Emerging forest-peatland bistability and resilience of European peatland carbon stores.

Northern peatlands store large amounts of carbon. Observations indicate that forests and peatlands in northern biomes can be alternative stable states for a range of landscape settings. Climatic and hydrological changes may reduce the resilience of peatlands and forests, induce persistent shifts between these states, and release the carbon stored in peatlands. Here, we present a dynamic simulation model constrained and validated by a wide set of observations to quantify how feedbacks in water and carbon cycling control resilience of both peatlands and forests in northern landscapes. Our results show that 34% of Europe (area) has a climate that can currently sustain existing rainwater-fed peatlands (raised bogs). However, raised bog initiation and restoration by water conservation measures after the original peat soil has disappeared is only possible in 10% of Europe where the climate allows raised bogs to initiate and outcompete forests. Moreover, in another 10% of Europe, existing raised bogs (concerning ∼20% of the European raised bogs) are already affected by ongoing climate change. Here, forests may overgrow peatlands, which could potentially release in the order of 4% (∼24 Pg carbon) of the European soil organic carbon pool. Our study demonstrates quantitatively that preserving and restoring peatlands requires looking beyond peatland-specific processes and taking into account wider landscape-scale feedbacks with forest ecosystems.

Spatial early warning signals for impending regime shifts: A practical framework for application in real-world landscapes.

Prediction of ecosystem response to global environmental change is a pressing scientific challenge of major societal relevance. Many ecosystems display nonlinear responses to environmental change, and may even undergo practically irreversible 'regime shifts' that initiate ecosystem collapse. Recently, early warning signals based on spatiotemporal metrics have been proposed for the identification of impending regime shifts. The rapidly increasing availability of remotely sensed data provides excellent opportunities to apply such model-based spatial early warning signals in the real world, to assess ecosystem resilience and identify impending regime shifts induced by global change. Such information would allow land-managers and policy makers to interfere and avoid catastrophic shifts, but also to induce regime shifts that move ecosystems to a desired state. Here, we show that the application of spatial early warning signals in real-world landscapes presents unique and unexpected challenges, and may result in misleading conclusions when employed without careful consideration of the spatial data and processes at hand. We identify key practical and theoretical issues and provide guidelines for applying spatial early warning signals in heterogeneous, real-world landscapes based on literature review and examples from real-world data. Major identified issues include (1) spatial heterogeneity in real-world landscapes may enhance reversibility of regime shifts and boost landscape-level resilience to environmental change (2) ecosystem states are often difficult to define, while these definitions have great impact on spatial early warning signals and (3) spatial environmental variability and socio-economic factors may affect spatial patterns, spatial early warning signals and associated regime shift predictions. We propose a novel framework, shifting from an ecosystem perspective towards a landscape approach. The framework can be used to identify conditions under which resilience assessment with spatial remotely sensed data may be successful, to support well-informed application of spatial early warning signals, and to improve predictions of ecosystem responses to global environmental change.

Fast racemisation of chiral amines and alcohols by using cationic half-sandwich ruthena- and iridacycle catalysts.

The lipase-catalysed resolution of alcohols and amines yields only 50 % of the desired enantiopure product. However, addition of a racemisation catalyst leads to 100 % yield in what is called a dynamic kinetic resolution (DKR). There is a need for new racemisation catalysts that are fast and compatible with the conditions of the enzymatic reaction. We show that cationic half-sandwich ruthena- and iridacycle complexes are highly active and efficient in the racemisation of chiral alcohols and amines. Upon activation with base, these complexes are able to selectively racemise alcohols, whereas the non-activated complexes are selective catalysts for the racemisation of amines. We have applied the iridacycles in the DKR of racemic beta-chloroalcohols to produce chiral epoxides in a biphasic system in good yields and high ee (ee=enantiomeric excess).

Dynamic kinetic resolution of racemic beta-haloalcohols: direct access to enantioenriched epoxides.

The direct chemo-enzymatic DKR of racemic beta-haloalcohols is reported, yielding the corresponding optically active epoxides in a single step. The mutant haloalcohol dehalogenase HheC Cys153Ser Trp249Phe is used for the asymmetric ring closure, whereas racemization of the remaining enantiomer of the haloalcohol is achieved using the new iridacycle 3, one of the most effective racemization catalysts to date for beta-haloalcohols.

