Download The Avoidable Adverse Affects of Modern Urban Development on

The Avoidable Adverse Affects of Modern Urban Development on Trees.
Root Compation.
“All Trees, regardless of where they are growing, are genetically ‘Forest’ Trees….The
closer you can keep trees environment to that of a forest, the healthier the trees will be”
(Marx, 1995, p4)
Trees have been around for a very long time, well before the earth was physically altered
to suit our needs. They are complex biological systems in tune with the surrounding
environment, i.e. the forest. It has taken thousands of years for them to adapt and function
as part of their ecological niche. Since the arrival of man they have been interfered with.
Their natural environments have been changed or destroyed. Trees now have to contend
with and survive in a new type of environment; our environment, the Urban Forest.
Trees have not had time to change the biological defences and strategies that made them
so successful in the natural world, to cope with the stresses of the modern urban
environment. They face pollution, vandalism, solid soils, topping, lopping (amazingly a
practice still used by some local authorities) and other forms of mechanical damage. It is
no surprise that many street trees have a life expectancy that can be brought into a time
scale comparable with our own.
The aim of this paper is to explore the factors that affect tree roots in the urban
environment, and what measures can be put into place to control, treat and even prevent
the damage that can occur.
The subject of root damage is vast, so this paper will concentrate on one of the main
contributors to ill health in urban trees, that of soil compaction. This paper will ask the
question “How does soil compaction occur? How are Arboricultural departments in Local
Authorities treating or preventing it and are their methods successful?
To understand how and why tree roots are affected by the environment, it is important to
understand how they work.
A once common view of the tree was that the roots were a mirror image of the crown,
growing deep into the earth. We now know the reality to be somewhat different. A tree
and its roots (depending on the species) tend to take the form similar to that of a wine
glass. The roots take a shallow but wide spreading course through the soil. Putting down
deep root systems is time consuming, energy draining and physically difficult, so most
trees do not root themselves deeply, purely because they have no reason to.
Water and dissolved minerals are slow to move through the soil, so roots have to go to
them rather then wait for the water to come to the root. A large spreading root system
enables the tree to gather water from a large volume of soil and is important to the growth
and development of the tree.
Although roots do react to the affects of gravity, they do not grow towards anything or in
any particular direction. Roots are opportunistic and it is purely by chance rather than
design that they find their way to water supplies.
A tree’s roots system has two main functions; (i) to provide the tree with water, essential
elements and minerals and (ii) to hold the tree in place. In most environmental situations
water is normally found in quantity near the soil surface, from leaching and rainfall. The
soil surface is also where the bulk of organic matter breaks down and releases minerals
that are needed for the tree to carry out its’ biological functions. A large flat root plate is
also very good at providing stability for the tree. Deep roots would be of no benefit to the
tree.
Roots need a minimum of 3% oxygen in the soil to survive and function. 12% oxygen is
needed for the formation of new roots. Most soils are 20% oxygen, mostly contained at a
shallow depth where the tree can access it. If lack of oxygen where to occur, as in the
case of soil compaction, existing roots may survive, but new roots would fail to form
causing considerable stress to the tree. Roots have other functions besides stability and
water gathering. Roots are used for food storage and will play an important part in
determining the overall size of the tree. Roots on average will account for 20% to 30% of
a trees overall mass. (Trees and Their Natural History. Peter Thomas. Cambridge
University Press)
Roots develop internally rather than from buds, such as is the case on stems and
branches. In order to develop a root has several parts. The tip of the roots has a cap,
known as a root cap, which protects the root tip when it pushes through the soil. The tip
of the root is propelled through the soil by elongating tissues behind it. Old root cap cells
fall from the tip and are continually replaced by new cells. The new cells are supplied by
the Aprial maristan which also reproduces the area of elongation. The area of elongation
is where the active cell division takes place which pushes the root cap through the soil.
As the root becomes mature it divides into the epidermis and cartex. Root hair will then
form in the epidermis area of the root. In the area behind the root hairs lateral roots are
developed and …….. process of sending at root caps into fresh soil.
