How does the seed fate of Crotalaria podocarpa DC, a highly competitive herbaceous legume in arid rangelands, contribute to its establishment probability?

Fischer, Christina; Kollmann, Johannes; Wagner, Thomas C.

doi:10.1016/j.ppees.2015.07.002

Cited by 5 publications

(6 citation statements)

References 57 publications

(65 reference statements)

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“…Further, C . podocarpa has a considerable seed bank with more than 350 seeds/m 2 ([40], T. C. Wagner and C. Fischer, unpublished data) due to the high rainfalls over the last ten years (own rainfall data, Table A in S3 Table), its seeds remain viable for many years (T. C. Wagner and C. Fischer, unpublished data) and predicted higher rainfalls for the coming decades [55], make a fast decline of the species in the study region rather unlikely. Therefore it will probably take decades for the affected land to recover its former productivity.…”

Section: Discussionmentioning

confidence: 99%

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Herbaceous Legume Encroachment Reduces Grass Productivity and Density in Arid Rangelands

et al. 2016

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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.

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Section: Discussionmentioning

confidence: 99%

“…Wagner and C. Fischer, unpublished data). Seeds are produced in high numbers, are viable for over four years and build up a persistent soil seed bank [40]. Due to its content of pyrrolizidine alkaloids and flavonoids, C .…”

Section: Methodsmentioning

confidence: 99%

Herbaceous Legume Encroachment Reduces Grass Productivity and Density in Arid Rangelands

et al. 2016

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“…Its comparatively large and heavy seeds are primarily dispersed by explosive dehiscence and reach a distance of about 5 m around the mother plant. Secondary long‐distance dispersal is rare and probably related to scatter hoarding by ants and small mammals (Fischer, Kollmann, & Wagner, 2015). Crotalaria is part of the local plant community of Namibia’s escarpment region where it normally occurs in moderate numbers.…”

Section: Methodsmentioning

confidence: 99%

“…Higher rainfall is associated with higher seed production and generally better seedling survival of encroaching species (Kraaij & Ward, 2006;Oldeland, Dreber, & Wesuls, 2010;Roques, O'Connor, & Watkinson, 2001). Taking into account Crotalarias high seed production, its extended seed viability, and dispersal by explosive dehiscence (Fischer et al, 2015), above-average rainfall will automatically be linked to a higher probability and number of seeds ending up in grass tussocks after dispersal. Our pot experiment has shown that the grass tussocks facilitate early establishment of Crotalaria and that the grass is unable to outcom- Regarding a strengthening of Crotalarias competitive abilities, we may also speculate about a possible role of elevated atmospheric CO 2 levels.…”

Section: This Raises the Question Why Such A Dominance Shift Involvingmentioning

confidence: 99%

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

Wagner

Richter

Joubert

et al. 2018

Ecology and Evolution

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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.

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“…However, this assumption largely ignores 'cryptic consumers' like insects (sensu Andersen & Lonsdale 1990;Keesing 2000), which are abundant and diverse in Neotropical savannas (Marquis, Morais & Diniz 2002;Costa et al 2008;Mundim et al 2012). Indeed, the few invertebrate exclusion experiments conducted in savanna habitats in the Neotropics and elsewhere have indicated high rates of seed removal and seedling mortality by insects, although the impact of consumers varies substantially among plant species (between 10-95% and 20-30%, respectively; Shaw, Keesing & Ostfeld 2002;Goheen et al 2004;Moe et al 2009;Vaz-Ferreira, Bruna & Vasconcelos 2011;Fischer, Kollmann & Wagner 2015).…”

Section: Introductionmentioning

confidence: 99%

Biotic drivers of seedling establishment in Neotropical savannas: selective granivory and seedling herbivory by leaf‐cutter ants as an ecological filter

2016

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Summary 1.Herbivory has been shown to have prominent top-down effects on vegetation in Paleotropical savannas, where consumers of early stages of life history act as demographic bottlenecks. Such impact has been largely ignored in Neotropical savannas, however, despite insect consumption being linked to reduced recruitment of woody species. 2. We hypothesize that Atta leaf-cutter ants -the prevalent herbivores in the Neotropics -alter the establishment of woody plant seedlings in the Brazilian Cerrado by reducing seed availability and seedling survival. We also hypothesize that the demographic effects will be species-specific. 3. To test these hypotheses, we experimentally (i) compared seed removal in areas with and without Atta foraging and (ii) compared the survivorship of seedlings exposed to or protected from Atta foraging. Both experiments were performed with the same common Cerrado species which allowed us to evaluate the potential net effects of consumers at the population and community levels. 4. Overall seed removal rates in sites with Atta foraging were greater than those where ants were absent (59.2% and 39.2% of seeds removed, respectively). There were differences in removal for 10 of the 12 tested species, with per species removal 1.1-to 3.8-fold greater in areas with Atta foraging. On transplanted plants, 45% of seedlings exposed to Atta were attacked, resulting in a survival 7.6% lower than that of protected seedlings. Seedling survival was 11.8-31.5% lower in five species, with the largest differences in survival between treatments during the dry season. 5. Taken together, these results support the hypothesis that consumers can lead to demographic bottlenecks in Neotropical savanna plant species, with effects varying among life-history stages and between species. 6. Synthesis. Granivory by ants has been linked to reduced seedling recruitment of woody species in the Brazilian Cerrado. Our results show that leaf-cutter ants may largely limit early seedling establishment of woody species by reducing seed availability and seedling survival with differential species-specific effects. Atta ants may therefore be acting as an ecological filter, which coupled with potential selectivity in foraging on reproductive life-history stages, may ultimately influence the relative abundance of different species and hence the structure and composition of Cerrado vegetation.

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How does the seed fate of Crotalaria podocarpa DC, a highly competitive herbaceous legume in arid rangelands, contribute to its establishment probability?

Cited by 5 publications

References 57 publications

Herbaceous Legume Encroachment Reduces Grass Productivity and Density in Arid Rangelands

Herbaceous Legume Encroachment Reduces Grass Productivity and Density in Arid Rangelands

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

Biotic drivers of seedling establishment in Neotropical savannas: selective granivory and seedling herbivory by leaf‐cutter ants as an ecological filter

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How does the seed fate of Crotalaria podocarpa DC, a highly competitive herbaceous legume in arid rangelands, contribute to its establishment probability?

Cited by 5 publications

References 57 publications

Herbaceous Legume Encroachment Reduces Grass Productivity and Density in Arid Rangelands

Herbaceous Legume Encroachment Reduces Grass Productivity and Density in Arid Rangelands

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

Biotic drivers of seedling establishment in Neotropical savannas: selective granivory and seedling herbivory by leaf‐cutter ants as an ecological filter

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