Large trees, fertile islands, and birds in arid savanna

Dean, W. R.; Milton, Suzanne J.; Jeltsch, Florian

doi:10.1006/jare.1998.0455

Cited by 246 publications

(244 citation statements)

References 36 publications

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“…Woody vegetation is known to provide nest sites, shade, and food resources for animals in open-vegetation ecosystems and it is an important driver of avian diversity in grasslands (Dean et al, 1999;tews et al, 2004). Large clumps of cacti in our study area can reach 15-20 m 2 and be nearly 2 m high, providing shelter and perching sites for birds in grasslands.…”

Section: Discussionmentioning

confidence: 99%

Bird-habitat associations in coastal rangelands of southern Brazil

Dias

Bastazini

Gianuca

2014

Iheringia, Sér. Zool.

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ABSTRACT. Nearly all remnants of temperate grasslands in southeastern South America are used for livestock ranching and are subject to habitat degradation resulting from this activity. Exploring how habitat features affect the composition of grassland avifaunal communities is a first step to understand how current cattle-ranching management practices impact avian diversity. We used canonical ordination to test for relationships between five habitat variables and the composition of the bird community in coastal grasslands in southern Brazil. We sampled pastures with different heights, from overgrazed short-grass to tall herbaceous vegetation. We recorded 1,535 individuals and 27 species of birds. The first ordination axis indicated a strong contribution of mean vegetation height on the composition of the bird community, whereas the second axis revealed the influence of herbaceous vegetation patchiness and woody vegetation cover. Three groups of species were revealed by the ordination: one more diffuse associated with intermediate and tall herbaceous vegetation, another with short grass, and a third with vegetation patchiness and woody vegetation. Species restricted to tall herbaceous vegetation are negatively impacted from habitat degradation resulting from overgrazing and trampling by livestock, and mowing and burning of tall plants. Occurrence of these species in our study area is related with the presence of swales immediately behind the dune system and where remnants of tall vegetation persist. Birds of pastures with ample cover of short herbaceous plants, including one globally threatened species and six other restricted to short-grass habitat, apparently benefit from local livestock management practices. Woody vegetation possibly functions as a keystone structure, enabling the occurrence in grasslands of avian species that rely on shrubby habitat. Although livestock ranching promotes the diversity of habitats by creating distinct patches of vegetation height in grasslands, current management practices directed to the maintenance of short grass pastures may eliminate an entire subset of species, including regionally threatened taxa, and reduce avian diversity. The maintenance of large patches of tall herbaceous plants is needed to ensure the survival of species reliant on this type of grassland structure in our study area. KEYWORDS.Habitat use, livestock ranching, grassland management, canonical ordination, "Campos" grasslands.RESUMO. Associações entre aves e variáveis de habitat em campos costeiros do sul do Brasil. Praticamente todos os remanescentes de campo temperado no sudeste da América do Sul são usados para a pecuária e estão sujeitos à degradação de habitat resultante dessa atividade. Explorar como a composição das comunidades de aves é influenciada por características de habitat é um passo inicial para compreender o impacto que as atuais práticas de manejo da pecuária em campos nativos exercem sobre a biodiversidade. Nós usamos ordenação canônica para testar as relações entre cinco variáveis de hab...

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Section: Discussionmentioning

confidence: 99%

Bird-habitat associations in coastal rangelands of southern Brazil

Dias

Bastazini

Gianuca

2014

Iheringia, Sér. Zool.

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“…The underlying mechanisms of a larger canopy improving grass nutrients more strongly compared to smaller canopies are that larger shady areas might be more beneficial because of a more favourable microclimate. Further, ongoing processes of defecation by attracted herbivores (Dean et al 1999), hydraulic lift (Ludwig et al 2003), nutrient accumulation through the tree's root system (Ludwig et al 2004), and a higher amount of litter fall compared to smaller trees might have contributed to higher grass nutrients beneath larger trees. Our study shows that tree structure, such as size and canopy radius, are potentially important determinants of grass quality with respect to nutrient content and species composition; however, the underlying mechanisms still need to be experimentally tested.…”

Section: Tree Sizementioning

confidence: 99%

“…The mechanisms by which trees affect grass quality beneath their crowns are manifold. Trees can improve the conditions for grass layer growth directly, for example, by providing shade and reducing evapo-transpiration, by accumulating soil nutrients close to their root systems (Belsky 1994;Dean et al 1999), and by changing the micro-climate (Jackson et al 1990;Belsky 1994;Ludwig et al 2001;Power et al 2003). Recent studies have shown that both soil and grass nitrogen (N) and phosphorus (P) contents were elevated beneath tree canopies compared to inter-canopy sites (Ludwig et al 2004;Treydte et al 2007Treydte et al , 2008 whereas some studies did not find any or only slight differences in soil properties between canopy and inter-canopy sites (Witkowski and Garner 2000).…”

