What are the effects of substrate and grass removal on recruitment of Acacia mellifera seedlings in a semi-arid environment?

Ward, David; Esler, Karen J.

doi:10.1007/s11258-010-9818-6

Cited by 70 publications

(67 citation statements)

References 27 publications

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“…Grass competition emerged as the major factor limiting basal diameter growth and branch production inside glades, and to a lesser degree, it also limited growth outside glades. These results support previous studies in our system (Riginos and Young 2007;Riginos 2009) and other savannas (Knoop and Walker 1985;van der Waal et al 2009;Cramer et al 2010;Ward and Esler 2011) which found that grass competition can significantly restrict tree growth and recruitment. In a pot experiment, van der Waal et al (2011) went a step further by showing that competition between grasses and trees was more intense when plants were grown in glade-derived, nutrient-rich soil.…”

Section: Impacts Of Grasses and Browsingsupporting

confidence: 93%

Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots

Porensky

Veblen

2011

Oecologia

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Spatial heterogeneity in woody cover affects biodiversity and ecosystem function, and may be particularly influential in savanna ecosystems. Browsing and interactions with herbaceous plants can create and maintain heterogeneity in woody cover, but the relative importance of these drivers remains unclear, especially when considered across multiple edaphic contexts. In African savannas, abandoned temporary livestock corrals (bomas) develop into long-term, nutrient-rich ecosystem hotspots with unique vegetation. In central Kenya, abandoned corral sites persist for decades as treeless 'glades' in a wooded matrix. Though glades are treeless, areas between adjacent glades have higher tree densities than the background savanna or areas near isolated glades. The mechanisms maintaining these distinctive woody cover patterns remain unclear. We asked whether browsing or interactions with herbaceous plants help to maintain landscape heterogeneity by differentially impacting young trees in different locations. We planted the mono-dominant tree species (Acacia drepanolobium) in four locations: inside glades, far from glades, at edges of isolated glades and at edges between adjacent glades. Within each location, we assessed the separate and combined effects of herbivore exclusion (caging) and herbaceous plant removal (clearing) on tree survival and growth. Both caging and clearing improved tree survival and growth inside glades. When herbaceous plants were removed, trees inside glades grew more than trees in other locations, suggesting that glade soils were favorable for tree growth. Different types of glade edges (isolated vs. non-isolated) did not have significantly different impacts on tree performance. This represents one of the first field-based experiments testing the separate and interactive effects of browsing, grass competition and edaphic context on savanna tree performance. Our findings suggest that, by excluding trees from otherwise favorable sites, both herbaceous plants and herbivores help to maintain functionally important landscape heterogeneity in African savannas.

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Section: Impacts Of Grasses and Browsingsupporting

confidence: 93%

Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots

Porensky

Veblen

2011

Oecologia

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“…There was no effect of grass competition on RGR L of all species excluding A. nigrescens and Schotia brachypetala, RGR D of A. sieberiana, and LP of Strychnos spinosa and Combretum apiculatum. It is widely understood that grass competition is a fundamental hindrance to savanna tree sapling establishment (Riginos and Young 2007, Kambatuku et al 2011, Ward and Esler 2011, Grellier et al 2012. We further note that the grasses in our experiment were relatively young plants; older, established grasses would likely present a greater constraint on tree establishment (Jurena and Archer 2003, Ward 2005, Messier et al 2009).…”

Section: Notesmentioning

confidence: 70%

“…However, consistent with the findings of Vadigi and Ward (2012) for Acacia saplings, we found no nutrient effect on sapling survival in any species. Grass competition, identified as the greatest hurdle for sapling survival by Grellier et al (2012) and Ward and Esler (2011), also did not have an effect on sapling survival. Perhaps, in humid savannas, other environmental factors such as temperature, fire and herbivory play a more influential role in savanna sapling survival (Shaw et al 2002, Hoffmann and Solbrig 2003, Sharam et al 2006, Chidumayo 2008, Gignoux et al 2009).…”

Section: Sapling Survivalmentioning

confidence: 85%

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Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

Vadigi

Ward

2013

Ecosphere

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Citation: Vadigi, S., and D. Ward. 2013. Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna. Ecosphere 4(11):142. http://dx.doi.org/10.1890/ES13-00239.1Abstract. The structure and function of savannas is contingent upon tree sapling establishment into the ecosystem. Frequent rainfall, canopy shade and nutrient availability are the main factors influencing recruitment of tree saplings. Grass competition has potential to change the tree establishment dynamics by interacting with these factors. The factors affecting tree sapling establishment have been well studied and tested in arid and semi-arid environments (approx. 0.350-0.750 m mean annual precipitation [MAP]) compared to humid (.1.000 m MAP) savanna ecosystems. We studied the effects of water (frequent irrigation vs. rainfall), shade (presence vs. absence), nutrients (addition vs. no addition) and grass competition (presence vs. absence) on sapling survival and functional traits related to growth and size of four dominant humid savanna species and four dominant mesic savanna species (approx. 0.750 m MAP), under controlled field conditions in a humid South African savanna. None of the treatments had an effect on sapling survival. Growth of the saplings did not depend on the frequency of watering, probably due to high rainfall received in humid savannas. Shade had a strong negative effect on sapling growth, suppressing the beneficial effects of nutrient availability and absence of grass competition. Net positive effects of nutrient availability had less influence over sapling growth than the negative effects exerted by grass competition. Overall shoot growth increased with nutrient addition while investment in leaves drastically declined with grass competition. We conclude that canopy shade is a strong potential driver of structure and function of humid savanna ecosystems, although we recognize that nutrient limitation and grass competition additionally influence tree sapling establishment.

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“…Heavy grazing in mesic savannas reduces fuel load through consumption and trampling, thereby reducing fire frequency and more significantly fire intensity [1,31,36,44,53]. Overgrazing has also been reported to reduce the effect of grass competition on tree seedlings and saplings, as a dense grass layer can negatively affect tree growth and survival [54][55][56]. Long-term grazing trials in both mesic and arid systems have consistently reported increases in the density of woody plants over 5-40 year periods of observation [52].…”

Section: Grazers and Browsersmentioning

confidence: 99%

Woody Encroachment as a Social-Ecological Regime Shift

et al. 2018

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African savannas are increasingly affected by woody encroachment, an increase in the density of woody plants. Woody encroachment often occurs unexpectedly, is difficult to reverse, and has significant economic, cultural and ecological implications. The process of woody encroachment represents a so-called regime shift that results from feedback loops that link vegetation and variables such as fire, grazing and water availability. Much of the work on woody encroachment has focused on the direct drivers of the process, such as the role of fire or grazing in inhibiting or promoting encroachment. However, little work has been done on how ecological changes may provide feedback to affect some of the underlying social processes driving woody encroachment. In this paper, we build on the ecological literature on encroachment to present a qualitative systems analysis of woody encroachment as a social-ecological regime shift. Our analysis highlights the underlying indirect role of human population growth, and we distinguish the key social-ecological processes underlying woody encroachment in arid versus mesic African savannas. The analysis we present helps synthesize the impacts of encroachment, the drivers and feedbacks that play a key role and identify potential social and ecological leverage points to prevent or reverse the woody encroachment process.

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What are the effects of substrate and grass removal on recruitment of Acacia mellifera seedlings in a semi-arid environment?

Cited by 70 publications

References 27 publications

Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots

Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots

Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

Woody Encroachment as a Social-Ecological Regime Shift

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