Recent Widespread Tree Growth Decline Despite Increasing Atmospheric CO2

Background: The synergetic effects of recent rising atmospheric CO2 and temperature are expected to favor tree growth in boreal and temperate forests. However, recent dendrochronological studies have shown site-specific unprecedented growth enhancements or declines. The question of whether either of these trends is caused by changes in the atmosphere remains unanswered because dendrochronology alone has not been able to clarify the physiological basis of such trends. Methodology/Principal Findings: Here we combined standard dendrochronological methods with carbon isotopic analysis to investigate whether atmospheric changes enhanced water use efficiency (WUE) and growth of two deciduous and two coniferous tree species along a 9u latitudinal gradient across temperate and boreal forests in Ontario, Canada. Our results show that although trees have had around 53 % increases in WUE over the past century, growth decline (measured as a decrease in basal area increment – BAI) has been the prevalent response in recent decades irrespective of species identity and latitude. Since the 1950s, tree BAI was predominantly negatively correlated with warmer climates and/or positively correlated with precipitation, suggesting warming induced water stress. However, where growth declines were not explained by climate, WUE and BAI were linearly and positively correlated, showing that declines are not always attributable to warming induced stress and additional stressors may exist. Conclusions: Our results show an unexpected widespread tree growth decline in temperate and boreal forests due t

Majority-Vote Model on Scale-Free Hypergraphs

Majority-vote models on scale-free hypergraphs are investigated by means of numerical simulations with different variants of system dynamics. Hypergraphs are generalisations of ordinary graphs in which higher order of social organisation is included by introducing hyperedges corresponding to social groups, connecting more than two nodes. In the models under study, opinions of agents (two-state spins) placed in nodes are updated according to a probabilistic rule with control parameter representing social noise. The probability of a single spin flip depends on the average opinion within only one social group (hyperedge) the agent belongs to. This introduces an intermediate level of social interactions, in contrast with the case of networks, where the opinion of an agent usually depends on the average opinion of all neighbours. In all cases under consideration a second-order phase transition to a state with an uniform opinion was found as a function of the social noise, with the critical value of the control parameter and the critical exponents depending on the hypergraph topology and details of the system dynamics (node or hyperedge update)

Statistical Properties of the Proportional Voting Process

We propose a model of elections based on the Sznajd model of social interactions in the stochastic Ravasz-Barabási hierarchical network. The results of numerical calculations for the population described by this model were compared with statistical analysis of elections' results to the lower house of Parliament held in Poland in the years of 2001, 2005 and 2007. It is shown that the distribution of votes among candidates for the whole country as well as for the district of Warsaw has the log-normal character. The results from Poland are compared with other countries: Brazil and India. For all the cases log-normal distribution of votes for the candidates from each political party is shown. The probability of gaining a number of votes as a function of the number of the candidate on the list is presented

Statistical Properties of the Proportional Voting Process

We propose a model of elections based on the Sznajd model of social interactions in the stochastic Ravasz-Barabási hierarchical network. The results of numerical calculations for the population described by this model were compared with statistical analysis of elections' results to the lower house of Parliament held in Poland in the years of 2001, 2005 and 2007. It is shown that the distribution of votes among candidates for the whole country as well as for the district of Warsaw has the log-normal character. The results from Poland are compared with other countries: Brazil and India. For all the cases log-normal distribution of votes for the candidates from each political party is shown. The probability of gaining a number of votes as a function of the number of the candidate on the list is presented

Majority Vote Model on Multiplex Networks

Majority vote model on multiplex networks with two independently generated layers in the form of scale-free networks is investigated by means of Monte Carlo simulations and heterogeneous mean-field approximation. In a version of the model under study each agent with probability 1-q (0≤q≤1/2) follows the opinions of the majorities of her neighbors within both layers if these opinions are identical; otherwise, she makes decision randomly. The model exhibits second-order ferromagnetic transition as q, the parameter measuring the level of internal noise, is decreased, with critical exponents depending on the details of the degree distributions in the layers. The critical value q_{c} of the parameter q evaluated in the heterogeneous mean-field approximation shows quantitative agreement with that obtained from numerical simulations for a broad range of parameters characterizing the degree distributions of the layers

q-Neighbor Ising model on random networks

A modified kinetic Ising model with Metropolis dynamics, so-called [Formula: see text]-neighbor Ising model, is investigated on random graphs. In this model, each spin interacts only with [Formula: see text] spins randomly chosen from its neighborhood. Investigations are performed by means of Monte Carlo (MC) simulations and the analytic pair approximation (PA). The range of parameters such as the size of the [Formula: see text]-neighborhood and the mean degree of nodes of the random graph is determined for which the model exhibits continuous or discontinuous ferromagnetic (FM) phase transition with decreasing temperature. It is also shown that, in the case of discontinuous transition for large enough and fixed mean degree of nodes, the width of the hysteresis loop oscillates with the parameter [Formula: see text], expanding for even and shrinking for odd values of [Formula: see text]. Predictions of the PA show satisfactory quantitative agreement with results of MC simulations. </jats:p