Sunday, September 28, 2008

Regional Variation in C-value. Why?

The Rotala ramosior post of a few days ago got me thinking. Scott mentioned that it has a C-value of 10 in the Chicago region. In Missouri, it has a C-value of 4. In Indiana it has a C-value of 2. Swink and Wilhelm say, as reiterated by Scott, that it occurs in ditches and areas of recent excavation. Unless ditches and recently excavated sites are pristine, pre-settlement natural community types, I don't see how it could possibly be a 10. Even Running Buffalo Clover, a disturbance dependant species, has a C-value of 3 in Indiana.

Because of this regional bias, I am increasingly convinced that C-values should not be regionally specific. In my opinion, a species’ behavior, as expressed by its fidelity to undisturbed habitat, does not vary from region to region. Rotala ramosior is as weedy in Missouri as it is in the Chicago region and Indiana. Therefore it should have the same C-value regardless of region. Am I missing something? Can anyone give me a reason to believe otherwise?

22 comments:

ben said...

Wouldn't plants have a higher C value on the edge of their range and lower towards the middle? I read about species like Platanthera ciliata that I have seen growing on mowed roadsides in the Great Smokey Mountain NP, but *only grow in bogs in IN. Very interesting idea, though. Of course C-values are imperfectly assigned by flawed, imperfect botanists.

Scott said...

WARNING: THE FOLLOWING ARE THE RAMBLINGS AND FREE FLOWING THOUGHTS I HAD ON THIS TOPIC. I'M NOT SURE THAT I COME TO ANY CONCLUSION ONE WAY OR THE OTHER...

Interesting topic. Justin, I know we've talked about this before. I think there are two issues at hand... that of whether there should be only one C-value for each species regardless of region, and that of whether early successional species should receive high C-values.

Regional C-Values:
This probably all boils down to how you define the conservatism of a species. According to Swink and Wilhelm, the conservatism is both the tolerance to disturbance and the degree of fidelity to specific habitat integrity. Given this definition, prior to settlement, would every species have been given a C-value of 10? Were there any "weeds" at that time? Would early successional species have lower C-values because they are more tolerant to disturbance?

As Ben said, as you move to the edge of the range of a species, it becomes less common and often (but not always) more confined to pristine or more natural areas, and people in that region could therefore consider it more conservative because it's only found in the best remnants of that given plant community. However, as you move to the interior of its range, a species often becomes more common. Does this necessarily mean that it is less conservative? It's important not to confuse "common" and "conservative", I think.

Is a mesic prairie community in Indiana identical to a mesic prairie community in Kansas? They're similar, but there are also many species that are characteristic of that community in one region that are not found in another region. Are all species that are restricted to the mesic prairie in Indiana only found in mesic prairie communities in other regions? Maybe... I'm not sure. I would have to think that there are regional differences in the flora that would make it so that a plant with high community fidelity and low resistance to disturbance could have a different reaction in another region. I guess I don't consider myself an expert in our national flora and plant community dynamics. I am really only familiar with those in northern Indiana. That's why when I go to another region and look at a similar plant community to one in Indiana, I don't feel comfortable doing a detailed plant inventory... there are species that occur in the other region that don't occur in Indiana, and the plants occur in different assemblages than they do here.

