Development of White Spruce Tree Islands in the Shrub Zone of the Forest-tundra

The growth of four white spruce (Picea glauca) clonal islands ranging in age from ca. 98 years to more than 400 years was investigated in the shrub zone of the forest-tundra east of Churchill, Manitoba, Canada. The elongation of 20 similar-aged stems in each of the three youngest islands was monitored during 1988 and 1989, along with ground and air temperatures. Stems in the younger islands showed a more flexible response to both daily and annual variation in temperature. Younger islands showed faster recovery from frost events during elongation and longer periods of elongation in cooler years. Early spring warming that caused snowmelt to occur before the growing season appeared to result in moisture stress later in the period of elongation. In stems of spruce shrub, the branches are concentrated near ground level because growth is slow and adventitious buds develop on the stem after repeated loss of stem terminals through snow abrasion. In young trees, shading and increased moisture from trapped snow coincide with feather moss establishment and a deep active layer, resulting in higher ground temperatures and faster tree growth. It is during this early period of development that a tree may be best able to develop an erect stem. In later development, the lowest branches of trees become appressed, grow roots, and become second-order stems, and this process continues outward from the central stem. In older tree islands, peat accumulation and needle abrasion can lead to conditions less favourable for growth and maintenance of needles. Consequently, the canopy may thin, which reduces its ability to trap snow. When snow cover is reduced, lichen-heath establishes and permafrost intrudes into the mound. Subsequent growth of the secondary stems on the mound may be too slow to enable successful development of an erect stem. Thus, island development is largely dependent on changing ground temperatures, which become colder as peat accumulates and frost heaving elevates the mound. Warm spring and summer conditions appear to lead to unfavourable conditions for tree islands.On a étudié la croissance de quatre îlots de clones d'épinettes blanches (Picea glauca) dont l'âge allait d'environ 98 à plus de 400 ans, situés dans la zone de végétation arbustive de la toundra forestière, à l'est de Churchill (Manitoba) au Canada. En 1988 et 1989, on a surveillé l'élongation de vingt tiges du même âge dans chacun des trois plus jeunes îlots ainsi que la température du sol et celle de l'air. Les tiges des plus jeunes îlots réagissaient avec plus de souplesse aux variations de température quotidienne comme annuelle. Les îlots les plus jeunes affichaient une aptitude à la réparation plus rapide suite à des périodes de gel durant l'élongation, ainsi que des périodes d'élongation plus longues durant les années plus fraîches. Un réchauffement printanier précoce provoquant la fonte des neiges avant la saison de croissance semblait résulter en une tension hydrique plus tard durant la période d'élongation. Sur les troncs d'épinettes arbustives, les branches sont concentrées près du niveau du sol en raison de la lenteur de la croissance et du développement de bourgeons adventifs sur le tronc après la perte répétée des terminaisons de tiges causée par l'abrasion nivale. Chez les jeunes arbres, l'ombrage et l'augmentation d'humidité provenant de la neige emprisonnée coïncident avec l'établissement d'hypnacées et une épaisse couche active, ce qui résulte en des températures au sol plus élevées et une croissance des arbres plus rapide. C'est durant cette phase initiale de développement qu'un arbre se révèle capable de développer un tronc droit. Au cours de la croissance ultérieure, les branches les plus basses deviennent appressées, prennent racine, devenant troncs secondaires, et le processus se poursuit en rayonnant à partir du tronc central. Dans les îlots d'arbres plus vieux, l'accumulation de tourbe et l'abrasion des aiguilles peuvent aboutir à des conditions moins favorables pour la croissance et le maintien des aiguilles. Il en résulte que le houppier peut s'éclaircir, ce qui réduit sa capacité à emprisonner la neige. Quand le couvert nival est réduit, il s'établit une bruyère à lichens et le pergélisol pénètre dans le monticule. Il peut arriver que la croissance ultérieure des tiges secondaires sur le monticule soit trop lente pour permettre le développement d'un tronc droit. Le développement d'un îlot est donc en grande partie dépendant de la variabilité de la température au sol, qui baisse à mesure que s'accumule la tourbe et que le gonflement dû au gel soulève le monticule. Il semble qu'un printemps chaud et la saison estivale résultent en des conditions défavorables pour les îlots arbustifs

Influences of Wind and Snow on Northern Tree-Line Environments at Churchill, Manitoba, Canada

