Sammanfattning

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.

Nyckelord

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

Publicerad i

Journal of Vegetation Science
2008, Volym: 19, nummer: 5, sidor: 633-644
Utgivare: OPULUS PRESS UPPSALA AB

SLU författare

• Fridman, Jonas

  • Institutionen för skoglig resurshushållning, Sveriges lantbruksuniversitet
    

Associerade SLU-program

Skog


UKÄ forskningsämne

Miljö- och naturvårdsvetenskap


Publikationens identifierare

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

Permanent länk till denna sida (URI)

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