Early Successional C-Values:
It is obvious that Swink and Wilhelm, and the other botanists who contributed to the C-value discussion for the Chicago Region, felt that Rotala ramosior was a species that could not withstand unnatural disturbance and site degradation, at least to the point where that unnatural disturbance didn't mimic the natural form of disturbance. I would tend to agree with this, though maybe not to the extent of giving it a 10 C-value. It needs disturbance of some kind to exist. It was likely a naturally occurring species in flooded areas (or in areas where mastodons created a footprint) - areas that received periodic natural disturbances. Without disturbance, this species would cease to exist. Does that mean that it should have a C-value of 0 because it not only withstands disturbance, but requires it? What, then, about prairie obligates? Should they, too, all have C-values of 0 because they withstand and in fact require disturbance (often fire), without which they would disappear? Going back to Rotala, yes, it occurs in excavated ditches. However, when that excavated ditch becomes more disturbed than just being excavated (nutrient increase, invasive species pressure, etc.), Rotala seems to disappear... at least in Indiana. I do not see it often, and only in wet, recently excavated sand. How about Brad's Echinodorus tenellus and Schoenoplectus hallii? Those two species are similar to Rotala ramosior in the sense that they are early successional and need disturbance. How are they different from Rotala? Why are they more rare than Rotala? I sure don't have the answers. Is it simply because they are more "conservative" by nature? What about Drosera spp., Calopogon tuberosus, Aletris farinosa, Xyris torta, and Polygala cruciata? I would consider these all fairly conservative species. They all show up when sand in NW Indiana is excavated to the water table. There are several nature preserves in NW Indiana that have the plant communities that they do because they were mined for sand. Maybe Indiana is just so messed up (okay, I'm just talking floristically) that C-values don't really apply here. Or maybe my sense of what comprises a natural community is out of whack based on what is considered "natural" in the area where I do most of my work. Who knows.

So all that, and I don't really have an opinion either way. How about that? Thanks for reading all of this. As I said before, it's an interesting topic, and one that deserves a lot more attention. Let's keep discussing this...

Justin said...

Just as every species has evolved a specific morphology, every species has evolved habitat requirements to which their autecology is attuned. Said requirements do not change because of geographical location (with the exception of variation consequent to local genotype). A plant occurs in an area simply because it finds the place habitable (where habitable refers to the location having the necessary requirements). Plants with a larger ecological amplitude (i.e. less specific habitat requirements) will inhabit a wider range of habitats. If C-value represents this ecological amplitude as well as the species sensitivity to disturbance, which is inherent to ecological amplitude, then regardless of range, the C-value should be the same.

Scott asked about natural weeds. There are many. Take Common Ragweed for example. It has a very wide ecological amplitude and thus is found in a wide range of habitats. It also increases with certain types of disturbance. Somewhere in Canada, at the edge of its range, it should not grade to a C=10. It doesn't magically become more picky about where it lives. Rather, the habitat it can tolerate becomes less common.

Ben provides an interesting scenario with Platanthera ciliata. However, orchid distribution and habitat fidelity is often dictated by their fungal associates.

Rotala ramosior, throughout its range, is more often associated with disturbed and degraded habitats than otherwise. This doesn't mean it can't occur in quality habitats. Quality habitats have many ecological players from weeds to whatever the opposite of a weed is.

I would say that species like Scirpus hallii, should not have a high C-value. Anything that can persist in agricultural fields for decades is not sensitive to disturbance. This does not mean it isn't rare and special or that it shouldn't be protected. It just means that C-values shouldn't be used to reflect the desire to conserve entities, but rather should exhibit the true ecological character of the species involved.

Perhaps the best way to test this is to try and think of a plant that behaves like a weed in one area and a conservative species in another. I can think of none.

Scott said...

How about exotic species? Are there any exotics that do not act aggressively in their native regions and that are actually conservative there, but that do act aggressively and grow anywhere (including disturbed conditions) in the US?

How about invasive natives? I'm thinking specifically about species that are thought of as undesirable in some regions because they "invade" early successional communities. Populus balsamifera, for example, is being controlled in places futher north, where it is thought of as a weed. In the Chicago Region, it is given a C-value of 10 because it is thought to only occur in the highest quality boreal communities. I wonder, though, if this is given a 10 more because of its rarity in the Region (due to edge of range distribution).

