Soil texture mediates tree responses to rainfall intensity in African savannas

Case, Madelon F.; Staver, A. Carla

doi:10.1111/nph.15254

Cited by 50 publications

(61 citation statements)

References 88 publications

(172 reference statements)

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“…Grass biomass was modelled as the response variable, with rainfall, previous‐year rainfall, temperature, year, and geologic substrate as fixed effects and plot identity as a random effect. We also considered possible interactions between rainfall and geologic substrate (because soil texture has been proposed to mediate rainfall availability; e.g., Case & Staver, ) and between rainfall and previous‐year rainfall (because these have been proposed to interact; see Sala et al, ). Thus, the most complicated possible model is given by

\begin{matrix} grass biomass rainfall + previous - year \\ rainfall + substrate + temparature + year + rainfall \times \\ substrate + rainfall \times previous - year rainfall + (random effect = site) \end{matrix}

…”

Section: Methodsmentioning

confidence: 99%

Grazer movements exacerbate grass declines during drought in an African savanna

2018

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Rainfall variability, including drought, consistently translates into variation in grass productivity in savanna ecosystems. These variations in grass availability can impact grazer populations, which can in turn shape grass responses to drought. However, despite clear evidence that grazers move freely in response to forage and water availability, drought ecologists have mostly failed to consider how these landscape‐scale processes influence vegetation responses to drought. Here, we ask how rainfall variability and drought, in interaction with population dynamics and movement of large grazers, shape grass layer responses in extant savanna landscapes in Kruger National Park in South Africa. We found that grass biomass was highly responsive to rainfall variability; notably, grass declines during drought were especially severe—even more severe than previously documented elsewhere. Grazing probably contributed, exacerbating drought effects in droughted regions and spatially extending them into non‐droughted refugia, where grass biomass also declined. Moreover, the existence of these refugia potentially prevented a grazer population crash. Synthesis. Our current focus on the physiological effects of drought is probably insufficient for understanding vegetation responses to drought. A renewed focus on landscape processes, including animal movement, will be critical to predicting savanna responses to increasingly frequent extreme events.

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\begin{matrix} grass biomass rainfall + previous - year \\ rainfall + substrate + temparature + year + rainfall \times \\ substrate + rainfall \times previous - year rainfall + (random effect = site) \end{matrix}

…”

Section: Methodsmentioning

confidence: 99%

Grazer movements exacerbate grass declines during drought in an African savanna

2018

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“…Climate generally varies over large spatial scales, but substantial variation in tree cover (from open grassland to forest) can occur at much finer scales (0.01-0.1 km), distances over which climate is essentially invariant. At these scales, the effects of soils on soil moisture availability, interacting with intraannual variation in precipitation (Case & Staver, 2018;D'Onofrio, Sweeney, von Hardenberg, & Baudena, 2019), likely play a larger role. In particular, there is a well-known association between tree cover and soil texture in savannas, with coarse-textured soils tending to favour trees over grasses (Case & Staver, 2018;Fensham, Butler, & Foley, 2015;Knoop & Walker, 1985;Noy-Meir, 1973;Sankaran, Ratnam, & Hanan, 2008;Staver, Botha, & Hedin, 2017), implying a texture-driven hydrological mechanism.…”

Section: Introductionmentioning

confidence: 99%

Spatial transitions in tree cover are associated with soil hydrology, but not with grass biomass, fire frequency, or herbivore biomass in Serengeti savannahs

