Exotic Vegetation

Clarens News Exotic vegetation in Clarens

During the mid nineteenth century the first English settlers in the Eastern Free State planted two ornamental tree species that have since become synonymous with this area.  Few brochures or paintings of the area exclude images of the weeping willow or the Lombardy poplar; such is the attention that both photographer and artist give these attractive trees.  Both trees have also received significant attention from the Department of Environmental Affairs and appear in the publications concerning alien and invasive tree species.

Our photograph this week shows a cluster of cosmos flowering blithely against the backdrop of a high ridge on the Rooiberg range which overlooks Clarens. In the late summer months these resilient flowers enrich our roadsides with their shades of pink, red, mauve and white, their distribution extending from the midlands, through the northern parts of KwaZulu-Natal, the Drakensberg foothills, the Eastern Free State and many parts of the highveld.  The story of their introduction to South Africa is well documented.

During the Anglo Boer War the British imported bags of horse-feed from Mexico and these turned out to be contaminated with cosmos seeds, a plant indigenous to central America.  All along the routes that the British soldiers rode the cosmos seeds fell and, for more than a century since, these flowers have bloomed reliably, year after year.

Cosmos, closely related to the sunflower, is listed as an alien plant species in South Africa.  Once exotic plants become ‘naturalised’ and able to propagate and establish themselves in natural surroundings, their impact or potential impact should be assessed and, where necessary, management procedures put in place.

The weeping willow is a Category 2 Invader Plant and is subject to a set of management criteria that should be followed in the event of weeping willows being retained in an area as opposed to being eradicated from that area.  SANParks conservation management at Golden Gate has opted to retain a weeping willow population in a controlled area for its aesthetic value and historical significance, and is thus obliged to employ the prescribed management criteria to secure its isolated status.

The Lombardy poplar is listed as an ‘invasive threat’, which loosely means that its impact is on the increase and that it is subject to ongoing research and monitoring.

There are many other trees, some growing in the Clarens area, that are deemed equally threatening, or more so, than our willow.  Examples are the eucalyptus, pine and wattle, where certain species of wattle are listed as Category 1 Invader Plants, which means that they should to be removed and destroyed immediately.

But why all the purist fuss about alien species?  In the main, it is because of the threat they pose to the integrity of our natural biodiversity and because of the ongoing impact they have on our limited water resources.  For this reason invasive plant legislation and related management protocols have been developed.  Alien vegetation displaces indigenous vegetation, which in turn impacts on and displaces other, often mutually dependent, indigenous species, which could include animal species.  This, unchecked and on a large scale, impacts negatively, and quite significantly so, on our natural biodiversity.  Often exotic trees have evolved in climates with far greater water availability and therefore require a greater intake of water to survive than do our indigenous species, resulting in the unaffordable loss of water.

Southern Africa is largely a warm dry region with low precipitation and high evaporation.  There is a high demand for water and a low supply of it.  Typically, periodic storms result in much of the annual rainfall occurring during heavy downpours, with long dry, sunny spells in between, resulting in potentially high runoff followed by high evaporation rates.

If you look at the lower reaches of the mountains in our area you will see the thickly vegetated watercourses, in the folds of the slopes, descending into the valleys.  This natural vegetation is well adapted to our conditions and the short scrubby bush and tightly knit undergrowth stabilises the watercourse bed and banks and is a natural filter in the water catchments, retaining a high level of moisture.  They thus also act as natural fire barriers.

Colonies of willows, or indeed other alien species, along our streams and riverbanks, alter how our streams and rivers function.  The exotic species will draw an excessive amount of water and nutrients from the soil, thus compromising the growth of the natural vegetation.  In times of heavy rainfall and accelerated flow, the river margin runoff coupled with conditions of exposed and unfortified soil, results in soil erosion along the banks, as well as river silting.  This in turn reduces the carrying capacity of the river, resulting in even further damage, including damage further downstream.

In permitting the current levels of human and human introduced impacts on the environment to continue, we further sacrifice the quality and integrity of our natural biodiversity; in giving priority to aesthetics and sentimentality, even apathy, we endanger our limited natural resources to an even greater degree.

These are the difficult dilemmas faced by the environmentalists, who must find the right compromise and balance – to satisfy a population with a range of demands, and to secure a sustainable future for their children.

And what of the cosmos?  Thus far these bold and cheerful little flowers have not appeared on any lists that prescribe specific management criteria.  In all likelihood they will continue to flower on our roadsides and veld paths, in quiet tribute to those British soldiers and their horses of so long ago, for multitudes of summers to come.

The picture insert features in the 2014 calendar produced by and sold in aid of Cluny Animal Trust.  Calendars can be purchased at Clarens Gallery, Clementines Restaurant and the Old Stone Bottle Store, in Clarens.  Alternatively they can be ordered from Katherine on 0827886287, Jan on 0782462553, Helen on 0582230918 or by email tojansander22@gmail.com

Mary Walker

 

Article and photograph by Mary Walker
Clarens News: September 2013
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