Test of observer variability in measuring riparian shrub twig length

Measurement of riparian shrub twig length before and after use should yield a useful utilization index. A first step towards determining utilization is measurement of twig length. This study appraised variability between 15 observers for measuring dormant season twig length on riparian alder (Alnus incana (L.) Moench) shrubs. Ten streamside shrubs were selected on which 5 branches consisting of 5 twigs each were tagged below the fifth twig for a total of 250 twigs. Fifteen experienced people independently measured twigs on the same day after instruction in the method. Data were analyzed by hierarchical analysis of variance for length of twigs by branches, by shrubs, and by observers. Variation among observers within a branch was about twice the size of variability among shrubs and represented 20% of the total variation. Items contributing to observer variability were measurement of dieback, selecting the twig end or live bud for measurement, inclusion of short lateral leaf stubs in measurements, and selection of a crotch from where the twig is measured. These results clearly illustrate major difficulties in trying to measure riparian shrub utilization.