Fact or Fad
an article by Lawrie Metcalf AHRIH
From the New Zealand
Garden Journal (Journal of the Royal New Zealand Institute of Horticulture),
Volume 5, Number 1, June 2002, pp. 10-11.
Among those keen on growing
native plants, particularly for regeneration and similar projects,
the term "ecosource" has become quite a "buzz word". But what does
it mean? Quite simply, it refers to plants that have been sourced
and propagated from those that grow naturally in the local area,
so that they can be used for replanting somewhere in that area.
If, for some reason or another, there are few or no suitable species
actually growing in that area then the usual practice
is to obtain propagation material from the nearest available source.
the reasons for ecosourcing plants?
Firstly, it is generally
supposed that because certain plants occur naturally in a particular
area or region, that they must be the forms best suited for that
area or region because they have evolved there. Therefore, it stands
to reason, so the supposition goes, that plants propagated from
local sources must automatically be the best ones to plant in that
area or region.
Secondly, there is the
"dreaded" problem of what is known as "genetic pollution". By bringing
in plants from outside the local area they are going to cross with
the local forms, thereby upsetting the whole gene structure of the
local populations. Actually, "genetic pollution" has also become
another "buzz word'' phrase.
Let us look at the common
supposition that, because plants have had hundreds or maybe even
thousands of years to adapt to the local climate and conditions
that they must be the best ones for that particular area. In some
instances that may be true, but in others it is not necessarily
so. Have you ever noticed how, in some districts, particular species
of native plants never look completely happy. They may have sparse
growth and lack vigour, or may be prone to the attacks of a particular
pest. Presumably they have had hundreds of years to adapt to local
conditions and yet their growth is not what might be expected of
a plant that is supposed to be so well adapted to local conditions.
Plants brought-in from outside may sometimes outperform plants of
local origin. For example, during last summer's (2000-2001) disastrous
drought many trees of Pittosporum eugenioides all around
the Nelson area were showing signs of stress. In my own garden P.
eugenioides that I had brought up from Southland, apart from
a few fallen leaves, which is quite normal in dry conditions, showed
no signs of stress whatsoever. I hasten to add, that one specimen
in particular is in the driest part of the garden where it has a
great deal of root competition and yet it appeared to be quite happy.
Even local plants do not always fare well in times of climatic diversity.
In any case, my own observations have been that local plants of
Pittosporum eugenioides do not appear to show any noticeable
superiority over those from other sources.
"Genetic pollution" is
another buzz term that has gained quite a lot of currency. It is
a term that strikes fear into the hearts of some. Firstly, there
is a widely held assumption that different clones of a species must
be genetically distinct. For example, those who hold this to be
true, automatically assume that Pittosporum eugenioides from
Marlborough, for example, must be genetically distinct from the
same species obtained from other localities. It is very easy to
make such assumptions, but the truth of the matter may be very different.
Pittosporum eugenioides genetically distinct from Nelson-sourced
P. eugenioides, or are they just adaptations to their local
areas? The only way to be certain would be to carry out DNA tests
on the 7 various clones to find out if it was so and I am not certain
that any such work has actually been done. Just because one clone
has a different appearance from another, it is all too easy to assume
that the two must be genetically distinct. Genetically, there may
be no noticeable difference. It may well be that they are nothing
more than just climatic or habitat variations. The case has yet
to be proven.
In any case, have those
who vigorously defend the theory of genetic purity never heard of
hybrid vigour? It may well be that the out-crossing of a species
of local origin with a clone from another locality may not be the
bad thing that we are told that it could be. Such a cross may well
produce a plant that is more vigorous and better adapted to the
local conditions. Therefore, introducing different clones, even
if they are genetically different, may not necessarily be a bad
This so-called genetic
pollution already exists on quite a wide scale over large areas
of the country. Just to give an example, let us consider Riccarton
Bush, in Christchurch. Riccarton Bush is an isolated pocket of the
ancient swamp forest that once covered parts of the Canterbury Plains,
particularly in the vicinity of Christchurch. With the passage of
time it has become completely surrounded by suburbia. Especially
in the older gardens that bordered the bush there was always a strong
accent on the planting of native plants. Presumably, most of those
planted were obtained from local nurseries and were of unknown provenance.
Certainly, many probably originated from sources outside of the
These older gardens actually
back onto the boundary fence of the Riccarton Bush, and after a
period of 70 years or more, the native plants in those gardens have
had ample opportunity to cross with the same species that grow over
the bush fenceline. Pittosporum tenuifolium, P. eugenioides,
Myrsine australis, Podocarpus totara, Plagianthus regius, Coprosma
robusta, Cordyline australis and Sophora microphylla are
just some of those growing in adjoining gardens. In my association
with Riccarton Bush I cannot say that I have ever noticed any obvious
ill effects from the crossing of all of these "foreign" clones with
the species naturally growing in the bush. I would venture to state
that probably nobody else has either. And yet when any regenerative
planting is carried out in the
bush, those responsible take pains to ensure that such plants are
sourced from mother stock growing within the bush.
Do the birds and insects
that live in the bush voluntarily confine themselves to the bush
and not venture into neighbouring gardens in search of food? I feel
that any honest person would have to agree that they do not. Therefore,
the chances of so-called "genetic pollution" occurring must happen
I think that it would
be true to state that so-called "genetic pollution" has been going
on since time immemorial. Regardless of what gardeners and scientists
may feel, plants have been doing their best to seek pollen from
sources other than their own. Think of all those species that have
wind pollinated flowers. They do not set up barriers to prevent
their flowers from being pollinated by pollen from the adjoining
valley, the other side of the mountain range, or from an off-shore
I must add that where
a revegetation project is to be undertaken in a natural area, such
as a national park, scenic reserve or similar type of situation,
I feel that it is preferable that all material planted be propagated
from locally sourced material. It is only when people are undertaking
planting on farms, in their own gardens, maybe even in towns and
cities that I feel that insistence on using eco-sourced material
really has to be queried. To conclude, it is yet to be proven that
local variants of most species of plants are so genetically distinct
that they should be kept segregated from each other.
of wind pollinated plants?
of eco-sourcing also apparently overlook the effects of wind
pollination on genetic purity. Pollen is known to travel great
distances, as evidenced by the fact that podocarp pollen is
found in Chatham Islands peat deposits and yet podocarps do
not grow on the Chathams. The Chatham Islands lie some 800
kilometres to the east of mainland New Zealand, which gives
an indication of just how far pollen can travel. Therefore,
it is ludicrous to imagine that, for example, podocarps growing
along the eastern side of the country are going to be pollinated
only by those growing locally, thus preserving their genetic
purity, and are going to reject the vast quantities of pollen
that blows across from western districts, every time there
is a westerly air system. The simple fact of the matter is
that for thousands of years they have been pollinated from
whatever pollen happens to blow their way.
and bird pollinated plants
travel up to about five kilometres to collect nectar and to
gather pollen. It is not inconceivable that, as a result of
their foraging activities, they could easily transfer pollen
from a supposedly different clone of a particular species
to another clone of the same species in an adjoining locality.
Similarly, birds have the ability to travel great distances
from one food source to another. Those native birds that so
many people are actively trying to attract to their properties,
by planting suitable food plants, do not have any qualms about
travelling to whatever food source is available. They are
not going to say that because nearby areas may contain genetically
different clones that they will not go and feed off them.
Tuis and bellbirds, in particular, can be quite migratory,
ranging far and wide to visit particular food sources when
they are in season.