N. C. Kenkel

Applying Metric and Nonmetric Multidimensional Scaling to Ecological Studies: Some New Results

Metric (eigenanalysis) and nonmetric multidimensional scaling strategies for ecological ordination were compared. The results, based on simulated coenoplane data showing varying degrees of species turnover on two independent environmental axes, suggested some strong differences between metric and nonmetric scaling methods in their ability to recover underlying nonlinear data structures. Prior data standardization had important effects on the results of both metric and nonmetric scaling, though the effect varied with the ordination method used. Nonmetric multidimensional scaling based on Euclidean distance following stand norm standardization proved to be the best strategy for re- covering simulated coenoplane data. Of the metric strategies compared, correspondence analysis and the detrended form were the most successful. While detrending improved ordination configurations in some cases, in others it led to a distortion of results. It is suggested that none of the currently available ordination strategies is appropriate under all circumstances, and that future research in ordination methodology should emphasize a statistical rather than empirical approach.

A long‐term study of Pinus banksiana population dynamics

. The long-term population dynamics of a pure, naturally-established stand of Pinus banksiana (jack pine) in southeastern Manitoba, Canada is described. The study was initiated at stand age 15, when all 468 trees in a plot were mapped and their trunk diameter at breast height (DBH) measured. The plot was remeasured eight times — every five years (six years in one case) — and mortality and DBH changes recorded. Total mortality over the 41-yr study period was ca. 84 %. Mortality was initially very low, increased once the stand entered the self-thinning stage from ages 25–46, and declined at ages 46–56. Mortality was restricted to the smallest size classes throughout. The stand reached the self-thinning line at ca. age 30. The self-thinning slope was significantly less steep than the theoretically expected value of −0.5. The distribution of DBH values was initially symmetric, showed increasing positive skewness during the period of highest mortality, and became symmetric again at later stages. Size (DBH) inequality was highest just prior to the onset of density-dependent mortality, and subsequently declined. Tree DBH values were positively autocorrelated both initially and at later stages of stand development, but were spatially independent during the period of highest density-dependent mortality. The stand initially had a strongly clumped pattern at all spatial scales. Patterns of mortality were non-random during stand development, however, resulting in increased spatial regularity over time. Mortality was initially restricted to high density patches of the stand, but occurred throughout the plot once the self-thinning line was reached. Mortality during the self-thinning stage deviated from random expectation at local spatial scales (1–2m radius), suggesting that individuals were competing with their immediate neighbours. It is argued that an integrated approach, incorporating both population size and spatial structures, is essential in improving our understanding of long-term plant population dynamics.

Review: Multivariate analysis in weed science research

Abstract Data containing many variables are often collected in weed science research, but until recently few weed scientists have used multivariate statistical methods to examine such data. Multivariate analysis can be used for both descriptive and predictive modeling. This paper provides an intuitive geometric introduction to the more commonly used and relevant multivariate methods in weed science research, including ordination, discriminant analysis, and canonical analysis. These methods are illustrated using a simple artificial data set consisting of abundance measures of six weed species and two soil variables over 12 sample plots.

Impacts of land use on riparian forest along an urban – rural gradient in southern Manitoba

Extensive landscape modification by humans has led to the fragmentation of riparian forests across North America. We compared the vegetation of extant riparian forest along an urban-rural disturbance gradient. In 1999, twenty-five sites along Assiniboine River in Manitoba, Canada were categorized according to land use: urban, suburban, high intensity rural, low intensity rural, and relatively high quality reference forest. Differences in herbaceous, shrub, and tree species composition and diversity were related to the proportion of surrounding land use, forest patch size, connectivity, and area:perimeter ratio. Urban riparian forests were more disturbed and isolated. They were smaller and characterized by drier, more alkaline soils. Moreover, they had significantly lower native and overall understorey species diversity, and had a higher proportion of exotics including Solanum dulcamara and Hesperis matronalis. Suburban forests were less disturbed, faced greater development pressure, and had sandier soils. Although suburban understorey diversity was similar to that of rural forests, suburban sites had a higher proportion of exotic species, especially escaped horticultural and invasive species including Caragana arborescens and Rhamnus cathartica. Reference sites were relatively large and exhibited greater connectivity, but there was little difference in species composition and diversity among high intensity rural, low intensity rural, and reference sites. These site types were less disturbed than either urban or suburban forests, and reference sites were characterized by hydrophilic species including Scirpus fluviatilis and Carex aquatilis. Our results suggest that landscape measures of disturbance, and related changes in environment, may be confidently used to assess impacts of land use on vegetation along urban-rural gradients.

