CONTINUING EDUCATION UNIT
Root Management Challenges
in Urban Sites: Achieving a
Healthy Root–Crown Balance
By Angela Hewitt
LEARNING OBJECTIVES
Figure 1. Loss of soil structure can result in poor root
development and reduced
tree growth. These two pots
were filled with the same
topsoil. Soil structure was
destroyed in the soil on the
right.
The arborist will be able to
• describe the physical, biologic, and nutrient properties of
healthy soil
• explain the relationship between tree vigor and soil root volume
• describe the built infrastructure and soil management
practices that promote healthy trees
CEUs for this article apply to Certified Arborist, Utility
Specialist, Municipal Specialist, Tree/Worker Climber,
and the BCMA management category.
Many consider that 80 percent of urban tree problems originate below
ground. Urban environments must provide the same essential soil
resources as natural environments if trees are to maintain a healthy
balance between the crown and root system. By understanding what
soil resources trees rely on, we can manage the urban environment to
optimize these features to the extent possible.
When trees are well-adapted to their environment, they maintain
a natural balance between the crown (consumer of water and nutrients,
producer of carbohydrates and energy) and the root system (producer
of water and nutrients, consumer of carbohydrates and energy). The
root system must have enough capacity to absorb water that is lost from
the leaves through transpiration on the hottest days, along with mineral
nutrients used for growth and maintenance throughout the tree. In
urban situations, site limitations that limit root development are the
greatest threat to maintaining balance. The management challenge is
to provide an urban soil environment that functions like the natural
environment, though its appearance may be quite different.
Soil as a Medium for Root Growth
Physical Properties
Fine roots absorb water and nutrients from the soil to support the
crown, and their development is influenced by soil physical properties.
Soil texture (the proportion of sand, silt, and clay particles), soil structure (how those particles are held together in larger aggregates), and the
associated properties of density, penetration resistance, and pore space
all affect fine root development.
Roots often branch extensively in loamy textured soils with high
organic matter content because these are typically well aerated, have a
high water-holding capacity, and are rich in nutrients. Sandy soils, on
the other hand, are relatively low in moisture and nutrient holding
capacity and roots often extend with little branching in order to
access needed resources from a larger volume of soil. Clay soils are
16
often poorly aerated and are easily compacted, resulting in high penetration resistance that can severely limit root growth.
Soil structure is important for the macropore space it provides for
aeration and root penetration. When soil structure is lost, pore space
and aeration are reduced, and penetration resistance may be increased.
Loss of soil structure can result in poor root development and reduced
tree growth even in otherwise high-quality soil (Figure 1).
Surface layers of natural soils have a tendency to possess less clay
and have better structure and higher organic matter content. This
environment can support the greatest fine root development and
diversity of soil microorganisms that are part of a healthy soil. In disturbed urban soils, rich surface soils can be lacking, or are replaced
with a thin layer of topsoil of lesser quality. Development of roots
and associated microorganisms are directly affected.
It is not easy to repair a soil that took centuries to develop. If the
natural soil has been disturbed, quality of the surface soil and drainage may be the two factors most altered from the natural state, and
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Nutrient Availability
Figure 2. Soil in the root zone can be decompacted
and amended without harming roots, using pneumatic
excavation tools.
most limiting of root growth. Surface soils can be improved through
tilling and the addition of organic matter. Around existing tree roots,
pneumatic excavation tools can be used to loosen the soil and incorporate organic matter with minimal damage to roots. Avoid depressions
where water can accumulate. A drain system may need to be installed
to improve drainage below the improved surface soil.
Soil Biology
Healthy soil contains many beneficial bacteria, fungi, protozoa, nematodes, arthropods, and earthworms. They all contribute to soil chemical
processes, aeration, and the accumulation of organic matter. In nature,
forest soils have lower pH and higher levels of fungi, compared to grassland soils. Many urban landscapes trees are surrounded by lawn grasses
that foster very different soil biology than the forest environment to
which the trees area adapted.
Many products have been marketed as biostimulants that can be
applied to the soil to increase soil biological activity and root growth.
Most are based on sound scientific concepts, but not specifically formulated to mimic the forest soil environment, and are unproven as treatments for trees in urban situations. Application of organic products, such
as humates and plant extracts, has not consistently improved root growth
of trees, although species appear to vary widely in their response.
Compost teas are liquids containing soluble nutrients and species
of bacteria, fungi, protozoa, and nematodes extracted from compost.
Compost teas are being used to enhance soil biology and provide
nutrients, sometimes as an alternative to fertilization, but research
support for their effectiveness is lacking.
