Herbaceous Legume Encroachment Reduces Grass Productivity and Density in Arid Rangelands

Abstract

Worldwide savannas and arid grasslands are mainly used for livestock grazing, providing livelihood to over a billion people. While normally dominated by perennial C4 grasses, these rangelands are increasingly affected by the massive spread of native, mainly woody legumes. The consequences are often a repression of grass cover and productivity, leading to a reduced carrying capacity. While such encroachment by woody plants has been extensively researched, studies on similar processes involving herbaceous species are rare. We studied the impact of a sustained and massive spread of the native herbaceous legume Crotalaria podocarpa in Namibia’s escarpment region on the locally dominant fodder grasses Stipagrostis ciliata and Stipagrostis uniplumis. We measured tussock densities, biomass production of individual tussocks and tussock dormancy state of Stipagrostis on ten 10 m x 10 m plots affected and ten similarly-sized plots unaffected by C. podocarpa over eight consecutive years and under different seasonal rainfalls and estimated the potential relative productivity of the land. We found the percentage of active Stipagrostis tussocks and the biomass production of individual tussocks to increase asymptotically with higher seasonal rainfall reaching a maximum around 300 mm while the land’s relative productivity under average local rainfall conditions reached only 40% of its potential. Crotalaria podocarpa encroachment had no effect on the proportion of productive grass tussocks, but reduced he productivity of individual Stipagrostis tussocks by a third. This effect of C. podocarpa on grass productivity was immediate and direct and was not compensated for by above-average rainfall. Besides this immediate effect, over time, the density of grass tussocks declined by more than 50% in areas encroached by C. podocarpa further and lastingly reducing the lands carrying capacity. The effects of C. podocarpa on grass productivity hereby resemble those of woody encroachers. Therefore, against the background of global change, the spread of herbaceous legumes and the underlying patterns needs to be further investigated to develop adequate counter measures for a sustainable land use.

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

“…in Namibia’s lower great escarpment. Soils are predominantly shallow, nutrient‐poor calcisols with a coarse texture and a high proportion of stones and sand (Wagner et al., 2016). The climate is hot‐arid (Mendelsohn, Jarvis, Robertson, & Roberts, 2009).…”

“…Together with the related S. uniplumis, it constitutes the main source of forage in Namibia’s escarpment region (Juergens, Oldeland, Hachfeld, Erb, & Schultz, 2013; Müller, 2007). On the study site, Stipagrostis forms a light matrix of 1.5–2 tussocks/m 2 (Wagner et al., 2016) and its cover is considerably varying with seasonal rainfall. Loosely interspersed are perennial shrubs and occasionally trees.…”

“…Crotalaria is part of the local plant community of Namibia’s escarpment region where it normally occurs in moderate numbers. Starting in 2008, in the course of several years of above‐average rainfall, a considerable proliferation of Crotalaria has been observed in Namibia’s escarpment region (Wagner et al., 2016). …”

“…Our field observation was carried out on 20 long‐term observation plots of 10 × 10 m half of which were affected by Crotalaria encroachment (Wagner et al., 2016). All plots had identical soil types, similar soil texture, and initially the same vegetation composition and structure (Table S1) with a Stipagrostis tussock density of about 2 tussocks/m².…”

“…Likewise, the native herbaceous legume Crotalaria podocarpa has encroached parts of Namibia’s escarpment region with considerable impact on fodder grass production and the lands carrying capacity (Wagner, Hane, Joubert, & Fischer, 2016). With our study, we investigate the competitive balance between this legume and the dominant local fodder grass Stipagrostis ciliata and try to establish the causes for this recent spread of Crotalaria podocarpa .…”