et al. 2019

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Although there is a well‐known association between tree cover and soil texture in savannahs, the hydrological drivers of tree cover variation have not been systematically explored, particularly in parallel with factors such as fire, herbivory, and tree–grass interactions. The relationship between hydrological factors and tree cover is important for resolving the relative contribution of bottom‐up versus top‐down factors in structuring savannah vegetation. We quantified soil moisture dynamics across eight 1‐km transects spanning tree cover gradients from open to woody savannah in Serengeti National Park in Tanzania using soil moisture sensors coupled with dataloggers. We mapped tree cover at two spatial scales through supervised classification of high‐resolution satellite imagery. We simultaneously produced water retention curves in open and woody habitats within each transect to compare soil hydrological properties and to convert volumetric water content (θ) from dataloggers to plant‐available water over the course of an annual cycle. We also quantified grass biomass at 100 locations per transect, estimated fire frequency from MODIS satellite data, and quantified herbivore occupancy with paired camera traps situated in open and woody habitats within each transect. We found a positive relationship between tree cover and soil moisture drainage rate, and found that open habitats had more negative water potentials than woody habitats for a given value of θ. In contrast, we found no evidence for a consistent relationship between grass biomass or fire frequency and tree cover. We found evidence for higher browser occupancy in woody than open habitats, but no habitat effects on herbivores as a group (browsers plus grazers), suggesting that herbivory is unlikely to be the dominant factor explaining variation in tree cover. Synthesis. Our results suggest that variation in tree cover is partly driven by hydrological (edaphic) factors unrelated to fire, herbivory, tree–grass interactions or mean annual precipitation at these spatial scales in Serengeti. We contrast our findings with previous work attributing tree cover shifts in Serengeti to precipitation gradients.

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“…The causes of shrub encroachment are diverse, from overgrazing and fire suppression (Case & Staver, 2017;Stevens et al, 2017;Venter et al, 2018) to changing precipitation patterns to increased CO 2 concentrations (Case & Staver, 2018). Furthermore, these factors are often interrelated and are likely to interact (Archer et al, 2017).…”

Section: Evidence and Consequences Of Semi-arid Biome Shiftsmentioning

confidence: 99%

“…the timing, intensity, seasonality, amount of precipitation), vegetation (community type, species traits), soil type, topography, and human management (e.g. fire, grazing) and feedbacks among these factors (Case & Staver, 2018;Touboul et al, 2018;Venter et al, 2018). As a result, many different conceptual approaches have been developed to understand these systems (Archer et al, 2017;Bestelmeyer et al, 2018;Peters et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

Forecasting semi‐arid biome shifts in the Anthropocene

Kulmatiski

Mackay

et al. 2020

New Phytologist

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Summary Shrub encroachment, forest decline and wildfires have caused large‐scale changes in semi‐arid vegetation over the past 50 years. Climate is a primary determinant of plant growth in semi‐arid ecosystems, yet it remains difficult to forecast large‐scale vegetation shifts (i.e. biome shifts) in response to climate change. We highlight recent advances from four conceptual perspectives that are improving forecasts of semi‐arid biome shifts. Moving from small to large scales, first, tree‐level models that simulate the carbon costs of drought‐induced plant hydraulic failure are improving predictions of delayed‐mortality responses to drought. Second, tracer‐informed water flow models are improving predictions of species coexistence as a function of climate. Third, new applications of ecohydrological models are beginning to simulate small‐scale water movement processes at large scales. Fourth, remotely‐sensed measurements of plant traits such as relative canopy moisture are providing early‐warning signals that predict forest mortality more than a year in advance. We suggest that a community of researchers using modeling approaches (e.g. machine learning) that can integrate these perspectives will rapidly improve forecasts of semi‐arid biome shifts. Better forecasts can be expected to help prevent catastrophic changes in vegetation states by identifying improved monitoring approaches and by prioritizing high‐risk areas for management.

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Soil texture mediates tree responses to rainfall intensity in African savannas

Cited by 50 publications

References 88 publications

Grazer movements exacerbate grass declines during drought in an African savanna

Grazer movements exacerbate grass declines during drought in an African savanna

Spatial transitions in tree cover are associated with soil hydrology, but not with grass biomass, fire frequency, or herbivore biomass in Serengeti savannahs

Forecasting semi‐arid biome shifts in the Anthropocene

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