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Protecting our precious places

Can natives be weeds?

By Charlie Palmer

This article is not so much a 'tips and techniques' treatise but a comment on an issue that managers of natural ecosystems are increasingly asked to deal with. It is a vexing issue with a philosophical basis that is not easy for many people to find a definitive answer to. I have always been intrigued by the anomalous value judgements that are placed on plants in natural ecosystems. 

The term 'weed' is of course subjective. A plant that one person finds abhorrent and thus regards as a weed will be valued by someone else. The situation is not made any easier by simply regarding introduced or exotic plants as weeds and natives as non-weed or desired plants. 

Divergence of opinion over the value of manuka and kanuka illustrates the issue well. Many regard these native species as being highly valuable for their role in forest dynamics. They are good colonisers of open or disturbed areas and prepare the environment for other, more long lasting, forest species by providing shelter. However, to a farmer in dry hill country the invasive growth of manuka and kanuka robs them of valuable pasture. Thus these scrub species are undesirable.

Clearly the native/exotic division is not a suitable means by which to classify plants in natural ecosystems as weeds. The scientific community has an answer to this problem.

However, as is usually the case it is not a particularly practical one - it is more a matter of definition. 

Scientists regard plants as weeds not through value judgements of their desirability but by their characteristics. Generally plants with high reproductive ability (seed or fruit output), ability to germinate and establish in a variety of environments, fast growth and often, short lives are called weeds.

This definition clearly includes many native species - a situation many people will find unsatisfactory. Not only may manuka and kanuka fit into this definition as weeds, but so do other plants that are desirable in natural ecosystems; tauhinu (or cottonwood), tutu, wineberry and fuchsia to name a few of the woody native species that could be regarded as having 'weedy' characteristics.

Unfortunately, the scientific definition does nothing to solve the problem of what to do about native species in natural environments that are showing weedy tendencies. Does the fact that karaka, a species planted extensively as a food crop by Maori and now invasive in many forest areas, mean that it should be controlled in the same way as exotic species such as elderberry? 

Maybe the answer lies in examining the causes of the infestation - and reconciling ourselves to accept that not every forest will look diverse and have random distribution of tree species. 

In the case of karaka, the ability of the seeds of this species to germinate and grow in the deep shade of the parent means that it often forms dense groves that exclude other species. What might from the outset look like an infestation is in fact a natural part of the life history of this tree.

In a similar way, nikau palms often form dense groves that can look like they are taking over the forest. This situation is nothing new for those who are familiar with some beech forests, where the diversity of species is very low, and beech forms a canopy unbroken by the intrusion of any other species - similar in many ways to an exotic pine plantation.

Confounding the issue is a native climbing vine, Muehlenbeckia, which is encouraged by conditions to its liking in some forest remnants and wetlands and is smothering huge areas. 

Quite how to deal with this species is not only a philosophical problem, but also a practical one. The factors that have encouraged this growth are unknown, and may in fact be different in different areas. Thus the problem of preventing such infestations is a dilemma.

The practical task of controlling infestations is as mammoth as any exotic climbing vine can create - destruction of trees under the vine, extensive areas and revegetation. 

This of course all leaves aside the philosophical issue of whether or not it is desirable to actively control a species that is native to the area and simply responding to conditions that suit it very well.

Open SpaceTM Magazine No. 47, March 2000 © QEII National Trust


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