedling root was penetrating the soil, not the boundary between the tube wall and the soil. 
After thinning, the seedlings 

were allowed to grow for an additional 90 days. Watering was continued to maintain soil water potential near - If3 bar, and the soil 

tubes were examined periodically to 

ensure that roots were growing in 

the soil. After 90 days, a few seedling roots had penetrated the entire 

length of the soil column, and the 

experiment was terminated. 

Growth Variables 

The seedlings were then removed from the tube by washing 

the soil from the roots, starting 

from the bottom of the tube. Measurements of growth variables were 

then made on the harvested seedlings. 

We measured root penetration 

depth by noting the depth of soil 

left when a root was first encountered as the soil was washed 

away from the bottom of the tube. 

After the soil was removed, we immediately measured the shoot 

height and the extended length of 

the root. Each seedling was separated into top and root portions. 

Each portion was dried in a forced 

draft oven at 80°C for 48 hours and 

then weighed. 

Analysis of Growth Data 

We used analysis of variance to 

test the effects and interactions of 

growth factors. Each factor was 

analyzed separately. There were statistically significant differences in 

the effects of soil type, soil compaction level, and tree species on 

each of the growth variables (Table 

1). The interactions for shoot growth 

were not significant but there were 

significant interactions for all root 

growth variables. All simple effects, 

two-way interactions, and the threeway interactions were significant for 

root weight. The significant threeway effects show that mean root 

weight for each soil type, soil density, and tree species combination 

must be kept separate. Other work 

showed growth reduced as soil 

compaction level increased, but interpretations were confounded by 

interactions and there were no significant differences in establishment or growth until the highest 

bulk density was reached (Zisa et al. 

1 980). 

In this experiment, the primary 

objective was to identify the growth 

response most related to soil compaction level, Information from the 

analysis of variance was used to determine how much of the variance 

in growth response was due to soil 

compaction level. The variance due 

to soil compaction is measured by 

the intraclass correlation coefficient. Shoot height, root length, and 

shoot weight variations were very 

poorly related to compaction level 

(Table 2). Root weight was weakly 

correlated with compaction. Root 

penetration depth was well correlated with soil compaction level, 

with a intraclass correlation exceeding 0.94 (Table 2). The analysis 

showed that the root penetration 

growth response was more related 

to stresses due to compaction than 

to any other response we measured. 

Details of the analysis are in the Appendix. 

Shoot height is a nondestructive measure of growth response and was investigated for use 

as an indirect measure of seedling 

response. In this experiment, shoot 

height was poorly correlated with 

soil compaction level for individual 

seedlings. Shoot height of individual seedlings also was poorly related to seedling root penetration 

depth. Consequently, height could 

not be used to predict performance 

of an individual seedling. 

Table I .--Analysis of variance of seedling growth variables 

Source Degrees Growth variable 

of o f Shoot Root Shoot Root Root penetration 

variation freedom height length weight weight depth 

Replicate 

Soil type (T) 

Soil density (D) 

Tree species (S) 

TxD 

TxS 

DxS 

TxDxS 

Individual plant 

------------- Mean squares - - - 

**Significant at the 0.01 level. 
Table 2.-Variance components and intracllass correlation 

for growth variables 

Growth variable 

Component 

Shoot Root Shoot Root Root penetration 

height length weight weight depth 

Table 3.-Average value of growth variables of conifer seedlings 

at four levels of soil compaction 

Soil Average Growth response 

density soil Shoot Root Shoot Root Root penetration 

level density height length weight weight depth 

1 1.2 5.42 18.50 97.65 138.12 14.50 

2 1.4 4.62 13.64 92.12 87.66 8.85 

3 1.6 4.84 14.18 88.88 79.15 4.77 

4 1.8 4.33 12.77 66.52 53.40 1.73 

mean 4.80 14.78 86.29 89.58 7.46 

We also analyzed the mean a population of trees is related to 

height growth of our seedling popu- other growth responses. 

lation, two pines and a spruce, versus the mean each of the other The regression equations for 

measures of growth response (Table predicting a mean growth response 

3). We found that although shoot for our species and soils from averheight does not predict other age height and the coefficients of 

growth variables on individual trees, determination (R2) are: 

the mean shoot growth response of 

Variable Equation R2 (among means) 

Mean root length j7 = 14.78 + 5.99 (h * - 4.80) 1.09 

Mean shoot weight E = 86.29 + 25.05 (h * - 4.80) 0.89 

Mean root weight y = 89.58 + 81.25 (h * - 4.80) 1.03 

Mean root depth j7 = 7.46 c 11 '62 (h * - 4.80) 0.93 

where h* is average seedling height. 

