Comparison of data from two vegetation monitoring methods in semi-natural grasslands

Abstract

Two vegetation-monitoring methods were compared: subplot frequency analysis (SF) and visual estimation of percentage cover (VE). Two independent observers collected data from two semi-natural, species-rich grasslands on three different occasions during the growth-season. During the last data collection period, survey times were also recorded. The two different data sets from the two methods were compared using partial Redundancy Analyses. The purpose of the comparison was to identify the method that explains most of the relevant variation in biodiversity-monitoring (inter- and intra-site variation), and the variation irrelevant when evaluating data (systematic inter-observer variation and variation due to phenological changes). Compared with VE data, more variation in SF data could be explained by spatial variables, while less variation depended on the observer and time of year surveyed. SF also found more species per plot but took on average five times longer to complete than VE. In conclusion, the different methods are suitable for different purposes: SF is more suitable for purposes demanding high accuracy and high precision, such as long-term biodiversity-monitoring when the identification of small changes has high priority, while VE might be more suitable for a one-time mapping of a large area.

In the eye of the beholder: bias and stochastic variation in cover estimates

“…Økland 1988Økland , Kercher et al 2003. The reliability of visual cover estimates has been the focus for a number of studies (Sykes et al 1983, Jukola-Sulonen and Salemaa 1985, Floyd and Anderson 1987, Kennedy and Addison 1987, Mitchell et al 1988, Dethier et al 1993, Bråkenhielm & Qinghong 1995, Carlsson et al 2005, Vittoz & Guisan 2007. Still, the contribution of bias due to individual observers versus other unexplained sources of error has rarely been reported (van Hees & Mead 2000, Milberg et al 2008.…”

Cover estimation versus density counting in species-rich pasture under different grazing intensities

“…The cover estimation method was less timeconsuming than plant counting, as in comparison of cover estimation to other methods (Brakenhielm and Liu 1995;Carlsson et al 2005). Detailed counting of each grass tiller and stolon growing-point for Trifolium repens was the main reason for making the plant counting method more time consuming than cover estimation.…”

“…The problem of cover estimation concerned those species that were either small, had a high abundance or were winding plants (Carlsson et al 2005). It should be noted that humans visually perceive cover on a geometric scale rather than on a linear scale, because human visualization is attuned to doubling and can more easily discern the difference between values of 1% and 2% than between 31% and 32% (Bonham et al 2004).…”

“…In addition to point-frequency (Brakenhielm and Liu 1995) and subplot-frequency analysis (Carlsson et al 2005), the counting density of individual plants (Brock et al 1996) is another widespread quantitative description of sward to provide specific information about grassland. In comparison with cover estimation, this technique reduces bias arising from assumptions of each observer.…”

Systematic monitoring of heathy woodlands in a Mediterranean climate—a practical assessment of methods

“…Replication of plots is required to capture habitat variation and ensure statistical significance (Legg and Nagy 2006;Reinke and Jones 2006). Replication must tradeoff with available resources, spatial coverage of plot networks and the need for more costly methods that produce precise data (Carlsson et al 2005;Reinke and Jones 2006). For remote sensing validation, plots are best located within homogeneous (or evenly mixed) habitat at least a pixel width from the edge of the patch (Reinke and Jones 2006).…”