On sampling procedures in population and community ecology

In this paper we emphasize that sampling decisions in population and community ecology are context dependent. Thus, the selection of an appropriate sampling procedure should follow directly from considerations of the objectives of an investigation. We recognize eight sampling alternatives, which arise as a result of three basic dichotomies: parameter estimation versus pattern detection, univariate versus multivariate, and a discrete versus continuous sampling universe. These eight alternative sampling procedures are discussed as they relate to decisions regarding the required empirical sample size, the selection or arrangement of sampling units, and plot size and shape. Our results indicate that the decision-making process in sampling must be viewed as a flexible exercise, dictated not by generalized recommendations but by specific objectives: there is no panacea in ecological sampling. We also point to a number of unresolved sampling problems in ecology.

Revegetation dynamics of cliff faces in abandoned limestone quarries

1. Revegetation dynamics on vertical cliff faces in abandoned limestone quarries were examined at 18 sites in southern Ontario, Canada. 2. A wide range of biotic and abiotic variables were measured in randomly positioned plots at each site. 3. Quarry walls ranged in age from 17 to 92 years since abandonment and this known chronosequence was used to investigate successional patterns in the vegetation. 4. Site age and the density of trees adjacent to the quarry walls had the strongest influence on vegetation community composition and abundance. 5. During the first 70 years of abandonment, species richness was high and variable. Community composition was also extremely variable during this period. 6. After 70 years, species richness on quarry walls dropped suddenly and the variation in vegetation composition was reduced. 7. This period of sudden change correlates with the growth of neighbouring trees on the quarry floor that start to shade the quarry walls at about 60 years since abandonment. 8. The final form of the vegetation community on the vertical cliffs in long-abandoned quarries is similar to the naturally occurring community on cliffs of the Niagara Escarpment. 9. The results of this study suggest that natural successional processes can be used to rehabilitate the vertical cliff faces that are produced in limestone quarry operations. 10. The benefits of allowing natural processes to rehabilitate the walls of abandoned limestone quarries are: the recruitment of an uncommon, but native component of biodiversity; a reduced initial cost; and a reduced long-term maintenance cost. 11. It is recommended that abandoned limestone quarries should not be rehabilitated by backfilling of exposed quarry walls.

Clonal integration facilitates the proliferation of smooth brome clones invading northern fescue prairies

Predicting exotic invaders and reducing their impacts on the biodiversity and function of native ecosystems require understanding of the mechanisms that facilitate their success during key stages of invasion. We determined whether clonal growth, characteristic of the majority of successful invaders of natural areas, facilitates the proliferation of Bromus inermis (smooth brome), an exotic grass invading prairie ecosystems across the Great Plains. By manipulating the below-ground connections of proliferating rhizomes as well as the levels of soil nitrogen along the margins of clones invading northern fescue prairies in Manitoba, Canada, we hypothesized that physiological integration would most benefit ramets invading low resource environments. Severing clonal connections reduced the mass of smooth brome shoots invading native prairies and was exacerbated by the immobilization of soil nutrients with glucose. Clonal connections were equally important in the maintenance of smooth brome density and the horizontal proliferation of ramets. Our results demonstrate the role of physiological integration in the proliferation of a clonal exotic invader and may help explain the success of clonal invaders in other regions. Although integration among invading ramets suggests several possibilities for successful management, future research must continue to elucidate differences in the invasiveness of native versus exotic species as well as the persistence of clonal connections among exotic invaders.

Spatial Competition Models for Plant Populations

This review summarizes the major spatial approaches to modelling intraspecific interactions in plants. Spatial models recognize that interactions in plant populations are localized, and therefore use the number, size, and proximity of neighbouring plants to predict an individual’s performance. Such models have been used to develop interaction (or competition) indices, to assess the relative importance of spatial patterning on individual plant performance, and to examine mortality patterns and changes in the size distribution of individuals in field, laboratory, and computer-simulated plant populations. The review begins by outlining the competition (or interference) indices developed by foresters and ecologists, and discusses their utility and limitations. Next the ‘zone of influence’ model, which utilizes overlapping circles in quantifying intraspecific interactions, is reviewed. Specific applications of this model are outlined, examples given, and limitations discussed. This is followed by a detailed outline of spatial tessellation models, which have only recently been applied to examine spatial processes in plant populations. The paper concludes with a discussion of future directions, in which it is argued that tessellation methods should be more widely used in modelling dynamic interactions in plant populations.

Plot size and estimation efficiency in plant community studies

We examine the effects of changing plot size on parameter estimation efficiency in multivariate (community-level) ecological studies, where estimation efficiency is defined in terms relating to the statistical precision of estimates of all variables (e.g. species) in a data set. Three ‘efficiency criteria’ for multivariate estimation are developed, and the relationship between estimation efficiency and plot size examined using three field data sets (deciduous understory, coniferous understory, and mire vegetation) from central Canada. For all three communities, estimation efficiency was found to increase monotonically with increasing plot size. However, relative gains in efficiency at larger plot sizes were offset by substantial increases in sampling effort (enumeration time per plot). Our results indicate that the largest plot size possible, given the constraints of time, should be used for parameter estimation in plant communities. Also, plots that are larger than the mean patch size should be utilized when sampling heterogeneous vegetation.

Edge Effects in the Use of Area Polygons to Study Competition

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