There are three major stresses that spekboom cuttings need to survive: drought, herbivory and frost. The last stress has been discussed in previous posts (see Rob Duker's "Thicket restoration and frost: the forgotten enemies") and can be overcome quite simply by avoiding areas that experience frosts.
Drought survival was the focus of initial spekboom restoration research. This suggested that smaller cuttings (so-called "fingerlings") had extremely low survival rates, and survival increased with the size of the cutting. This led to the suggestion of the current truncheon size of ~>4 cm stem diameter as part of the restoration protocol.
These truncheons are planted to a depth of ~10-20 cm (sometimes deeper) sticking upright out of the ground. In a previous post ("To the root of the problem with Spekboom restoration"), I highlighted that the roots tend to only grow from the cut base, which has been been buried deep in the soil and limiting the access the plant has to soil moisture after light rainfall events. However, an additional problem is that these cuttings take a long time to develop a root-to-shoot ratio that is sufficient to survive drought, but more importantly, withstand herbivory.
A root-to-shoot ratio describes the amount of plant tissue that has supportive functions (i.e. the roots) relative to the amount of tissue that has growth functions (i.e. above-ground stems and leaves). Roots allow a plant to absorb water and nutrients from the surrounding soil, and thus root-to-shoot ratios are usually discussed in terms of plant health and water/nutrient absorption. This is certainly an important consideration for spekboom truncheons, but what is of greater importance is the effect that this low root-to-shoot ratio has on herbivory survival.
Without an established root system (relative to the above-ground plant), a spekboom truncheon is more susceptible to browsing and catastrophic herbivory-related uprooting. Browsers eating spekboom generally include a pulling motion after the bite; this pulling motion can easily uproot a truncheon. (Note that baboons have also been observed pulling up truncheons).
Spekboom has evolved to withstand heavy browsing from indigenous game. Even in an area with arguably the highest elephant-browsing density in the world — at Hapoor Dam in Addo Elephant National Park where the elephants have decimated the thicket vegetation surrounding the dam (Landman et al. 2012) — spekboom is one of the first species to return to canopy dominance as you move away from the dam. Also, in relation to its annual rainfall, AENP supports the highest biomass of herbivores anywhere in Africa (Mills et al. 2014), and likely the world. As an aside, an untested hypothesis is that the primary dispersal agents of spekboom are elephants as there habit of walking while munching on a spekboom branch results in a rain of vegetative parts that can propagate new individuals.
So, why are spekboom truncheons susceptible to indigenous herbivory? The root-to-shoot ratio is wrong. There is not enough going on under the ground to support what is going on above the ground.
We need the truncheons to be large to withstand drought-stress, but being large makes them more apparent in the landscape, take a long time to develop the necessary roots, and ultimately highly susceptible to herbivory.
How can we deal with this catch twenty-two?
My suggestion is to return to smaller truncheon sizes (the "fingerlings"). The fingerlings:
- are less apparent in the landscape, and
- can develop the right root-to-shoot ratio in a shorter time period.
Thus, this still leaves the problem of drought survival. A lone spekboom fingerling out in the open has little chance of survival. However, there are a few things that can be done to improve these chances:
- Planting spekboom close together in a contoured band has been shown to have outstanding results at the Camdeboo National Park restoration sites established by Peter Burdett (results shared by Bruce Taplin at the Thicket Forum 2015).
- The pure mechanical effect of creating a contour line, by rip-ploughing, soil bags, car tyres etc. creates drainage lines of increased soil moisture. If positioned correctly, these lines can also create an ameliorated microclimate for a small plant.
- Loosening the soil prior to planting has also been reported (reference needed) to improve truncheon survival and growth.
- Hydromulch, a woodfibre mulch applied with water and a tackifier (natural glue), traps water and dew and creates an excellent medium for encouraging root development in fingerlings (see the images in my post about the rooting window hypothesis).
Thus, the combination of mechanical contour ploughing that loosens the soil (or some other water-trapping technique) and creates microclimates (on south-facing micro-slopes in the ditch and in the soil), hydromulch and a high density of spekboom fingerlings could very well be the recipe for seeding the landscape with spekboom resilient to drought and herbivory. As the mechanical structure of the contour breaks down from erosion, the spekboom will likely take over as a biological contour, filtering water and erosion and increasing water infiltration.