Revisiting the Two-Layer Hypothesis: Coexistence of Alternative Functional Rooting Strategies in Savannas

Holdø, Ricardo M.

doi:10.1371/journal.pone.0069625

Cited by 64 publications

(85 citation statements)

References 59 publications

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“…tree sites). This observation clearly contradicts the widely discussed two-layer hypothesis, proposing independent ecological niches for root water uptake of trees and understorey plants in savannahs in order to avoid competition (Hipondoka et al, 2003;Holdo and Planque, 2013;Kulmatiski et al, 2010;Walter et al, 1971). Moreover, exponential soil profiles of plant available nitrogen cause a coupled water and nutrient competition between herbs and trees in this ecosystem during spring (Dubbert et al, 2014b).…”

Section: Dynamic Responses Of Event Water Use and Plasticity Of Watercontrasting

confidence: 60%

Quantification of dynamic soil–vegetation feedbacks following an isotopically labelled precipitation pulse

Piayda¹,

Dubbert

Siegwolf

et al. 2017

Biogeosciences

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Abstract. The presence of vegetation alters hydrological cycles of ecosystems. Complex plant-soil interactions govern the fate of precipitation input and water transitions through ecosystem compartments. Disentangling these interactions is a major challenge in the field of ecohydrology and a pivotal foundation for understanding the carbon cycle of semiarid ecosystems. Stable water isotopes can be used in this context as tracer to quantify water movement through soilvegetation-atmosphere interfaces.The aim of this study is to disentangle vegetation effects on soil water infiltration and distribution as well as dynamics of soil evaporation and grassland water use in a Mediterranean cork oak woodland during dry conditions. An irrigation experiment using δ 18 O labelled water was carried out in order to quantify distinct effects of tree and herbaceous vegetation on the infiltration and distribution of event water in the soil profile. Dynamic responses of soil and herbaceous vegetation fluxes to precipitation regarding event water use, water uptake depth plasticity, and contribution to ecosystem soil evaporation and transpiration were quantified.Total water loss to the atmosphere from bare soil was as high as from vegetated soil, utilizing large amounts of unproductive evaporation for transpiration, but infiltration rates decreased. No adjustments of main root water uptake depth to changes in water availability could be observed during the experiment. This forces understorey plants to compete with adjacent trees for water in deeper soil layers at the onset of summer. Thus, understorey plants are subjected to chronic water deficits faster, leading to premature senescence at the onset of drought. Despite this water competition, the presence of cork oak trees fosters infiltration and reduces evapotranspirative water losses from the understorey and the soil, both due to altered microclimatic conditions under crown shading. This study highlights complex soilplant-atmosphere and inter-species interactions controlling rain pulse transitions through a typical Mediterranean savannah ecosystem, disentangled by the use of stable water isotopes.

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Section: Dynamic Responses Of Event Water Use and Plasticity Of Watercontrasting

confidence: 60%

Quantification of dynamic soil–vegetation feedbacks following an isotopically labelled precipitation pulse

Piayda¹,

Dubbert

Siegwolf

et al. 2017

Biogeosciences

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“…This idea has led to the view that plants may utilize different niches (Silvertown et al, 2015) by partitioning their roots according to the hydrological conditions of different layers (e.g., Walter's two-layer hypothesis). Specifically, Walter's hypothesis states, in part, that shallow and deeply rooted plants do not compete for the same water resources (Walter, 1939;Weltzin and McPherson, 1997;Schenk and Jackson, 2002a;Schwinning, 2008;Holdo, 2013;Ward et al, 2013).…”

