Plant functional constraints on foliar N:P ratios in a tropical forest landscape

Although large scale analyses of foliar N:P ratios suggest and overall pattern of P limitation in tropical forests, analyses within the biome are less indicative of a consistent pattern of nutrient limitation. High tree species diversity and soil heterogeneity are important factors driving the variability of foliar chemistry in the tropics; however, this variability could be reduced at the level of the functional characteristics of the species present at a site. In this study it is hypothesized that foliar N:P ratios would be more constrained when the species are grouped according to functional characteristics, and would reveal patterns of nutrient limitation. The study was conducted in a tropical forest landscape of the Porce region in Colombia, which consists of patches of primary and secondary forests of different ages. The functional groupings revealed consistent patterns of conservative N cycling in species present at young secondary sites, as well as conservative P cycling of the species present later in succession and in primary forests. Although the observed data have limitations in terms of capturing the overall variability for each functional group, the classification used here provided support for the proposed hypotheses. It is concluded that functional composition, rather than taxonomic composition, can potentially improve our understanding of nutrient cycling in tropical forests

Strongly nonlinear thermovoltage and heat dissipation in interacting quantum dots

We investigate the nonlinear regime of charge and energy transport through Coulomb-blockaded quantum dots. We discuss crossed effects that arise when electrons move in response to thermal gradients (Seebeck effect) or energy flows in reaction to voltage differences (Peltier effect). We find that the differential thermoelectric conductance shows a characteristic Coulomb butterfly structure due to charging effects. Importantly, we show that experimentally observed thermovoltage zeros are caused by the activation of Coulomb resonances at large thermal shifts. Furthermore, the power dissipation asymmetry between the two attached electrodes can be manipulated with the applied voltage, which has implications for the efficient design of nanoscale coolers.Comment: 6 pages, 4 figure

Heat current through an artificial Kondo impurity beyond linear response

We investigate the heat current of a strongly interacting quantum dot in the presence of a voltage bias in the Kondo regime. Using the slave- boson mean-field theory, we discuss the behavior of the energy flow and the Joule heating. We find that both contributions to the heat current dis- play interesting symmetry properties under reversal of the applied dc bias. We show that the symmetries arise from the behavior of the dot trans- mission function. Importantly, the transmission probability is a function of both energy and voltage. This allows us to analyze the heat current in the nonlinear regime of transport. We observe that nonlinearities ap- pear already for voltages smaller than the Kondo temperature. Finally, we suggest to use the contact and electric symmetry coefficients as a way to measure pure energy currents.Comment: 9 pages, 2 figures, proceeding of the Low Temperature Physics Conferenc

Life on the South Side of Chambersburg Street, 1910

The people of Gettysburg, Pennsylvania heralded in the year of 1911 and reflected on their accomplishments throughout the past year. With pealing bells, tooting whistles and noisy revolvers...in a more vigorous way than has been witnessed here for many years, this New Year’s Eve celebration recognized the past year as it welcomed the new year to come. The entire town took part and its faculties were utilized in the festivities of the night, including the Court House bell and those of the St. James and College Lutheran churches...engines added their quota of noise and all over town men brought into use guns and revolvers. The year of 1910 was a noteworthy year for the town and larger county. The citizens witnessed the erection of a large number of handsome homes, a sure sign of prosperity. Business firms developed and the county saw an outstanding apple crop and tourist season. In general, the year of 1910 was proudly characterized and recorded by Gettysburg’s constituents as a great place to call home

Effects of processes at the population and community level on carbon dynamics of an ecosystem model

Ecological processes at the population and community level are often ignored in biogeochemical models, however, the effects of excluding these processes at the ecosystem level is uncertain. In this study we analyzed the set of behaviors that emerge after introducing population and community processes into an ecosystem carbon model. We used STANDCARB, a hybrid model that incorporates population, community, and ecosystem processes to predict carbon dynamics over time. Our simulations showed that at the population level, colonization and mortality rates can limit the maximum biomass achieved during a successional sequence. Specifically, colonization rates control temporal lags in the initiation of carbon accumulation, and mortality rates can have important effects on annual variation in live biomass. At the community level, differences in species traits and changes in species composition over time introduced significant changes in carbon dynamics. Species with different set of parameters, such as growth and mortality rates, introduce patterns of carbon accumulation that could not be reproduced using a single species with the average of parameters of multiple species or by simulating the most abundant species (strategies commonly employed in terrestrial biogeochemical models). We conclude that omitting population and community processes from biogeochemical models introduces an important source of uncertainty that can impose important limitations for predictions of future carbon balances