Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

Large single-standing trees are rapidly declining in savannahs, ecosystems supporting a high diversity of large herbivorous mammals. Savannah trees are important as they support both a unique flora and fauna. The herbaceous layer in particular responds to the structural and functional properties of a tree. As shrubland expands stem thickening occurs and large trees are replaced by smaller trees. Here we examine whether small trees are as effective in providing advantages for grasses growing beneath their crowns as large trees are. The role of herbivory in this positive tree-grass interaction is also investigated. We assessed soil and grass nutrient content, structural properties, and herbaceous species composition beneath trees of three size classes and under two grazing regimes in a South African savannah. We found that grass leaf content (N and P) beneath the crowns of particularly large (ca. 3. 5 m) and very large trees (ca. 9 m) was as much as 40% greater than the same grass species not growing under a tree canopy, whereas nutrient contents of grasses did not differ beneath small trees

Generalización de un algoritmo de inferencia basado en gramáticas de grafos

Se presentó en [Je 95] un Algoritmo de Inferencia, el cual posee como entrada un conjunto finito de figuras o modelos, que se pueden representar como Grafos y como salida un tipo de Gramáticas de Grafos, llamadas Gramáticas de Reemplazo de Hiperlíneas. en [Je 96] se demostró que este Algoritmo es altamente no-determinístico. De manera que en este trabajo se presenta una generalización del Algoritmo de Inferencia y una alternativa semántica para restringir el flujo exponencial de gramáticas inferidas, la que se refiere a exigirle a los grafos ciertas propiedades, que llamaremos compatibles. En especial se considera un conjunto finito de ejemplos "negativos"que son análogos a los modelos ingresados, pero que en la generación del lenguaje de grafos no aparecen.Eje: TeoríaRed de Universidades con Carreras en Informática (RedUNCI

Generalización de un algoritmo de inferencia basado en gramáticas de grafos

Se presentó en [Je 95] un Algoritmo de Inferencia, el cual posee como entrada un conjunto finito de figuras o modelos, que se pueden representar como Grafos y como salida un tipo de Gramáticas de Grafos, llamadas Gramáticas de Reemplazo de Hiperlíneas. en [Je 96] se demostró que este Algoritmo es altamente no-determinístico. De manera que en este trabajo se presenta una generalización del Algoritmo de Inferencia y una alternativa semántica para restringir el flujo exponencial de gramáticas inferidas, la que se refiere a exigirle a los grafos ciertas propiedades, que llamaremos compatibles. En especial se considera un conjunto finito de ejemplos "negativos"que son análogos a los modelos ingresados, pero que en la generación del lenguaje de grafos no aparecen.Eje: TeoríaRed de Universidades con Carreras en Informática (RedUNCI

The role of climate change and human pressure on forest decline in the early Holocene on Qinghai-Tibetan Plateau – an ecohydrological modelling approach

Abstract HKT-ISTP 2013 B

Simulated stability of the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum

The response of the Atlantic Meridional Overturning Circulation (AMOC) to freshwater perturbations critically depends on its mean state. Large swaths of icebergs melting in the North Atlantic during the last deglaciation constituted such perturbations and can, thus, provide important constraints on the stability of the AMOC. However, the mean AMOC state during the Last Glacial Maximum (LGM), preceding the rapid disintegration of the ice sheets during the deglaciation, as well as its response to these perturbations remain debated. Here, we investigate the evolution of the AMOC as it responds to freshwater perturbations under improved LGM boundary conditions in the Bern3D intermediate complexity model. Particularly, we consider the effect of an open versus a closed Bering Strait and the effect of increased tidal dissipation as a result of the altered bathymetry due to the lower glacial sea level stand. The vigorous and deep AMOC under these glacial boundary conditions, consistent with previous simulations with different models, reacts more strongly to North Atlantic freshwater forcings than under preindustrial conditions. This increased sensitivity is mostly related to the closed Bering Strait that cuts off the freshwater escape route through the Arctic into the Pacific, thereby facilitating faster accumulation of freshwater in the North Atlantic and halting deep-water formation. Proxy reconstructions of the LGM AMOC instead indicate a weaker and possibly shallower AMOC than today, which is in conflict with the particularly strong and deep circulation states coherently simulated with ocean circulation models for the LGM. Simulations with reduced North Atlantic deep-water formation, as a consequence of potentially increased continental runoff from ice sheet melt and imposed changes in the hydrological cycle, more closely resemble the overturning circulation inferred from proxies. These circulation states also show bistable behavior, where the AMOC does not recover after North Atlantic freshwater hosing. However, no AMOC states are found here that either comprise an extreme shoaling or vigorous and concurrent shallow overturning as previously proposed based on paleoceanographic data

Modeling transcription factor binding events to DNA using a random walker/jumper representation on a 1D/2D lattice with different affinity sites

Surviving in a diverse environment requires corresponding organism responses. At the cellular level, such adjustment relies on the transcription factors (TFs) which must rapidly find their target sequences amidst a vast amount of non-relevant sequences on DNA molecules. Whether these transcription factors locate their target sites through a 1D or 3D pathway is still a matter of speculation. It has been suggested that the optimum search time is when the protein equally shares its search time between 1D and 3D diffusions. In this paper, we study the above problem using a Monte Carlo simulation by considering a very simple physical model. A 1D strip, representing a DNA, with a number of low affinity sites, corresponding to non-target sites, and high affinity sites, corresponding to target sites, is considered and later extended to a 2D strip. We study the 1D and 3D exploration pathways, and combinations of the two modes by considering three different types of molecules: a walker that randomly walks along the strip with no dissociation; a jumper that represents dissociation and then re-association of a TF with the strip at later time at a distant site; and a hopper that is similar to the jumper but it dissociates and then re-associates at a faster rate than the jumper. We analyze the final probability distribution of molecules for each case and find that TFs can locate their targets fast enough even if they spend 15% of their search time diffusing freely in the solution. This indeed agrees with recent experimental results obtained by Elf et al. 2007 and is in contrast with theoretical expectation.Comment: 24 pages, 9 figure

The Caulobacter crescentus DNA-(adenine-N6)-methyltransferase CcrM methylates DNA in a distributive manner

The specificity and processivity of DNA methyltransferases have important implications regarding their biological functions. We have investigated the sequence specificity of CcrM and show here that the enzyme has a high specificity for GANTC sites, with only minor preferences at the central position. It slightly prefers hemimethylated DNA, which represents the physiological substrate. In a previous work, CcrM was reported to be highly processive [Berdis et al. (1998) Proc. Natl Acad. Sci. USA 95: 2874–2879]. However upon review of this work, we identified a technical error in the setup of a crucial experiment in this publication, which prohibits making any statement about the processivity of CcrM. In this study, we performed a series of in vitro experiments to study CcrM processivity. We show that it distributively methylates six target sites on the pUC19 plasmid as well as two target sites located on a 129-mer DNA fragment both in unmethylated and hemimethylated state. Reaction quenching experiments confirmed the lack of processivity. We conclude that the original statement that CcrM is processive is no longer valid