The benefits of mycorrhizal fungi associations of tree roots are well
established, but research has failed to show consistent benefits of
applying commercial products in urban situations. The poor quality
of urban soils often limit mycorrhizae development more than a lack
of fungal inoculum in the soil.
Soils with favorable physical and chemical characteristics will naturally support active soil biology. Replace the lawn grass with mulch
december 2012
Deficiency symptoms occur
when there is an imbalance
between absorption by the
roots and use by the crown.
If a balance is restored by
roots exploring a larger soil
volume or top growth rate
adjusting to available supply,
the tree can overcome the
deficiency and symptoms
may dissipate.
Nutrient availability varies with soil pH. Subsoils are
often more alkaline than surface soils because base-forming cations (Mg++, Ca++, K+, Na+) are leached.
Nutrient deficiencies are more likely to occur in urban trees than in
nature, primarily because the sites are highly disturbed and more alkaline soils are closer to the surface, making some nutrients, such as iron
and manganese, less available for plant uptake. Modifying pH, especially
lowering it, can be difficult. The large amount of sulfur or aluminum
compounds required to alter soil pH can cause direct damage to the
plants themselves. Very low pH can be harmful to roots because of
aluminum toxicity.
Nutrient availability can affect root development in several ways.
Contrary to traditional wisdom, the addition of phosphorous does
not stimulate root growth unless low levels are already limiting root
growth. A phosphorous deficiency could actually stimulate root growth
in order to explore larger volumes of soil to acquire additional nutrients
and restore balance.
Increasing nutrient levels in the soil above normal levels will not
affect root growth. The one exception may be nitrogen. Nitrogen
applied to a limited area (e.g., liquid injection) can concentrate fine
roots in that area, but may not increase overall fine root development
for the tree. Very high levels of nitrogen can cause an increase in top
growth without increasing root growth, resulting in a crown that may
not be adequately balanced with the root system and increase vulnerability to water stress.
The wide variety of species planted in urban landscapes makes nutrient deficiencies difficult to diagnose. Characteristic deficiency symptoms
can be reliable indicators of a nutrient deficiency, but they do vary somewhat by species. Foliar analysis can be difficult to interpret because of
the wide variation in species, age, time of year, moisture stress, and vigor
of urban trees. Soil analysis provides a measure of nutrients available
to the tree, but is not a direct measure of a status within the tree. In
addition, a poorly developed root system may be unable to absorb the
nutrients available, resulting in a deficiency even though sufficient quantities are present in the soil. In the short term, identified soil nutrient
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u
17
CONTINUING EDUCATION UNIT
and plants similar to the forest understory. There is not
yet any clear evidence that
biostimulants, compost teas,
humates, and other soil additives alone can reliably enhance
tree-compatible soil microorganisms with the lawn still
in place.
Providing Adequate Root Space
Few trees have truly unlimited root space. Even forest trees must compete with surrounding trees for root space. This likely will limit the size
of their root system but their crowns are also limited by competition.
The closed canopy of the forest creates a consistent, protected environment and minimizes extremes. The natural physiological balance
between the crown and root system can be easily maintained.
In contrast, most urban trees have root spaces limited by pavements
and structures to at least some degree. Even the relatively generous
space of residential lots will eventually limit the root system of larger
trees. Streets, parking lot islands, and pavement cutouts severely limit
the root system of trees. Crowns of urban trees are open grown and
larger, and therefore more exposed to sun and wind compared to forest trees. The hard surfaces around them can reflect heat and light,
increasing exposure. These trees need large root systems that have the
ability to supply the crown under the most extreme conditions (hot
and windy) of maximum transpiration, even though this full capacity
may only be used a few days each year.
Soil quality, root competition, frequency of watering, and other
factors can affect functional soil volume - the capacity of soil to supply
necessary resources for tree growth. Poor quality soil can limit root
development, which has the same effect as reducing soil volume. The
presence of turfgrass roots, which compete more aggressively for soil
water and nutrients, can also effectively decrease functional soil volume
for tree roots. Greater crown size and exposure, combined with smaller
root spaces and reduced soil quality, make achieving a sustainable root–
crown balance difficult on most urban sites.
As a general guideline, if above- and belowground environmental
extremes are not severe, the root space should be approximately two
cubic feet (56.6 L) of soil for each square foot of the expected mature
tree crown projection area (area within the dripline). With this soil
volume for root growth, a tree would require supplemental irrigation
only occasionally during periods of drought. In a small pavement cutout
that has only one-quarter of this soil volume, the same size tree would
need irrigation at least once a week. By this guideline, a six-foot (1.8 m)
square, two-foot (60 cm) deep planting pit would only be able to support a tree with a crown diameter of seven feet (2.1 m) without regular
irrigation.
