WV Southern Forest

Research Note

Experiment Station

so-314

August 1995

The Roots of Plantation Cottonwood: Their Characteristics and Properties

John K. Francis

SUMMARY

The root biomass and its distribution and the

growth rate of roots of pulpwood-size cottonwood

(Popolus deltoides) in plantations were estimated

by excavation and sampling. About 27 percent of

the total biomass was in root tissue. Equations for

predicting stump-taproot dry weight from d.b.h. and

top dry weight were derived. Lateral roots in two

plantations increased in length until dieback, and

regrowth produced a lateral root of semistable average length to a little more than half the distance

between trees.

Additional keywords: Populus deltoides, root

competition, root biomass.

INTRODUCTION

Cottonwood have been well studied above ground,

but examination of the “dark continent” below

ground has hardly begun. Tree roots are not only

difficult to expose but also lack present commercial

markets. Despite small human interest, roots, to the

tree, are as indispensable as the top. Roots are a

principal source of interaction with the environment.

While supporting the cottonwood, they acquire moisture and nutrients from the soil. Roots also require

carbohydrates and nutrients for growth.

Roots contain significant amounts of biomass. The

cellulose concentration in roots, at least in the stump

and taproot, is probably similar to that in the bole.

However, because of the difficulty in removing associated dirt, fuel may be the most feasible use.

Utilizing this formerly wasted biomass could be less

expensive than growing additional wood.

A few investigators have studied cottonwood and

hybrid poplar roots. Baker and Blackman (1977)

found that roots of l-year-old planted cottonwood

contained 30 percent of the biomass of the plantation and that most of the root biomass was located

in the upper 8 inches of soil. Sprackling and Read

(1979) described the root system #of naturally grown

plains cottonwood (P. deltoides’ var. occidentalis

Rydb.). Hansen (1981) measured the lateral extension of hybrid poplars (P. deltoides X nigra) in plantations. Faulkner and Fayle (1979) reported the horizontal and vertical development of the root systems

of poplar hybrids and observed that repeated cultivation appeared to reduce the extent of the surface

root system. The net primary productivity and biomass distribution, including roots, in a plantation

of hybrid poplars are presented by Marcelli et al.

(1980).

The study reported here (1980-83) was confined

to pulpwood-size cottonwoods in plantations established from cuttings, Research was guided by the

following questions: How are the biomass and nutrients in cottonwood root systems distributed? Is

the weight of the harvestable portion (stump + taproot) of the root system predictable? How rapidly

do roots extend outward to occupy the space

between trees?

METHODS

This investigation was conducted in three parts.

In the first, measurements of the biomass and nutrient distribution were conducted in an 8-year-old

plantation on Alligator clay soil. This soil supports

Southern Forest Experiment Station/l-102lO U.S. Postal Services Bldg., 701 Loyola Avenue, New Orleans, La. 70113

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Louisiana, Mississippi, E. Oklahoma, Tennessee, E. Texas, Puerto Rico, U.S. Virgin Islands

1
natural cottonwood [site index (30 years) = 90 ft]

but is not currently recommended for cottonwood

plantations. The trees were spaced at 10 by 20 feet

(after one thinning) and averaged 5.4 inches d.b.h.

and 40 feet tall. Sample trees were selected to reflect the diameter range of the plantation. Ten trees

were excavated; the process was as follows. Each

tree was felled and weighed green. About 5 percent

of the top and bole was dried (7OOC) and weighed

to obtain a factor for converting to total dry weight.

A trench was dug with a backhoe around each

stump, leaving a 4- by 4-foot pedestal of soil. The

soil was carefully picked away from the roots.

The root system was divided into stump-taproot,

lateral roots, and fine roots (<7 mm). The weight of

roots at a distance greater than 4 feet from the

stump was estimated from the roots in l-foot-square,

2-foot-deep samples of soil dug at 4 feet out from

the stump within-row and 9 feet out between rows.

Samples of stump wood and bark, lateral-root wood

and bark, and fine roots were analyzed in the laboratory for concentrations of N, P, K, Ca, and Mg.

Nitrogen concentration was determined by the Kjeldahl method. The molybdenum blue calorimetric

method was used for P determination. Concentrations of K, Ca, and Mg were measured by atomic

absorption spectroscopy after being taken up from

ashed material in 1N HCI .

To establish a relationship between stump-taproot

dry weight and properties of the aboveground portion

of the tree, 25 trees were excavated and evaluated.