The linear relationships between the mean of each of the 

growth variables and mean shoot 

height are strong. Two of the estimated correlations exceed 1.0, 

which may be questioned. However, 

these values are estimates and sample correlations among means as 

based on the estimated values of 

the variance and covariance components can exceed 1.0. Details of this 

analysis are in the Appendix. 

Discussion 

The analysis of variance 

showed that soil type, soil compaction level, and species were significant factors affecting seedling 

growth response. Partitioning the 

variance among the various main effects and interactions suggested 

that soil compaction levels had the 

greatest impact on root penetration 

depth since more of the variance 

was explained by this model (Table 

7 ). 

Removing the effect of species 

and soil type confirmed the suggestion that root penetration depth is 

highly correlated with soil compaction stress. The correlation values 

(Table 2) showed that average root 

penetration depth was highly related 

to average soil compaction level. 

None of the other responses in 

growth variables were well related 

to soil compaction stress. Soil compaction effects did not account for 

much of the variation in seedling 

height. 

Depth of root penetration is not 

a traditional measure of seedling response to stress. For example, in a 

study of western conifer seedling 

germination and establishment, Noble and others (1979) found that 

height, root elongation, and biomass were not significantly related 

to stresses imposed by different 

soil moisture levels during the first 

20 weeks of growth. Although the 

study of seedling response to water 

stress is not directly related to this 

work, it is significant that traditional 

measures of seedling response 

were not well correlated with stress 

during a growing period of similar 
length, In tests with loblolly pine, 

Foil and Ralston (1967) also found 

poor correlations between root 

weight and root length with soil 

bulk density, though the growth period was a full season. 

The results of this study and interpretation of some reported results suggest that seedling response to soil compaction stress is 

poorly indexed by most common 

measures of growth. In this study, 

root penetration of the soil was the 

"best" measure of seedling reponse. There is no evidence that 

these measurements would be the 

best to use in future studies of soil 

compaction stress, though other 

types of measurement did not perform as expected. It may be necessary to conduct a pilot or companion test to determine the proper 

growth measurement that should be 

used. 

Because measuring seedling 

height is nondestructive, we estimated the regression equation between mean seedling height and the 

mean of each of the other growth 

variables. The relationship with root 

penetration depth is of greatest interest, because predicting this variable from a nondestructive measurement will indicate how well the 

seedlings are performing on compacted soils. These equations 

showed strong relationships among 

the mean of the growth variables. 

The correlations among the means 

suggest that mean height growth 

might be used to predict the mean 

response in seedling 'studies, but 

only if equations are developed between the variables. Developing the 

relationships also would require a 

separate experiment. 

The equations were derived for 

the mean response of our population of trees. For example, if we 

duplicated the experiment with the 

same species, and had height measurements from a large number of 

seedlings grown on a compacted 

soil, we could obtain good estimates of the mean of each of the 

other growth variables by using the 

prediction equations. 

If we had only a few seedling 

height measurements from trees 

grown on soil with a known compaction level, the best estimate of 

the several growth variables would 

be the mean growth response at 

that compaction level (Table 3). 

Acknowledgment 

The work reported was supported by the Northeastern Forest 

Experiment Station and the New 

Jersey Agricultural Experiment Station. 

Literature Cited 

Foil, R. Rodney; Ralston, C. W. The 

establishment and growth of loblolly pine seedlings on compacted 

soils. Soil Sci. Soc. Am. Proc. 31; 

565-568; 1967. 

Gerhold, H. D.; Long, A. J.; Demeritt, 

Jr., M. E. Genetic information 

needed for metropolitan trees. 

J. For. 73(3):150- 153; 1975. 

Himelick, E. B. Disease stresses of 

urban trees. In: Better trees for 

metropolitan landscapes: 

symposium proceedings. 1976; 

USDA For. Serv. Gen. Tech. Rep. 

NE-22. p. 113-125. 

Little, Silas; Trew, I. Frederick. Pitch 

x loblolly pine hybrids: loblollies 

for the North? J. For. 77(11); 

709-713; 9979. 

Minore, Don; Smith, Clark E.; Wollard, Robert F. Effects of high soil 

density on seedling root growth of 

seven northwestern tree species. 

1969; USDA For. Serv. Res. Note 

PNW-112. 6 p. 

Noble, Daniel L.; Edminster, Carleton B.; Shepperd, Wayne D. Effects of watering treatments on 

germination, sunrival, and growth 

of ponderosa pine: a greenhouse 

study. 1979; USDA For. Serv. Res. 

Note RM-369. 6 p. 

Patterson, James. C. Soil compaction and its effects upon urban 

vegetation. In: Better trees for 

metropolitan landscapes: 

symposium proceedings. 1976; 

USDA For. Serv. Gen. Tech. Rep. 

N E-2231- 102. 