Figure 3. This parkway was doubled in
width to provide more root space for the
trees by eliminating a lane of parking.
18
Figure 4. The trees on the left are growing in quality soil
under a suspended pavement. Root and crown growth are
better than in a conventional tree pit, but remain inferior to
those growing open space conditions (right).
Urban Horticulture Institute
CONTINUING EDUCATION UNIT
deficiencies can be corrected by fertilization. Do not apply fertilizers
just as a routine maintenance procedure, assuming that no harm can
be done.
If root space around existing trees is limited laterally, there are few
options to increase it. Expanding root space by eliminating pavements
or structures is an unlikely but sometimes possible solution (Figure 3).
Soil quality can be improved to support more root growth in the existing root space.
A high level of maintenance, especially irrigation, can soften the
impact of limited soil volume, but only to a point. If soil moisture and
nutrients are kept at optimum levels, fine root density will increase and
absorb more water and nutrients, effectively increasing functional soil
volume. In extreme situations, such as small planting pits, this approach
may mean watering every few days. This is risky, since a very short interruption in the maintenance regime – even just one day – could result
in damaging drought stress levels.
Increasing usable depth may be possible in new construction, especially in planting pits. Roots will proliferate as deeply as soil conditions
will permit it, aeration and penetrability being the key factors. Good
aeration and drainage can allow roots to develop at least three feet (0.9 m)
deep.
Pavement systems can be designed to allow roots to develop underneath them. If pavements can be suspended without depending on
compacted soil beneath them for support, the soil can be of high quality for good root growth. Suspended pavements range from engineered
designs that are constructed on site, to simpler and smaller precast concrete structures and commercial cell systems. Although better than a
small pit, this approach is still not equal to the same volume of open
soil (Figure 4).
Soils designed to support pavement while providing a favorable environment for root growth without settling are often called load-bearing,
skeletal, or structural soils. Stone–soil mixes are most commonly used.
These mixes can be a useful compromise in situations where high quality non-compacted soils cannot be used, but they will not produce the
same results as the same volume of quality soil. Trees may be more stable
in structural soils than traditional tree pits because roots must extend
farther to gain access to the limited soil in the stone–soil mix. Permeable
pavement may be desirable for improved aeration and water movement.
Root paths provide a narrow channel of good quality soil that
roots can grow under pavement to reach additional root space on the
other side. Paths can also connect individual planting pits (Figure 5).
However, it can take several years for roots to grow through the paths
and develop in the soil beyond the pavement. This concept has not
yet been supported with research.
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Figure 6. A layer of mulch reproduces many
of the soil benefits found in the forest.
When pits are combined and shared by several trees, the performance
of the trees seems to be better than when trees are placed in several
smaller, individual planting pits. Similar to the forest, the closely spaced
canopies of the trees limit their size and help protect each other and the
soil from sun and wind. Below ground, roots can spread over a larger
area. Though not much additional soil is available for each tree, the larger
shared root space provides a more consistent environment for the roots.
The best long-term solution is a combination of restoring the soil
environment so that it can provide the resources trees need naturally
without supplemental fertilization, and choosing species that can tolerate the site conditions. Around existing trees, organic mulch (Figure 6)
decomposition can supply a steady source of nutrients and organic
matter. On new sites, compost can be incorporated into the surface
soil before mulching.
The soil environment is integral to maintaining the natural balance between the crown and root system. Disturbing the soil can disrupt this delicate balance
and trigger stress that can
reduce a tree’s defenses
against pests, and lead to
decline and even death. Preventing soil damage is critical. Once a soil is disturbed,
it will take many decades to
recover, if it ever can. The
same careful preservation of
soil resources that has
become standard practice in
farming should be practiced
in urban developments.
Angela Hewitt is a research
specialist with The Morton
Arboretum (Lisle, IL).
CEU TEST QUESTIONS
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arbNewsQuizzes.aspx). If you need a username and password, promply send us an e-mail ([email protected]).
