Preventing Grade Change Damage to Trees E. Thomas Smiley, PhD

RESEARCH LABORATORY

TECHNICAL REPORT

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Preventing Grade Change Damage to Trees

E. Thomas Smiley, PhD, Urban Forestry

During building or landscape construction, the grade

of the land frequently is altered to form a more level site. Grade

changes made near trees will seriously disturb the delicate relationship between roots and the surrounding soil. This

results in root mortality, decline in vigor and frequently, death of the tree.

Symptoms of Damage

Symptoms of damage from grade changes can appear

as a progressive decline of the crown occurring over a

period of several months to several years. Initial

symptoms generally include delayed budbreak,

reduced growth, stunted light green to yellow leaves,

crown thinness, and premature fall coloration and leaf

abscission. Epicormic sprouts might form on the trunk

and large limbs and twig dieback may occur. This may

be followed by dieback of large branches and entire

leaders and finally, tree death.

The

severity of the symptoms and the rate at which the

tree declines depend on the extent of the grade change,

any associated soil and root disturbances, tree species,

and the age, size and initial vigor of the tree. If root

damage is severe, tree mortality can result within one

year. If grade changes are slight, young, vigorous trees

might tolerate the disturbance and exhibit few

symptoms. Generally, mature or over-mature trees,

those in a weakened condition, and shallow-rooted

species are most sensitive to grade changes.

Evidence of grade changes also can be detected near

the base of the tree. Most healthy trees exhibit a flare

(widening) at the base of the trunk. Where soil or

other fill has been added over the root system to raise

the grade, little or no flare is present on the trunk

(Figure 1). Where soil has been removed to lower the

grade, abrupt changes in the normal grade of the land

Figure 1: Soil fill and turf up to base of tree. Note

lack of flare at base of trunk

Figure 2: Grade change around tree

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may be detected. In some instances, trees may

appear to be growing on small, raised beds or

mounds and buttress roots may be exposed,

unless backfilled (Figure 2).

Effects of Grade Changes

Lowering the grade

near trees causes root

destruction which reduces the water and

nutrient absorption capability of the tree and

reduces its structural integrity. Fine or feeder

roots, which are responsible for water and nutrient

absorption, occur primarily within the upper six to

eight inches of soil. Lowering the grade by only a

few inches will cause extensive fine root destruction.

Drastic grade reductions may destroy large

support roots which can predispose trees to

windthrow during storms.

Raising the grade near trees essentially

suffocates roots. Like all plant cells, root cells respire;

that is, they utilize oxygen and give off carbon

dioxide (CO

). Oxygen and CO

exchange occurs

between tree roots and pores within the

surrounding soil. The addition of fill compacts the

soil thereby reducing the amount of pore space.

Proper gaseous exchange between the roots and

surrounding soil is inhibited which results in root

suffocation (Figure 3). The extent of tree

damage

from raising the grade depends largely on the

depth of the fill and the soil type. The deeper the

fill, the more extensive the root destruction will be.

Fill composed of sandy soil, which is very porous, will

Preventing Grade Change Damage to Trees

affect trees much less than fill composed of heavy

clay soil.

Raising and lowering the grade also can affect trees by

altering the water table. The addition of fill causes the

water table to rise which may result in flooding and

subsequently, suffocation of the root system if the

water table is initially very high. Lowering the grade

will cause the water table to fall which may reduce the

amount of water available to the tree.

Preventing Damage from Grade Changes

Preventing tree damage from grade changes must be

undertaken before the grade of the land is actually

altere

d. Trees that are seriously declining due to grade

changes seldom respond to corrective measures

designed to save them.

Raising the Grade: Most young, vigorous trees can

tolerate shallow fills of up to four inches if the fill is

sandy. The same amount of clay fill, however, usually

causes tree decline. Certain species, however, such as

beech, yellow poplar, pines, and dogwood are very

sensitive to even small amounts of a sandy fill.

Developers and landscapers should consult a

competent arborist before placing even shallow fills

over tree root systems.

If more than a few inches of fill must be placed over

tree roots, a well and drainage system must be

installed. Figures 4 and 5 show the proper method of

constructing a dry well and drainage system around a

tree. The dry well must be large enough to allow for

future growth of the trunk. Four to six inch

agricultural drain tile should be placed on the natural

grade of the land. The tile should drain to a lower level

to prevent water from collecting within the well.

Cover the tile with six to eight inches of two- to three-

inch stone. Do not use limestone because this will

raise the soil pH and could adversely affect tree

growth. Connect vent tiles with drain tile to allow for

gaseous exchange between the root zone and

atmosphere. The fill should consist of a sandy soil,

including biochar as organic matter, in order to

allow maximum aeration of the root zone.

Figure 3: Root and soil disturbance

Preventing Grade Change Damage to Trees

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wering the Grade: All cuts in the natural grade

must be made outside the dripline of a tree. Figure 6

shows the proper method of lowering the grade

around trees.

Rais

ing and Lowering the Grade: Where trees are

growing on a slope, the landscape sometimes is cut and

filled in order to create a level site. Figure 7 illustrates

the proper method of raising and lowering the grade

around trees. Again, all grade changes should be made

outside the dripline of the tree. Specification for the

drain tile and fill soil are the same as those outlined

previously.

unded in 1926, The Bartlett Tree Research

Laboratories is the research wing of Bartlett Tree

Experts. Scientists here develop guidelines for all of

the Company’s services. The Lab also houses a state-

of-the-art plant diagnostic clinic and provides vital

technical support to Bartlett arborists and field staff

for the benefit of our clients.

Figure 4: Side view showing proper construction of

well and drainage system for use around trees

where soil fil

l has been added to raise grade

Figure 5: Top view showing proper construction of

well and drainage system for use around trees

where soil fil

l has been added to raise grade

Figure 7: Side view showing proper method of

raising and

lowering the grade around trees

Figure 6: Side view showing proper method of

lowering grade near trees