29 research outputs found
Mixed perturbative expansion: the validity of a model for the cascading
A new type of perturbative expansion is built in order to give a rigorous
derivation and to clarify the range of validity of some commonly used model
equations.
This model describes the evolution of the modulation of two short and
localized pulses, fundamental and second harmonic, propagating together in a
bulk uniaxial crystal with non-vanishing second order susceptibility
and interacting through the nonlinear effect known as ``cascading'' in
nonlinear optics.
The perturbative method mixes a multi-scale expansion with a power series
expansion of the susceptibility, and must be carefully adapted to the physical
situation. It allows the determination of the physical conditions under which
the model is valid: the order of magnitude of the walk-off, phase-mismatch,and
anisotropy must have determined values.Comment: arxiv version is already officia
Can management intensity be more important than environmental factors? A case study along an extreme elevation gradient from central Italian cereal fields
This paper aims to assess the importance of environmental and management factors determining the weed species
composition along a strong elevation gradient. A total of 76 cereal fields (39 low input and 37 intensively managed) were
sampled along an elevation gradient in central Italy. Explanatory variables were recorded for each field to elucidate the role of
large-scale spatial trends, of site-specific abiotic environmental conditions and of field management characters. Redundancy
analysis was used to assess the relative importance of each environmental variable in explaining the variation in species
composition. Our results indicate that variation in weed species composition is strongly determined by altitude, mean annual
precipitation, mean annual temperature and also by soil characteristics. However, the level of intensification proved to be the
most influential variable. There was a significant difference in species richness and composition between low-input and
intensively managed fields. Intensification leads to considerable species loss at both lower and higher elevations. Low-input
fields had 296 species in total, while intensively managed fields had only 196