To receive continuing education unit (CEU) credit (1 CEU) for home study of this article, after you have read it, darken the appropriate circles on the answer
form of the insert card in this issue of Arborist News. (A photocopy of the answer form is not acceptable.) A passing score for this test requires 24 correct
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Next, complete the registration information, including your certification number, on the answer form and send it to ISA, P.O. Box 3129, Champaign, IL
61826-3129. Answer forms for this test, Root Management Challenges in Urban Sites: Achieving a Healthy Root–Crown Balance, may be sent for the
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If you do not pass the quiz, ISA will send you a retake answer sheet. You may take the quiz as often as necessary to pass. If you pass, you will not be notified;
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CEUs for this article apply to Certified Arborist, Utility Specialist, Municipal Specialist, Tree Worker/Climber and Aerial Lift, and the BCMA
management category.
Questions on next page
u
december 2012
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19
CONTINUING EDUCATION UNIT
Bryan Kotwica
Figure 5. Root paths provide access to open soil
beyond the pavement and connect individual planting
pits as shared root space.
7.Which of the following methods measures
nutrients available to the tree?
a. visual symptoms
b. soil analysis
c. root sampling
d. foliar analysis
13.Small pavement cutouts and parking lot
islands affect tree growth by
a. limiting root space
b. increasing crown exposure to sun and wind
c. containing poor quality soil
d. all of the above
2.Soil texture is best described as the
a. number of soil particles
b. proportions of sand, silt, and clay
c. percentage of organic matter within the soil
d. size of soil particles
8.In some cases, a phosphorous deficiency can
affect roots by
a. increasing nitrogen absorption
b. decreasing root growth
c. raising soil alkalinity
d. increasing root growth
14.A tree’s functional soil volume can be
influenced by
a. root competition
b. soil quality
c. the frequency of watering
d. all of the above
9.Increasing nitrogen levels in the soil
a. will have no effect top growth or root
growth
b. increases root and top growth
c. increases top growth only
d. increases root growth only
15.The use of root paths in urban plantings is
intended to facilitate tree root growth by
providing
a. a path to water
b. paths for roots to grow into concrete
c. paths for utilities
d. a path for roots to grow under pavements
to reach additional soil space
3.What is soil structure?
a. How the soil is layered in its profile
b. The proportions of sand, silt, and clay
c. How individual soil particles are organized
and held together
d. The size of soil particles
4.Which of the following soil environments can
support the greatest fine root development?
a. clay subsoil
b. sandy soil
c. natural surface soil
d. all of the above
5.Fine root development is influenced by
which of these soil properties?
a. penetration resistance
b. texture and structure
c. density and pore space
d. all of the above
6.Identify the base-forming cations that are
leached out of naturally formed surface soil.
a. H, Cl, Na+, N
b. Mg++, K+, N, P
c. Na+, Ca++, Mg++, K+
d. Mg, Na+, P, Al
TREE HEALTH • ROPES & KNOTS • CLIMBING • PRUNING
CONTINUING EDUCATION UNIT
1.Approximately what percentage of urban tree
problems originates below ground?
a. 20 percent
b. 40 percent
c. 40 percent
d. 80 percent
20
10.Soil structure can be improved by
a. using air excavation tools to cultivate the
soil around roots
b. adding organic matter
c. tilling existing soil
d. all of the above
11.Compared to forest soils, grassland soils have
lower
a. bacteria populations
b. fungi populations
c. pH
d. organic matter levels
12.Soil biology more appropriate for trees can be
promoted by
a. maintaining a healthy green lawn under
the tree
b. establishing a layer of mulch under the tree
c. adding fertilizer
d. watering
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16.As a general guideline, the recommended soil
volume needed for each square foot of projected
tree crown is?
a. 2 cubic feet (56.6 L)
b. 4 cubic feet (113.3 L)
c. 5 cubic feet (141.6 L)
d. 10 cubic feet (283.2 L)
17.A planting pit soil with both good aeration
and drainage allows roots to develop to a
depth of approximately
a. 2 feet (0.6 m)
b. 3 feet (0.9 m)
c. 5 feet (1.5 m)
d. 10 feet (3 m)
18.Nutrient deficiency in trees seems to appear
a. when there is an imbalance between
absorption by the roots and use by the
crown
b. more in urban areas than natural areas
c. when growing in areas with high amounts
of H+ and AL+++
d. all of the above
19.In planting pits shared by several trees
a. all the trees grow better
b. the soil dries out faster
c. additional soil volume is required to equal
growth in individual pits
d. root paths will provide no additional
benefit
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20.If the natural soil profile has been disturbed,
what two factors may be most limiting to tree
growth?
a. quality of the surface soil and drainage
b. nutrient availability and drainage
c. quality of the surface soil and
restricted root space
d. restricted root space and nutrient
availability
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