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Research article - Peer-reviewed, 2008

Observer bias and random variation in vegetation monitoring data

Milberg, Per; Bergstedt, Johan; Fridman, Jonas; Odell, Gunnar; Westerberg, Lars

Abstract

Question: Detecting species presence in vegetation and making visual assessment of abundances involve a certain amount of skill, and therefore subjectivity. We evaluated the magnitude of the error in data, and its consequences for evaluating temporal trends.Location: Swedish forest vegetation.Methods: Vegetation data were collected independently by two observers in 342 permanent 100-m(2) plots in mature boreal forests. Each plot was visited by one observer from a group of 36 and one of two quality assessment observers. The cover class of 29 taxa was recorded, and presence/absence for an additional 50.Results: Overall, one third of each occurrence was missed by one of the two observers, but with large differences among species. There were more missed occurrences at low abundances. Species occurring at low abundance when present tended to be frequently overlooked. Variance component analyses indicated that cover data on 5 of 17 species had a significant observer bias. Observer-explained variance was < 10% in 15 of 17 species.Conclusion: The substantial number of missed occurrences suggests poor power in detecting changes based on presence/ absence data. The magnitude of observer bias in cover estimates was relatively small, compared with random error, and therefore potentially analytically tractable. Data in this monitoring system could be improved by a more structured working model during field work.

Keywords

forest; observer error; permanent plot; statistical power; Sweden

Published in

Journal of Vegetation Science
2008, volume: 19, number: 5, pages: 633-644
Publisher: OPULUS PRESS UPPSALA AB

Authors' information

Milberg, Per
Linköping University
Bergstedt, Johan
Linköping University
Swedish University of Agricultural Sciences, Department of Forest Resource Management
Westerberg, Lars
Linköping University

Associated SLU-program

Forest

UKÄ Subject classification

Environmental Sciences related to Agriculture and Land-use

Publication Identifiers

DOI: https://doi.org/10.3170/2008-8-18423

URI (permanent link to this page)

https://res.slu.se/id/publ/19901