Here's another example to think about... two species that are more conservative in Missouri than the Chicago Region. Sporobolus asper is given a C-value of 4 in Missouri (I think... I'm using a website to get this info), and Sporobolus vaginiflorus a 2. I saw these in glades in Missouri. In the Chicago Region, S. asper is considered by most to be non-native. In fact, it grows along the road in front of my house! Swink and Wilhelm say that it is acting as an invader, and is found along railroads, in disturbed ground, along roadsides (in nearly solid stands), and in calcareous prairies. S. vaginiflorus is given a C-value of 0 and is found in waste ground, along railroads, along roads, in road shoulders, and in parking lots. Justin, do you consider these two species to be less conservative than those who assigned C-values in Missouri? How do you address species that are native in one region and non-native in another, if there is one uniform C-value?

Here is another similar example... Diplachne acuminata and Distichlis stricta. Both of these are assigned C-values of 4 in Colorado. In the Chicago Region, they are both non-native weeds that occur in waste areas and along roads. Diplachne can form near monocultures along roadsides and in wet areas with high salinity levels. Maybe, again, it's a case of these plants being given too high of a C-value in Colorado.

Thoughts?

Scott said...

Here's what Paul Rothrock uses as justification for regional C-values (from Floristic Quality Assessment in Indiana: The Concept, Use, and Development of Coefficients of Conservatism)...

"...C values assigned to species need to reflect their observed behavior in the particular region... The differences in C values may be particularly pronounced when one compares populations near the periphery of a species' geographical range with populations from nearer the core of its range. Peripheral populations may be near their tolerance limits for important physiological traits. As a result, the necessary environmental conditions are supplied in more limited microclimates and disturbance of these microclimates can quickly lead to the extirpation of the population. One species that seems to exemplify this pattern is the circumboreal Linnaea borealis (twin flower) with a C of only 6 in Michigan compared to a C of 10 for populations in Indiana. While in a converse way, Carex jamesii, a species widespread in Indiana woodlands, has a C of 4 compared to 9 in Wisconsin where it reaches its northern limit. Consequently, compared to Wisconsin, the species Carex jamesii provides the Indiana user less confidence that the site is a low disturbance, remnant community."

I'm not saying that I agree with this, but just offering this as one expert's take on the issue. To me, this still sounds like C value is being interpreted as how common a plant is in a given region, and not its ability to withstand site degradation. Maybe Justin needs to author a paper on the true meaning and use of C values!

Brad said...

You guys made me look up this definition of what the C value represents for each particular plant species:

C is "a number on a scale from 0 to 10 that represents an estimated probability that a plant species is likely to occur in a landscape relatively unaltered from what is believed to be a pre-settlement condition" according to the Wisconsin FQA methodology, which is how we interpret C in Michigan, too.

I agree with Ben's comment, supported by Paul Rothrock's writing, that plant species naturally exhibit different behavior at the periphery of their ranges than in the core of their ranges. Since we track Rotala ramosior (until the new rare plant list becomes official), I can easily pull all of our records and determine how many occurrences are found in "natural" vs. "non-natural" habitats as a check of its mean C value in Michigan. We have a concentration of appropriate natural habitats for this species in western Lower Michigan, and the number of populations found in these areas far outnumbers the number of populations found in human-created habitats. Of course, in heavily developed Chicagoland, there is likely to be more anthropogenic habitat than natural habitat. So, based on a strict reading of the definition of C I posted above, we would expect different C values in different regions.

Another interesting question raised is what, exactly, a "relatively unaltered" habitat is. Certain species respond to precise ecological conditions (the S. hallii example) that may actually be mimicked by certain forms of anthropogenic disturbance. Other species are "punished" for competing well in a variety of habitats under a variety of conditions. I've worked on a project in Michigan where we have both FQA data and scored metrics assessing structural and functional quality of habitats. The results (not analyzed) suggest to me that mean C values do not necessarily respond to certain types of degradation.