Section: Introductionmentioning

confidence: 99%

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

2009

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Large single-standing trees are rapidly declining in savannahs, ecosystems supporting a high diversity of large herbivorous mammals. Savannah trees are important as they support both a unique flora and fauna. The herbaceous layer in particular responds to the structural and functional properties of a tree. As shrubland expands stem thickening occurs and large trees are replaced by smaller trees. Here we examine whether small trees are as effective in providing advantages for grasses growing beneath their crowns as large trees are. The role of herbivory in this positive tree-grass interaction is also investigated. We assessed soil and grass nutrient content, structural properties, and herbaceous species composition beneath trees of three size classes and under two grazing regimes in a South African savannah. We found that grass leaf content (N and P) beneath the crowns of particularly large (ca. 3.5 m) and very large trees (ca. 9 m) was as much as 40% greater than the same grass species not growing under a tree canopy, whereas nutrient contents of grasses did not differ beneath small trees (\2.3 m). Moderate herbivory enhanced these effects slightly. Grass species composition differed beneath and beyond the tree canopy but not between tree size classes. As large trees significantly improve the grass nutrient quality for grazers in contrast to smaller trees, the decline of the former should be halted. The presence of trees further increases grass species diversity and patchiness by favouring shade-tolerant species. Both grazing wildlife and livestock will benefit from the presence of large trees because of their structural and functional importance for savannahs.

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“…Th us, the presence of isolated trees in open pastures can help maintain biodiversity by facilitating the movements of dispersal agents between habitats (Benayas et al 2008;Arroyo-Rodríguez et al 2009;Cole et al 2010). Additionally, the structural characteristics of plants that growing in pastures (such as their heights, diameters, crown coverage, and densities) will influence seed dispersal as tall, broad trees can provide shelter, nesting, and foraging sites (McDonnell 1986;Dean et al 1999). Trees with wide and more highly branched crowns promote greater seed abundance beneath them (McDonnell & Stiles 1983;Holl 1998;Cole et al 2010;Derroire et al 2016) and can serve as attractive environments for seed dispersers and will provide the immediate microclimate for seed germination and seedling growth (Belsky et al 1989;Callaway 2007;Derroire et al 2016).…”

Section: Introductionmentioning

confidence: 99%

“…The effects of isolated tree densities and the structural characteristics of individual trees on seed dispersal processes in pastures have only been poorly investigated (Guevara et al 1992;Holl 1998;Dean et al 1999;Slocum & Horvitz 2000;Mazurek & Zielinski 2004;Cole et al 2010), and a better understanding of the characteristics of forest fragment connectivity in these areas could help efforts to improve seed dispersal and increase the biodiversity levels of those landscapes. We therefore sought to determine whether (i) the structural characteristics of trees growing in pastures, (ii) their densities, and (iii) their distances from forest fragments influence seed dispersal in pasture habitats.…”

Section: Introductionmentioning

confidence: 99%

Isolated trees with high crown coverage and densities increase pasture seed rain

et al. 2016

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Pasture and crop lands restrict seed dispersal near remnant forest fragments, especially by restricting the movements of dispersal agents and limiting propagule dispersal. Some factors can improve seed dispersal in open areas, such as the presence of high numbers of isolated trees in close proximity to forest fragments. We sought to determine if: (i) the structural characteristics and (ii) densities of isolated trees in pasture lands, and (iii) their distances from the forest fragments, infl uence seed dispersal. We installed 18 seed traps in each of six pastures (total=108 traps) bordering forest fragments distributed over 6 distance classes from the forest edges (0, 5, 10, 20, 40 and 80 m). We determined the characteristics of the plants surrounding the traps. GLM and GLMer analyses were performed and the best model was selected by AIC. We collected 8162 seeds (4722 anemochorous, 3304 epizoochorous, 72 autochorous, and 64 endozoochorous) belonging to 26 species. Our results showed that plants with high crown coverage close to forest fragments and at high densities in the pastures increased seed dispersal. Th ese results may aid future restoration of pasture lands by improving seed dispersal in this harsh habitat and promoting better connectivity between forest fragments.

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Large trees, fertile islands, and birds in arid savanna

Cited by 246 publications

References 36 publications

Bird-habitat associations in coastal rangelands of southern Brazil

Bird-habitat associations in coastal rangelands of southern Brazil

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

Isolated trees with high crown coverage and densities increase pasture seed rain

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