Two vegetation importance indices were calculated based on the above metrics. One value is based on a summation of density-frequency-dominance metrics and follows the idea of the DFD index (density-frequency-dominance) or its modification by Curtis and McIntosh, the (Vegetation) Importance index[i].
In this study, given the surveying approached followed, a average vegetation importance (AvgVI) index was derived using the average summation:
VI = %BA + %C + (%BA x %D-p).
The measurement of density is based on %BA x %D-p, which emphasises the contribution to the local density by the species’ relative basal area contribution. The frequency is given by the per cent constancy, and the dominance by the relative basal area. Like the DFD the vegetation importance used in this work is an open ended value unlike Curtis Importance index where any given species had a maximum index of 300.
The second importance index, the Basal Area Importance (BAI) Index, was calculated from the multiplication of the basal area of a species by its % Encounter (BAI = BA (m2/ha x %EBA). This metric, also open ended in nature, emphasises only the importance of the species to the basal area of the site in relationship to its encounter frequency, in other words it’s only a species dominance value.
[i]. Curtis, J. T., and McIntosh R.P. 1951. “An Upland Forest Continuum in the Prairie-Forest Region of Winconsin.” Ecology 32(3):476–496.