Shrubs in the cold : interactions between vegetation, permafrost and climate in Siberian tundra

The Arctic is experiencing strong increases in air temperature during the last decades. High-latitude tundra regions are very responsive to changes in temperature and may cause a shift in tundra vegetation composition towards greater dominance of deciduous shrubs. With increasing deciduous shrub cover, the surface albedo (proportion of sunlight that is reflected to the atmosphere) may be reduced and lead to air warming by trapping more solar radiation into the Arctic ecosystem. As a result of this warming, thawing of carbon-rich permafrost soils may increase and cause a large greenhouse gas flux to the atmosphere, thus contributing to global warming. In my thesis I studied how climate influences shrub growth in the Siberian tundra and how climate-induced changes in shrub cover affect summer permafrost thaw and surface albedo. I investigated these interactions between climate, permafrost and Arctic shrub growth using a combination of shrub ring width analysis, field experiments and remote sensing techniques. I measured and compared growth ring widths with meteorological station data and observed that shrub growth is stimulated by higher summer air temperatures. By performing a shrub removal experiment, I demonstrated that a temperature-induced increase in shrub cover may reduce summer permafrost thaw. Shading by the shrub canopy reduced the transfer of energy to the soil. A denser shrub cover thus effectively reduces summer permafrost thaw, despite leading at the same time to a lower surface albedo. These results indicate it is important to incorporate feedbacks between shrub growth, climate and permafrost thaw in model predictions on the Arctic climate and stability of permafrost in a future warmer world. </p

Exploiting Query Structure and Document Structure to Improve Document Retrieval Effectiveness

In this paper we present a systematic analysis of document retrieval using unstructured and structured queries within the score region algebra (SRA) structured retrieval framework. The behavior of di®erent retrieval models, namely Boolean, tf.idf, GPX, language models, and Okapi, is tested using the transparent SRA framework in our three-level structured retrieval system called TIJAH. The retrieval models are implemented along four elementary retrieval aspects: element and term selection, element score computation, score combination, and score propagation. The analysis is performed on a numerous experiments evaluated on TREC and CLEF collections, using manually generated unstructured and structured queries. Unstructured queries range from the short title queries to long title + description + narrative queries. For generating structured queries we exploit the knowledge of the document structure and the content used to semantically describe or classify documents. We show that such structured information can be utilized in retrieval engines to give more precise answers to user queries then when using unstructured queries

A-dependence of hadronization in nuclei

The A-dependence of models for the attenuation of hadron production in semi-inclusive deep-inelastic scattering on a nucleus is investigated for realistic matter distributions. It is shown that the dependence for a pure partonic (absorption) mechanism is more complicated than a simple $A^{2/3}$ ($A^{1/3}$) behavior, commonly found when using rectangular or Gaussian distributions, but that the A-dependence may still be indicative for the dominant mechanism of hadronization.Comment: 4 pages, 4 figure

Immediate Effects of Cervical Spine Thrust Joint Manipulation on Gait Parameters in Individuals with Neck Pain

Purpose/Hypotheses: 1. To investigate for any change in gait parameters in individuals with neck pain while walking with different functional neck conditions immediately following cervical thrust joint manipulation (TJM) versus a sham intervention. 2. To investigate any association between Global Rating of Change (GROC) scores and gait parameters immediately following cervical thrust joint manipulation versus a sham intervention. The hypotheses were that, a) cervical TJM would have an immediate effect on gait parameters during walking with the neck in at least one of three conditions (neutral, flexion/extension and rotation) among individuals with neck pain; and b) higher scores on the GROC would be associated with improved gait parameters post-intervention. Subjects: Convenience sample of 40 individuals (30 female; mean age 24.5 ± 6.78 years) with neck pain. To qualify, subjects had to have a score \u3e0 on the question of pain intensity in the neck on the Neck Disability Index (NDI) questionnaire and have no contraindications or precautions for cervical TJM. Materials/Methods: Subjects walked on a Zeno Walkway under the following conditions: 1) head in neutral; 2) head rotating from side-to-side, and 3) head nodding up and down. After completing 30 practice trials (10 in each condition), pre-intervention trial 1 gait parameters were recorded for each of the three neck conditions in a randomized order. After a 5-minute rest period, pre-intervention trial 2 was conducted for each condition in same order as trial 1. Subjects then received one of two randomly assigned interventions: cervical spine TJM or active cervical rotation. Immediately after the intervention, the subject returned to the Zeno Walkway for the post-intervention trial 3 in each of the three conditions, in the same order as their previous trials. Gait parameters of average step length, stride length, stride width, velocity, and cadence were analyzed using a 2x2 repeated measures ANOVA (of trials 2 and 3), as well as independent and paired t-tests, to determine if there were any significant changes based on intervention when comparing TJM to sham groups. Results: The results of the 2x2 ANOVA revealed significant interactions between group and time on average gait velocity (p=0.008), step length (p=\u3c0.001), and stride length (p=0.009) when the head was in a neutral position. The TJM group experienced significant increases from pre to post-intervention as shown by paired samples t-test for average gait velocity (p=0.003), step length (p\u3c0.001), and stride length (p=0.008). The sham group however, experienced no significant change in gait velocity (p = 0.290), average step length (p = 0.299), and stride length (p = 0.292). There was also a significant decrease in the Numeric Pain Rating Scale (NPRS) (mean decrease of 1.25; p=0.003) and the group that received cervical TJM reported an improved perception of change demonstrated by an average increase in GROC score by 2.85 (p=0.001). Conclusions: Although our results demonstrate a statistically significant improvement in three gait parameters following TJM while walking with the neck in a neutral position, the improvements are not clinically significant. At this time, there is no evidence-based indication for the clinical use of cervical TJM to improve gait parameters in individuals with neck pain. Our findings cannot confirm clinical significance for reduction of neck pain with cervical TJM based on NDI, NPRS, or GROC questionnaires