The influences of wind and snow distribution at the tree line near Churchill, Manitoba, were studied. Wind profiles above the snow surface, snow crystal size and quantity were examined during abrasion experiments with white spruce branchlets. For white spruce, the abrasion zone was evidently within 80 cm above the snow surface, and needle removal by abrasion was strongly influenced by branch age. Removal by abrasion of similar-aged needles was highest in new branches and declined with branch age up to 7 or 8 years, and then increased again with older branches. Trees that grew 80 cm in height in 7 or 8 years were successful in passing through the abrasion zone. In forest-tundra environments the matted forms of trees were snow covered early in winter and received little abrasion. Sparsely needled islands of trees were largely covered with snow at the base. More densely needled trees and some trees within woodlands were less exposed to abrasion. The blockage of air flow resulted in high-speed jets, which cleared out snow within a "horseshoe-shaped vortex" around the base of the tree. Both in much of the woodlands and in the open forest, snow abrasion was minimal or non-existent and snow load on the branches (qali), becomes the prevalent stress. During winter, a large proportion of the snowfall was initially blown off the exposed surfaces of Hudson Bay and the coastal tundra regions, into the woodlands, and eventually across the tree line and into the open forest. As the woodlands filled up with snow in mid-winter, the rate of snow accumulation in the forest increased from double to triple the snowfall. Variations in the rate of accumulation occurred with wind speed and direction. ...Key words: tree line, wind, snow, white spruce, abrasionL'influence de la distribution du vent et de la neige à la limite forestière, près de Churchill, au Manitoba, constitue le thème de cette recherche. On a étudié le profil éolien au dessus de la surface de la neige, ainsi que la taille et la quantité des cristaux de neige au cours d'expériences d'abrasion faites sur des petites branches d'épinette blanche. Pour cette dernière, la zone d'abrasion se situait de toute évidence dans les 80 cm au-dessus de la surface de la neige et la disparition des aiguilles par abrasion était fortement influencée par l'âge de la branche. La disparition par abrasion des aiguilles d'âge semblable était plus élevée pour les nouvelles branches et diminuait avec l'âge de la branche, jusqu'à 7 ou 8 ans, pour augmenter à nouveau avec les branches plus vieilles. Les arbres qui poussaient de 80 cm en 7 ou 8 ans réussissaient à traverser la zone d'abrasion. Dans les environnements de toundra boisée, les arbres aux formes tassées et enchevêtrées étaient couverts de neige tôt en hiver et étaient peu sujets à l'abrasion. Les îlots d'arbres aux aiguilles clairsemées étaient largement couverts de neige à la base. Les arbres aux aiguilles plus fournies et quelques arbres de la forêt-parc étaient moins exposés à l'abrasion. Le blocage de la circulation d'air créait des courants très rapides qui soufflaient la neige en créant un tourbillon en fer à cheval au pied de l'arbre. Dans la plupart de la forêt-parc comme dans la forêt claire, l'abrasion par la neige était minime ou non existante et la charge nivale sur les branches (qali) était le principal agent de stress. Au cours de l'hiver, une grande quantité de la chute nivale était d'abord transportée par le vent, depuis la surface exposée des régions de la baie d'Hudson et de la toundra côtière, jusqu'à la forêt-parc, puis dépassait la limite forestière pour entrer dans la forêt claire. Comme la forêt-parc se remplissait de neige au milieu de l'hiver, le taux d'accumulation nivale dans la forêt passait du double au triple de la chute de neige. La vitesse et la direction du vent faisaient varier le taux d'accumulation. Durant le mois de mai, la fonte nivale se produisait d'abord dans la toundra et ruisselait en général sur la surface gelée. La fonte nivale se produisait environ 3 semaines plus tard dans la forêt claire et s'y caractérisait par une réduction du ruissellement, du fait que l'eau s'infiltrait plus facilement dans la tourbe en dégel. Les combes de neige tardives étaient caractéristiques de la toundra boisée de la forêt-parc et avaient généralement disparu au milieu de juillet. La forêt-parc était libre de neige pendant 1 mois et demi à 2 mois au cours d'une année, alors que la forêt claire était libre de neige durant environ 3 mois et la toundra pratiquement libre de neige durant au moins 6 mois.Mots clés : limite forestière, vent, neige, épinette blanche, abrasio