Germinated seedlings will produce a single root that will grow down and become a young
tap root. In some species where small seeds are produced – Picea, Tilia, and Salix
species, the tap root is small and plays only a small role in the trees overall development
and in some cases the tap root is so rudimentary that it is often described as a fibrous root
system. In other species, particularly Quercus species, the tap root grows very fast and
…. up most of the root system in the young sapling. Even so the tap root is still an
unessential part. The reason is that as a sapling grows it puts out a number of major frame
work roots, normally thought to be around 3 to 11 frame work roots (Trees and their
Natural History. Peter Thomas), which grow sideways from the top of the tap root, still
further diminishing its importance. These lateral roots may stay close to the surface of the
soil – such is the case for many Conifers. In the case of the Oak tree the lateral roots may
descend into the soil to a certain depth before continuing to spread and grow horizontally.
When a tree has failed and the root plate has lifted and become exposed it gives the
impression that the roots will spread only as far as the canopy, which is not the case. The
lateral roots often spread far beyond the main root plate.
The large woody roots of the root plate and outer roots will determine the overall size and
shape of the root system itself, but it is the fine roots that are actually in contact with the
earth. It is in the fine roots that a significant amount of water and essential elemental
intake occurs.
The most important section of the fine root systems are the root hairs. Root hairs
generally have a fairly short life span, usually only lasting a few weeks or even days
before they are lost. The tree will then cease to take up water and elements through them.
The root hair begins to develop more support and tissue, becoming more solid and rigid,
and will start to act as an anchorage and conducting root. Some species have an apparent
lack of root hairs, for example some gymosperms have no root hairs at all. In this case
mycorrhizas will come into play. Unfortunately mycorrihzal fungi are not found in the
majority of urban situations, due to soil compaction and lack of organic matter.
On the other hand there are some species that can retain root hairs for months or even
years, but such hairs become relatively ineffective as absorption.
Urban tree roots can be affected and damaged both directly and indirectly.
Direct root damaged is usually caused by machinery and includes tearing, crushing, and
stripping, cutting and general exposure of the roots themselves. Indirect damage is often
caused by a change in soil conditions. Soil change can occur when the normal process of
gaseous exchange within the atmosphere of the soil is interrupted, which will result in a
lack of oxygen within the soil and an increase in Carbon Dioxide and other such gases.
Examples of indirect damage include soil compaction, changes in soil moisture or
drainage patterns and general soil contamination.
Root problems can occur even before a tree has been planted in its permanent setting.
Roots can often be damaged during transportation and during lifting whilst still in the
nursery. However the largest killer or urban trees is the indirect damage caused by soil
compaction.
A recent survey of aborists estimated that “40% of commercial and residential properties
had an area of significant soil compaction near trees” (E Thomas Smiley (n.d)
mhtml://D:\Treating soil compaction near trees.mht)
In a recent journal Dr Kim D Coder states that the health and structure of trees are
reflections of soil health. The ecological processes which govern tree survival and growth
are concentrated around the soil or root interface. As soils and associated resources
change, tree systems must change to effectively utilize and tolerate changing resources
quantities and qualities, as well as the physical space available. Soil compaction is a
major tree limiting feature of community forest managers and arborists. (Dr Kim D Coder
1995. http://warnell.forestry.uga/edu.service/library/for00-003/node1.html)
In the urban environment trees are never far from soil compaction. Soil compaction
occurs by the simple act of applying pressure on the soil. Exerted pressure compacts soil
particles together eliminating pores spaces within the soil which drives out oxygen;
thereby creating a hardened environment in which roots struggle to maintain their
biological processes and systems and are ultimately unable to support the tree.
Once the marcopores in the soil have been crushed or lost completely the roots are no
longer able to penetrate the soil to any degree. Compacted soils are also prone to
droughts, pores that would normally hold pockets of air after heavy rain, fill with water.
Compacted soil is unable to do this and cannot retain sufficient amounts of water.
Compaction will also reduce ground cover vegetation such as grass. This can lead to
crusting of the soil surface and development of a hydrophobic layer that further hinders
water intake into the soil.
According to E Thomas Smiley in his website: “Treating soil compaction near trees”;
compacted soils can often have a lower essential element and organic matter level.
