You will hear it said that there is as much of a tree below
the ground as there is above. In some regards there is a truth to that. In fact
most tree roots grow in the top 60cm of soil. The roots of a tree are essential
to the health and stability of the tree. They draw up water and minerals and
provide anchorage and storage for the tree.
Roots also have requirements of their own. Being below
ground they lack the capacity for photosynthesis and therefore cannot produce
their own carbohydrates that have to be transported down from the leaves via
the phloem vessels just under the bark. They also respire, which means they
need oxygen. The oxygen comes from the air and makes its way to the roots
through pores in the soil which allow gaseous exchange through pores in the
roots called lenticels and along channels in the wood called medullary rays
(the prominent lines you can see in Oak timber). This is why you will rarely
find tree roots below 60cm. The oxygen just doesn`t diffuse that far.
Tree roots also rely on certain types of micro organisms to
help them with their job. The carbohydrates that are transported down from the
crown don`t all get used by the tree. Some pour out into the soil where they
are used by other organisms. In particular a type of fungus known as mycorrhizae
are essential to the health of the tree. Mycorrhizae (meaning fungus roots) are
a wide variety of fungi that have evolved alongside trees. Essentially they
grow either into the trees roots (endomycorrhizae) or form sheaths outside
(ectomycorrhizae) and they benefit from the tree`s carbohydrates whilst the
tree benefits from their greater ability to take up minerals from the soil. Mycorrhizae
are essential to the trees long term health.
The area where tree, soil and microorganisms interact is known as the
rhizosphere.
Rhizosphere
In the natural woodland environment trees will shed branches
and leaves which are broken down by various saprophytic (dead eating) fungi,
bacteria and animals. This increases the amount of organic material in the
soil. Organic matter helps to stabilize soil Ph, increases the nutrient holding
capacity of soil it also increases the water holding capacity of free draining
soils (sand, chalk etc) and increases the draining capacity of soil prone to water
logging (clay). In short it enhances soil texture and in the longer term, with
the help of small animals, enhances the structure. The other result of having
lots of little creatures moving around in the soil is they create channels for
gaseous exchange.
Over millions of years trees have evolved alongside the
creatures that live around them for mutual benefit.
In the garden environment soil can often be lacking organic
matter. Foot traffic and vehicle traffic (mowers etc) cause soil compaction
which crushes the channels in the soil that tree roots rely on for water and
air to reach them. Fertilizers and pesticides interfere with nutrient levels
and poison the tree. All this can make for a pretty tough environment for the
tree to grow. Next to roads or driveways can be far worse. Soil compaction can
be a significant problem and salt from winter gritting can cause serious issues
for tree and soil organisms alike.
A simple organic remedy
Using mulched wood chip can significantly enhance the soil
conditions over longer periods of time. It`s not the quick fix that fertilizer
and mycorrhizal injection offer nor does it deal with compaction as quickly as
using an air spade but in the long run it can provide significant benefits to
trees, shrubs and general soil health. A layer of mulched wood chip spread over
as much of the root crown as is possible and maintained at about 75mm deep will
almost certainly improve the health of your trees in the long term.
Won`t that encourage fungus into my garden?
A healthy tree should
be able to resist parasitic fungi. Healthy soil will contain a wide variety of
saprophytic and mycorrhizal fungi which compete with opportunistic parasitic
fungi like Honey Fungus. This is not a miracle cure, but the miracle cures
rarely retain their miraculous status for very long. This is based on good soil
science, trying to recreate the conditions that trees experience in the woods.
For more information please visit Tree Surgeon Cheltenham
