Figure 5 - uploaded by Lixin Wang
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1. The Kalahari sands and sampling locations of the soil profiles. The Kalahari sand sheet distribution is adopted from Thomas and Shaw (1991).
Citations
... The vegetation cover in Pandamatenga is a woodland savanna dominated by tree species such as Kirkia africana and grass species such as Schmidtia pappophoroides and Pogonarihria squarrosa. The soil type in this region is classified as " Shifting Sands " according to the USDA classification (Wang, 2008). The soil temperatures at the four sites were similar and the mean soil temperatures measured in January–February 2006 were 24.5°C, 23.7°C, 26.7°C and 25.4°C at Tshane, Ghanzi, Pandamatenga and Mongu, respectively. ...
Southern African savannas are mixed plant communities where C3 trees co-exist with C4 grasses. Here foliar δ15N and δ13C were used as indicators of nitrogen uptake and of water use efficiency to investigate the effect of the rainfall regime on the use of nitrogen and water by herbaceous and woody plants in both dry and wet seasons. Foliar δ15N increased as aridity rose for both C3 and C4 plants for both seasons, although the magnitude of the increase was different for C3 and C4 plants and for two seasons. Soil δ15N also significantly increased with aridity. Foliar δ13C increased with aridity for C3 plants in the wet season but not in the dry season, whereas in C4 plants the relationship was more complex and non-linear. The consistently higher foliar δ15N for C3 plants suggests that C4 plants may be a superior competitor for nitrogen. The different foliar δ13C relationships with rainfall may indicate that the C3 plants have an advantage when competing for water resources. The differences in water and nitrogen use likely collectively contribute to the tree–grass coexistence in savannas. Such differences facilitate interpretations of palaeo-vegetation composition variations and help predictions of vegetation composition changes under future climatic scenarios.
... The vegetation cover in Pandamatenga is a woodland savanna dominated by tree species such as Kirkia africana and grass species such as Schmidtia pappophoroides and Pogonarihria squarrosa. The soil type in this region is classified as "Shifting Sands" according to the USDA classification (Wang, 2008). The soil temperatures at the four sites were similar and the mean soil temperatures measured in January-February 2006 were 24.5°C, 23.7°C, 26.7°C and 25.4°C at Tshane, Ghanzi, Pandamatenga and Mongu, respectively. ...
... Regardless of the processes underlying tree–grass co-dominance, the belowground distributions of resources (e.g., nutrients) and belowground processes likely play important roles on vegetation dynamics and the composition and structure of vegetation should exhibit the imprints of these distributions and belowground processes. Vegetation distribution and surface soil biogeochemical properties along the KT have been documented by a number of studies (Caylor et al., 2003Caylor et al., , 2005 Privette et al., 2004; Ringrose et al., 1998; Scholes et al., 2002 Scholes et al., , 2004 Wang et al., 2007a; Wang, 2008). The plant–soil interactions along the KT have also been investigated (D'Odorico et al., 2007; Okin et al., 2008). ...
... The P-limitation is also supported by the observed high foliar N/P ratios (N/P z 20–30) for C 3 plants at the drier Kalahari sites in a wet year (Aranibar et al., 2003). Nitrogen is another potential factor that limits plant productivity in part of tropical savannas with higher water availability (Scanlon and Albertson, 2003) although recent studies show that in the KT N limitations are less important than water limitation (Wang, 2008). With the exception of Tshane, soil surface (top 20 cm) nitrate concentrations are much higher in the under canopy areas than the between canopy areas in the dry season but are similar in the wet season. ...
Savannas cover about 20% of the Earth's land area across a wide range of climatic conditions. As an important and distinct biome, savannas produce approximately 29% of global terrestrial net primary productivity. In these ecosystems the distribution of belowground resources remains poorly investigated and the relationship to the climatic conditions remains unclear. In the present study, vertical profiles of soil nutrients (chloride, nitrate, phosphate and sulfate) and nitrogen stable isotopes were analyzed at four sites along the Kalahari mega-transect, where a distinct rainfall gradient exists on a homogeneous soil substrate. The results show clear differences in nutrients and δ15N vertical distributions between wet and dry seasons. The results also show how the formation of “fertility islands” (i.e., the concentration of soil nutrients in the soils beneath tree canopies) is not necessarily coupled with belowground processes in that the distribution of soil nutrients at the surface does not match belowground patterns. The results also indicate that phosphorus may be a limiting nutrient in these savanna ecosystems with seasonal dynamics in its cycling.
