Replacing encroached bushes with tree forest offers option

Windhoek: Much of Namibia’s rangeland has become less productive and encroached by bushes due to interlinked causes related to mismanagement. An option that rarely receives consideration is to gradually replace encroached bushes with a food forest supported by rainwater harvesting.

This will improve the productivity of such land and increase its resilience to climate change. On Middelplaats, owned vy Uwe Kahl near Kalkfeld, a 30ha paddock dominated by Acacia erubescens (Withaak) in Namibia’s Thornbush Savanna was selected as an experimental site to demonstrate food security.

It has sandy loam soil sloping gently northwards at about 1-in-100 and used to receive 350mm mean annual rainfall. In September 2014, a bulldozer was used to clear strips for a grader to dig 12 contour ditches (swales) to a depth of roughly 11cm at height intervals of 0,5m within the 30ha paddock from one edge to the other.

Diversion ditches of roughly 7cm depth were also dug at a gradient of roughly 1-in-200 outside the paddock connected to some of the swales, to later bring extra rainwater flowing on surrounding land into the paddock.

The excavated soil was deposited on the downslope side over a rip line and married into the land surface below ground level to form the bund, which was hooked upwards at the ends of swales to prevent spillage down the roads surrounding the paddock.

A 10m section of bund nearby the end of each swale was lowered to serve as a spillway to slowly release excess water evenly over its cut edge, alternating between eastern and western ends of successive swales so that excess water would zigzag its way slowly down the landscape to maximise infiltration time.

Several lines were ripped at right angles across the roads surrounding the paddock wherever a diversion ditch connects to a swale, for constructing a larger bund to withstand the flow of diverted water into the swale. The greatest cost was for use of earthmoving equipment, estimated at N$ 110,000.

Tree seedlings were planted about 10m apart along the lower sides of several swales, roughly 30cm downslope from the bund. However, only very few of the seedlings managed to survive the combined challenges of drought, extreme heat, high pressure of grazing animals and attack by termites.

The selection of tree seedlings for the initial planting had been opportunistic, making use of old donated seedlings that had been root bound. In the meantime a nursery is being established on the farm.

Newly planted seedlings will be watered during the first, second and third seasons (every 10 days) and mulch will be tried on the soil around them. Later biochar produced on the farm, and conditioned with dung and urine in the cattle kraal, will be mixed into the soil surrounding tree seedlings to improve moisture retention.

Initially most of the trees are for improving the micro-environment for later planting of fruit trees. Tall canopy trees that improve soil fertility, such as Faidherbia albida (Anaboom) will nurse fruit trees, many of which evolved to grow under tall canopies in rain forests. These and other thorny trees will be pollarded at intervals when their shade becomes excessive and for the cut branches to provide bush fencing for fruit tree seedlings.

Thornless legumes of shorter trees, such as Bolusanthus speciosus (Vanwykshout) and Peltophoroum africanum (Huilboom), will be grown for “chop and drop” mulching and root exudate feeding of beneficial soil microorganisms. Selection of tree provenances and species will be based on those likely to care for themselves with minimal input and those providing multiple products and functions.

Animals and chicken tractors will be brought into the landscape at strategic times to benefit from the abundance of forage expected to grow there, and to serve functions such as nutrient cycling, pest control, seed dispersal, seedbed preparation and reduction of fuel load to reduce fire risk in dry seasons that follow good rain.