Will: Interesting question about how to sample a landscape. There is no right answer. In part the answer depends on the purpose of the study and questions to be answered.
In general, square or circular plots located randomly or on a systematic grid are the best ways to sample the landscape. Statistical tests do not require random samples, but they do require that samples are independent of one another, and people often make the assumption that randomly located plots are independent, but that is often not the case, as it depends on the spatial scale of the underlying natural processes being studied. There are usually a set of rules for the plots (i.e. they can't cross a river, or contain cliffs, etc., if so the plot is moved in a random direction). To compare areas with different histories, you need to include enough plots in each to allow for statistical comparisons, and its best to have at least two study sites for each category. Each plot needs to be large enough to have enough trees on it to comprise a meaningful sample, at least 12 trees for some purposes, but up to 50 trees if one is interested in variation in demography for stands across the landscape. For my Ph.D. project we set out 70 randomly located plots in the unlogged forests of Upper Michigan (mostly the Porcupine Mountains and Sylvania Wilderness, total about 50,000 acres). At each location we sampled a square 0.5 ha plot (70.0m or 232 feet square). For some plots we measured heights of all trees on a 10m wide transect crossing the plot, to get so-called height profiles. 70 plots within 50,000 acres turned out not to be independent for purposes of studying large-scale wind disturbance, so I had to develop covariance estimates to do statistics. For one of my Ph.D. students working in the Boundary Waters Big Blowdown, we set out 52 transects with plots every 25 m, and had a total of 741 plots. Each plot had all trees measured in a 12 m radius. It was so difficult to travel on this landscape, that transects starting at lakeshores were the only option, since canoe is the only mode of travel and even young graduate students can only walk 1-2 miles in an entire day. Sometimes it takes a half hour to walk across as 12 m radius plot. Therefore, we settled for plots along transects. Lee Will Blozan wrote: > > ENTS, > > We still have an open research permit for Congaree NP until end of > February 2010 (it can be extended as well). The last trip in 2009 > brought in some very successful hunts and measurements. This time I > would like to focus on a methodology to ascertain average canopy > height as well as continue to remeasure the currently identified > champion trees, both state and national champs but also ENTS > superlatives. With this in mind I would like to get input on the > following: > > 1) Who can attend and what dates would work. Let’s look at the > weekends in the last ½ of January through the middle of March. I would > be open to several dates to do as much as possible since Congaree is > not too far from me. Also, the leaf off, non super-buggy season is short… > > 2) Developing a methodology to assess canopy height in an unbiased and > repeatable manner. Ideally, I would like to see ENTS develop a system > that can be used or adapted to any site we wish. This would include > small sites and those with varied topography. The Rucker Index is a > great way to initially compare sites but does not show the forest on > average- just on the extreme. Thoughts to consider: > > Would it utilize transects or plots? What size or how long? > > What trees are measured? Just canopy dominant or all sizes/heights? > > Do we mix old-growth with selectively logged or formerly clearcut sites? > > How are the sites selected for measuring? Random or representative? > Age classes, species mix? > > Thoughts running through my mind include a defined study site of a > given size with random plots generated with coordinates and located > and sampled in the field. For example, a 10 ha site with subplots > within. How many and how big? Circular plots are easy to do with a > rangefinder; so would be 100 meter transects with a tape or > rangefinder on a random azimuth. > > Thoughts please! > > Will F. Blozan > > President, Eastern Native Tree Society > > President, Appalachian Arborists, Inc. > > //"No sympathy for apathy"// > > -- > Eastern Native Tree Society http://www.nativetreesociety.org > Send email to [email protected] > Visit this group at http://groups.google.com/group/entstrees?hl=en > To unsubscribe send email to [email protected] -- Eastern Native Tree Society http://www.nativetreesociety.org Send email to [email protected] Visit this group at http://groups.google.com/group/entstrees?hl=en To unsubscribe send email to [email protected]
