Selecting a Study Site

Transcription

1 Selecting a Study Site Vocabulary hectare (ha): A metric unit of area. 1 ha = 10,000 m 2 = 2.47 acres. 1 acre = 0.40 ha. meter (m): A metric unit of length. 1 m = 3.28 ft. 1 ft = m. 1 m x 1 m = 1 m 2. replication: Taking of multiple samples or measurements to obtain a better estimate of the properties of the environment under investigation. sample plot or quadrat: An area of the study site in which plants, trees or woody debris are measured. A fixed-size area (plot) often used in ecological studies. sampling: A process of taking representative measurements (samples) of what we study. study site: A location that is representative of the study area at which samples are taken. Overview Selecting a good study location is an essential step of every field scientific study. Scientists choose study areas that are characteristic of the environment they want to study. For example, if you want to know how forest vegetation changes during succession, you could compare vegetation at two study sites, one chosen in a clearcut and the other in a mature forest. Also, it is not necessary to identify all plants at your study sites. This would be an enormous task. Instead, scientists choose smaller subsets of the study site, called sample plots, which are representative of the area. In this activity, you will learn how to properly choose a study area and establish sample plots that you will use in subsequent activities to identify and sample trees and understory vegetation, and to collect woody debris. Sequence This is the initial activity. Students should complete this activity first since it lays the foundation for all other activities in this set. Once you have your forest maps, plan to spend at least one to two classroom sessions in which students will formulate a hypothesis and suggest locations where they can test them. The total length of time involved in field work will vary with the activity but generally plan on spending one field day to choose one study site with sample plots. Materials 50-m tape meter stick PVC pipe pencils forest maps surveyor flags and tape student journals calculators Work Safety All students should wear clothes and gear appropriate for field work in the woods. At a minimum, this includes hiking boots or sturdy shoes, long-sleeved shirts and long pants, rain gear, and a hat. Anti-tick and mosquito repellents may be applied as needed. None of the suggested tools and equipment require any special handling caution. 7

2 Background Selection of a study site is the first step in a field investigation of the forest environment. Obviously, scientists establish study sites in areas with properties they want to study. It is recommended that you select more than one site in order to replicate the study. A choice of several sites of different management history or successional stage will allow you to compare differences in what the students sample. Consult with Forest Service, certified foresters, or university scientists about appropriate locations for your study site. If you work at a private site make sure you obtain appropriate permits for use. If possible, choose a site that is close to the road for easy access. This can come handy in case of an emergency or if you want to visit the site more often to resample your plots. Students can build and post a simple sign at their study site to notify visitors of an ongoing investigation (Figure 1-3). Figure 1-3. A Please Do Not Disturb sign. When scientists investigate a forest it is not feasible to take measurements (sample) from the entire area. For example, it would expensive and timeconsuming and often unnecessary to measure all trees in a 50-acre forest to estimate the average diameter or age of trees in this area. Instead, scientists select sample plots or points. These are locations uniformly or randomly distributed throughout the study area. Scientists then take measurements or sample these areas and extrapolate their results to the entire study site and other similar areas. Study plots can be square, rectangular or circular in shape. Sometimes, scientists do not establish permanent plots but instead they sample along a transect line. For example, in vegetation studies, they may want to count the number of plants that touch a sampling line stretched for many meters through the forest. Keep in mind several basic rules that scientists use to obtain a more representative characterization of the vegetation they sample: Two-dimensional plots such as squares and circles give you a better information about the plant community at each point of observation than linear transect plots. You are better off if you sample several smaller plots than one large plot at your study site. If you want to increase the accuracy of your sampling then increase the number, not the size of your sample plots. Generally, the more samples you collect, the more confident you can be about the results you find. By taking measurements in several or many study plots (replicating), and averaging your results, you can account for the effect of natural variation (heterogeneity) present in the forest. This natural variation of the environment can be very misleading if you are not aware of it! For example, if you use only one quadrat to estimate fine woody debris and this plot happens to have an unusually low content of woody debris, you would be tempted to conclude that that your study site is generally low in woody debris. This, however, may not be correct -- a few feet away you could find huge amounts of woody debris that you never sampled. Remember, however, there is a trade off between the precision of the study and the cost of conducting an experiment. Working with more study plots requires more time, money and effort on the part of the scientist. 8

