Alien Plant Clearing – The threat to the Cape Floral Kingdom
Alien plant clearing is necessary to protect the Cape Province.
The Cape Province covers 717 800 square kilometers, approximately 60% of the area of the Republic of South Africa. It has been estimated that the Cape Province as a whole has up to 10 000 plant species, and that the South Western Cape comprising only one tenth of the area of the Cape, contains more than half this number. Fernkloof Nature reserve only covers 18 square kilometers and boasts around 2000 plant species. Fynbos vegetation in particular is at risk from alien infestation. The natural vegetation becomes displaced as the invaders form dense impenetrable ‘jungle thickets’ and absorb up to 60% more water than indigenous plants. The beauty of the delicate indigenous forests and fynbos ecosystems are being supplanted by monotonous seas of exotic species.
Alien Invaders need alien plant clearing
A total of 13 Australian species were introduced to the Western Cape in the first half of the last century to help stabilise dunes in coastal areas and to provide a source of wood fuel. Unfortunately these species have spread prolifically and now pose a serious threat to the local indigenous fynbos vegetation. The principal alien invasives are wattles, eucalyptus and hakeas from Australia and pines from the Mediterranean Basin and California. These areas all have a climate very similar to parts of the Cape Floristic Region. Acacias (Mimosaceae) have become naturalized in South Africa and are now declared invasive weeds.
Aliens in the Overberg
The Overberg area has many alien plants, Australian myrtle, pines, black wattle, blackwood, kariba weed, long-leaved wattle, port jackson, rooikrans, sesbania, saligna or blue gum to name a few. In all parts of SA, alien plants are gaining a foothold in varying degrees and they start to compete directly with the indigenous vegetation.Aliens generally thrive in foreign lands because they don’t have the predators or control measures required to control numbers like they have in their country of origin.
Some Pine species were most likely introduced to South Africa by the French Huguents. A plantation was established at Genadendal in 1825 and the species has been used extensively for afforestation ever since. They have become naturalized in Mountain Fynbos and Lowland Fynbos vegetation groups and have the potential to spread throughout the Fynbos. Wind and baboon dispersal of seeds also plays a role in their spread as well as the rapid germination of seeds after fire. Dense stands establish within a few years as they bear seed from their 6th year. The natural vegetation becomes closed coniferous forest, with streamflow from catchments reduced, fynbos vegetation suppressed and in many cases eradicated.
Two of the most successful invasive plant species in the fynbos are Acacia cyclops (Rooikrans) and Acacia saligna (Port Jackson willow). They are exceptionally well adapted to local conditions, thriving on the nutrient poor soils of the Cape and proliferating after the regular fires typical of fynbos landscapes.
Port Jackson thrives on acidic mountain soils while Rooikrans is most at home on alkaline coastal soils.Acacia saligna or the Port Jackson willow has become one of the most widespread and destructive invasive species in the fynbos region is a very adaptable and fast growing species. It is a very difficult species to manage as it is able to resprout from its roots after being cut down and creates massive, long-lived seed banks. The difficulty and cost of eradicating this fast spreading species led to it being targeted for biological control measures.
Biological Controls used in alien plant clearing
A few years ago a multivoltine midge (Dasineura dielsi) that induces galls on the ovary of its host , was introduced from Australia for the biological control of Acacia cyclops in South Africa. It spread extremely rapidly and now colonizes virtually all mature Acacia cyclops in the region. By inducing the gall it stops the flower from forming thereby drastically reduces the seeds produced.
The gall forming rust fungus Uromycladium tepperianum has proven very successful at controlling reproduction and seed set in Port Jackson.The rust fungus is wind dispersed and is now found wherever Port Jackson is growing in fynbos and it has been found to lower population densities by at least 80% in the absence of fire. The rust fungus kills the pants after a few years by reducing the flowering and seed set on Port Jackson.
These two biological control agents have proven very useful in the battle to control alien invasive species in the fynbos, as part of alien plant clearing. Because cluster and radiata pines are an important commercial forestry tree in South Africa no biological control in use and these are manually removed by hacking teams.
Mechanical Controls used in alien plant clearing
During the weekly botanical walk, on Wednesday the 10th August we covered the “Red” route offered on the Fernkloof hiking guide, and noticed quite a few alien Pine trees growing on the western slope of Fisherman’s Kloof.
One of the designated Fernkloof field rangers Morgan Hiki put his hand up and volunteered to help cut down the Pines growing in Fishermans Kloof, so we did a bit of Alien Plant Clearing. Because Pine Trees do not coppice, it is enough to chop them down as close to the base as possible ensuring that all growth buds and stems are removed, to ensure that they don’t re-grow.
Morgan and Ralph from the botanical society left the visitors center at 08h30 on Thursday 11th August armed with an axe and saw. The first task of the day was to chop down the big Pine (mother) on the crest of the western slope that was most likely the first tree that that was responsible for seeding all the others. We reached the crest of the westerns slope of Fishermans Kloof in just over one hour, and it took 30 minutes to chop the “mother” down. 100m further along the crest we found fairly big Pine not visible from the path below. Once at the crest it was easy to see many more small Pines that were also not visible from the “Red route” path. In total 14 alien Pines were removed that day and we noticed another 6 to 10 on the northern slope of Fishermans Kloof which will be removed at a later date. The work continues.
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