The interpretation of Hill's 'Two Term Local Quadrat Variance' analysis to detect the scale of spatial pattern in vegetation is improved by an equation that relates the block size of a variance peak to the scale of the pattern that gave rise to it. (Contrary to previous belief, the two are not the same, especially for large block sizes.) Deviations of pattern from a perfectly regular alternation of equally sized gaps and patches of uniform density cause changes in the variance. To aid in the interpretation of these changes, two indices of pattern regularity are proposed, one based on density and one based on presence/absence, in order to distinguish the effects of irregularities of patch density from irregularities of patch size and position.These methods are applied to a study of primary succession on glacial moraines near Mt. Robson, British Columbia, Canada, in order to evaluate certain hypotheses about the development of pattern. Other researchers have proposed that during succession, the pattern at first intensifies at the scales initially observed, then as succession proceeds some scales of pattern are lost due to coalescence of patches and eventually the intensity of those that remain decreases as the patterns become more and more irregular. The vegetation on the Mt. Robson moraines confirms this sequence of changes in vegetation pattern, only to the extent that patterns intensify initially in the chronosequence; the number of scales of pattern in the vegetation remains about the same throughout and there is no evidence that the patterns become more irregular.The variance-block size graphs derived from presence / absence data matched those from density data well, indicating that the simpler data, in this case, are almost as informative about pattern as the more detailed data.
A study of nucleation during primary succession was carried out on age sequences of communities at two sites in the Canadian Rocky Mountains: one at the Mount Robson moraines, British Columbia, the other at Southeast Lyell Glacier, Alberta. The study concentrated on the associations of species with the nitrogen-fixing plants Hedysarum boreale var. mackenzii at Mount Robson moraines and Dryas drummondii at Southeast Lyell Glacier because those plants might serve as nuclei for colonization by other species, thus facilitating succession. The data show that recruitment of later successional species is greater in patches of the two pioneer species, but the fact that recruitment takes place away from the plants also suggests that although there is nucleation, it is not necessary for succession at these sites. Key words: colonization, nitrogen fixation, nucleation, succession.
Abstract. The traditional approach to the analysis of species association within a community, based upon co‐occurrence in sampling units such as quadrats, has been to test all pairs of species, using a 2 × 2 contingency table for each pair. It has long been recognised that all these tests are not independent of each other, but there is an additional problem in that the association between any particular pair may depend on the combination of the other species that are present or on the environmental factors that determine that combination. We use a 2k contingency table to examine this problem and find that pairwise associations are not independent of the other species. The second problem that we consider is the effect of spatial autocorrelation in the data which makes the statistical tests too liberal. In the absence of a derived solution for a deflation factor to correct the test statistic calculated from a 2k table, we describe a Monte Carlo approach that provides an approximate solution to this problem. In our data the amount of deflation that is necessary for a 2k table is small compared to the amount required for the 2 × 2 tables used to test pairwise association.
Objectives: We hypothesize that prosthetic valve thrombogenic potential may be inferred by valve regional flow velocity. This metric was used to compare two clinical valves and two experimental prototypes. Methods: Four our clinical and experimental prototype valve models were tested in aortic and mitral sites under pulsatile circulation in a pulse duplicator. An optical approach measuring projected dynamic valve area to gauge valve motion was implemented. Pulsatile pressures and flow rates were measured by conventional techniques and a quasi-steady flow tester was used to measure valve leakage. Regurgitant flow velocity was derived using time-dependent volumetric flow rate / projected dynamic valve area. Since flow velocity and fluid shear force are related through flow velocity gradient, thrombogenic potential for valves that achieve near closure during the forward flow deceleration phase were determined as regurgitant flow velocities relative to the control mechanical valve regurgitant flow velocity of -126 m/s. Analysis of the flow velocity data made use of Vioplot R* and VISIO software packages to provide split violin plots that represent probability of recurrence of data. Results: Thrombogenic potential was made dimensionless and ranged between -0.45 and +1.0. Negative thrombogenic potentials arise when transient rebound of valve occluder is accompanied by water-hammer phenomena. Positive thrombogenic potentials occur during decelerating forward flow. Bioprostheses had lowest thrombogenic potential transient of 0.15. A mock-transcatheter aortic valve replacement incorporating by design a trivial paravalvular leak (~1.35 ml/s) demonstrated a high transient thrombogenic potential of 0.95. Conclusions: Our data reveals distinct thrombogenic potential profile differences between valve models. If our study methods are verifiable, the design of future valves may utilize currently available experimental tools to produce advanced devices with significantly reduced thrombogenic potential.
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