Ten of these were trees from the first part of the investigation. Five each were from ll-, 17-, and 23-

year-old plantations. The diameters of the 25 trees

ranged from 5 to 15 inches. Heights were about 40 to

80 feet. Because of thinnings and mortality at the time

the trees were sampled, spacings varied considerably. All the plantations were on Alligator clay soil.

The latter 15 trees were measured for diameter and

height, felled, and the tops weighed green (leaves

present). An approximate 5-percent sample of each

top was collected and dried. From this information,

aboveground dry weight was calculated. To extract

the stump, the upper lateral roots were cut, a chain

was attached to the stump, and the stump was then

pulled up with a tractor. All remaining lateral roots

were then trimmed off. The stumps were dried whole

or in pieces in a walk-in oven for 2 weeks to a

month, depending on stump size. The relationships

between stump-taproot dry weight and aboveground properties of the trees were calculated by

regression,

In the third part of the investigation, the extension

of major lateral roots was measured. Three stands

were utilized. One was the same stand that was described in the first investigation (lo- by 20-foot spacing). The roots were excavated at age 9. Another

plantation was on Commerce silty clay loam soil.

This plantation was 8 years old, and trees were

spaced at 10 by 10 feet. The third plantation was

adjacent to the second and was the same age, but

trees were spaced at 40 by 40 feet. Half of the

widely spaced trees sampled were pruned to onehalf of their height and half were unpruned. The

textural contrast (clay vs. silty clay loam) between

the 9- and 8-year-old plantations prohibits perfect

comparisons. However, texture probably did not

seriously alter root extension,

Two major lateral roots on each sample tree were

severed from the taproot and excavated with a

shovel to their full length or until they exceeded

3 feet deep. (Deep or steeply descending cottonwood roots usually branch profusely and end in a

short distance.) The bias introduced by assuming

no further extension after the 3-foot depth was probably minor. Total root length, root extension (radial

distance to root tip or farthest extension), crown

radius, and d.bh. were measured. The roots were

sectioned and the rings counted on a shaved end.

Many roots in the two more closely spaced plantations died back and lost some of their former length.

Consequently, average root length in the first and

second plantations between year 4 and excavation

age could only be estimated. Fifteen trees were

examined in the first stand, 11 in the second, and

10 in the third. Total root length and root extension were related to crown radius and d.b.h. by

regression.

RESULTS AND DISCUSSION

In the 10 original cottonwoods excavated, which

ranged from 5.3 to 6 3 inches d.b.h., 72.8 percent

of the biomass was in the top and bole (leaves

present) and 27.2 percent in the total root system,

including a 2-inch-high stump. Root tissue ranged

from 19.0 to 37.1 percent of the whole trees. The

various root parts, as a percentage of the whole

tree, averaged as follows: stump-taproot, 6.8 percent; close laterals (in a 4-foot square around

stump), 8.2 percent; close fine roots, 0.7 percent;

and extended roots, 11.6 percent. The taproots of

John K. Francis is principal soil scientist at the Southern Hardwoods Laboratory, which is maintained at Stoneville, Mississippi,

by the Southern Forest Experiment Station, USDA Forest Service, in cooperation with the Mississippi Agricultural and Forestry Experiment Station and the Southern Hardwood Forest Research Group.

2
these trees averaged 15 inches long-about the

length the original cutting extended below ground.

Apparently, the taproot grew as a thickening of the

cutting. Lateral roots radiated out, particularly just

below the groundline and near the bottom of the

cutting. Most of the root biomass was between 3

and 12 inches deep in this clay soil, although a few

small roots extended to the 4-foot depth.

On the average, nutrients were a little more concentrated in root tissue than in aboveground portions. The roots, comprising 27 percent of the

biomass of the total tree, contained 26 percent of

the N, 34 percent of the P, 43 percent of the K, 29

percent of the Ca, and 34 percent of the Mg. The

concentrations of the five nutrients in root wood,root

bark, and small roots are given in table 1. [As with

their aboveground counterparts (bark and twigs),

root bark and small roots generally have nutrient

concentrations several times greater than the wood

of large roots.] Lateral roots averaged 25-percent

bark, and the thicker stump-taproot averaged 18-

percent bark. By comparison, the aboveground portion of the tree averaged 24-percent bark.

In the second part of the investigation, the dry

weights of the stump-taproots of 25 trees were determined. The average ratio of stump-taproot to

total top was 1 :lO, or the stump-taproots were about

10 percent as heavy as the aboveground portions

of the tree. Although the sample trees ranged from

8 to 23 years old and represented a mixture of

clones, variation was small (C.V. = 0.186). Four additional trees were sampled from a soil with somewhat lighter texture (Tunica series). Their stumptaproot to top ratio was also 1 :lO.

