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161.
Christian Damgaard 《Oikos》2004,107(2):225-230
Two-species plant competition experiments are often drawn on in an attempt to understand and possibly to predict the formation of plant community structures. Frequently, such competition experiments use a non-random spatial design but is analysed in models where a random spatial design is implicitly assumed. Here, the effect of spatial covariance in plant competition experiments on the inferred ecological conclusions is demonstrated. It is shown that spatial covariance, which is dependent on the competition kernel and the experimental design, may affect the estimation of parameters of interest. Based on the same set of experimental data, the predicted outcome of competition between two Arabidopsis thaliana genotypes can vary between (almost certain) coexistence and (almost certain) exclusion of one genotype, depending on the assumptions made about the spatial scale of competition between the plants . The predicted outcome of competition depended strongly on the mean interaction distance of the competition kernel, whereas the functional shape of the competition kernel was less important. It is suggested that plants in manipulated competition experiments should be placed randomly, such that the design of the competition experiment is in agreement with the model used in the analysis. 相似文献
162.
Davide Malatesta David Simar Yves Dauvilliers Robin Candau Fabio Borrani Christian Prefaut Corinne Caillaud 《Journal of applied physiology》2003,95(6):2248-2256
This study tested whether the lower economy of walking in healthy elderly subjects is due to greater gait instability. We compared the energy cost of walking and gait instability (assessed by stride to stride changes in the stride time) in octogenarians (G80, n = 10), 65-yr-olds (G65, n = 10), and young controls (G25, n = 10) walking on a treadmill at six different speeds. The energy cost of walking was higher for G80 than for G25 across the different walking speeds (P < 0.05). Stride time variability at preferred walking speed was significantly greater in G80 (2.31 +/- 0.68%) and G65 (1.93 +/- 0.39%) compared with G25 (1.40 +/- 0.30%; P < 0.05). There was no significant correlation between gait instability and energy cost of walking at preferred walking speed. These findings demonstrated greater energy expenditure in healthy elderly subjects while walking and increased gait instability. However, no relationship was noted between these two variables. The increase in energy cost is probably multifactorial, and our results suggest that gait instability is probably not the main contributing factor in this population. We thus concluded that other mechanisms, such as the energy expenditure associated with walking movements and related to mechanical work, or neuromuscular factors, are more likely involved in the higher cost of walking in elderly people. 相似文献
163.
Thomas F Floyd James M Clark Robert Gelfand John A Detre Sarah Ratcliffe Dimitri Guvakov Christian J Lambertsen Roderic G Eckenhoff 《Journal of applied physiology》2003,95(6):2453-2461
Breathing 100% O2 at 1 atmosphere absolute (ATA) is known to be associated with a decrease in cerebral blood flow (CBF). It is also accompanied by a fall in arterial Pco2 leading to uncertainty as to whether the cerebral vasoconstriction is totally or only in part caused by arterial hypocapnia. We tested the hypothesis that the increase in arterial Po2 while O2 was breathed at 1.0 ATA decreases CBF independently of a concurrent fall in arterial Pco2. CBF was measured in seven healthy men aged 21-62 yr by using noninvasive continuous arterial spin-labeled-perfusion MRI. The tracer in this technique, magnetically labeled protons in blood, has a half-life of seconds, allowing repetitive measurements over short time frames without contamination. CBF and arterial blood gases were measured while breathing air, 100% O2, and 4 and 6% CO2 in air and O2 backgrounds. Arterial Po2 increased from 91.7 +/- 6.8 Torr in air to 576.7 +/- 18.9 Torr in O2. Arterial Pco2 fell from 43.3 +/- 1.8 Torr in air to 40.2 +/- 3.3 Torr in O2. CBF-arterial Pco2 response curves for the air and hyperoxic runs were nearly parallel and separated by a distance representing a 28.7-32.6% decrement in CBF. Regression analysis confirmed the independent cerebral vasoconstrictive effect of increased arterial Po2. The present results also demonstrate that the magnitude of this effect at 1.0 ATA is greater than previously measured. 相似文献
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166.
Michael Lippert Karl Steiner Hans-Dieter Payer Sabina Simons Christian Langebartels Heinrich Sandermann Jr. 《Trees - Structure and Function》1996,10(4):268-275
An exposure — response study with proportionalto-ambient ozone levels was conducted in closed chambers on 3-year-old European beech (Fagus sylvatica L.) of montane origin. The fumigation started in April 1990 and lasted for a single growing season. Climate data and ozone concentrations monitored at an experimental station of the Institute for Applied Plant Biology, Schönenbuch, Switzerland were simulated in the exposure chambers 12 days later (1*O3). To test exposure-response relations three additional treatments were applied, subambient (0.2*O3) and two proportionally increased ozone treatments (1.5*O3 and 2*O3). The photosynthetic behaviour of the trees in August revealed the light reactions to be less affected than parameters which are related to the dark reactions of photosynthesis. Assimilation (A350), apparent carboxylation efficiency (CE), and maximum photosynthetic capacity (A2500) were reduced with increasing ozone concentration. For the ozone response of CE and A2500 Critical Levels were calculated. 相似文献
167.
Thomas Wachtler Christian Wehrhahn Barry B. Lee 《Journal of computational neuroscience》1996,3(1):73-82
We developed a physiologically plausible model of the first steps of spatial visual information processing in the fovea of the human retina. With the predictions of this model we could support the hypothesis that, for moderate contrasts ( 40%), hyperacuity is mediated by the magnocellular (MC-) pathway. Despite the lower sampling density in the MC pathway, as compared to the parvocellular (PC-) pathway, the information that is transferred by the MC ganglion cells is sufficient to achieve thresholds comparable to those of human subjects in psychophysical tasks. This is a result of the much higher signal-to-noise ratio of the MC pathway cell signals. The PC pathway cells do not transfer enough information for hyperacuity thresholds. 相似文献
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170.
Seasonal metabolism of a small, arboreal monitor lizard, Varanus scalaris, in tropical Australia 总被引:1,自引:1,他引:0
The field metabolic rates (FMR) and water fluxes of Varanus scalaris were measured during the wet and dry seasons by the doubly-labelled water technique. Seasonal measurements of standard (night-time) metabolism (SMR) and resting (daytime) metabolism (RMR) were made in the laboratory at 18, 24, 30 and 36°C, and maximal oxygen consumption was measured at 36°C on a motorized treadmill. This population was active throughout the year. In the wet season, the mean FMR was 7.8 kJ day−1 (128.0 kJkg−1 day−1 ; mean mass = 66.4 g, n = 13), and during the dry season the mean was 5.0 kJ day−1 (67.6 kJ kg−1 day−1 ; mean mass = 77.4 g, n = 17). The mean water flux rates for these animals were 3.6 and 1.2 ml day−1 , respectively (60.4 and 16.6 ml kg−1 day−1 ). The seasonal means of FMR and water flux were significantly different by ANCOVA ( P < 0.0001). Measurements of SMR and RMR were significantly higher in the wet season (ANCOVA: P < 0.0001), but we found no difference in the maximal oxygen consumption between seasons (ANCOVA: P = 0.6). The maximal oxygen consumption of the lizards on the treadmill (2.9 ml min−1 = 1.8 ml g−1 h−1 ), mean mass = 97.4 g, n = 16) was 20 times that of the SMR at the same temperature during the dry season, and 11 times that of the SMR during the wet season. The seasonal differences in FMR were attributable to: changes in SMR (12.2%) and RMR (16.4%); differences in night-time body temperatures (11.3) and daytime body temperatures (16.4%); and activity (broadly defined to include locomotion, digestion, and reproductive costs (43.7%). 相似文献