Disturbances on a shrubby steppe of the Monte phytogeographical province : changes in vegetation

En una estepa arbustiva del Monte oriental rionegrino se evaluó el efecto producido por la acción individual y combinada del fuego, desmonte mecánico y pastoreo doméstico sobre la vegetación que sustenta la producción ganadera de la región. Se instalaron clausuras en sitios afectados por grandes disturbios (fuego, desmonte, y ambos), y se tomaron mediciones fuera y dentro de ellas con el fin de evaluar estos disturbios. Se estudiaron diversas características estructurales (densidad y cobertura basal de los arbustos, biomasa aérea y cobertura basal del estrato herbáceo y de la costra biológica) y una funcional (fenología de las cinco gramíneas y los dos arbustos dominantes). El fuego y el desmonte redujeron la densidad y la superficie ocupada por los arbustos, y también la cobertura de costra biológica. A su vez, estos efectos se profundizaron cuando ambos disturbios actuaron en conjunto. El fuego modificó cualitativamente la composición florística del estrato herbáceo al alterar la dominancia de Stipa tenuis Phil. a favor de una especie más palatable como es Stipa papposa Nees. El pastoreo redujo de manera homogénea la biomasa aérea del estrato herbáceo, pero prolongó el período vegetativo de las gramíneas y aumentó el tamaño de ese compartimiento en desmedro del reproductivo, lo cual resultó en una mejor calidad de forraje disponible para los herbívoros. La ausencia general de interacciones entre el fuego o el desmonte con el pastoreo permitiría predecir la dirección de cambio de este sistema frente a cada disturbio en particular.We evaluated the effects of individual and joint effects of fire, mechanical removal of shrubs and grazing on natural vegetation of the eastern Monte Phytogeographical Province in Rio Negro, Argentina. Four enclosures were constructed in sites where large disturbances (fire, mechanical removal and both) had taken place. Measurements were made both inside and outside them in order to evaluate not only the grazing effects in each site, but also possible interactions between the latter and large disturbances, which have not usually been reported. Several structural characteristics (shrub density and cover, aerial biomass and basal cover of herbaceous layer, and of biological crust), plus a functional one (phenology of five grasses and two shrubs) were studied. Both the fire and the mechanical removal reduced the density and area covered by the shrubby layer, and also the biological crust cover, with the effects being larger at the site were both had taken place. Bare soil was higher in the disturbed sites respect to the control due to the disappearance of the biological crust. Fire produced a qualitative change in the floristic composition of the herbaceous layer through altering the dominance from Stipa tenuis Phil. to a more palatable species as it is Stipa papposa Nees. Grazing reduced homogeneously aboveground biomass of the herbaceous layer, but extended the vegetative period of grasses and increased the size of that budget though reducing the reproductive one. The last two effects will be beneficial for secondary production since they increase the energy flux to herbivory. Large disturbances, however, could have negative effects through the reduction of the biological crust, which plays an important role in seed germination, seedlings establishment and water balance, thus contributing to ecosystems sustainability

Effect of burn severity on vegetation recovery in the Austral Monte

El fuego es un fenómeno recurrente en los ecosistemas del norte de la Patagonia y ocurren, fundamentalmente, luego de una serie de años húmedos que promueve el crecimiento de pastos. La acumulación de material combustible fino que resulta de esta secuencia de años húme-dos facilita la ocurrencia de los incendios. Nuestro conocimiento sobre cómo es la regeneración de la vegetación es limitado. El objetivo de este trabajo fue evaluar el efecto de la severidad de quemado (IS) sobre la evolución post-fuego de distintos grupos funcionales de la vegetación: arbustos, pastos perennes y pastos anuales. Durante dos estaciones de crecimiento posteriores al fuego, en un área ubicada en el este de la provincia de Río Negro (Provincia Fitogeográfica del Monte), evaluamos la cobertura de estos grupos. La cobertura de pastos anuales superó a los testigos no incendiados en la primera estación de crecimiento posterior al incendio en los sitios con menor IS, mientras que en los sitios con mayor IS esto se produjo durante la segunda estación. La cobertura de pastos perennes se regeneró al segundo año en todas las severidades de quemado, mientras que los arbustos recuperaron menos de la mitad de su cobertura original. El aumento inicial de la relación pastos/arbustos producida luego del incendio favorecería la actividad ganadera, sin embargo pueden existir impactos negativos sobre el suelo (erosión, pérdida de fertilidad) que deberían evaluarse antes de generar prácticas de manejo del fuego.Fire has been a recurrent phenomenon in Northeast region of Patagonia and it takes place, mainly, after humid years that promote the growth of the grasses. The consequent accumulation of fine combustible material favors the propagation of fire. However, little is known about the post-fire evolution of the vegetation in relation to the magnitude of the damage caused by this disturbance. The objective of this work was to evaluate the effect of Burn Severity (IS) over the post-fire evolution of different functional groups of the vegetation (shrubs, perennial grasses and annual grasses) in two growing seasons after fire in an area located in the East of Río Negro province, inside the Monte Phytogeographic Province. The cover of annual grasses in the areas with lower IS overcome the non-burn sites in the first post fire growing season, while in the areas with higher IS this took place during the second season. The perennial grasses cover regenerated in the second year in all IS classes, while shrubs recovered less than half of their original cover. The initial increase in the grasses/shrubs relationship produced after the fire would be more favorable for cattle, however negative impacts on the soil can exist (erosion, loss of fertility) that should be evaluated before generating practices of fire management