... The vegetation cover in Pandamatenga is a woodland savanna dominated by tree species such as Kirkia africana and grass species such as Schmidtia pappophoroides and Pogonarihria squarrosa. The soil type in this region is classified as "Shifting Sands" according to the USDA classification ( Wang, 2008). The soil temperatures at the four sites were similar and the mean soil temperatures measured in January-February 2006 were 24.5°C, 23.7°C, 26.7°C and 25.4°C at Tshane, Ghanzi, Pandamatenga and Mongu, respectively. ...
Southern African savannas are mixed plant communities where C 3 trees co-exist with C 4 grasses. Here foliar δ 15 N and δ 13 C were used as indicators of nitrogen uptake and of water use efficiency to investigate the effect of the rainfall regime on the use of nitrogen and water by herbaceous and woody plants in both dry and wet seasons. Foliar δ 15 N increased as aridity rose for both C 3 and C 4 plants for both seasons, although the magnitude of the increase was different for C 3 and C 4 plants and for two seasons. Soil δ 15 N also significantly increased with aridity. Foliar δ 13 C increased with aridity for C 3 plants in the wet season but not in the dry season, whereas in C 4 plants the relationship was more complex and non-linear. The consistently higher foliar δ 15 N for C 3 plants suggests that C 4 plants may be a superior competitor for nitrogen. The different foliar δ 13 C relationships with rainfall may indicate that the C 3 plants have an advantage when competing for water resources. The differences in water and nitrogen use likely collectively contribute to the tree–grass coexistence in savannas. Such differences facilitate interpretations of palaeo-vegetation composition variations and help predictions of vegetation composition changes under future climatic scenarios.
... The vegetation cover in Pandamatenga is a woodland savanna dominated by tree species such as Kirkia africana and grass species such as Schmidtia pappophoroides and Pogonarihria squarrosa. The soil type in this region is classified as "Shifting Sands" according to the USDA classification (Wang, 2008). The soil temperatures at the four sites were similar and the mean soil temperatures measured in January-February 2006 were 24.5°C, 23.7°C, 26.7°C and 25.4°C at Tshane, Ghanzi, Pandamatenga and Mongu, respectively. ...
Southern African savannas are mixed plant communities where C3 trees co-exist with C4 grasses. Owing to differences in their morphology and physiology, trees and grasses have different access to nutrients and water and different efficiency in the use of these resources. It is still unclear how climate variables such as the mean annual precipitation may affect the relative efficiency of grasses and trees in the use of water and soil nutrients such as nitrogen. In this study, the foliar delta15N and delta13C were used as indicators of nitrogen uptake and of water use efficiency, respectively, to investigate the effect of the rainfall regime on the use of nitrogen and water by herbaceous and woody plants. To this end, patterns of foliar delta15N and delta13C for both C3 and C4 plants as well as patterns of soil delta15N and delta13C in canopy and intercanopy areas were investigated both in the dry and in the wet season along the Kalahari megatransect, where a distinct rainfall gradient exists on a homogeneous soil substrate. Foliar delta15N signatures increased as aridity heightened for both C3 and C4 plants in both seasons, although the magnitude of the increase was different for these two plant functional types. Soil delta15N also significantly increased with aridity. Foliar delta13C signatures increased with aridity for C3 plants in the wet season but not in the dry season, while in C4 plants the relation between foliar delta13C signatures and aridity was more complex and non-linear in both seasons. The consistent higher foliar delta15N for C3 plants suggests that C4 plants are superior competitor for N. The different foliar delta13C relationships with rainfall for the C3 plants and C4 plants may indicate that the C3 plants have an advantage over C4 plants when competing for water resources. The differences in water and nitrogen use between C3 and C4 plants likely collectively contribute to the tree-grass coexistence in savannas.
Savanna ecosystems are mixed plant communities in which trees and grasses co-exist thereby providing a heterogeneous landscape
with a mosaic of tree-dominated and grass-dominated soil patches. Despite the important role that nutrient availability plays
in these systems, detailed knowledge of differences in carbon and nitrogen cycling in soil patches predominantly covered by
tree canopies or by grasses is still lacking. In this study, a process-based model was used to investigate the carbon and
nitrogen dynamics in soil plots located in grass-dominated and tree/shrub-dominated soil patches along the Kalahari Transect
(KT). The KT in southern Africa traverses a dramatic aridity gradient, across relatively homogenous soils, providing an ideal
setting for global change studies. Here we show that there are distinctly different dynamics for soil moisture, decomposition
and nitrogen mineralization between soil plots located under tree canopies and in open canopy areas, especially at the dryer
end of the KT. Such differences diminished when approaching the wetter end of this transect. This study shows that in savanna
ecosystems, water availability determines the patterns and rates of nutrient cycling at large scales, while at the local scales,
vegetation patchiness plays an important role in nutrient cycling. Savannas are relatively stable ecosystems, resilient to
small rainfall modifications, although irreversible changes may occur with stronger shifts in climate conditions.
