IS THE USE OF SYCAMORE IN AGROFORESTRY JUSTIFIED?
by P. Binggeli published in 1995 in Agroforestry Forum 6(2),
2-3.
In the British Isles sycamore (Acer
pseudoplatanus L.) is one of the main species used in an
experimental set up to promote agroforestry as an alternative form of
land use. Trees are a major component in this system which interact
with a number of others including climate, soil type and grazing
animals. All components in the system must either supply a product,
which is useful and/or valuable, or enhance other components of the
system. It is necessary that the system is economically viable in the
long term.
When an agroforestry system is
proposed for a particular region where it has not been major part of
the land use like the British Isles, every component should be
carefully assessed before experimental plots are set up. One component
is the choice of tree or shrub species to be used. Assuming the purpose
of growing trees in agroforestry is the production of merchantable
timber then it is necessary to determine whether a particular European
broadleaf species will supply quality timber within a required period
of time. This is relatively easy as much is known about European
broadleaves since they have been investigated in much detail for over
100 years. Undoubtedly, language difficulties restrict access to this
information.
An assessment of the agroforestry
programme in the British Isles is not intended, rather I want to
appraise the value of sycamore as an agroforestry tree and particularly
in relation to the Silvopastoral National Network Experiment. I assume
that the reason for using sycamore is to produce quality timber and
ultimately use the species as an important source of revenue for
farmers. There does not appear to have been any detailed assessment as
to why sycamore should be used in the agroforestry network prior to the
silvopastoral scheme being set up. Thus it is not entirely clear why
the species was chosen in the first place although Teklehaimanot &
Sinclair (1993) state that sycamore "was chosen as the reference
species for the National Network because it was suitable across the
range of site types represented in the experiment". The common
occurrence of the tree around farmsteads throughout most of the British
Isles in conjunction with the species reported fast early growth rates
may be the main reasons for its suitability.
My assessment of the species is based
on its biology and ecology with particular emphasis on tree growth and
architecture. Below I give the reasons why sycamore, as currently
planted, is not a tree which can be used in agroforestry systems.
- tree architecture: tree growth and
architecture are strongly affected by a number of environmental and
developmental factors including wind, frost, salt, competition and
flowering. Wind and frost, particularly in open and semi-open
situations, frequently kill terminal buds and even shoots which induces
stem forking. Flowering also induces stem forking and reduces extension
growth rates (young non flowering trees can grow more than a meter a
year under appropriate conditions whereas heavily flowering trees grow
no more than 10 cm per year). When inter-crown competition is limited
or absent trees start flowering at a very early age. In open situations
trees also lack apical dominance and many branches try to become
leaders. As a result trees in hedges and in the open are relatively
small with low canopies and large lower branches. The resulting stem
and crown architecture are ideal for the study of the tree's
reproductive biology but the opposite to that required for timber
production. The amount of timber available in most of these trees is
limited to a few meters of stem above ground level.
- In the British Isles little or no
attention is paid to the source of the sycamores planted. It appears
that all available planting material used as somehow it is assumed that
all sycamore trees are the same. Sycamore has traditionally been
imported from northern continental Europe and current provenances of
sycamore seed from Forestart are from England, Germany and Hungary
(Anon 1993). It is likely that most of the sycamore planting undertaken
in recent years is from provenances different from that of the mature
trees found in the British countryside. Considering the large variation
reported in budding time and increment growth between provenances the
current planting practices can be described as indiscriminate.
- In grassland the species performs
poorly when young as it does not compete well with grass. The base of
the tree must remain free of vegetation.
- Optimum growth of sycamore only
occurs in moist, fertile and nutrient rich soils. Elsewhere sycamore
will grow but the growth rates will be too small for a profitable tree
crop.
- Sycamore is highly susceptible to
grey squirrel damage. Many areas are still free of grey squirrels, but
it is only a matter of time before they spread to the whole of the
British Isles. Where grey squirrels are present they may cause a lot of
damage (debarking) to trees often resulting in crown dieback. This
results in poor stem quality and probably lower growth rates.
The criticisms made above also apply to the use of
sycamore in forestry. However in forestry trees grow in stands where
they are strongly affected by competition. Competition and the use of
appropriate silvicultural practices can alleviate some of the
sycamore's poor growth habits and improve growth rates. This is much
more difficult to achieve in an agroforestry system, where crown
competition is minimal. Regular pruning should help, but this is both
time-consuming and expensive and may result in much sprouting.
This negative assessment of the value
of sycamore as a tree for agroforestry systems does not mean that the
species should be discarded. In fact sycamore has great potential as a
timber tree in forestry and probably in agroforestry too. Sycamore is a
highly variable species which is generally poorly understood despite
being one of the best investigated European broadleaves. Unfortunately
it has received limited attention from tree breeders and the potential
for selecting and breeding of fast growing sycamore with high quality
timber is untapped.
In agroforestry, like in any system,
it is essential to understand the individual components (i.e. tree
species) prior to investigate how they interact. Somehow research is
carried out in the opposite manner where the individual components are
only looked at in detail when the system fails to deliver the expected
goods.
References
Anon. (1993) Tree and shrub seed price
list 1993-1994. Forestart, Shrewsbury.
Teklehaimanot, Z. & Sinclair, F.L.
(1993) Establishment of the Silvopastoral National Network Experiment
site, Henfaes, Bangor. Agroforestry Forum 4(2), 18-21.
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