Effects of sampling time, species richness and observer on the exhaustiveness of plant censuses

Question: How may sampling time affect exhaustiveness of vegetation censuses in interaction with observer effect and quadrat species richness? Location: French lowland forests. Methods: Two data sets comprised of 75 timed, one-hour censuses of vascular plants carried out by five observers on 24 400-m(2) forest quadrats were analysed using mixed-effect models. Results: The level of exhaustiveness increased in a semilogarithmic way with sampling time and decreased with quadrat species richness. After one hour, 20 to 30% of the species remained undetected by single observers. This proportion varied among observers and the discrepancy increased with increasing sampling time. Fixing the sampling time may make richness estimates vary less between observers but the time limit should be at least 30 min to reduce the bias in exhaustiveness between rich and poor quadrats. Conclusions: We advocate the use of sampling methods based on spatially or temporally-replicated censuses and statistical analyses that correct for the lack of census exhaustiveness in vegetation studies.