A High-Resolution Rotation Curve of NGC 6822: A Test-case for Cold Dark Matter

We present high resolution rotation curves of the local group dwarf irregular galaxy NGC 6822 obtained with the Australia Telescope Compact Array. Our best curves have an angular resolution of 8'' or 20 pc and contain some 250 independent points. The stellar and gas components of NGC 6822 cannot explain the shape of the curve, except for the very inner regions, and NGC 6822 is consequently very dark matter dominated. There is no evidence for the presence of a steep density cusp down to scales of ~20 pc, contrary to the predictions of Cold Dark Matter.Comment: Accepted for publication in MNRA

Mass Density Profiles of LSB Galaxies

We derive the mass density profiles of dark matter halos that are implied by high spatial resolution rotation curves of low surface brightness galaxies. We find that at small radii, the mass density distribution is dominated by a nearly constant density core with a core radius of a few kpc. For rho(r) ~ r^a, the distribution of inner slopes a is strongly peaked around a = -0.2. This is significantly shallower than the cuspy a < -1 halos found in CDM simulations. While the observed distribution of alpha does have a tail towards such extreme values, the derived value of alpha is found to depend on the spatial resolution of the rotation curves: a ~ -1 is found only for the least well resolved galaxies. Even for these galaxies, our data are also consistent with constant density cores (a = 0) of modest (~ 1 kpc) core radius, which can give the illusion of steep cusps when insufficiently resolved. Consequently, there is no clear evidence for a cuspy halo in any of the low surface brightness galaxies observed.Comment: To be published in ApJ Letters. 6 pages. Uses aastex and emulateapj5.sty Typo in Eq 1 fixe

Free-Floating HI Clouds in the M 81 Group

Recent VLA observations pointed at dwarf spheroidal (dSph) galaxies in the M 81 group reveal a hitherto hidden population of extremely low mass (~1e5 Msol) HI clouds with no obvious optical counterparts. We have searched 10 fields in the M81 group totalling 2.2 square degree, both targeting known dwarf spheroidal galaxies and blank fields around the central triplet. Our observations show that the new population of low-mass HI clouds appears to be confined to a region toward the South-East of the central triplet (at distances of ~100 kpc from M 81). Possible explanations for these free-floating HI clouds are that they are related to the dSphs found to the South-East of M 81, that they belong to the galaxies of the M 81 triplet (equivalent to HVCs), that they are of primordial nature and provide fresh, unenriched material falling into the M 81 group, or that they are tidal debris from the 3-body interaction involving M 81-M 82-NGC 3077. Based on circumstantial evidence, we currently favour the latter explanation.Comment: To appear in the Proceedings of IAU Symp. 244 on "Dark Galaxies and Lost Baryons", eds. J. I. Davies & M. J. Disne

The Effects of Starburst Activity on Low Surface Brightness Disk Galaxies

Although numerous simulations have been done to understand the effects of intense bursts of star formation on high surface brightness galaxies, few attempts have been made to understand how localized starbursts would affect both the color and surface brightness of low surface brightness (LSB) galaxies. To remedy this, we have run 53 simulations involving bursts of star formation activity on LSB galaxies, varying both the underlying galaxy properties and the parameters describing the starbursts. We discovered that although changing the total color of a galaxy was fairly straightforward, it was virtually impossible to alter a galaxy's central surface brightness and thereby remove it from the LSB galaxy classification without placing a high (and fairly artificial) threshold for the underlying gas density. The primary effect of large amounts of induced star formation was to produce a centralized core (bulge) component which is generally not observed in LSB galaxies. The noisy morphological appearance of LSB galaxies as well as their noisy surface brightness profiles can be reproduced by considering small bursts of star formation that are localized within the disk. The trigger mechanism for such bursts is likely distant/weak tidal encounters. The stability of disk central surface brightness to these periods of star formation argues that the large space density of LSB galaxies at z = 0 should hold to substantially higher redshifts.Comment: 38 pages, 5 figures, 4 tables, tarred and compressed Also available on http://guernsey.uoregon.edu/~kare