Wood in streams – size really does matter
Brenda Baillie
New
Zealand Tree Grower February 2005
When the time comes for harvesting woodlots, most of us are aware of
the risks associated with leaving large amounts of logging slash in
streams. This is reflected in regional council and forest company
rules, which usually advocate the removal of stem and branch material
from streams after logging. What is often overlooked is the ecological
benefit of wood in stream ecosystems.
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Large woody debris storing
sediment, wood and finer organic material.
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Prior to the arrival of humans, much of the land was covered in
forests. Wood was an integral part of stream ecosystems, and
still is in remaining indigenous forests. So where is the balance
between managing wood in streams for risk, especially after harvest,
and the ecological benefit?
Ecological role of wood in stream ecosystems
What are the ecological benefits of leaving wood in streams? In
forested stream ecosystems, wood plays a multitude of roles in stream
processes and stream habitat. Wood-
- Slows down the stream flow, dissipating energy
- Helps aerate the water
- Diverts and deflects water flow
- Increases flow complexity
- Increases the number and variety of pools
- Protects bank edges
- Retains and slows down the movement of sediment, fine
organic matter, and twigs and leaves through the stream system,
increasing retention time for in stream processing by micro-organisms
such as fungi and bacteria and aquatic insects
- Provides food resources and increases habitat diversity for
native fish, trout, koura, aquatic insects and birds such the
endangered blue duck
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| Influence of wood on stream channels.
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What sort of wood is most beneficial?
A look at our indigenous forest streams can provide a few clues.
Factors contributing to the stability and longevity of wood include
species, piece size, length, abundance, position in the channel, degree
of burial and how well the piece is anchored. These key structural
pieces have the greatest influence on the stream ecosystem and are
often referred to as large woody debris.
Channel width also has a strong influence on the amount, distribution
and influence of wood in the stream channel. The wood loadings
influence on the stream system is higher in smaller streams. As the
stream width and flow increase, wood is more easily dislodged
and transported during floods, so volumes tend to decrease and the
average size of the remaining pieces increases. Less wood remains in
the main stream channel and more is stacked up along the bank edge.
It is important to remember that streams are not static systems. It is
a natural process for wood to fall into streams, where it gradually
breaks down, mainly through physical abrasion and decay as it is
transported down through the stream system, with some pieces eventually
ending up in lakes, estuaries, and along coastlines.
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| Contribution of large woody debris to
stream habitat. |
Wood in plantation forest streams
As plantation stands mature, windthrown trees often fall across and
into the stream channel. While they may not be as large as some of the
large woody debris found in native forest streams, these logs have all
the characteristics of the key structural pieces in native forest
streams. They are large, often straddling the stream channel, and
usually have a partially intact root system which increases their
resistance to movement. If left in place these pieces will gradually
decay and enter the stream ecosystem.
Prior to harvest, most of the wood in pine plantation streams is
composed of large woody debris, similar to what we find in native
forest streams. Because most of the large merchantable pieces of wood
are removed at harvest, a lot of smaller logs, branches and needles can
end up in the stream channel. Most of the wood is less than 10 cm in
diameter. This logging slash is very mobile during high flows and it is
common practice to remove this material from the stream system to
prevent it forming debris dams and causing damage downstream.
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| Large woody debris aerating the water
and forming a plunge pool. |
In the smaller, more stable streams, there is the potential to balance
post-harvest woody debris stream-cleaning practices. This can be done
by leaving behind large key structural pieces such as windthrown stems,
and removing the majority of the finer logging slash, achieving a
similar appearance and function to that found in native forest streams.
There is no cost to the landowner to leave these pieces in place and
there may even be cost savings if these pieces are
currently removed as part of routine stream-cleaning practices.
However, this should be managed on a site-by-site basis, and in
consultation with pertinent local authorities, taking into account
factors such as hydrology, flood risk, geology, and stream size, and
regional council rules. Some forest companies are already doing this.
Brenda Baillie works for the Centre
for Sustainable Forest Management, Forest Research
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