Nouvelle méthode de mesure de la réponse impulsionnelle des fibres optiques.

A new technique for measuring the temporal transfer function of optical fibers is described. The method consists of placing the fiber under test in one arm of a Mach-Zehnder interferometer excited by a broadband source. The temporal impulse response is obtained from a holographic reconstruction. The method requires only short lengths of single-mode or multimode fibers (less than 1 m). We have measured a dispersion of 0.3 nsec/km.nm at 0.59 microm with a single-mode fiber, in good agreement with theory. The arrival times of the various modes of multimode fibers are resolved.

Guidance of surface waves by multilayer coatings.

A periodic sequence of layers with alternately high and low refractive indices can guide loosely bound surface waves parallel to the layers. Most of the power flows in free space, and, thus, the losses may be considerably smaller than the bulk losses of the dielectric materials used. Possible applications are briefly discussed.

Modes in helical gas lenses.

Helical gas lenses incorporate four coaxial helices at temperatures +T, -T, +T, and -T, respectively. Because of the resulting change in refractive index of the gas filling the space inside the helices, optical beams can be guided by this system over long distances. A general expression for the modes of propagation is given; it involves Hermite polynomials in two complex variables. For small temperature differences the mode fields reduce to Laguerre-Gauss functions. Calculated irradiance patterns are shown for various mode numbers and various values of T.

Degenerate Optical Cavities. III: Effect of Aberrations.

The capability of degenerate optical cavities to transmit faithfully incident optical signals with arbitrary wavefronts is limited primarily by geometrical optics aberrations. This capability is expressed by an acceptance factor which is calculated for various types of cavities lacking first-order degeneracy, or suffering from primary aberrations. It is found that the acceptance factors of spherically symmetric cavities are larger, by orders of magnitude, than the acceptance factors of cavities possessing only rotational symmetry (such as the well-known confocal cavity). The correction of primary aberrations for both types of cavities is discussed. Acceptance factors of the order of 107 with finesses of the order of 100 can be obtained. The mechanical accuracy required is, however, a few orders of magnitude higher than in conventional optical instruments.

Focusing and deflection of optical beams by cylindrical mirrors.

An optical system incorporating two closely spaced cylindrical mirrors is described. By properly orienting the mirrors in space, an incident beam can be focused at any desired point within a large volume. This simple periscopic system, which provides variable focal lengths and deflection angles, is applicable to optical or millimeter wave transmission systems lying along irregular paths. A paraxial ray theory of the system is given, as well as experimental results.

Degenerate optical cavities.

An optical cavity is degenerate when an arbitrary ray retraces its own path after a single round trip. The condition for degeneracy is given for ring type cavities incorporating internal lenses, using geometrical optics methods. The simplest linear configurations require a spherical mirror or a corner cube, a thin lens, and a plane mirror. Planar rings with four plane mirrors require at least three thin focusing elements. A nonplanar ring is discussed which requires only two thin lenses. The alignment of degenerate cavities is, in general, as critical as the alignment of plane Fabry-Perot.

Gaussian light beams with general astigmatism.

This paper considers the propagation and diffraction of coherent light beams through nonorthogonal optical systems such as sequences of astigmatic lenses oriented at oblique angles to each other. The fundamental (gaussian) mode has elliptical light spots in each beam cross section and ellipsoidal (or hyperboloidal) wavefronts near the axis. It is found that the orientation of the light spot differs from that of the wavefront, and changes continuously by as much as pi radians as the beam propagates through free space. A theory of these general astigmatic beams is given and simple experimental observations are described. The coupling factor between two such beams is also given.

Degenerate Optical Cavities. II: Effect of Misalignments.

A simple expression is given for the response of degenerate cavities suffering from arbitrary misalignments, and numerical results are presented. The method of resonance excitation is carried out analytically with the help of a complex ray representation of gaussian beams. It is first shown that the modulus and phase of such complex rays can be identified with, respectively, the beam radius and the phase of the on-axis field. This identification simplifies the calculation of the coupling factor between two gaussian beams, which is needed in deriving the expression for the response. For the case of conventional cavities, the results are in exact agreement with results derived from the Laguerre-Gauss or Hermite-Gauss mode theory. The case of degenerate cavities with large and possibly nonorthogonal misalignments, of interest in various nonresonant multipath systems, is also discussed.