20,677 research outputs found
REMOTE SENSING OF FOLIAR NITROGEN IN CULTIVATED GRASSLANDS OF HUMAN DOMINATED LANDSCAPES
Foliar nitrogen (N) concentration of plant canopies plays a central role in a number of important ecosystem processes and continues to be an active subject in the field of remote sensing. Previous efforts to estimate foliar N at the landscape scale have primarily focused on intact forests and grasslands using aircraft imaging spectrometry and various techniques of statistical calibration and modeling. The present study was designed to extend this work by examining the potential to estimate the foliar N concentration of residential, agricultural and other cultivated grassland areas within a suburbanizing watershed. In conjunction with ground-based vegetation sampling, we developed Partial Least Squares (PLS) models for predicting mass-based foliar N across management types using input from airborne and field based imaging spectrometers. Results yielded strong predictive relationships for both ground- and aircraft-based sensors across sites that included turf grass, grazed pasture, hayfields and fallow fields. We also report on relationships between imaging spectrometer data and other important variables such as canopy height, biomass, and water content, results from which show strong promise for detection with high quality imaging spectrometry data and suggest that cultivated grassland offer opportunity for empirical study of canopy light dynamics. Finally, we discuss the potential for application of our results, and potential challenges, with data from the planned HyspIRI satellite, which will provide global coverage of data useful for vegetation N estimation
Phantom distribution functions for some stationary sequences
The notion of a phantom distribution function (phdf) was introduced by
O'Brien (1987). We show that the existence of a phdf is a quite common
phenomenon for stationary weakly dependent sequences. It is proved that any
-mixing stationary sequence with continuous marginals admits a
continuous phdf. Sufficient conditions are given for stationary sequences
exhibiting weak dependence, what allows the use of attractive models beyond
mixing. The case of discontinuous marginals is also discussed for
-mixing.
Special attention is paid to examples of processes which admit a continuous
phantom distribution function while their extremal index is zero. We show that
Asmussen (1998) and Roberts et al. (2006) provide natural examples of such
processes. We also construct a non-ergodic stationary process of this type
Holography for inflation using conformal perturbation theory
We provide a precise and quantitative holographic description of a class of
inflationary slow-roll models. The dual QFT is a deformation of a
three-dimensional CFT by a nearly marginal operator, which, in the models we
consider, generates an RG flow to a nearby IR fixed point. These models
describe hilltop inflation, where the inflaton rolls from a local maximum of
the potential in the infinite past (corresponding to the IR fixed point of the
dual QFT) to reach a nearby local minimum in the infinite future (corresponding
to the UV of the dual QFT). Through purely holographic means, we compute the
spectra and bispectra of scalar and tensor cosmological perturbations. The QFT
correlators to which these observables map holographically may be calculated
using conformal perturbation theory, even when the dual QFT is strongly
coupled. Both the spectra and the bispectra may be expressed this way in terms
of CFT correlators that are fixed, up to a few constants, by conformal
invariance. The form of slow-roll inflationary correlators is thus determined
by the perturbative breaking of the de Sitter isometries away from the fixed
point. Setting the constants to their values obtained by AdS/CFT at the fixed
point, we find exact agreement with known expressions for the slow-roll power
spectra and non-Gaussianities.Comment: 44 pp, 3 fig
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