Permafrost Development in the Intertidal Zone at Churchill, Manitoba: A Possible Mechanism for Accelerated Beach Lift

Boreholes drilled in the Beech Bay area during July to November 1929 indicated that a sill of permafrost had extended below the high-water line, tapering in depth towards low water. The boreholes revealed thick layers of fine sediments on top of deep underlain bedrock. Recent borings determined the upper limits of permafrost in 1981. Examination of the data shows that there has been a permafrost expansion into the emerging tidal zone. These observations suggest an additional mechanism for accelerated uplift of coastal exposed "soft" sediments: the vertical expansion of refrozen, water-saturated silts and clays as new permafrost forms. The existing rates of isostatic uplift are enhanced by the process.Key words: permafrost, active zone, isostatic uplift, Breech Bay, Churchill, Hudson BayDes carottes prises dans la région de la baie Beech entre juillet et novembre 1929 ont indiqué qu'un seuil de pergélisol s'étendait au-dessous du niveau des hautes mers, diminuant en profondeur en approchant le niveau des basses mers. Les carottes révèlant des couches épaisses de fins sédiments reposant sur un profond soubassement. Des carottes récentes ont déterminé les limites supérieures du pergélisol en 1981. L'étude des données signales une expansion du pergélisol dans la zone intertidale surgissante. Ces observations suggèrent un mécanisme additionnel accélérant la levée des sédiments "mous" exposés le long des côtes par l'expansion verticale de vases et de glaises saturés d'eau et congelées à nouveau sous de nouvelles formes de pergélisol. Le taux de levée isostatique est augmenté par le processus.Mots clés: pergélisol, zone active, levée isostatique, baie Beech, la baie d'Hudso

Two-dimensional Packing in Prolate Granular Materials

We investigate the two-dimensional packing of extremely prolate (aspect ratio $\alpha=L/D>10$) granular materials, comparing experiments with Monte-Carlo simulations. The average packing fraction of particles with aspect ratio $\alpha=12$ is $0.68\pm0.03$. We quantify the orientational correlation of particles and find a correlation length of two particle lengths. The functional form of the decay of orientational correlation is the same in both experiments and simulations spanning three orders of magnitude in aspect ratio. This function decays over a distance of two particle lengths. It is possible to identify voids in the pile with sizes ranging over two orders of magnitude. The experimental void distribution function is a power law with exponent $-\beta=-2.43\pm0.08$. Void distributions in simulated piles do not decay as a power law, but do show a broad tail. We extend the simulation to investigate the scaling at very large aspect ratios. A geometric argument predicts the pile number density to scale as $\alpha^{-2}$. Simulations do indeed scale this way, but particle alignment complicates the picture, and the actual number densities are quite a bit larger than predicted.Comment: 6 pages + 10 ps/eps figure

Continuous Quantum Measurement and the Quantum to Classical Transition

While ultimately they are described by quantum mechanics, macroscopic mechanical systems are nevertheless observed to follow the trajectories predicted by classical mechanics. Hence, in the regime defining macroscopic physics, the trajectories of the correct classical motion must emerge from quantum mechanics, a process referred to as the quantum to classical transition. Extending previous work [Bhattacharya, Habib, and Jacobs, Phys. Rev. Lett. {\bf 85}, 4852 (2000)], here we elucidate this transition in some detail, showing that once the measurement processes which affect all macroscopic systems are taken into account, quantum mechanics indeed predicts the emergence of classical motion. We derive inequalities that describe the parameter regime in which classical motion is obtained, and provide numerical examples. We also demonstrate two further important properties of the classical limit. First, that multiple observers all agree on the motion of an object, and second, that classical statistical inference may be used to correctly track the classical motion.Comment: 12 pages, 4 figures, Revtex

The delta-function-kicked rotor: Momentum diffusion and the quantum-classical boundary

We investigate the quantum-classical transition in the delta-kicked rotor and the attainment of the classical limit in terms of measurement-induced state-localization. It is possible to study the transition by fixing the environmentally induced disturbance at a sufficiently small value, and examining the dynamics as the system is made more macroscopic. When the system action is relatively small, the dynamics is quantum mechanical and when the system action is sufficiently large there is a transition to classical behavior. The dynamics of the rotor in the region of transition, characterized by the late-time momentum diffusion coefficient, can be strikingly different from both the purely quantum and classical results. Remarkably, the early time diffusive behavior of the quantum system, even when different from its classical counterpart, is stabilized by the continuous measurement process. This shows that such measurements can succeed in extracting essentially quantum effects. The transition regime studied in this paper is accessible in ongoing experiments.Comment: 8 pages, 4 figures, revtex4 (revised version contains much more introductory material