Section: Hypothesismentioning

confidence: 99%

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

et al. 2017

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Abstract. Trees, the most successful biological power plants on earth, build and plumb the critical zone (CZ) in ways that we do not yet understand. To encourage exploration of the character and implications of interactions between trees and soil in the CZ, we propose nine hypotheses that can be tested at diverse settings. The hypotheses are roughly divided into those about the architecture (building) and those about the water (plumbing) in the CZ, but the two functions are intertwined. Depending upon one's disciplinary background, many of the nine hypotheses listed below may appear obviously true or obviously false. (1) Tree roots can only physically penetrate and biogeochemically comminute the immobile substrate underlying mobile soil where that underlying substrate is fractured or pre-weathered. (2) In settings where the thickness of weathered material, H , is large, trees primarily shape the CZ through biogeochemical reactions within the rooting zone. (3) In forested uplands, the thickness of mobile soil, h, can evolve toward a steady state because of feedbacks related to root disruption and tree throw. (4) In settings where h H and the rates of uplift and erosion are low, the uptake of phosphorus into trees is buffered by the fine-grained fraction of the soil, and the ultimate source of this phosphorus is dust. (5) In settings of limited water availability, trees maintain the highest length density of functional roots at depths where water can be extracted over most of the growing season with the least amount of energy expenditure. (6) Trees grow the majority of their roots in the zone where the most growth-limiting resource is abundant, but they also grow roots at other depths to forage for other resources and to hydraulically redistribute those resources to depths where they can be taken up more efficiently. (7) Trees rely on matrix water in the unsaturated zone that at times may have anPublished by Copernicus Publications on behalf of the European Geosciences Union. 5116 S. L. Brantley et al.: Reviews and syntheses: on the roles trees play in building and plumbing the critical zone isotopic composition distinct from the gravity-drained water that transits from the hillslope to groundwater and streamflow. (8) Mycorrhizal fungi can use matrix water directly, but trees can only use this water by accessing it indirectly through the fungi. (9) Even trees growing well above the valley floor of a catchment can directly affect stream chemistry where changes in permeability near the rooting zone promote intermittent zones of water saturation and downslope flow of water to the stream. By testing these nine hypotheses, we will generate important new cross-disciplinary insights that advance CZ science.

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“…(Walker and Noy-Meir, 1982;Eagleson and Segarra, 1985). Recent research indicates that woody plants and grasses can coexist even with substantial rooting overlap because of difference in resource uptake (Holdo et al, 2013;Ward et al, 2013). My results suggest that woody plants and grasses can coexist with substantial root overlapping (α 1 /α 2 ) in relatively dry environments (Figure 4.2a, b).…”

Section: Case Study: Kalahari Transect In Southern Africamentioning

confidence: 59%

“…and (Scholes and Walker, 1993;Holdo et al, 2013), assuming the ratio of aboveground biomass to total woody biomass to be 0.6.…”

Section: Case Study: Kalahari Transect In Southern Africamentioning

confidence: 99%

“…Walter's two layer hypothesis has been supported in some ecosystems (Knoop and Walker, 1985;Sala et al, 1989;Weltzin and McPherson, 1997) but not in others (Hipondoka et al, 2003;Sankaran et al, 2004;Beckage et al, 2009). Recent studies revisited this hypothesis and found that difference in resource uptake between woody plants and grasses could prevent competitive exclusion even with substantial root overlap (Holdo et al, 2013;Ward et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

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Global Environmental Change in Drylands: Ecohydrological Controls on Plant Communities

Yu¹

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Drylands cover much of the terrestrial land surface and many people depend on them as sources of their livelihoods. In the past few decades human activities associated with fossil fuel burning, fertilizer applications, and land use change have dramatically increased atmospheric CO2 concentrations and other trace gases (i.e., NO and NO2) as well as atmospheric temperature, a trend that is expected to continue in the decades to come. Global climate models predict increased precipitation variability at intra-annual, interannual and decadal time scales, especially in dryland regions. These changes in environmental conditions combined with human activities have led and will lead to significant changes in vegetation cover and plant community composition, with important impacts on ecohydrological and geochemical processes, regional

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Revisiting the Two-Layer Hypothesis: Coexistence of Alternative Functional Rooting Strategies in Savannas

Cited by 64 publications

References 59 publications

Quantification of dynamic soil–vegetation feedbacks following an isotopically labelled precipitation pulse

Quantification of dynamic soil–vegetation feedbacks following an isotopically labelled precipitation pulse

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

Global Environmental Change in Drylands: Ecohydrological Controls on Plant Communities

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