Decision Tree on How to Control Acacia mellifera

Modified version by Dave Joubert & Ibo Zimmerman

 

Background

Obviously there is no clear division between these and, as witnessed in 2004, 2006 and 2008 the Hot Dry Season can actually be a very wet season! Recent droughts had hardly any rain in the hot, wet season. Nevertheless, because the growth and phenology of the grass and bush is linked to these seasons, it is useful to distinguish between them.

Background


Saplings (small plants but not seedlings) of Acacia mellifera become resistant to fire quite quickly and alternative measures might be necessary to control them. Read more

Background

Research has shown that mature A. mellifera (S. mellifera) in the highland savanna only produce significant amounts of seeds after a season of exceptionally good rainfall (Joubert, Rothauge and Smit, 2008; Joubert, Smit and Hoffman, 2013). Read more

Background

If a lot of viable seeds of A. mellifera (S. mellifera) are produced, they will germinate in January - March next year. Research shows that seeds do not carry over from one seasons to the next (they either get eaten, germinate and survive as seedlings, germinate and die through lack of rain, or browsed by hares and other small mammals. Read more

Background

The diebacks of the aboveground parts of mature shrubs occasionally occur, through severe drought, severe frost, fungal dieback, or combinations of the above. The resprout at the base grows slowly and typically after one year is less than 30 cm high. Read more

Decision 2: No worries for bush establishment, but consider de-stocking and monitoring

Background

If the seedlings died out then they are no longer a threat. If there is germination, survival will only occur if there is well spaced good effective rainfall. Read More

Background

High perennial grass density will provide competition with seedlings or fuel for a fire to kill them. Low perennial grass density is likely to allow seedlings to become established unless there is sufficient dry annual grass to allow a fire.  

Background

Even a tonne of dry matter per hectare may be able to kill all seedlings, during the hot, dry windy October, as fuel is also dry.
This fire will kill all seedlings. In addition, if there are mature skeletons (after drought or frost) with a little bit of regrowth, this fire will ignite the skeletons which will allow a longer burn, thus damaging and probably killing the regrowth. Animal trampling will prevent soil capping. If perennial grass density is low, ensure that seeds have been shed before burning.

Decison 3: Burn at end of dry season and allow animals to graze and trample for first half of growing season, then rest.

This fire will kill all seedlings. In addition, if there are mature skeletons (after drought or frost) with a little bit of regrowth, this fire will ignite the skeletons which will allow a longer burn, thus damaging and probably killing the regrowth. Animal trampling will prevent soil capping. If perennial grass density is low, ensure that seeds have been shed before burning.

Decison 3: Burn at end of dry season and allow animals to graze and trample for first half of growing season, then rest.

Because of the poor perennial grass cover, the seedlings should be visible. Because fire is not an option and because the grass offers no competition to the seedlings, the seedlings are likely to establish successfully. There is little option other than to treat the symptom of the excessive bush seedlings to get rid of most of them. Although pulling them out with pliers requires a lot of work, it is less than would be required to remove older bushes if the seedlings are left to establish themselves. Ongoing good grazing management should then be applied to encourage perennial grass cover and prevent bush seedlings establishing in future. Guidelines for recovery periods, if rain continues, are about 110 days on loamy soil and 120 days on sandy soil.

Decision 4: Pull out most surviving seedlings with pliers, or use a hoe to chop them out, and rest the veld for the remainder of the growing season, in order that a hot fire can be implemented in the following year. Unfortunately you will also not be able to burn regrowth by burning the mature skeletons as there is insufficient fuel.

Hopefully, an entire year of rest will allow the grass to recover sufficiently to suppress growth and be sufficient for a fire next dry season which will kill the rest of the seedlings, and the resprout from mature shrub skeletons top killed from frost or drought. If unable to rest for the whole growing season, then low stocking rate should be applied. This will hopefully allow the grass to produce sufficient biomass (depending upon the rainfall of the next season) which will allow a fire to kill the surviving seedlings and the resprout from mature shrub skeletons topkilled from frost or drought, in the following dry season and possibly suppress the growth of the bush seedlings.

