Fact: Dominant or not, herb and shrub species tend, on average, to have substantially narrow distributions along environmental gradients than trees. Consequently, if one is interested in gradient analysis, or in placing plots along environmental gradients, one should quantify herb and shrub abundance.
Fact: Quantifying herb and shrub densities (viz., stems/hectare) is a fool's mission. In forest understories, where most leaves are broad and nearly horizontal, estimating the proportion of ground covered by different species can – after calibration and checks for consistency – be estimated visually in large numbers (say, 10-50 per 0.1 ha forest plot) per stand fairly quickly. See the guidelines for sampling forests that Bob Peet assembled for the collaborative survey of NC forests. In prairies, woodlands, and savannas, many herbs are grasses or sedges, with very numerous and very narrow but long leaves; estimating total leaf cover under such circumstances is another fool's mission. Most people either estimate the proportion of ground occupied by such plants or, if destructive harvest is permitted, clip, dry, and weigh samples. Another option my lab has used successfully in such systems is to estimate LAI (leaf area index) by measuring light levels above the ground layer and at ground level (three replicates per 1 m2) using a Decagon ceptometer (wand of light meters). Such measurements can be made relatively rapidly, and if made ± 2 hrs around midday, appear to be highly replicable except on partly cloudy days. Ceptometers, however, do NOT permit the estimation of abundance of individual species, only the aggregate in a layer. Fact: Herbs tend to vary quite substantially in leaf phenology in many systems. It is imperative in such systems (e.g., eastern deciduous forests, mixed and tall grass prairie, Midwestern oak savannas) to re-census sites several times per year. Visits in mid spring, early summer, and late summer are needed to quantify fairly the abundance of species whose cover peaks at different times of year. Fact: Long, narrow quadrats will tend to include more species than square or round ones of the same area, due to the nearly universal spatial autocorrelation of plant coverage and environmental variables. But the effect is so small in comparing square vs. circular quadrats as to be negligible. Square quadrats are often easier to sample than circular quadrats. Cheers, Tom On 06/02/13, "David M. Lawrence" wrote: > 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 -- Thomas J. Givnish Henry Allan Gleason Professor of Botany University of Wisconsin [email protected] http://botany.wisc.edu/givnish/Givnish/Welcome.html