When the optimal is not the best: parameter estimation in complex biological models

Background: The vast computational resources that became available during the past decade enabled the development and simulation of increasingly complex mathematical models of cancer growth. These models typically involve many free parameters whose determination is a substantial obstacle to model development. Direct measurement of biochemical parameters in vivo is often difficult and sometimes impracticable, while fitting them under data-poor conditions may result in biologically implausible values. Results: We discuss different methodological approaches to estimate parameters in complex biological models. We make use of the high computational power of the Blue Gene technology to perform an extensive study of the parameter space in a model of avascular tumor growth. We explicitly show that the landscape of the cost function used to optimize the model to the data has a very rugged surface in parameter space. This cost function has many local minima with unrealistic solutions, including the global minimum corresponding to the best fit. Conclusions: The case studied in this paper shows one example in which model parameters that optimally fit the data are not necessarily the best ones from a biological point of view. To avoid force-fitting a model to a dataset, we propose that the best model parameters should be found by choosing, among suboptimal parameters, those that match criteria other than the ones used to fit the model. We also conclude that the model, data and optimization approach form a new complex system, and point to the need of a theory that addresses this problem more generally

Identification of heavy-flavour jets with the CMS detector in pp collisions at 13 TeV

Many measurements and searches for physics beyond the standard model at the LHC rely on the efficient identification of heavy-flavour jets, i.e. jets originating from bottom or charm quarks. In this paper, the discriminating variables and the algorithms used for heavy-flavour jet identification during the first years of operation of the CMS experiment in proton-proton collisions at a centre-of-mass energy of 13 TeV, are presented. Heavy-flavour jet identification algorithms have been improved compared to those used previously at centre-of-mass energies of 7 and 8 TeV. For jets with transverse momenta in the range expected in simulated $\mathrm{t}\overline{\mathrm{t}}$ events, these new developments result in an efficiency of 68% for the correct identification of a b jet for a probability of 1% of misidentifying a light-flavour jet. The improvement in relative efficiency at this misidentification probability is about 15%, compared to previous CMS algorithms. In addition, for the first time algorithms have been developed to identify jets containing two b hadrons in Lorentz-boosted event topologies, as well as to tag c jets. The large data sample recorded in 2016 at a centre-of-mass energy of 13 TeV has also allowed the development of new methods to measure the efficiency and misidentification probability of heavy-flavour jet identification algorithms. The heavy-flavour jet identification efficiency is measured with a precision of a few per cent at moderate jet transverse momenta (between 30 and 300 GeV) and about 5% at the highest jet transverse momenta (between 500 and 1000 GeV)

Search for heavy resonances decaying to a top quark and a bottom quark in the lepton+jets final state in proton–proton collisions at 13 TeV

info:eu-repo/semantics/publishe

Evidence for the Higgs boson decay to a bottom quark–antiquark pair

info:eu-repo/semantics/publishe

Measurement of t(t)over-bar normalised multi-differential cross sections in pp collisions at root s=13 TeV, and simultaneous determination of the strong coupling strength, top quark pole mass, and parton distribution functions

Peer reviewe

Search for Physics beyond the Standard Model in Events with Overlapping Photons and Jets

Results are reported from a search for new particles that decay into a photon and two gluons, in events with jets. Novel jet substructure techniques are developed that allow photons to be identified in an environment densely populated with hadrons. The analyzed proton-proton collision data were collected by the CMS experiment at the LHC, in 2016 at root s = 13 TeV, and correspond to an integrated luminosity of 35.9 fb(-1). The spectra of total transverse hadronic energy of candidate events are examined for deviations from the standard model predictions. No statistically significant excess is observed over the expected background. The first cross section limits on new physics processes resulting in such events are set. The results are interpreted as upper limits on the rate of gluino pair production, utilizing a simplified stealth supersymmetry model. The excluded gluino masses extend up to 1.7 TeV, for a neutralino mass of 200 GeV and exceed previous mass constraints set by analyses targeting events with isolated photons.Peer reviewe

An embedding technique to determine ττ backgrounds in proton-proton collision data

An embedding technique is presented to estimate standard model tau tau backgrounds from data with minimal simulation input. In the data, the muons are removed from reconstructed mu mu events and replaced with simulated tau leptons with the same kinematic properties. In this way, a set of hybrid events is obtained that does not rely on simulation except for the decay of the tau leptons. The challenges in describing the underlying event or the production of associated jets in the simulation are avoided. The technique described in this paper was developed for CMS. Its validation and the inherent uncertainties are also discussed. The demonstration of the performance of the technique is based on a sample of proton-proton collisions collected by CMS in 2017 at root s = 13 TeV corresponding to an integrated luminosity of 41.5 fb(-1).Peer reviewe