Evaluation of a measurement method for forest vegetation in a large-scale ecological survey

We evaluate a field method for determining species richness and canopy cover of vascular plants for the Forest Health Monitoring Program (FHM), an ecological survey of U.S. forests. Measurements are taken within 12 1-m(2) quadrats on 1/15 ha plots in FHM. Species richness and cover are determined for four height classes (strata) within each quadrat and aggregated by stratum over the entire plot. We estimated (1) the agreement between experienced trainers and inexperienced technicians who collected the data on this survey (accuracy) and (2) the agreement among the technicians (precision) for results on species richness and cover from 3 test plots at 3 time intervals. The methods appear to be highly precise, although some discrepancies with the values obtained by the trainers were found. Trainers found significantly more species in the ground stratum (0-0.6 m) and measured significantly more cover in the uppermost stratum (>4.9 m). The proportion of variation due to measurement error and temporal variability was less than 13% for species richness (all strata) and cover (all but one stratum). This indicates that the method is suitable for monitoring changes in species richness and canopy cover for a large-scale synoptic monitoring project such as FHM.