3 Procedures 1. Form groups of 4-5 students. 2. Study your maps and identify areas suitable for your project. Select two study sites. Try to find locations that are different from each other, for example, if you are doing a forest study select two areas represent a young stand and older, mature stand. This will allow you to make comparisons and see whether you see any differences. 3. Once at your study site, begin by establishing a transect line. You will use this as a baseline for measuring out your study plots (Figure 1-1). Find a point that is in the center of the area. Find a means to randomly select the starting point of the transect; for example, throw a stick behind your back into the forest stand. At the point where the stick hits the ground establish a starting point for your transect line. Mark this area with a flag or a wooden stake. Choose a direction of your line so that it cuts through the area that you want to sample. 4. Using your 50-m tape, measure out eters along the transect line and mark this Transect line Starting point of your transect SMALL FIXED AREA PLOT (1 m 2 ) LARGE FIXED AREA PLOTS (100 m 2 ) Figure 1-1. Establishment of a transect line with study plots location. Then measure out perpendicularly to your transect line. Mark this location. This will be the center of your first large sample plot. Label your transect with a 2 letter location code (e.g., if you are in an old growth stand, use OG). 5. Establish your first large sample plot (0.025 or 0.10 acre). You can choose between a square or circular shape for your plot. Look up dimension for your plot in Table 1-1. After you have measured out the plot, permanently mark its corners or the circumference so you can return to your plot in the future. Label your first plot with the number Continue to measure along the transect line until you establish 2 to 5 sample plots (Figure 1-1). Keep the plots at least eters apart. This will allow you to spread the sample plots over a larger area, which will give a better representation of the forest environment. You will later use the large study plots to identify and measure trees and to sample large woody debris. 9

4 7. Establish 5-10 small 1m 2 quadrats within your larger plots that you measured out along the transect line. You can use a meter stick to define the boundaries of your quadrat plot, or you can build a 1-m square frame out of PVC pipe (Figure 1-2) and use it as a template. Position this frame within your plots and mark the corners with flags. You will later use these small fixed area plots to measure herbaceous vegetation. 8. Fill out the Tree Data Sheet with your information Figure 1-2. A PVC-pipe template to measure out sample quadrats. Table 1-1. Sample Plot Dimensions. Study Plot Area Measurements for square plots Measurements for circular plots Where to use them? 1 m 2 1 x 1 m radius = 56 cm Sampling herbaceous vegetation 100 m 2 (0.025 acre) 10 x 10 m radius = 5.6 m Sampling of shrubs and trees 400 m 2 (0.10 acre) 20 x radius = 11.3 m Sampling of shrubs and trees General Vegetation Sampling Guidelines: Tree sampling If the site is less than an acre, you can probably measure all of the mature trees and subsample the shrubs and herbs. If the site is more than an acre, and has less than 50 mature trees present, you can probably measure all of the mature trees and sub-sample the shrubs and herbs. Otherwise, you will sub-sample the trees, shrubs and herbs. Shrub and herb sampling You can set up fixed quadrats, line transects, or belt transects (a long narrow strip, usually meters long and 1 meter on each side of the measuring tape), depending on the study design. Monumenting / labeling If you are planning on studying changes in the site over time, then you will need to set your plots up so that the corners or centers are permanently marked. This way, you can revisit them repeatedly and compare changes in those specific areas over time. Likewise, if you are going study tree growth, you will need to permanently label each tree (using metal 10

5 numbered tags, nailed into the tree at DBH height (see Activity Section 3), or use some other method to re-find each tree (using a known location, compass bearing and distance). Each of these decisions has consequences for time, layout, and data management. You can use GPS units to pin point the latitude and longitude of your plot corners; however, keep in mind that often the units do not work well in thick forest canopy cover or steep canyons. 11