A dominance shift in arid savanna: An herbaceous legume outcompetes local C₄ grasses

Abstract

The characteristic vegetation structure of arid savannas with a dominant layer of perennial grass is maintained by the putative competitive superiority of the C4 grasses. When this competitive balance is disturbed by weakening the grasses or favoring the recruitment of other species, trees, shrubs, single grass, or forb species can increase and initiate sudden dominance shifts. Such shifts involving woody species, often termed “shrub encroachment”, or the mass spreading of so‐called increaser species have been extensively researched, but studies on similar processes without obvious preceding disturbance are rare. In Namibia, the native herbaceous legume Crotalaria podocarpa has recently encroached parts of the escarpment region, seriously affecting the productivity of local fodder grasses. Here, we studied the interaction between seedlings of the legume and the dominant local fodder grass (Stipagrostis ciliata). We used a pot experiment to test seedling survival and to investigate the growth of Crotalaria in competition with Stipagrostis. Additional field observations were conducted to quantify the interactive effect. We found germination and growth of the legume seedlings to be facilitated by inactive (dead or dormant) grass tussocks and unhindered by active ones. Seedling survival was three times higher in inactive tussocks and Crotalaria grew taller. In the field, high densities of the legume had a clear negative effect on productivity of the grass. The C4 grass was unable to limit the recruitment and spread of the legume, and Crotalaria did outcompete the putative more competitive grass. Hence, the legume is able to spread and establish itself in large numbers and initiate a dominance shift in savannas, similar to shrub encroachment.

Vegetation state changes in the course of shrub encroachment in an African savanna since about 1850 CE and their potential drivers

“…This improved ratio of carbon assimilation-to-conductance results in an increase of transpiration efficiency (Evans et al, 2000), which in turn leads to 1 H enrichment in leaves and thus a decrease in δD values. Such higher atmospheric CO 2 concentrations may have favored the propagation of Vachellia and Senegalia toward the present day, because Fabaceae species are expected to benefit from such conditions in nutrient-limited savanna environments (Wagner et al, 2018;Ward, Hoffman, & Collocott, 2014). As observed by Evans et al (2000), Acacia species increase their nitrogen content per unit foliage area when growing under a higher CO 2 concentration.…”

Growth and competition between a native leguminous forb and an alien grass from the Cerrado under elevated CO2

“…The great benefit demonstrated by S. capitata under high [CO 2 ] is likely due to its C 3 carbon metabolism and symbiosis with N 2 fixing bacteria. The increase in atmospheric [CO 2 ] can influence competition between C 3 and C 4 species by decreasing the photosynthetic disadvantage of C 3 relative C 4 plants (Bond & Midgley, 2000;Ward et al 2014;Wagner et al 2018). Therefore, under elevated [CO 2 ] S. capitata may show improved morphophysiological traits related to growth benefitting it in competition with this invasive species in natural areas of Cerrado.…”

“…2014; Wagner et al . 2018). Therefore, under elevated [CO 2 ] S. capitata may show improved morphophysiological traits related to growth benefitting it in competition with this invasive species in natural areas of Cerrado.…”

“…Predicted climate changes will have a significant influence on the growth and yield of leguminous species (Sulieman & Tran 2015). Several studies (Higgins & Scheiter 2012;Kulmatiski & Beard 2013;O'Connor et al 2014;Ward et al 2014) have shown that legumes cultivated under high atmospheric CO 2 concentration ([CO 2 ]) have a competitive advantage over non-leguminous plants due to their capacity for symbiotic nitrogen (N 2 ) fixation (Rogers et al 2009;Sanz-s aez et al 2013;Wagner et al 2018). With an increase in atmospheric [CO 2 ], these species maximise the mobilisation of carbohydrates to the root nodules, resulting in higher available energy and C for N-fixing bacteria and nitrogenase activity (Irigoyen et al 2014).…”

“…2013; Wagner et al . 2018). With an increase in atmospheric [CO 2 ], these species maximise the mobilisation of carbohydrates to the root nodules, resulting in higher available energy and C for N‐fixing bacteria and nitrogenase activity (Irigoyen et al .…”