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
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David M. Lawrence | Home: (804) 559-9786
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"We have met the enemy and he is us." -- Pogo
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