Organic matter within the soil is part of the building blocks that hold soil particles
together in granules which help resist compaction.
Soil compaction affects trees by disrupting the respiration process that fuels every
biological function of a tree. A trees defensive triggers, growth functions, food storage
and movements, and all life processes are disrupted. The actual structure of the tree can
be damaged by soil compaction. Root decline and death can cause massive failure within
a tree. Cells can die and the compartmentalisation (c.o.d.i.t) that follows can cause
mechanical faults. Reaction wood is laid down and root collar taper occurs. Canopy and
root plate die back can also be prelevant in a tree affected by soil compaction. (Dr D
Coder. Soil Compaction and trees: causes, symptoms and Effects. 1995) All of the above
will cause a tree to become severely stressed and unable to cope with the condition
created by compaction which leads to a tee rapidly falling into decline and ultimately,
fail.
Soil compaction can be caused by many different agents. It has even been known that
trees can cause their own downfall. In some instances, the sheer weight of certain species
when reaching full maturity, or becoming over mature, pushes the tree down and creates
pressure and compacting the soil. However the most common causes of soil compaction
in the urban environment is that of people and development work. Any hard surface such
as car parks and pavements cause soil compaction by blocking oxygen and moisture entry
into the soil. Constant traffic from vehicles and pedestrian traffic on hard surfaces further
hardens the soil, destroying the pore spaces in the soil, creating a harsh environment for
roots to grow. Even in areas where there is no man made surface such as parks there is
compaction. Constant use of foot paths and desire lines that run near trees can have
effects on roots. Clear examples of this can be seen in a number of trees in Bovey Park,
Bovey Tracey, Devon.
Add pictures
These photos show a heavily used foot path in a town park. The foot path runs alongside
trees and excessive wear has exposed the roots of many of the trees and the ground is
heavily compacted. This is known as the “bistic factor.” The poplar nearest the path has
failed to flush fully and shows clear signs of decline.
Construction and development sites are also a major problem for the urban tree and soil
compaction. Healthy vigourous trees can become stressed and will succumb to the affects
of compaction, caused by a constant stream of heavy goods traffic and construction
techniques used on site. Construction and development work is probably the most
harmful and damaging activity that a tree can face in and urban setting. The effects of
construction on a tree can be swift, destructive and extremely harmful. The full extent of
the damage caused by compaction may not become apparent until some time after
development has completed. Figure … depicts the process of soil compaction and it
effect on trees. (D.B=soil density)
Add diagram
The full extent of soil compaction can be measured. One of the most common methods
used to measure soil compaction is to use a penetrometer. A penetrometer is a probe that
measures resistance when pushed into the ground. (mhtml: file://D:\soil compaction facts
(nd)) Another method of measuring soil compaction is by measuring the change in the
soils bulk density. Bulk density is the weight of the soil per unit volume (usually
measured in g/cc). As bulk density increases, total pore space declines and aeration pore
space is destroyed. In one soil for example, 20% increase in bulk density initiated a 68%
loss of aeration pores and an increase in 7% capillary pore space. Bulk density as a
measure of soil compaction rapidly increases with the first few impacts on the soil
surfaces, then levels off. Soils can be compacted to 90-95% of their maximum
compaction in as little as 3-4 trips over a single site. “In other words, it is not years of
traffic , but the first four trips that does the majority of the compaction.” (Dr Kim D
Codar (n.d) http://warnell.forestry.uga.edu/service/library/fr00-003/node12.html)
Soil compaction is everywhere. Where ever a tree encounters the human environment it
encounters the human environment it encounters compaction. Compaction is an
unavoidable part of urban expansion and there are very few places left where a tree is
able to grow in a habitat that is completely free from some form of soil compaction. Even
in a natural “forest” environment there is to some extent soil compaction. Compaction is
a natural process. It has only become unnatural because we, as humans have allowed it to.
Knowing what compaction is and knowing what causes it is the first step to preventing,
alleviating and treating it. It must be noted at this point, that once soil compaction has
occurred and a tree and it’s root system has become affected, it is not possible to revert
the soil back to it’s original, natural biological state. However there are ways to help
alleviate the problem.