SCIENTIFIC METHODS FOR STUDYING VEGETATION

     Biologists and plant ecologists use a number of methods to study vegetation communities. The exact sampling method that is employed depends on the objective of the study. Most sampling methods are quantitative, which involves collecting numerical data on numbers of individuals, plant sizes, and the amount of space occupied by target plants. Qualitative methods are also occasionally used, especially by experienced researchers who want to quickly define a plant community simply by observation.

      The most common quantitative sampling methods are the quadrat method and the transect method. The quadrat method allows the user to define a fixed area, called a plot, within which plant characters can be measured. Usually, a rectangular quadrat frame, such as the one shown in Figure 1, is used to define the sampling area, although a quadrat can also be a permanently established area within a site. Although the exact experimental design will determine where and how many samples are taken, the procedure always involves measuring plant characters of only those plants inside the quadrat. Quadrat sampling usually attempts to define plant community characteristics for an area much larger than the actual area sampled. For this reason, care must be taken to obtain samples that represent the entire habitat and that eliminate the human factor. Usually this means employing an experimental design that ensures random placement of the frame or permanent quadrat.

      The transect method simply refers to the establishment of a baseline along which sampling is conducted. This method is typically used when there are apparent vegetation differences from one point of interest to another within a sampling site. For example, when sampling an area containing a river, wetlands, and uplands, establishing a transect line that traverses these distinct habitats is a reliable means of collecting representative data.
Sometimes the two methods are used together. In Figure 2, several transect lines, marked by rows of stakes, were established across a coastal hummock to study the effects of cattle grazing on vegetation. Sampling was conducted by placing the quadrat frame in the same position at each stake along each transect line, giving representative samples of the entire hummock community (the astute reader may complain that since each sampling location was fixed, randomness was not achieved. But in this design, the "grazed" and "ungrazed" sectors were randomly determined. And, since the study involved assessing temporal change, it was desirable to sample exactly the same quadrats from one period to the next).

      Some, but by no means all, of the quantitative characters that can be measured by these methods are:

      Abundance - estimates plentifulness of a species or target group according to a predefined scale, such as rare, infrequent, abundant, etc.

     Density - abundance, measured by actual count, per unit area. Counts are averaged when more than one sample is taken.

     Frequency - the percentage of sample plots in which a species or target group appears. Frequency data are often grouped into classes, such as A = 1-20%, B = 21-40%, C = 41-60%, D = 61-80%, E = 81-100%.
     
      Cover - commonly measured as "Total Cover", which is the percentage of all vegetation covering the ground surface inside the quadrat, or as "Species Cover", which is the percentage of the target species covering the ground surface inside the quadrat.
     
      Richness - the number of species occurring in a quadrat or study area.

      Biomass - the dry weight of the total vegetation or target species inside a quadrat. Sampling consists of clipping the vegetation to ground level, sorting by species if required, oven drying the sorted clippings, and weighing. Figure 3 shows new growth inside a quadrat that had previously been clipped.

     Data collected in the field are usually subjected to some type of statistical analysis. Statistical methods range from simple to complex, with the exact method chosen depending on the objective of the study and the original experimental design.