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.
Highlights
The savannas and grasslands of arid and semi-arid drylands in the subtropics and tropics make up more than 20% of the earth’s land surface
Our study provides evidence that the massive spread of the herbaceous legume C. podocarpa has a significant impact on Stipagrostis biomass production
The presence of high numbers of C. podocarpa did not affect the percentage of active grass tussocks, but decreased Stipagrostis productivity by 30% and, over time, reduced grass tussock density on the affected sites to less than 50% of its maximum
Summary
The savannas and grasslands of arid and semi-arid drylands in the subtropics and tropics make up more than 20% of the earth’s land surface. An increasing number of these rangelands are affected by the recent and massive proliferation of C3 species [2,3] These encroachers are typically native woody, perennial legumes but occasionally herbaceous plants or invasive alien species [4,5,6], that lastingly change the community structure and repress the formerly dominant grasses [7,8]. As over 60% of the agricultural production in Namibia originates from livestock farming [23] the annual losses are estimated to amount to over 70 M US$ [22], and similar numbers apply for South Africa [24] or Uganda [25] This negative effect of encroachment on pastoral production has so far mainly been attributed to the encroachers’ spatial demands [21,26]. We ask the following question: How does encroachment by C. podocarpa affect dormancy state, individual tussock productivity and tussock density of Stipagrostis under natural rainfall conditions?
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