The Residual Stress Relaxation Behavior of Weldments During Cyclic Loading

Accurate measurement of residual stress is necessary to obtain reliable predictions of fatigue lifetime and enable estimation of time-to-facture for any given stress level. In this article, relaxation of welding residual stresses as a function of cyclic loading was documented on three common steels: AISI 1008, ASTM A572, and AISI 4142. Welded specimens were subjected to cyclic bending (R = 0.1) at different applied stresses, and the residual stress relaxation existing near the welds was measured as a function of cycles. The steels exhibited very different stress relaxation behaviors during cyclic loadings, which can be related to the differences in the microstructures of the specimens. A phenomenological model, which treats dislocation motion during cyclic loading as being analogous to creep of dislocations, is proposed for estimation of the residual stress relaxation

Search for supersymmetry with a dominant R-parity violating LQDbar couplings in e+e- collisions at centre-of-mass energies of 130GeV to 172 GeV

A search for pair-production of supersymmetric particles under the assumption that R-parity is violated via a dominant LQDbar coupling has been performed using the data collected by ALEPH at centre-of-mass energies of 130-172 GeV. The observed candidate events in the data are in agreement with the Standard Model expectation. This result is translated into lower limits on the masses of charginos, neutralinos, sleptons, sneutrinos and squarks. For instance, for m_0=500 GeV/c^2 and tan(beta)=sqrt(2) charginos with masses smaller than 81 GeV/c^2 and neutralinos with masses smaller than 29 GeV/c^2 are excluded at the 95% confidence level for any generation structure of the LQDbar coupling.Comment: 32 pages, 30 figure

Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at sNN=2.76\sqrt{s_{\rm NN}}=2.76 TeV

The elliptic, $v_2$, triangular, $v_3$, and quadrangular, $v_4$, azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range $|\eta|<0.8$ at different collision centralities and as a function of transverse momentum, $p_{\rm T}$, out to $p_{\rm T}=20$ GeV/$c$. The observed non-zero elliptic and triangular flow depends only weakly on transverse momentum for $p_{\rm T}>8$ GeV/$c$. The small $p_{\rm T}$ dependence of the difference between elliptic flow results obtained from the event plane and four-particle cumulant methods suggests a common origin of flow fluctuations up to $p_{\rm T}=8$ GeV/$c$. The magnitude of the (anti-)proton elliptic and triangular flow is larger than that of pions out to at least $p_{\rm T}=8$ GeV/$c$ indicating that the particle type dependence persists out to high $p_{\rm T}$.Comment: 16 pages, 5 captioned figures, authors from page 11, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/186

Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at sNN=2.76\sqrt{s_{\rm{NN}}} = 2.76 TeV

The inclusive transverse momentum ($p_{\rm T}$) distributions of primary charged particles are measured in the pseudo-rapidity range $|\eta|<0.8$ as a function of event centrality in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}=2.76$ TeV with ALICE at the LHC. The data are presented in the $p_{\rm T}$ range $0.15<p_{\rm T}<50$ GeV/$c$ for nine centrality intervals from 70-80% to 0-5%. The Pb-Pb spectra are presented in terms of the nuclear modification factor $R_{\rm{AA}}$ using a pp reference spectrum measured at the same collision energy. We observe that the suppression of high-$p_{\rm T}$ particles strongly depends on event centrality. In central collisions (0-5%) the yield is most suppressed with $R_{\rm{AA}}\approx0.13$ at $p_{\rm T}=6$-7 GeV/$c$. Above $p_{\rm T}=7$ GeV/$c$, there is a significant rise in the nuclear modification factor, which reaches $R_{\rm{AA}} \approx0.4$ for $p_{\rm T}>30$ GeV/$c$. In peripheral collisions (70-80%), the suppression is weaker with $R_{\rm{AA}} \approx 0.7$ almost independently of $p_{\rm T}$. The measured nuclear modification factors are compared to other measurements and model calculations.Comment: 17 pages, 4 captioned figures, 2 tables, authors from page 12, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/284