The stump-taproot dry weight of pulpwood-size

cottonwood growing on clayey soil may be predicted

by the following relationships:

Y = -17.386 + 1.328 D1.7 r2 = 0.964

where D = d.b.h.

Y = -0.168 + 0.0968 T r2 = 0.940

where T = aboveground dry weight.

Tree height was not a good predictor of stumptaproot dry weight.

The root tissue most likely to be used eventually

on a commercial basis is the stump-taproot because

Table 1 .-Concentrations of five essential nutrients in three root

tissues of &?-year-old cottonwoods in plantations

Tissue N P K C a Mg

---_-____-- percent of dry weight--- --------

Root wood 0.12 0.06 0.27 0.16 0.04

Root bark 0.48 0.09 0.79 1.65 0.14

Small roots’ 0.43 0.14 0.75 1.32 0.20

..__.__

I<7 mm thick.

of its size, compactness, and wood quality. The

technology for harvesting stumps is already available (Koch 1976), although cost of extraction could

be a prohibitive factor. A lo-percent increase in harvested raw material would not be the only benefit

of harvesting stumps. Site preparation for the next

rotation would be easier if the stumps were gone.

The increase in length of lateral roots of trees in

the three plantations began at similar rates (fig. 1).

As root closure began to occur, root-length increase

slowed, beginning first in the closer-spaced plantations. The lo- by IO-foot plantation suffered severe

dieback of roots in about year 6. This was observed

in all the roots excavated. It is interesting that in

year 5, aboveground growth rate of the trees

dropped sharply. (Root dieback is probably a result

of competition-induced stress-possibly a shortage

of carbohydrate to support an extensive and growing root system.) After the dieback in year 5, the

roots began to branch and slowly grow out again.

(Thinning before the onset of root dieback or using

a wider initial spacing are suggested as preventive

measures.) At year 9, the lateral roots of the plantation spaced at 10 by 20 feet were extending very

slowly and suffering some minor dieback. Roots in

the 40- by 40-foot plantation had closed between

rows, yet the spacing was so great that roots were

still widely dispersed. There was no sign of dieback

of roots in this plantation.

b-

>-

)-

) -

r? 0

IO I20

. . . . . . . . . . . . . . . . . . 1 . . . .

I 2 3 4 5 6 7 0 9

AGE (YEARS)

Figure l.-Root length by year of cottonwood plantations at

three spacings. l Estimated period; exact root length

could not be measured because of root dieback.

3
It appears that root length tends toward a quasiequilibrium at a little more than half the plantation

spacing. Root extension, the distance between tree

trunk and root tip, is slightly shorter than, but highly

correlated with, root length. Thus root extension is

ultimately controlled by spacing. There were several

weak correlations (r = approximately 0.5) of d.b.h.

or crown radius with root length or root extent. Although pruning in the 40- by 40-foot plantation had

reduced diameter growth, it had no significant effect

on root length or root extension.

LITERATURE CITED

Baker, J. B.; Blackman, 8. G. Biomass and nutrient

accumulation in a cottonwood plantation-the

first growing season. Soil Science Society of

America Journal 41(3): 632-636; 1977.

Faulkner, H.; Fayle, D. C. Root development of three

5year-old Euroamerican poplar clones at two

plantation sites. Report 8. Poplar Research, Management and Utilization in Canada. Information

Paper 102. Ontario Ministry of Natural Resources . 1979. 200 p,

Hansen, Edward A. Root length in young hybrid

populus plantations: Its implications for border

width of research plots. Forest Science. 27(4):

808-814; 1981.

Koch, P. New technique for harvesting southern

pines with taproot attached can extend pulpwood

resources significantly. In: Applied Polymer Science Symposium 28. New York, NY: John Wiley

and Sons, Inc. 403-420; 1976.

Marcelli, R.; Duranti, G.; Giulimandi, G. The productivity of a poplar plantation. II. Net primary productivity of a poplar plantation. Pubblicazioni del

Centro di Sperimentazioni Agricola e Forestale

13(3) : 203-227; 1980.

Sprackling, J. A.; Read, R. A. Tree root systems in

eastern Nebraska. Nebraska Conservation Bulletin 37. Institute of Agriculture and Natural Resources, University of Nebraska; Lincoln, NE.

1979. 73 p.