Photosynthetic properties of C4 plants growing in an African savanna/wetland mosaic

Photosynthesis rates and photosynthesis-leaf nutrient relationships were analysed in nine tropical grass and sedge species growing in three different ecosystems: a rain-fed grassland, a seasonal floodplain, and a permanent swamp, located along a hydrological gradient in the Okavango Delta, Botswana. These investigations were conducted during the rainy season, at a time of the year when differences in growth conditions between the sites were relatively uniform. At the permanent swamp, the largest variations were found for area-based leaf nitrogen contents, from 20 mmol m(-2) to 140 mmol m(-2), nitrogen use efficiencies (NUE), from 0.2 mmol (C) mol(-1) (N) s(-1) to 2.0 mmol (C) mol(-1) (N) s(-1), and specific leaf areas (SLA), from 50 cm(2) g(-1) to 400 cm(2) g(-1). For the vegetation growing at the rain-fed grassland, the highest leaf gas exchange rates, high leaf nutrient levels, a low ratio of intercellular to ambient CO(2) concentration, and high carboxylation efficiency were found. Taken together, these observations indicate a very efficient growth strategy that is required for survival and reproduction during the relatively brief period of water availability. The overall lowest values of light-saturated photosynthesis (A(sat)) were observed at the seasonal floodplain; around 25 micromol m(-2) s(-1) and 30 micromol m(-2) s(-1). To place these observations into the broader context of functional leaf trait analysis, relationships of photosynthesis rates, specific leaf area, and foliar nutrient levels were plotted, in the same way as was done for previously published 'scaling relationships' that are based largely on C(3) plants, noting the differences in the analyses between this study and the previous study. The within- and across-species variation in both A(sat) and SLA appeared better predicted by foliar phosphorus content (dry mass or area basis) rather than by foliar nitrogen concentrations, possibly because the availability of phosphorus is even more critical than the availability of nitrogen in the studied relatively oligotrophic ecosystems.

SOIL FEEDBACK OF EXOTIC SAVANNA GRASS RELATES TO PATHOGEN ABSENCE AND MYCORRHIZAL SELECTIVITY

Enemy release of exotic plants from soil pathogens has been tested by examining plant-soil feedback effects in repetitive growth cycles. However, positive soil feedback may also be due to enhanced benefit from the local arbuscular mycorrhizal fungi (AMF). Few studies actually have tested pathogen effects, and none of them did so in arid savannas. In the Kalahari savanna in Botswana, we compared the soil feedback of the exotic grass Cenchrus biflorus with that of two dominant native grasses, Eragrostis lehmanniana and Aristida meridionalis. The exotic grass had neutral to positive soil feedback, whereas both native grasses showed neutral to negative feedback effects. Isolation and testing of root-inhabiting fungi of E. lehmanniana yielded two host-specific pathogens that did not influence the exotic C. biflorus or the other native grass, A. meridionalis. None of the grasses was affected by the fungi that were isolated from the roots of the exotic C. biflorus. We isolated and compared the AMF community of the native and exotic grasses by polymerase chain reaction-denaturing gradient gel elecrophoresis (PCR-DGGE), targeting AMF 18S rRNA. We used roots from monospecific field stands and from plants grown in pots with mixtures of soils from the monospecific field stands. Three-quarters of the root samples of the exotic grass had two nearly identical sequences, showing 99% similarity with Glomus versiforme. The two native grasses were also associated with distinct bands, but each of these bands occurred in only a fraction of the root samples. The native grasses contained a higher diversity of AMF bands than the exotic grass. Canonical correspondence analyses of the AMF band patterns revealed almost as much difference between the native and exotic grasses as between the native grasses. In conclusion, our results support the hypothesis that release from soil-borne enemies may facilitate local abundance of exotic plants, and we provide the first evidence that these processes may occur in arid savanna ecosystems. Pathogenicity tests implicated the involvement of soil pathogens in the soil feedback responses, and further studies should reveal the functional consequences of the observed high infection with a low diversity of AMF in the roots of exotic plants.

Vesicular-arbuscular mycorrhizal infection of grass seedlings in a degraded semi-arid savanna in Botswana

Infection of eight indigenous grasses with vesicular-arbuscular mycorrhizal (VAM) fungi was studied in a savanna in South-eastern Botswana. Infection of seedlings with VAM fungi was rapid just after germination at the onset of the rainfall season. Particularly high infection levels were found in the primary roots of seedlings but levels temporarily dropped with the development of the secondary roots. In 30 % of all cases germinating spores were found to be the source of infection, suggesting an important role for them in semi-arid grasslands. The inoculum potential of soils from eroded and non-eroded sites in the study area was compared. Subsoil was found to have a lower inoculum potential than topsoil. Topsoil from an eroded site without vegetation showed a lowered inoculum potential compared to sites with vegetation present. Topsoil from unvegetated sites where soil material was deposited however, did not always show a lower inoculum potential. This indicates that the transport and redistribution of water and soil material does not automatically result in lower infection levels.