I have received several interesting responses from readers and I agree with what they all have to say about sampling philosophy. I agree that relevance, consistency, and context are all important, but I haven't even started on those potential problems with this protocol. My first question concerns whether the protocol insures that data are accurate and precise without regard to whether they are relevant. However, I think I may not have clearly stated my question regarding this. The protocol being used does measure cover for all plant species (herbaceous and woody), based on point data along a transect, i.e., if 50 of 100 points touches species x, then species x has 50% cover. I have no problem with that part of the protocol, although it is very time consuming to gather only 100 points of data, the data can be collected subjectively with little observer bias. My main concern is with the second part of the protocol, where woody species density is determined. In that protocol, woody plants rooted outside a 100m x 6m plot are being tallied, when any part of the plant overtops the plot. It seems to me that including those plants would over-estimate woody species density, since density is the number of plants per unit area (with area defined by the plot boundary). For example, if 50 stems (trunks) of white oak are rooted in the 600m2 plot, there are 833 stems/ha (50*10,000/600). However, if an additional 25 stems rooted outside the plot are included, then density would be 1,252 stems/ha (75*10,000/600). So, even if this protocol is applied consistently, the answer will be consistently wrong and fraught with sampling error due to the difficulty of large, tall, far way trees being consistently included in counts.
Including plants rooted outside the plot when determining cover (proportion ground covered) would not be problematic, but cover isn't being measured in the second part of the protocol. -----Original Message----- From: Ecological Society of America: grants, jobs, news [mailto:[email protected]] On Behalf Of Wayne Tyson Sent: Sunday, June 02, 2013 11:04 AM To: [email protected] Subject: Re: [ECOLOG-L] plot sampling for density To RELEVANCE, RELEVANCE, RELAVANCE, I suppose should have added "CONTEXT, CONTEXT, CONTEXT, and I thought I make it clear the PURPOSE, PURPOSE, PURPOSE. IF, and only if, one is restricting one's questions to some subset of the whole, one can simplify (actually reduce) the kinds and numbers of factors observed and recorded. As to edge-effect "error," I would add CONSISTENCY, CONSISTENCY, CONSISTENCY. How much does it matter whether or not something is counted that is partly "in" and partly "out" as long as it is consistently done and is an actual stated part of the protocol? These are ESTIMATES, they are SAMPLES, and nothing more. Now, how GOOD a sample, how truly representative it is of the whole or how well it reflects variations and changes--that's a matter for clarity in the research design and its claimed results and the confidence in conclusions--find THAT in most research designs. How many study results, how many conclusions, should be replicated by others and carefully examined by other researchers before we place confidence in them. And how many should be thrown out or done it the first place? WT ----- Original Message ----- From: "David M. Lawrence" <[email protected]> To: <[email protected]> Sent: Sunday, June 02, 2013 5:17 AM Subject: Re: [ECOLOG-L] plot sampling for density > If you're working in the mixed-grass prairie of Oklahoma or the Chihuahuan > Desert of southern Arizona, Wayne, I would agree that one should include > grasses and other small-stature species -- because they are the dominant > vegetation in the area. > > In forests, though, trees are typically the dominants in terms of stature, > biomass, consumption or capture of resources such as light, etc., so that > those other species may not need to be measured. (That is not to say > their presence is irrelevant, just that it may not be as important to > those trying to characterize the system.) > > Rick, in most of the work I've done in forests, we use tenth-hectare plots > and count, measure the DBH of all trees rooted inside the plot, and count > saplings (using a pre-determined size criteria) within the plot overall or > within smaller subplots. > > If your metric is based on canopy cover, however, then what you are asked > to do sounds reasonable, even though I agree with you that it would be > fraught with error. I have enough time standing straight up -- > I usually stand and walk with a persistent list -- and things get really > screwy when I've bent my neck to look vertical rather than horizontal. > > The method you are asked to use may be based on of those good theoretical > ideas gone bad. I remember inhaling when I saw some scholarship that > circular plots are better than rectangular plots because they minimized > edge error -- by minimizing the amount of edge per area of plot. It > sounded great until I got in the field and had to figure out where the > edge was when deciding what was in or out. It's hard to do when your edge > is a continuous curve. I've been committed to rectangular plots from then > on. > > Dave > > > On 6/2/2013 12:55 AM, Wayne Tyson wrote: >> Rick/Ecolog: >> >> I think you are absolutely right to question these procedures; in fact >> your post set off so many bells in what's left of my mind that it looked >> like the Fourth of July and New Year's combined! I hesitate to make any >> remarks at all unless there is enough interest to get into a lot of >> specifics, and I'm sure there would be a lot of disagreement amongst >> subscribers, as "sampling" procedures are the Holy Grail for a lot of >> people. >> >> You do not want comments on anything but density, and I think you are >> right on this too, but I again hesitate to comment because it would take >> a long time and exchange of emails to resolve the sticky issues that will >> arise. Suffice it to say that a lot of "protocols" have, if any, >> foundations that are highly suspect to me; hence, I share your instincts >> (which, I suspect, are only a sample of the entire thicket of stickery >> issues. >> >> I believe that it is simply lazy, if not downright