A depth‐controlled tracer technique measures vertical, horizontal and temporal patterns of water use by trees and grasses in a subtropical savanna

Kulmatiski, Andrew; Beard, Karen H.; Verweij, Richard J. T.; February, Edmund C.

doi:10.1111/j.1469-8137.2010.03338.x

Cited by 132 publications

(169 citation statements)

References 58 publications

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“…The source-water partitioning between C. drummondii and A. gerardii is consistent with a large number of studies that have found a vertical separation of tree and grass root systems (Jackson et al 1996, Weltzin andMcPherson 1997, but see Scholes and Archer 1997, Bond 2008, Kulmatiski et al 2010. However, it is surprising that C. drummondii clones measured in the grassland matrix were accessing the same soil water source as the mature shrubs, even up to 5 m from the center of the shrub island ( p .…”

Section: Discussionsupporting

confidence: 86%

Positive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie

et al. 2011

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Abstract. Over the last century, many grasslands worldwide have transitioned from a graminoid to a tree/shrub-dominated state in a short period of time, a phenomenon referred to as woody encroachment. Positive feedbacks and bi-stability are thought to be important drivers of woody encroachment, but there is little empirical evidence to suggest that positive feedbacks accelerate the woody encroachment of mesic grasslands. In mesic tallgrass prairie, shrub establishment does not directly facilitate seedling establishment. Yet, shrub establishment may facilitate the clonal spread of existing shrubs into nearby patches, because clonal reproduction might circumvent barriers that typically limit woody seedlings. Our results show that when Cornus drummondii (the predominate encroacher of mesic tallgrass prairie) extends rhizomatous stems into open grasslands, these stems use the same deep soil water sources as mature stems-thereby avoiding competition with grasses and gaining access to a reliable water source. In addition, herbaceous fuel concentrations are lower at the shrub/grass interface than in open grasslands, reducing the potential impacts of subsequent grassland fires. We propose that the release from resource and fire limitation results in a positive feedback loop as clonal stems are able to extend into surrounding patches, circumvent demographic barriers, mature, and spread by developing their own clonal stems. Long-term data on site (26 years) corroborates this interpretation: the size of deep-rooted clonal shrub species has increased 16-fold and their cover has increased from 0 to 27%, whereas the cover of shallowrooted species (both clonal and non-clonal) has only increased marginally. Together, these results suggest that (1) positive feedbacks can facilitate mesic woody encroachment and (2) bi-stability exists in mesic tallgrass prairie.

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Section: Discussionsupporting

confidence: 86%

Positive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie

et al. 2011

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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 Watermentioning

confidence: 59%

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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“…1a), and that deep soil layers retain soil moisture for a significant proportion of the growing season. Previous work using tracers has revealed that trees and grasses differ (clearly but subtly) in terms of functional rooting depth in a similar site in nearby Kruger National Park (Kulmatiski et al 2010, Kulmatiski andBeard 2013a). These authors also found that trees (but not grasses) appear to be highly plastic in their uptake patterns, using shallow soil moisture early in the season and shifting to sources below 30 cm depth later in the season (Kulmatiski and Beard 2013a).…”

Section: Discussionmentioning

confidence: 84%

Transpiration dynamics support resource partitioning in African savanna trees and grasses

Holdø

Nippert

2015

Ecology

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Abstract. It is still far from clear whether and to what extent trees and grasses partition soil moisture in tropical savannas. A major reason for this is that we don't know how snapshot data on rooting differences translate into ecologically relevant patterns of water use at seasonal scales. We used stable isotopes in soil and stem water to quantify functional rooting profiles in grasses and two tree species in a South African savanna. Concurrently, we measured tree sap-flow velocity, grass canopy temperature (a transpiration correlate), and soil moisture content at multiple depths over the course of a growing season. We used lasso regression to identify the dominant soil moisture layers driving daily variation in tree and grass water-use metrics while controlling for weather variables. We found clear rooting depth differences between grasses (shallow) and trees (deep) from the isotopic data, and these patterns were strongly supported by the water-use data, which showed that grasses and trees predominantly responded to soil moisture availability at 5 and 40 cm depth, respectively. Our results provide a rare example of mechanistic support for the resource partitioning hypothesis in savannas, with important implications for our understanding of tree-grass dynamics under altered precipitation regimes.

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A depth‐controlled tracer technique measures vertical, horizontal and temporal patterns of water use by trees and grasses in a subtropical savanna

Cited by 132 publications

References 58 publications

Positive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie

Positive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie

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

Transpiration dynamics support resource partitioning in African savanna trees and grasses

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