Beefwood trees – a blessing or a curse?

I am an engineer and not a botanist, but the nature of my work brings me into close contact with many Western Cape Rivers. My recent observations while working to stabilize rivers, that the spread of Beefwood trees (Casuarina cunninghamiana) in Western Cape  rivers is increasing at an alarming rate, and I see it as a new threat to the stability of these rivers, and has motivated me to write this blog.

Beefwood trees have become a popular choice for windbreak tree amongst farmers in the Western Cape over the last 20 years. When I started working as Soil Conservation Engineer for the Department of Agriculture 30 years ago, Dutch Alders and various pines were common windbreaks. The reason for the change is probably because the beefwood grows more quickly, and when they are planted close together the form a very effective windbreak.

Part of my role as Soil Conservation Engineer was to oversee the design of agricultural sub-surface drainage. Once beefwoods made their appearance, we quickly learned that buried drainage pipes had to be kept at least 20m away from beefwood trees, otherwise the roots would rapidly invade and block the pipe. This is one of the dis-advantages of beefwood trees – but not the only one. As Soil Conservation Engineer I had realized early in my career that one of the most serious threats to the soil of commercial farmers was the erosion of riverbanks during floods. In the Western Cape often the best agricultural soil is the deep alluvial deposits in river flood plains. One of the factors driving the un-natural erosion of river banks (river instability) is the presence of woody alien vegetation on the banks or in the river itself. These trees disturb rivers by creating blockages (flood debris packs against the rigid trees) and redirecting the flow of the river into the river bank. Alternatively the trees get washed out and leave large patches of soil without vegetative cover. In both cases the alien trees cause erosion in the rivers which leads to river instability.

For long I (and many others) were under the impression that Beefwood seeds are sterile and they will not spread to rivers. This is definitely not the case. In the last ten to fifteen years I have seen a marked increase in the numbers of beefwoods growing wild in rivers such as the Riviersonderend, Breede, Buffeljags, Duiwenhoks and Goukou, and I foresee that where the Black Wattle and Port Jackson Willow have been linked to a lot of the flood damage along rivers from the early 1990’s till the 2012/13 flood, the Beefwood trees will start playing a significant role when future flood damage is experienced.

Beefwoods growing wild in the Duiwenhoks River

A very interesting and insightful paper about the spread of Beefwood trees in the Western Cape by Potgieter, Richardson and Wilson (2014) titled “Casuarina cunninghamiana in the Western Cape, South Africa: Determinants of naturalisation and invasion, and options for management” can be found on the following link: –https://www.sciencedirect.com/science/article/pii/S0254629914000416

Three important recommendations that they made were that: – 

  1. Only male plants should be sold from nurseries.
  2. Any female Beefwood trees already planted closer than 100 m to a watercourse (and I would add that the definition of watercourse should include “drainage channels leading to a watercourse”), need to be removed.
  3. A national strategy for controlling the spread of Beefwood trees needs to be implemented.
The seed pods that identify the female tree

The Soil Conservation Act (Act 43 0f 1983 CARA) also addresses the issue of Beefwood trees.

In the Regulations of the “Soil Conservation Act” – (GN R. 1048 dated 25 May 1984), a list of declared weeds is given (Table 3). The table refers to 3 categories of weeds and this indicates how they must be treated. Category 1 weeds are totally forbidden. Category 2 weeds may be allowed under strict conditions – and the Beefwood is listed as a category 2 weed.

As I see it (but I am not a legal expert), providing that there is a water users license for the property where the beefwood is planted (regulation 15B(2)(b)), and providing that trees are not planted closer than 30 m to the 1:50 year flood line of a “river, stream, spring or natural channel”, a land user may plant beefwood trees. Note that clause 15(8) obliges a land user to control any category 2 plants that occur on any” land or inland water surface” in contravention of these regulations, in other words trees that have been planted, or established naturally within 30 m of the 1:50 year flood line, or on farm land where a water users license has not been granted.

 

I am very much aware of farmer’s needs to contain costs these days. I understand farmer’s needs for cost-effective windbreaks, but I can’t help thinking that the use of Beefwoods is very short-sighted. It is a typical scenario where tomorrow’s farmer will have to pay for the poor choices made by today’s farmer. We all can see the problems caused by Black Wattles and Port Jackson Willows in rivers, why then are we allowing Beefwoods to spread in the rivers the way they are? I cannot agree more with the recommendations of Potgieter, Richardson and Wilson. Planting only male trees would solve most of the problems. If that is not done, some other effective means of control must be found.

Beefwoods sprouting in a stream adjacent to a Beefwood windbreak