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Systematic and random variation in vegetation monitoring data
Linköping University, Department of Physics, Chemistry and Biology, Ecology . Linköping University, The Institute of Technology.ORCID iD: 0000-0001-6128-1051
Linköping University, Department of Physics, Chemistry and Biology, Ecology . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
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2008 (English)In: Journal of Vegetation Science, ISSN 1100-9233, E-ISSN 1654-1103, Vol. 19, 633-644 p.Article in journal (Refereed) Published
Abstract [en]

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-m2 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.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2008. Vol. 19, 633-644 p.
Keyword [en]
Forest, Observer error, Permanent plot, Statistical power, Sweden
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-11872DOI: 10.3170/2008-8-18423OAI: oai:DiVA.org:liu-11872DiVA: diva2:18275
Note
Original publication: Milberg, P., Bergstedt, J., Fridman, J., Odell, G & Westerberg, L., Systematic and random variation in vegetation monitoring data, 2008, Journal of Vegetation Science, (19), 633-644. http://dx.doi.org/10.3170/2008-8-18423. Copyright: Opulus Press, http://www.opuluspress.se/index.phpAvailable from: 2008-05-22 Created: 2008-05-22 Last updated: 2017-12-13
In thesis
1. Boreal vegetation responses to forestry as reflected in field trial and survey data and the quality of cover estimates and presence/absence in vegetation inventory
Open this publication in new window or tab >>Boreal vegetation responses to forestry as reflected in field trial and survey data and the quality of cover estimates and presence/absence in vegetation inventory
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

Den här avhandlingen belyser hur avverkning och markberedning påverkar markfloran i den svenska barrskogen. Dessutom utvärderas två inventeringsmetoder som används inom växtekologin. Vid arbetet har både rikstäckande inventeringsdata och fältförsök använts och de likartade resultaten tyder på att rikstäckande inventeringar är en underutnyttjad resurs i forskningen.

Ju större andel av träden som avverkas desto större blir förändringen av markflorans sammansättning. Vissa arter, som lingon, ljung, etc., verkar dock inte påverkas i nämnvärd omfattning, medan andra, som blåbär, minskar i relation till hur mycket som avverkats. Gräs och mjölkört ökar efter avverkning, dock visar sig vissa gräs och mjölkört inte reagera om inte avverkningen överskrider ett tröskelvärde på ca 80 %. Avverkning har en liten, men signifikant, effekt på antalet arter, medan artomsättning, d.v.s. arters etablering på och/eller försvinnande från provytorna, framförallt påverkas av andel gran innan avverkning, markens produktionsförmåga och först därefter av hur stor andel av träden som avverkas. Det var också uppenbart att markberedning har en stark effekt som skiljer sig från avverkning. Framförallt gynnas björnmossor av markberedning men även vårfryle, kruståtel och mjölkört. Arter som missgynnas av markberedning var bl.a., en levermossa, lingon, väggmossa och kråkbär.

I växtekologi är visuell täckningsbedömning, d.v.s. hur stor del av en provyta som täcks av en växtart, och registrering av förekomst/icke förekomst, d.v.s. finns en växtart på en provyta eller inte, de två vanligaste metoderna vid vegetationsinventering.

Vid registrering av förekomst/icke förekomst missas upp till en tredjedel av förekomsterna, vanligaste orsaken till missade registreringar verkar vara att man inte upptäcker arten snarare än att den inte kan identifieras. Det var stora variationer mellan arter, där arter med få exemplar på provytan missas oftare.

Både den visuella täckningsbedömningen och förekomst/icke förekomst visar sig ha personberoende fel, d.v.s. att olika personer genomgående ger högre eller lägre värden än andra. Trots det personberoende felet visar sig täckningsbedömningar ha ett större informationsvärde än registrering av förekomst/icke förekomst när det gäller att särskilja olika typer av vegetation. Erfarenhet har en förvånansvärt liten effekt på kvaliteten av täckningsbedömningar.

Abstract [en]

This thesis has two main focuses; first, the response of forest ground layer flora on forestry, mainly harvesting and secondly, the quality of the vegetation assessment methods, cover estimates by eye and presence/absence data.

The effect of harvesting intensity was evaluated with survey data from permanent plots as well as vegetation data from a field trial fourteen years after harvesting. Both data sets confirmed that response of ground layer flora increased with increasing logging intensity. Thereby, indicating that survey data is possible to use in research. From the survey data set, existence of a time lag was evident for several species and also a threshold level was evident in cutting intensity needed to affect a number of species. Logging had a modest, but significant positive effect on the change in species number per plot. Species turnover was influenced by the proportion of Picea abies in the tree canopy; site productivity; and logging intensity. In the field trial scarification had a strong effect that was different from the one created by cutting.

In plant ecology cover estimate by eye and presence/absence recording are the two most frequent methods used. The methods were evaluated with survey data and a field trial.

In the first data set vegetation was recorded independently by two observers in 342 permanent 100-m2 plots. Overall, one third of each occurrence was missed by one of the two observers, but with large differences among species. Species occurring at low abundance tended to be frequently overlooked. Observer-explained variance in cover estimates was <10% in 15 of 17 species.

In the second data set, 10 observers independently estimated cover in sixteen 100-m2 plots in two different vegetation types. The bias connected to observer varied substantially between species. The estimates of missing field and bottom layer had the highest bias, indicating that missing layers are problematic to use in analysis of change. Experience had a surprisingly small impact on the bias connected to observer. Analyses revealed that for the statistical power, cover estimates by eye carries a higher information value than do presence/absence data when distinguishing between vegetation types, differences between observers is negligible, and using more than one observer had little effect.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2008
Series
Linköping Studies in Management and Economics. Dissertations, ISSN 0347-8920 ; 1172
Keyword
Boreal forest, cutting intensity, scarification, cover estimates, presence/absence
National Category
Ecology
Identifiers
urn:nbn:se:liu:diva-11750 (URN)978-91-7393-939-3 (ISBN)
Public defence
2008-04-25, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2008-05-07 Created: 2008-05-07 Last updated: 2014-10-08

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Milberg, PerBergstedt, JohanWesterberg, Lars

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