Surveyor consistency in presence/absence sampling for monitoring vegetation in a boreal forest

Vegetation assessments are a central part of many large-scale monitoring programmes. To accurately estimate change over time, consistent field methods are important. Presence/absence (P/A) sampling is considered to be less susceptible to judgement and measurement errors in comparison with visual cover assessment, although errors also occur with this method in complete species inventories. Few studies have evaluated surveyor consistency in P/A sampling with a limited list of species. In this study, the consistency of results in P/A sampling was evaluated in a field test, with different surveyors assessing the same sample plots. The results indicated a good consistency between surveyors and high accuracy according to a reference survey for many of the tested species, although some differences both between surveyors and in comparison with the reference survey were found. Comparing a group of surveyors with a larger experience of vegetation assessments with a group of surveyors with less experience indicated that the former were more accurate and consistent. No clear differences were found between different plot sizes tested. The main conclusion from the study is that P/A sampling is slightly affected by observer judgement bias, but that in comparison with the consistency of visual cover assessment observed in other studies, the difference between surveyors for many species is reasonably low.