Do fungal pathogens drive density-dependent mortality in established seedlings of two dominant African rain forest trees?

Where one or a few tree species reach local high abundance, different ecological factors may variously facilitate or hinder their regeneration. Plant pathogens are thought to be one of those possible agents which drive intraspecific density-dependent mortality of tree seedlings in tropical forests. Experimental evidence for this is scarce, however. In an African rain forest at Korup, we manipulated the density of recently established seedlings (~5–8 wk old; low vs. high-density) of two dominant species of contrasting recruitment potential, and altered their exposure to pathogens using a broad-spectrum fungicide. Seedling mortality of the abundantly recruiting subcanopy tree Oubanguia alata was strongly density-dependent after 7 mo, yet fungicide-treated seedlings had slightly higher mortality than controls. By contrast, seedling mortality of the poorly recruiting large canopy-emergent tree Microberlinia bisulcata was unaffected by density or fungicide. Ectomycorrhizal colonization of M. bisulcata was not affected by density or fungicide either. For O. alata, adverse effects of fungicide on its vesicular arbuscular mycorrhizas may have offset any possible benefit of pathogen removal. We tentatively conclude that fungal pathogens are not a likely major cause of density dependence in O. alata, or of early post-establishment mortality in M. bisulcata. They do not explain the latter's currently very low recruitment rate at Korup.

Longer term clinical and virological outcome of sub-Saharan African participants on antiretroviral treatment in the Swiss HIV Cohort Study

Persons from sub-Saharan Africa (SSA) are increasingly enrolled in the Swiss HIV Cohort Study (SHCS). Cohorts from other European countries showed higher rates of viral failure among their SSA participants. We analyzed long-term outcomes of SSA versus North Western European participants.

Improved antiretroviral treatment outcome in a rural African setting is associated with cART initiation at higher CD4 cell counts and better general health condition

Background Data on combination antiretroviral therapy (cART) in remote rural African regions is increasing. Methods We assessed prospectively initial cART in HIV-infected adults treated from 2005 to 2008 at St. Francis Designated District Hospital, Ifakara, Tanzania. Adherence was assisted by personal adherence supporters. We estimated risk factors of death or loss to follow-up by Cox regression during the first 12 months of cART. Results Overall, 1,463 individuals initiated cART, which was nevirapine-based in 84.6%. The median age was 40 years (IQR 34-47), 35.4% were males, 7.6% had proven tuberculosis. Median CD4 cell count was 131 cells/μl and 24.8% had WHO stage 4. Median CD4 cell count increased by 61 and 130 cells/μl after 6 and 12 months, respectively. 215 (14.7%) patients modified their treatment, mostly due to toxicity (56%), in particular polyneuropathy and anemia. Overall, 129 patients died (8.8%) and 189 (12.9%) were lost to follow-up. In a multivariate analysis, low CD4 cells at starting cART were associated with poorer survival and loss to follow-up (HR 1.77, 95% CI 1.15-2.75, p = 0.009; for CD4 <50 compared to >100 cells/μl). Higher weight was strongly associated with better survival (HR 0.63, 95% CI 0.51-0.76, p < 0.001 per 10 kg increase). Conclusions cART initiation at higher CD4 cell counts and better general health condition reduces HIV related mortality in a rural African setting. Efforts must be made to promote earlier HIV diagnosis to start cART timely. More research is needed to evaluate effective strategies to follow cART at a peripheral level with limited technical possibilities.

Seed fate and seedling dynamics after masting in two African rain forest trees

How the effects of biotic factors are moderated by abiotic factors, and their consequences for species interactions, is generally understudied in ecology. A key abiotic feature of forests is regular canopy disturbances that create temporary patches, or “gaps,” of above-average light availability. Co-occurring in lowland primary forest of Korup National Park (Cameroon), Microberlinia bisulcata and Tetraberlinia bifoliolata are locally dominant, ectomycorrhizal trees whose seeds share predator guilds in masting years. Here, we experimentally tested the impact of small mammal predators upon seedling abundance, growth, and survivorship. In 2007, we added a fixed density of seeds of each species to exclosures at 48 gap–understory locations across 82.5 ha within a large Microberlinia grove, and at 15 locations outside it. For both species, small mammals removed more seeds in gaps than in understory, whereas this was reversed for seeds killed by invertebrates. Nonetheless, Microberlinia lost twice as many seeds to small mammals, and more to invertebrates in exclosures, than Tetraberlinia, which was more prone to a pathogenic white fungus. After six weeks, both species had greater seedling establishment in gaps than understory, and in exclosures outside compared to exclosures inside the grove. In the subsequent two-year period, seedling growth and survivorship peaked in exclosures in gaps, but Microberlinia had more seedlings' stems clipped by animals than Tetraberlinia, and more than twice the percentage of leaf area damaged. Whereas Microberlinia seedling performance in gaps was inferior to Tetraberlinia inside the grove, outside it Microberlinia had reduced leaf damage, grew taller, and had many more leaves than Tetraberlinia. No evidence was found for “apparent mutualism” in the understory as seedling establishment of both species increased away from (>25 m) large stems of either species, pointing to “apparent competition” instead. In gaps, Microberlinia seedling establishment was lower near Tetraberlinia than conspecific adults because of context-dependent small mammal satiation. Stage-matrix analysis suggested that protecting Microberlinia from small mammals could increase its population growth rate by 0.06. In the light of prior research we conclude that small mammals and canopy gaps play an important role in promoting species coexistence in this forest, and that their strong interaction contributes to Microberlinia's currently very poor regeneration.

Effects of an overnight intravenous lipid infusion on intramyocellular lipid content and insulin sensitivity in African-American versus Caucasian adolescents

To explain the predisposition for insulin resistance among African American (AA) adolescents, this study aimed to: 1) examine changes in intramyocellular lipid content (IMCL), and insulin sensitivity with intralipid (IL) infusion; and 2) determine whether the increase in IMCL is comparable between AA and Caucasian adolescents.