Analysis of ecological thresholds in a temperate forest undergoing dieback.

Positive feedbacks in drivers of degradation can cause threshold responses in natural ecosystems. Though threshold responses have received much attention in studies of aquatic ecosystems, they have been neglected in terrestrial systems, such as forests, where the long time-scales required for monitoring have impeded research. In this study we explored the role of positive feedbacks in a temperate forest that has been monitored for 50 years and is undergoing dieback, largely as a result of death of the canopy dominant species (Fagus sylvatica, beech). Statistical analyses showed strong non-linear losses in basal area for some plots, while others showed relatively gradual change. Beech seedling density was positively related to canopy openness, but a similar relationship was not observed for saplings, suggesting a feedback whereby mortality in areas with high canopy openness was elevated. We combined this observation with empirical data on size- and growth-mediated mortality of trees to produce an individual-based model of forest dynamics. We used this model to simulate changes in the structure of the forest over 100 years under scenarios with different juvenile and mature mortality probabilities, as well as a positive feedback between seedling and mature tree mortality. This model produced declines in forest basal area when critical juvenile and mature mortality probabilities were exceeded. Feedbacks in juvenile mortality caused a greater reduction in basal area relative to scenarios with no feedback. Non-linear, concave declines of basal area occurred only when mature tree mortality was 3-5 times higher than rates observed in the field. Our results indicate that the longevity of trees may help to buffer forests against environmental change and that the maintenance of old, large trees may aid the resilience of forest stands. In addition, our work suggests that dieback of forests may be avoidable providing pressures on mature and juvenile trees do not pass critical thresholds

The formation mechanism of aluminum oxide tunnel barriers: Three-dimensional atom probe analysis

Magnetic tunnel junctions have applications in a range of spin-electronic devices. The functional properties of such devices are critically dependant on the nanoscale morphology of the insulating barrier (usually only a few atomic layers thick) that separates two ferromagnetic layers. Here we report atomic-resolution three-dimensional visualization of magnetic tunnel junctions with an aluminum oxide barrier, using three-dimensional atom probe analysis and cross-sectional high resolution electron microscopy. Our results on barriers with a range of degrees of oxidation have enabled a mechanism for the oxidation to be proposed. Low oxidation times result in discrete oxide islands and further oxidation leads to a more continuous, but nonstoichiometric, barrier. Post-deposition annealing leads to an increase in the barrier area and near stoichiometric chemistry with evidence that oxidation proceeds along the top of grain boundaries in the underlying CoFe. Our three-dimensional visualization enables us to show that even perforated barriers (formed at shorter oxidation times) can have reasonable transport properties. © 2005 American Institute of Physics

Development of a fish-based index to assess the eutrophication status of European lakes

The use of the CEN (European Committee for Standardization) standard method for sampling fish in lakes using multi-mesh gillnets allowed the collection of fish assemblages of 445 European lakes in 12 countries. The lakes were additionally characterised by environmental drivers and eutrophication proxies. Following a site-specific approach including a validation procedure, a fish index including two abundance metrics (catch per unit effort expressed as fish number and biomass) and one functional metric of composition (abundance of omnivorous fish) was developed. Correlated with the proxy of eutrophication, this index discriminates between heavily and moderately impacted lakes. Additional analyses on a subset of data from Nordic lakes revealed a stronger correlation between the new fish index and the pressure data. Despite an uneven geographical distribution of the lakes and certain shortcomings in the environmental and pressure data, the fish index proved to be useful for ecological status assessment of lakes applying standardised protocols and thus supports the development of national lake fish assessment tools in line with the European Water Framework Directive

Confidence regions for the intensity function of a cyclic Poisson process

Poisson process, Intensity function, Cyclic intensity function, Periodic intensity function, Kernel density estimation, Confidence intervals, Confidence bands, Extreme value distribution, Gumbel distribution, Primary 60F05, 60F17, Secondary 62E20,

Integrating transcriptional activity in genome-scale models of metabolism

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