**2.3. Role in soil fertility**

The effect of *V. karroo,* and other tree species, on herbaceous species (and grasses) may not always be negative, and there is evidence that trees may actually have a beneficial effect on neighbouring plants. For example, increased herbaceous layer productivity has been reported under tree canopies, due to favourable conditions such as improved soil water status and soil fertility [38, 39]. Likewise, *V. karroo* has various favourable influences on herbaceous production. First, *V. karroo* is a leguminous tree known to form root nodules [40], which are swellings on the root that contain nitrogen-fixing microorganisms (bacteria) known as *Rhizobium. Rhizobium* possesses the enzyme systems (including nitrogenase enzyme complex) that convert atmospheric nitrogen to nitrogen compounds useful to plants [41, 42]. Legumes like *V. karroo* then use the compounds to construct amino acids and protein [41, 43]. This ability of *V. karroo* to fix nitrogen is beneficial to other plants as well, mainly because the nitrogen content in the soil increases, and soil fertility is thus enhanced under these trees. In addition, *V. karroo* is able to use water and nutrients from deep underground because it has a long taproot, and this again leads to grasses and other plants thriving in its shade [6]. The ability of *V. karroo* to use water from deep underground means that it can grow in arid and otherwise inhospitable environments, as long as there is an assured supply of underground water [27]. It hence also acts as an indicator of surface and underground water, especially in arid land [2, 5]. The tree is further considered an indicator of sweet veld, which is highly valued for good grazing and fertile soils [5, 6]. This is due to the beneficial effects such as provision of shade, improved soil fertility, and water availability, which lead to the development of palatable and nutritious grasses under the *V. karroo* trees [18].

Several studies have been conducted on the positive effect of woody plants on grasses. In southern African savannas, *Panicum maximum* is well known to be associated with tree canopies, especially those of several *Vachellia* species. *P. maximum* is one of the most important fodder grass species in many savanna areas, mainly because it is highly palatable to cattle and other grazers, and it also has a high production potential [44]. The grass is strongly associated with tree canopy cover; it is common under trees but seldom occurs in the open [45]. Smit and Swart [46] suggest that such grass-tree associations, which exist in many semi-arid savanna areas, warrant that bush control measures should not simply imply a complete removal of woody plants but rather tree thinning with a view to reducing negative competition effects. This kind of approach can ensure that the important forage contribution by *P. maximum* is maintained.

This association is likely due to enhanced supply of nutrients such as nitrogen and phosphorus under tree canopies and suitable germination conditions for *P. maximum* seeds due to the relative abundance of litter and low temperatures under tree canopies [1]. A study investigating the relation between tree height of *V. karroo* and *V. tortilis* and the associated occurrence of *P. maximum* in the Sourish Mixed Bushveld [27] of Limpopo Province indicated that *P. maximum* mainly occurs under larger trees, but the grass attained pure stands under smaller *V. tortilis* trees of >2.0 m height [44]. In the False Thornveld of the Eastern Cape [27], Stuart-Hill et al. [47] proposed that the net effect of the favourable or unfavourable influences of *V. karroo* on grass production is dependent on tree density. It was observed that in situations where there were a few *V. karroo* trees, grass production was greater than where there were no trees but declined as tree density increased beyond a critical level.
