A Web‐based rainfall atlas for Southern Africa

Zucchini, Walter; Nenadić, Oleg

doi:10.1002/env.748

Cited by 18 publications

(13 citation statements)

References 17 publications

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“…I assumed a mean annual rainfall of 650 mm for Nylsvlei. Although more realistic tools are available for modeling Southern African rainfall [47], the approach I used enabled me to derive rainfall sequences with reasonable event depths and interarrival times as a function of a single independent variable (MAP).…”

Section: Methodsmentioning

confidence: 99%

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

Holdø

2013

PLoS ONE

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The two-layer hypothesis of tree-grass coexistence posits that trees and grasses differ in rooting depth, with grasses exploiting soil moisture in shallow layers while trees have exclusive access to deep water. The lack of clear differences in maximum rooting depth between these two functional groups, however, has caused this model to fall out of favor. The alternative model, the demographic bottleneck hypothesis, suggests that trees and grasses occupy overlapping rooting niches, and that stochastic events such as fires and droughts result in episodic tree mortality at various life stages, thus preventing trees from otherwise displacing grasses, at least in mesic savannas. Two potential problems with this view are: 1) we lack data on functional rooting profiles in trees and grasses, and these profiles are not necessarily reflected by differences in maximum or physical rooting depth, and 2) subtle, difficult-to-detect differences in rooting profiles between the two functional groups may be sufficient to result in coexistence in many situations. To tackle this question, I coupled a plant uptake model with a soil moisture dynamics model to explore the environmental conditions under which functional rooting profiles with equal rooting depth but different depth distributions (i.e., shapes) can coexist when competing for water. I show that, as long as rainfall inputs are stochastic, coexistence based on rooting differences is viable under a wide range of conditions, even when these differences are subtle. The results also indicate that coexistence mechanisms based on rooting niche differentiation are more viable under some climatic and edaphic conditions than others. This suggests that the two-layer model is both viable and stochastic in nature, and that a full understanding of tree-grass coexistence and dynamics may require incorporating fine-scale rooting differences between these functional groups and realistic stochastic climate drivers into future models.

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Section: Methodsmentioning

confidence: 99%

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

Holdø

2013

PLoS ONE

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“…We created the aspect, elevation and slope layers using ESRI Arc Map and the national 1:50,000 topographic map layers. For rainfall patterns, we used the South African rainfall atlas map (Zucchini and Nenadi c 2006). We used the map with the v www.esajournals.org smallest raster squares as a base map to clip the other maps in ESRI Arc Map, which resulted in the maps having a raster cell size of 45.1 3 45.1 m.…”

Section: Methodsmentioning

confidence: 99%

Using Maximum Entropy modeling to predict the potential distributions of large trees for conservation planning

et al. 2012

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Citation: Smith, A., B. Page, K. Duffy, and R. Slotow. 2012. Using Maximum Entropy modeling to predict the potential distributions of large trees for conservation planning. Ecosphere 3(6):56. http://dx.doi.org/10.1890/ES12-00053.1Abstract. Large trees, as keystone structures, are functionally important in savanna ecosystems, and low recruitment and slow growth makes their conservation important. Understanding factors influencing their distribution is essential for mitigation of excessive mortality, for example from management fires or large herbivores. We recorded the locations of large trees in Hluhluwe-Imfolozi Park (HiP) using GPS to record trees along 43 km of 10 m-wide transects. Maximum entropy modeling (MaxEnt) uses niche modeling to predict the distribution of a species from the probability of finding it within raster squares, based on environmental variables and recorded locations. MaxEnt is typically applied at a regional spatial scale, and here we assessed its usefulness when predicting the distribution of species at a small (local) scale. HiP has variable topography, heterogeneous soils, and a strong rainfall gradient, resulting in a wide variety of habitat types. We used locations of 179 Acacia nigrescens and 106 Sclerocarya birrea (large trees ! 5m), and raster environmental layers for: aspect, elevation, geology, annual rainfall, slope, soil and vegetation. A. nigrescens was largely restricted to the Imfolozi section, while S. birrea had a wider distribution across the reserve. Understanding the interaction of environmental variables dictating tree distribution may facilitate habitat restoration, and will assist planning decisions for persistence of large trees within reserves, including options to reduce fire frequency or herbivore impacts. Though the AUC (Area Under the Curve) values used to test model predictions were high for both species, the ground truthing test data showed that distribution for A. nigrescens was more accurate than that for S. birrea, highlighting the need for independent test data to assess model accuracy. We emphasize that MaxEnt can be used at finer spatial scales than those typically used for species occurrence, but models must be tested using spatially independent test data.

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“…Wetlands were classified as areas where the water table was near or periodically above the soil surface, with poorly drained hydric soils such as Vertic A and G horizons and the graminoid hydrophyte Imperata cylindrica (L.) Raeuschel was present. Mean annual rainfall for Gauteng Province was derived from the South African Rain Atlas (Zucchini and Nenadić 2006). Soil erosion was classified according to Tongway and Hindley (1995), with the following erosion types: crust brokenness, terracettes, pedestals, rills, gullies and deposits.…”

Section: Environmental Variablesmentioning

confidence: 99%

Are environmental factors important facilitators of pompom weed (Campuloclinium macrocephalum) invasion in South African rangelands?

et al. 2011

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The absence of natural enemies being keystone to the success of invasive alien plants (IAPs) can only be accepted once all the factors governing the invader and the ecosystems it invades have been established. Few studies have attempted this approach. This study reports on the relations between the invasive alien forb Campuloclinium macrocephalum (Less.) DC. (pompom weed), herbivory and the ecology of the rangelands invaded in South Africa. Eighty invaded rangelands in Gauteng Province were studied. Pompom weed herbivory was insignificant with the greatest damage being caused by native grasshoppers. Rainfall, topography and soil texture were the primary drivers of vegetation pattern. Pompom weed exhibited disturbance-mediated invasion strategies, favouring rangelands affected by non-sustainable commercial grazing practices, abandoned agricultural fields and drained wetlands. It invaded vegetation in poor condition with sward basal cover \19%. Weed density was exacerbated by high fire frequency. Vegetation susceptible to pompom weed was also invaded by other alien and native pasture weeds. The absence of natural enemies could give pompom weed a competitive advantage over native pioneer cohorts in disturbed rangelands, but is unlikely to increase fitness enabling its establishment in vegetation in good condition. Grazing strategies that promote the dominance of grass species adapted to frequent non-selective defoliation appear to restrict pompom weed better than degraded rangelands that are not utilised. Biotic resistance of rangelands to pompom weed invasion will be greatest in vegetation in good condition and under good management. The low abundance of pompom weed in rangelands in communal areas and fence-line contrasts in invaded and un-invaded road reserves warrants further investigation.

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A Web‐based rainfall atlas for Southern Africa

Cited by 18 publications

References 17 publications

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

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

Using Maximum Entropy modeling to predict the potential distributions of large trees for conservation planning

Are environmental factors important facilitators of pompom weed (Campuloclinium macrocephalum) invasion in South African rangelands?

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