Vittoz, P., Bayfield, N.G., Brooker, R.W., Elston, D.A., Duff, E.I., Theurillat, J-P. and Guisan, A.
Journal of Vegetation Science 21(6), 1035-1047.
Biodiversity monitoring, GLORIA protocol, Permanent plot, Point method, Pseudoturnover, Sampling error, Statistical power, Visual cover estimate
||Question: When multiple observers record the same spatial units of alpine vegetation, how much variation is there in the records and what are the consequences of this variation for monitoring schemes to detect change?
Location: One test summit in Switzerland (Alps) and one test summit in Scotland (Cairngorm Mountains).
Method: Eight observers used the GLORIA protocols for species composition and visual cover estimates in percent on large summit sections (>100 m2) and species composition and frequency in nested quadrats (1 m2).
Results: The multiple records from the same spatial unit for species composition and species cover showed considerable variation in the two countries. Estimates of pseudoturnover of composition and coefficients of variation of cover estimates for vascular plant species in 1m x 1m quadrats showed less variation than in previously published reports whereas our results in larger sections were broadly in line with previous reports. In Scotland, estimates for bryophytes and lichens were more variable than for vascular plants.
Conclusions: Statistical power calculations indicated that, unless large numbers of plots were used, changes in cover or frequency were only likely to be detected for abundant species (exceeding 10% cover) or if relative changes were large (50% or more). Lower variation could be reached with the point methods and with larger numbers of small plots. However, as summits often strongly differ from each other, supplementary summits cannot be considered as a way of increasing statistical power without introducing a supplementary component of variance into the analysis and hence the power calculations.