fraudulent, to leave >> "minor" (grasses, cryptobiotic communities and species) out of most >> studies--unless the purposes of the study are stated up front to be >> looking only for data on a limited fragment of the ecosystem. Apart from >> that, I believe that there is often little value to any survey data >> without long-term replication that would reveal at least interesting >> changes and trends, but how many actually do that. And when they do, what >> is actually DONE with the data? All too often it seems to me to be in the >> realm of "employment act" stuff . . . >> >> I will say only this for now: RELEVANCE, RELEVANCE, RELEVANCE! >> >> WT >> >> PS: I ran many, many miles of transects during my brief Farce Service >> "career" back in the last century, and I think we produced some very good >> data and mapping, with little relevant "error." I re-visited my old >> stomping grounds a few years ago, and was kindly allowed to poke around >> in the storeroom of the Supervisor's Office, where I found our old field >> notes and maps (apparently undisturbed, and probably un-analyzed). >> >> As to density--like "cover," I don't think it reveals much except when >> done over time like your case, it might provide interesting data on the >> maximum productive (carrying) capacity potential for the vegetation being >> studied, as well as recovery times following perturbations like fire and >> logging. >> >> ----- Original Message ----- From: "Rheinhardt, Rick" >> <[email protected]> >> To: <[email protected]> >> Sent: Saturday, June 01, 2013 4:43 PM >> Subject: [ECOLOG-L] plot sampling for density >> >> >> I recently started re-sampling vegetation in fixed plots on a U.S. Army >> base. This base, and I presume many others in the U.S., use a standard >> protocol for collecting vegetation data. There are many plots on this >> base that are marked with benchmarks so that they can be re-sampled at >> intervals of years to decades. I presume that the data obtained from >> these plots will be used to monitor vegetation changes (structure and >> composition) through time. One aspect of the sampling protocol is a >> straightforward line-intercept method: a 100-m tape line is laid out in a >> straight line and the height and species of all plants that touch a >> vertical rod are recorded at 1-m intervals along the line. This method >> provides a fairly objective measure for cover but cannot provide any >> information on density. A second aspect of the protocol is designed to >> obtain density data for woody species > 1-m tall. This protocol >> essentially involves delimiting a 100-m-long plot using a range pole to >> determine the width of the plot, walking along the 100-m tape line from >> one end to the other, and recording woody plants, by height category and >> species, within the pre-determined horizontal distance delineated using >> the range pole. Usually, the predetermined distance (plot width) is 6 m, >> which delineates a 600-m2 plot (6 m x 100 m). Horizontal distance can be >> altered, based on perceived stem density. >> >> The question I would like to submit to LISTSERV subscribers concerns the >> methods used to apply the density sampling protocol. In all the plot work >> I have done, I have always recorded only woody plants (stems in the >> vegetation ecology vernacular) that are rooted within (or mostly within) >> the fixed plot of interest. That is, plants rooted outside the plot, but >> with canopy overhanging the plot boundaries, are not counted. However, >> the protocol we have been asked to apply involves also recording plants >> whose canopies overtop the plot even though they are rooted outside the >> plot. I believe that counting plants rooted outside the plots severely >> compromises both the accuracy and precision of the data, i.e., accuracy >> is compromised in that the plots are no longer of a fixed size, and >> precision is compromised in that there is much room for observer error >> when determining whether canopies from large far off trees are >> overhanging the plot (because the observer has to be in the middle of the >> plot to hold the ranging pole in place). >> >> If we were measuring cover, then it would be immaterial whether a plant >> were rooted inside or outside of a plot, since canopy overtopping the >> plots would be the parameter of interest. Part of the confusion may be >> due to the terminology used in explaining the protocol. The protocol says >> that woody "stems" are to be recorded in the plot. To me, the term "stem" >> refers to the main stem (trunk for a tree) that directly attaches to the >> roots, but I think the term may have been misinterpreted to include >> branches and secondary branches of plants. >> >> My concern that the density data we collect will be a nightmare to >> interpret, and worse, will not measure what it is intended to measure. >> Unfortunately, in searching the web, searching papers, and even looking >> through plant ecology texts, I have not found any guidance concerning >> what plants should be counted in plot work (plants rooted outside vs. >> insides of plots). Is this because protocol writers assume that everyone >> knows how to do it? Could there be there a potential problem with density >> data in the peer-review and/or gray literature? How much of a problem >> could misapplied protocols be having on data collected by natural >> resource programs? Should the word "stem" be defined every time it is >> used in describing a protocol? >> >> Rick Rheinhardt >> ECU >> >> >> ----- >> No virus found in this message. >> Checked by AVG - www.avg.com >> Version: 10.0.1432 / Virus Database: 3184/5874 - Release Date: 06/01/13 > > -- > ------------------------------------------------------ > David M. Lawrence | Home: (804) 559-9786 > 6467 Hanna Drive | Cell: (804) 305-5234 > Mechanicsville, VA 23111 | Email: [email protected] > USA | http: http://fuzzo.com > ------------------------------------------------------ > > "All drains lead to the ocean." -- Gill, Finding Nemo > > "We have met the enemy and he is us." -- Pogo > > "No trespassing > 4/17 of a haiku" -- Richard Brautigan > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 10.0.1432 / Virus Database: 3184/5876 - Release Date: 06/02/13 >
