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Plant species richness is defined as the total number of species in an area and is one indicator of biodiversity. There are several different methods for determining or estimating plant species richness.
The Modified Whittaker approach is based on Stohlgren et al. (1995) and Bull et al.(1998), which can be found in Herrick et al. 2005. This method is very time intensive. The Modified Whittaker approach calls for three 10m by 30m plots set up in a spoke design (figure 1). Within each of these plots, a 2m b 5m subplot and four 0.5m by 2m subplots are established. Starting with the smallest subplots, the subplots and plots are searched and the species found recorded. Richness is calculated based on the assumption that there is a linear relationship between the number of species and the log of the area searched. See Herrick et al. (2005) for detailed instructions on how to implement the Modified Whittaker approach and make richness estimates from the collected data.
An alternative, rapid approach to estimating species richness is to use the observations made along line-point intercept transects. Species richness is estimated as the number of species encountered over all of the point intercepts made in a given area (i.e., across all transects). This method is much quicker than the Modified Whittaker approach, but will tend only to capture the most common species in the area.
The Modified Whittaker method is time consuming and has high sampling costs associated with it. To save time and cost it is possible to sample fewer transects (one can be sufficient) at more locations.
Guide. U.S. Department of Agriculture, Forest Service. Washington, D.C.
Because the Modified Whittaker approach is time intensive, the simplest alternative is to use the minimum estimate of species richness calculated from Line-point intercept method. However, this will miss most species. The Line-point intercept estimate of species richness can be further supplemented by conducting a thorough search for exotics and other species of interest throughout the plot area. Another alternative is to search the 10 x 30 m plot without subplots. This is appropriate if the species-area curve is not required.