Pham, C. H.; Halverson, Howard G.; 

Heisler, Gordon M. Red Maple 

(Acer rubrum L.) growth and foliar 

nutrient responses to soil fertility 

level and water regime. 1978; 

USDA For. Serv. Res. Pap. 

NE-412. 7 p. 

Zisa, Robert P.; Halverson, Howard 

G.; Stout, Benjamin B, Establishment and early growth of urban 

conifers on compact soils. 1980; 

USDA For. Serv. Res. Pap. 

N E-451. 8 p. 
Appendix 

Analysis of Growth Response 

The analysis is based on the 

variances and covariances of the 

growth variables. The soil compaction level mean square for shoot 

height is MSD = 5.11 and the mean 

square for individuals (error) is MS, 

= 0.49 (Table 1). For a model with 

all fixed treatment effects, the expected value of MSD is 0: + 2406 

and the expected value of MSe is 03 

(Table 4). Equating the mean 

squares to their expected values 

and solving for the estimators (S2) of 

o2 as: 

Table 4.-Expected mean squares for a model with 

all fixed treatment effects - -- 

Source of Degrees Expected 

variation of freedom mean square 

Soil type (T) 

Soil density (D) 

Tree species (S) 

TxD 

TxS 

DxS 

TxDxS 

Individual plant 

and 

The estimates of a$ and oi for 

shoot height (H) based on the 

variance table are: 

Similarly, the variance of the response at the four soil compaction 

levels for each of the other growth 

variables (Y) is estimated by: 

Table 5.-Covariance components between shoot height (ti) 

and growth variable (Y) 

Growth variable (Y) Covariance 

component shoot Root Root Root penetration 

weight weight length depth 

and error by: 

In a future experiment in which 

soil compaction level is the only The amount of variation ac- The values of the variances and 

factor, it is expected that the total counted for by the soil compaction R2(Y) are given in Table 2 and show 

variation in the measurement of the effects is measured by the intra- that root penetration depth variaith growth variable will be: class correlation coefficient tions are closely related to soil c-ompactiotl level. 

s~(Y) = s&Y) + Sg(Y) R2(Y) = s$(Y) I s~(Y) 
Correlations Among Growth 

Responses 

Covariances were used in this The regression of the mean of 

experiment to develop realistic esti- growth variable (Y) on the mean 

mates of correlation coefficients be- shoot height in a new sample (h*) is 

tween mean shoot height and the estimated by 

mean response in other variables. 

The estimate of the ovar ria ti on where j?* = predicted mean 

between mean seedling height (H) - growth response 

and the mean of growth variables (Y) = estimated grand mean 

is - of growth response (Y) 

h* = mean shoot height of 

MSD(H, Y) - MSe(H, Y) - new sample s~(H5 Y) = 24 h = estimated grand mean 

shoot height 

The covariation due to individual tree effects is and bD(H, Y) = SD(H, Y) I s$(H) 

Se (H, Y) = MSe(H, Y) and, the correlation among the 

means is given by 

The total covariance between 

top growth and growth variable (Y) R~(H, Y) = S~H, Y) 

is estimated by (Table 5) TS;(H) S$(Y)] 'I2 
Halverson, Howard G.; Zisa, Robert P. Measuring the response of conifer seedlings to soil compaction stress. 

Broomali, PA: Northeast. For. Exp. Stn.; 1982; USDA For. 

Serv. Res. Pap. NE-509. 6 p. 

Evaluation of several growth responses showed that root 

penetration depth was best related to soil compaction stress. 

Conventional measures such as seedling height were not well 

related to soil compaction level, but seedling height can be 

used to predict root penetration. Selecting the proper growth 

variable is an important step in any seedling study. 

114.1 1 

Keywords: urban trees, growth form, soil properties 
Headquarters of the Northeastern Forest Experiment Station are in 

Broomall, Pa. Field laboratories are maintained at: 

9 Amherst, Massachusetts, in cooperation with the University of 

Massachusetts. * Berea, Kentucky, in cooperation with Berea College. 

Burlington, Vermont, in cooperation with the University of 

Vermont. 

@ Delaware, Ohio. 

Durham, New Hampshire, in cooperation with the University of 

New Mannipshim. 

@ Hamden, Connecticut, in cooperation with Yale University. 

Morgantown, West Virginia, in cooperation with West Virginia 

University, Morgantown. 

@ Orono, Maine, in cooperation with the University of Maine, 

Orono. 

Parsons, West Virginia. 

Princeton, West Virginia. 

Syracuse, New York, in cooperation with the State University of 

New York College of Environmental Sciences and Forestry at 

Syracuse University, Syracuse. 

0 University Park, Pennsylvania, in cooperation with the 

Pennsylvania State University. 

Warren, Pennsylvania. 

'2 U.S. GOMWENT PRINTING OFFICE: 1982-505-012:29