Edible shrubs need different management to perennial grasses
When I visited South Africa in the late 1990’s, I met rangelands scientist Dr Louis du Pisani. He told me how he had discovered the difference between perennial grasses and perennial edible shrubs, in terms of how they store and utilise energy reserves. He explained that what happens in the roots is different.
With ongoing grazing pressure that depletes energy reserves, old man saltbush starts to grow very small leaves which is a sign it is close to dying.
His research into the Karoo Bush, which is similar to our perennial saltbushes, showed that when the shrub called on energy reserves, the root volume did not reduce. The energy reserves were in the centre of the roots and not part of the structure. The ability of the Karoo Bush to maintain root volume, after calling on energy reserves, means it can keep sourcing moisture and nutrients below the roots of perennial grasses.
In good years, annuals utilise excess moisture lying between perennial grasses to add to carbon flows, while in dry times, edible shrubs utilise moisture that is below perennial grasses to provide some carbon flows when perennial grasses are dormant. But, it’s critical that both the grass and the shrubs are managed according to their needs.
Like perennial grasses, if the energy reserves become depleted, then edible shrubs become weak and die. What alerted Louis to the different storage process, was that the Karoo Bushes that had died from overgrazing, had the normal root volume, i.e. the roots did not reduce with the depletion of the energy reserves.
Another lesson in timing
His other important discovery was that, unlike perennial grasses, which replenish energy reserves before reaching maturity, the shrubs made the main transfer of plant sap out of the leaves and down to the roots, with the onset of a dry season, but little before. We know that in Australia, Old Man Saltbush has the ability to transfer plant sap from leaves to roots.
Because of the timing of the transfer from leaves to roots, the complete defoliation of edible shrubs before the onset of dry times, if allowed to happen regularly and then followed by continuous grazing, can see the death of shrubs through depleted root reserves.
A blessing and a curse
It is the ability of shrubs to grow in dry times, that puts them at risk if not managed properly. A shrub producing new growth in a dry time from subsoil moisture, after it has been completely defoliated back to stems, is no different to a perennial grass plant that is starting to grow from rain after being dormant. In both cases, energy reserves will start to run down if animals keep them defoliated as they keep trying to grow.
In a mixed pasture of grass and edible shrubs, poor grass managers transfer grazing pressure onto the shrubs too early in the drought cycle. If animals are only removing some of the leaves on the shrubs, to source trace elements, then there is not a problem.
Appreciating that edible shrubs have a different process to perennial grasses, is important for the management of saltbush plantations. It also explains why established plantations of Old Man Saltbush perform well for some and not others.
Francis Ratcliffe’s novel, “Flying Fox And Drifting Sands”, published in 1938, documented how over grazing early in the twentieth century in South Australia, saw the demise of saltbushes in large sections of the arid areas.
While shrubs have the ability to produce carbon flows in dry times, they do need to be managed differently to grasses.