Decision 5: Rest the veld for the whole year and pull out seedlings. You have reached a final decision regarding adaptive management.

Background

There is no specific threshold here. An area is termed to be bush encroached if the numerical density of 1.5 m high bush equivalence (BE) exceeds twice the average annual rainfall in mm. Read more

Background

If the trees are dying back by themselves, it may not be necessary to apply treatment to the other trees, even if the density of trees and shrubs is high enough to warrant concern. Read more

Background

There are numerous reasons you may not want to thin the bush at this stage. Select no to see Decision 6.

Background

If this question is asked in the dry season, consider leaving this area for the wet season (the following year) and then ask the question again.

Background

The quality of fodder that can be harvested from short bushes is better than harvestable fodder from bigger bushes that have excessive lignin. This is because there is a higher proportion of leaf: wood biomass in smaller bushes (on the left).

Background

The use of arboricides is strongly questioned today, but it is quick to apply and relatively inexpensive, if the bush is not to be utilised for value chains. See the next page.

Background

There may be many reasons for you to decide one way or the other. To assist in this, refer to the Harvesting Guidelines on Forestry and Environmental authorisation process for Bush Harvesting Projects (2017) for valuable details on bush thinning. Bear in mind that you might have reached this decision without wanting to utilise the bush. Bush thinning without value addition is more expensive than the likely gains made through improvements in production.
This decision might have been reached because the bush density is too low, or because the dieback is already high. But you also might have reached the decision if there was, in your opinion, a very high density and cover of bushes. This then might seem like a contradictory decision for bush encroachment management. However, we have included it explicitly, since the decision to leave the bushes might be difficult for an individual farmer, given that there is currently a sense of urgency in the country, and an individual farmer might feel obliged to comply with this. Studies have shown that mature thickets consisting of mature trees are thinning out through natural mortality. If a farmer is able to make a living without harvesting the bushes, and is reluctant to explore avenues of value chain, he should rather focus on ongoing good management as well as adaptive management. In addition, the farmer may be clearing in another camp or area, and thus might want to defer the decision to remove and harvest for another year or more. The farmer may also want to monitor the effects of harvesting before making a decision in this camp. Dense bush thickets can act as fire breaks between areas that are to be burnt. This saves the farmer time and money.
Application of herbicide, such as Access, to surfaces of cut stems (left), prevents the bushes regrowing into a thick mass of coppicing branches, but is relatively expensive and time consuming. However, if the arboricides is effective, no aftercare will be necessary on resprouts. On the other hand, it may be more economical to wait for a year and then only apply herbicide to those bushes that have regrown, as some bushes may die after being chopped, so some herbicide could be saved. Chopped bushes can be used for charcoal since they are not poisoned. Application of soil based arboricides to the soil around trees (centre and right) is a much more rapid method. The side effects of this on the ecosystem are not well known though, and caution should be applied. See “THE PROS AND 49 CONS OF USING ARBORICIDES VS MANUAL LABOUR”. Bushes killed through this method should not be utilised.
The increase in production is less than the costs of bush thinning if value addition chains are not utilized. However, leaving the bushes in situ, although not providing any direct economic benefits, will enhance the soil, and act as a nursery for any seeds of perennial grasses that blow there. If runoff occurs during intense rain, then it is strongly recommended to apply the bush control treatment along contour strips, to regenerate water and nutrient cycling. Only in situations where no runoff occurs, should the treated strips rather be aligned at right angles to the prevailing wind.. In 48 addition, the overseeding with climax perennial grasses, in these bushes, will enhance long term improvements in productivity and species composition. Aftercare will be necessary as most stumps will resprout.

Chop the bushes or use light duty mechanized equipment such as horizontal saw cutters preferably in strips, and leave them in situ (consider reseeding), or Proceed to Decision 14.