Leaf-nutrient accumulation and turnover at three stages of succession from heathland to forest

Abstract. Accumulation of nutrients in leaves of the dominating species of three ecosystems, characterizing the secondary succession from Genisto-Callunetum heathland through Leucobryo-Pinetum birch-pine woodland to mature Querco-Fagetum oak-beech forest, as well as nutrient turnover within these ecosystems was studied. The objective of the study was to establish potential variations in quantity and quality of nutrient supply to the plants with respect to succession dynamics. The results show very low leaf nutrient concentrations of all species investigated, coinciding with low nutrient availability in the soil. However, the nutrient content of leaves and leaf litter of Quercus petraea and Fagus sylvatica, which dominate the late succession stages, and in Betulapéndula are higher than in the photosynthetic organs (leaves and young shoots) of Calluna vulgaris and Pinus sylvestris. The combination of the higher nutrient content of the leaves and an increasing leaf-litter production during succession results in an increased nutrient turnover via leaf-litter fall. However, due to the high leaf biomass, the storage of nutrients in the leaf biomass is highest within the birch-pine woodland. From this, it may be assumed that the low demand and the low loss of nutrients via leaf-litter fall are favourable for Pinus at the early stages of forest succession on poor sandy soils. In contrast, Quercus and Fagus are provided with better growth conditions at the later stages of succession resulting from the accumulation of plant-available nutrients in the ecosystem by Pinus sylvestris, combined with a higher nutrient turnover as compared with the heathland.     

Small‐scale spatial dynamics of vegetation in a grazed Uruguayan grassland

We explored the small‐scale plant species mobility in a subhumid native grassland subjected to grazing by cattle in south‐western Uruguay. We established four permanent plots of 40 × 40 cm, divided in 16 × 16 cells. In each cell, the presence of species was seasonally recorded for 2 years and annually recorded for 4 years. By nesting the cells, we studied the mobility at different scales, from 6.25 cm² to 400 cm². At each scale we measured species richness, cumulative richness and the turnover rates of the dominant species. We found that the cumulative species richness was an increasing power function, with higher accumulation rates with smaller spatial scale. Although species richness showed seasonal fluctuations, the mean species richness was constant during the study period. We detected significant spatio‐temporal variability in mobility patterns among species. Certain species showed a high capacity to colonize new sites, whereas other species rotate among sites that they previously occupied. Grazed communities in Uruguayan Campos are structured as a dense matrix of perennials grasses and forbs, where vegetative propagation is the main form of growth of the species. The small‐scale dynamics and the high variability in the mobility characteristics could be linked with the diversity of growth forms and spatial strategies of the species in this community. We believe that a high degree of small‐scale spatial dynamics contribute to explain the species coexistence and the apparent stability of communities at local scales.     

c-Jun N-terminal kinase (JNK) signaling: Recent advances and challenges

c-Jun N-terminal kinases (JNKs), first characterized as stress-activated members of the mitogen-activated protein kinase (MAPK) family, have become a focus of inhibitor screening strategies following studies that have shown their critical roles in the development of a number of diseases, such as diabetes, neurodegeneration and liver disease. We discuss recent advances in the discovery and development of ATP-competitive and ATP-noncompetitive JNK inhibitors. Because understanding the modes of actions of these inhibitors and improving their properties will rely on a better understanding of JNK structure, JNK catalytic mechanisms and substrates, recent advances in these areas of JNK biochemistry are also considered. In addition, the use of JNK gene knockout animals is continuing to reveal in vivo functions for these kinases, with tissue-specific roles now being dissected with tissue-specific knockouts. These latest advances highlight the many challenges now faced, particularly in the directed targeting of the JNK isoforms in specific tissues.     

All-trans retinoic acid (ATRA) induces miR-23a expression,decreases CTSC expression and granzyme B activity leading to impaired NK cell cytotoxicity

NK cell is an innate immune system lymphocyte lineage with natural cytotoxicity. Its optimal use in the clinic requires in vitro expansion and activation. Cytokines and encounter with target cells activate NK cells and induce proliferation, and this could depend on the presence of other immune cells. Here we activated PBMCs during 5 days with IL-2, with IL-2 plus the tumor cell line K562 and with the lymphoblastoid cell line R69 and perform integrated analyses of microRNA and mRNA expression profiles of purified NK cells. The samples cluster depending on the stimuli and not on the donor, indicating that the pattern of NK cell stimulation is acutely well conserved between individuals. Regulation of mRNA expression is tighter than that of miRNA expression. All stimuli induce a common preserved genetic remodeling. In addition, encounter with target cells mainly activates pathways related to metabolism. Different target cells induce different NK cell remodeling which affects cytokine response and cytotoxicity, supporting the notion that encounter with different target cells significantly changing the activation pattern. We validate our analysis by showing that activation down regulates miR-23a, which is a negative regulator of cathepsin C (CTSC) mRNA, a gene up regulated by all stimuli. The peptidase CTSC activates the granzymes, the main effector proteases involved in NK cell cytotoxicity. All-trans retinoic acid (ATRA), which induces miR-23a expression, decreases CTSC expression and granzyme B activity leading to impaired NK cell cytotoxicity in an in vivo mouse model.     

Biodiversity in a complex world: consolidation and progress in functional biodiversity research

The global decline of biodiversity caused by human domination of ecosystems worldwide is supposed to alter important process rates and state variables in these ecosystems. However, there is considerable debate on the prevalence and importance of biodiversity effects on ecosystem function (BDEF). Here, we argue that much of the debate stems from two major shortcomings. First, most studies do not directly link the traits leading to increased or decreased function to the traits needed for species coexistence and dominance. We argue that implementing a trait-based approach and broadening the perception of diversity to include trait dissimilarity or trait divergence will result in more realistic predictions on the consequences of altered biodiversity. Second, the empirical and theoretical studies do not reflect the complexity of natural ecosystems, which makes it difficult to transfer the results to natural situations of species loss. We review how different aspects of complexity (trophic structure, multifunctionality, spatial or temporal heterogeneity, and spatial population dynamics) alter our perception of BDEF. We propose future research avenues concisely testing whether acknowledging this complexity will strengthen the observed biodiversity effects. Finally, we propose that a major future task is to disentangle biodiversity effects on ecosystem function from direct changes in function due to human alterations of abiotic constraints.