Accounting for large-scale factors in the study of understory vegetation using a conditional logistic model
Journal article, 2007

Local-scale and large-scale factors can affect the presence of a species of understory vegetation in the forest. Local-scale factors may be the influence of surrounding trees, while climate and latitude are typically considered large-scale factors. A model for the presence of a species needs to take into account both scales. A conditional logistic model is proposed for those studies where only the local-scale factors are of interest and that avoids estimating the large-scale parameters. Conditioning is carried out by the number of quadrats in the plot where the vegetation is found. As the latter is a sufficient statistic for the large-scale factors, a model free from these parameters is obtained. Data gathered in the permanent sample plots of the 1985-1986 National Forest Inventory of Finland is used for illustration, where the local-scale factor of interest is the influence of the trees, quantified by an index based on the size and location of the trees. The model fitted to Vaccinium vitis-idaea showed a significant and positive influence of Scots pine on the presence of this species, while for Calamagrostis arundinacea, a decrease in the odds ratio was observed due to the influence of Norway spruce.

local scale

odds ratio


nuisance parameters


influence potential


sufficient statistics






S. Kuhlmann-Berenzon


Swedish Institute for Infectious Disease Control

Urban Hjorth

University of Gothenburg

Chalmers, Mathematical Sciences

Environmental and Ecological Statistics

1352-8505 (ISSN) 1573-3009 (eISSN)

Vol. 14 2 149-159

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