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Eucalyptus and Mimosa – Portugal's Australian problem

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A typical cultural landscape in Coimbra district. The distant hills would be about 90% eucalpytus
Just had two weeks in central Portugal, near Coimbra. The Iberian peninsula is not somewhere I am that familiar with, but would increasingly like to be. It is home to some 6,000 native flowering plant species, scattered over an amazingly wide range of habitats. A brief foray into Spain last spring made me feel very optimistic about spending more time in the region. Central and northern Portugal however been a bit of a reality check. It seems to be home to one of the biggest accidental experiments in ecology I have ever seen. One which looks disastrous and which has had amazingly little publicity, at least outside the country.
The trees in the rear include cork oak, some pine but also the characteristic cones of middle-aged eucalypts. The trees appear in many village contexts not just in formal plantations in the hills
The problem is Australians. Not the people I hasten to add, but eucalyptus and Acacia dealbata – the familiar mimosa and Acacia melanoxylon. And a New Zealander – Pittosporum undulatum, and increasingly the South American Cortaderia selloana – pampas grass. I have, as many of you may be aware, often been pretty sceptical about much of the currentdiscussion of invasive aliens. I have always felt that people in Britain who worry about impatiens or Japanese knotweed have very little idea of the damage that really invasive aliens can do; and that many ‘invasives’ are actually not so bad. Increasingly there is evidence that alien species can even play a positive role in the development of novel ecosystems. Portugal is a good example of where things can go really really wrong, but also just how complex these issues are.
To start with the deliberately spread alien, the eucalyptus, mostly E. globulus. Almost any vista in the region between the mountainous east and the coast, north of Lisbon, that we drove through included it, in vast quantities, the distinctively bunchy growth of the outermost branches being particularly conspicuous in silhouette. Almost all the hills are covered with it – nearly all planted as a forestry crop for the paper pulp industry, although it also has some capacity to spread by seed too. The story is that much of this region has granite or other acidic soils, and is not much good for the pastoral agriculture that one might expect in hilly regions, or indeed for cork oak, which is a major form of land use in the warmer and more calcareous south which the tree prefers. Historically, these hills were dominated by oak and chestnut but centuries of deforestation resulted in them being covered in scrub: gorse, heathers, cistus and suchlike. Economically pretty useless. Pine was often planted or spread naturally. But during the 20th century eucalyptus was introduced and promoted under the Salazar regime (always nice to have a fascist dictator to blame!). The paper pulp industry continues to promote planting the tree. The result is an oppressive monoculture, which with the decline of the pulp industry (now moving to South America), is going to be increasingly worthless. To say nothing of the fire risk, posed by this infamously inflammable tree. A eucalypt fire can turn whole landscapes to ash.
Eucalyptus is a controversial crop. One can’t blame poor rural regions for wanting to earn money from forestry. And in fact in terms of the big environmental picture it is actually a good thing. The vast area under the tree here must have soaked up a huge amount of CO2,done much to help reduce soil erosion and hold water in the ground. There is a widespread belief in much of the world that the trees dry the soil out, but in fact there is little evidence that this is the case. In very poor regions their presence can actually help protect native forests by being a superior source of firewood and timber, e.g Bolivia.
Eucalyptus plantations have been accused of being ‘green deserts’. This is not necessarily the case either, as from what I have seen in most unmanaged plantations is that amongst older trees there is extensive undergrowth in the form of gorse and heather or bracken (western Europe’s main invasive non-alien). The problem is that the trees themselves do not support any biodiversity, unlike native pines or even better, oak. One of their worst aspects is that they are more or less indestructible; fell them or burn them and they simply pop again from the base, getting way ahead of any pine or oak which might compete with them. Planting them has been an almost irrevocable decision. The result is a lifeless green coating over almost all the hills. It is as if the country has signed a Faustian pact with a malign fairy, who agreed to reforest it, but with a green monster which will never go away and supports no life.
Mimosa – notice how closely packed these young trees are – they stay like this with little competition between each other, suppressing all other plants

Worst still are the uninvited aliens, the escapees from ornamental cultivation which in the moist mild climate are seeding and spreading at an incredible rate. Mimosa is a particular menace in the central part of the country. It is the perfect example of the worst kind of invasive alien: rapid-growing, rapid-seeding, nitrogen-fixing and almost totally suppressing all other plantlife. Talking to locals in the area about Lousã it appears that in the last ten years they have spread along roads and streamsides to form a kind of foreground to the eucalyptus. They root into cracks in rock, into banks, over streep, into established maquis vegetation, and then grow incredibly densely. The shade they cast is so dark almost nothing will grow beneath them, killing off entire ecosystems. In many circumstances, these short-lived pioneer species would be replaced in due course by longer-lived canopy trees but there is so little here to seed into them that that is simply not going to happen. The danger is that the tree will become self-perpetuating, smothering what is currently the main refuge for much native vegetation along the eucalyptus forest margins.

The bio-desert beneath a mimosa canopy
 Environmental activists have long been warning about eucalyptus. There does seem to be a growing awareness, but once the tide of opinion has turned, it is going to be an almost superhuman struggle for a not very wealthy country to manage this gigantic and multi-faceted problem.

Links:
https://www.facebook.com/Floresta-Portuguesa-Sustent%C3%A1vel-Sustainable-Forests-for-Portugal-345039065588647/
 

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