Methods of Measuring the Proportions of Plant Species Present in Forest and Their Effect on Estimates of Bird Preferences for Plant Species

Stem density, basal area, vegetation cover and vegetation surface area were compared as measures of the proportions of plant species present in North Okarito Forest, South Westland, for use in determining bird preferences for plant species. In general, stem density estimates of the proportions of canopy species were about 10 times lower than basal area estimates. The converse was true for estimates of the proportions of sub-canopy and understorey species. The proportions estimated from vegetation cover and vegetation surface area were similar for most species, and were intermediate between the proportions estimated from stem density and basal area. However, in the upper forest tiers, vegetation cover gave lower estimates for the proportions of canopy species and higher estimates for the proportions of sub-canopy species than given by vegetation surface area. These differences affect calculation of bird preferences for plant species. We recommend vegetation surface area as a measure of the proportional availability of plant species to birds because it is appropriate to most birds in New Zealand forests, is likely to be more accurate than visual estimates of vegetation cover, and can be measured on the same plots separately for trunks, branches, foliage, and fruit.