THE USE OF SYCAMORE IN AGROFORESTRY JUSTIFIED?
by P. Binggeli published in 1995 in Agroforestry Forum 6(2), 2-3.
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.
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.
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.
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.
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
- 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.
grassland the species performs poorly when young as it does not
compete well with grass. The base of the tree must remain free of
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.
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.
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.
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.
Tree and shrub seed price list 1993-1994. Forestart, Shrewsbury.
Z. & Sinclair, F.L. (1993) Establishment of the Silvopastoral
National Network Experiment site, Henfaes, Bangor. Agroforestry Forum