That said, I think the main purpose of the C values is to develop the mean C value for a site to assess its natural integrity. I still think the tool, in general, works well to differentiate sites with relatively intact soil and vegetative structure vs. sites that have been tilled or where soil has been removed or covered. After that, it's more of a crapshoot.

I rambled and lost focus, but my overall points are that (1) I expect plants to be found with greater concentration in more-or-less natural habitats at the edges of their ranges where they can only compete under specific ecological conditions and (2) mean C values, and not individual C values, are what matter most.

B(S)S

Brad said...

One more issue with edge-of-range species is that we rarely know if these species are actually at their ecological limits where they occur or if we simply "caught" them during the wave of European settlement and fragmented the landscape, preventing their continued migration. If conditions were favorable for these species, I can envision a scenario in which several additional centuries minus European settlement would have led to increasing and coalescing populations and landscape saturation (i.e., broadening of ecological amplitude) due to increase in seed sources. We must now interpret what is going on these species in a heavily altered landscape on the scale of a human life or a few human lives.

Brad said...

To clarify my last comment, I mean edge of range species following climate south to north. The northern species are clearly relictual in relictual habitats.

Justin said...

I see where Ben, Scott, Rothrock and Brad are coming from, but still don’t agree. Where exactly does Carex leptalea, Solidago riddellii or Quercus alba become less conservative? Where does Ambrosia artemisiifolia, Amaranthus tuberculatus or Aster pilosus become more conservative? To suggest that a species can exhibit one behavior in one region and another elsewhere seems a bit far-fetched.

As to Brad’s Rotala example, In Missouri, it occurs along the margin of almost every farm pond. You can also find it along mudflats in Missouri's river systems where it associates with other native species of low C value. Just because the latter is its natural habitat and the former an adopted habitat doesn’t mean it should have two different C values (as is being suggested between the Chicago region and Michigan). Especially since the natural and artificial habitats are both disturbance based.

Similarly, I can find Ambrosia artemisiifolia from native prairies as well as agricultural fields. This is an artifact of the distribution of anthropogenic disturbance, not the behavior of the species. Regardless of where it is found, it behaves like a species with a low C value.

The Ozark endemic, Scutellaria bushii, is found on just about every dolomite glade of reasonable quality in Missouri and no where else in the world. This is high fidelity and thus should have a high C value.

Bottom line, for me: Just because a plant can be demonstrated to occur in a natural habitat (it wouldn’t be native if it didn’t) in one region and an anthropogenic one in another, does not necessarily mean it should have two different C values in said regions. This fallacy is probably why the Chicago numbers are so inflated to begin with.

Dana said...

I am enjoying this discussion and have one thought to add. I'm sure we have all had the urge, (although we of course fight it) to think of C values as measures of how common or rare a species is in a particular region. So, is this perhaps why we (and Rothrock) have a desire to assign different C values at edge of range?

Perhaps we need another set of numbers that estimates the commonness or rarity of a species IN A GIVEN REGION. So, we could have a C value for conservatism/affinity to an undisturbed natural community no matter where the plant occurs geographically, and then a second number that suggests how rare or common that plant is within an ecoregion (or within political boundaries, but yuck). Maybe this would alleviate some of the confusion?

ben said...

One thought. Not all plants act alike. Of course, we all know that. Maybe the example of Species A that is that is conservative throughout its range is equally accurate as the example for Species B that potentially behaves differently across its range. The argument that we cannot show where ragweed is conservative doesn't necessarily say anything about other species.

Justin said...

In response to Ben's last post:

Still no one has provided an example of a species that clearly behaves differently in two regions. I can provide numerous examples to the contrary.

In response to Scott's comments regarding Sporobolus:

If S. asper is considered exotic in the Chicago region then it doesn't get a C-value, so that isn't a problem. Sporobolus vaginiflorus is a 2 in Missouri because in addition to more degraded habitat it occurs on disturbed patches in otherwise undisturbed glades. It has a C value of 0 in the Chicago region, but notice that Swink and Wilhelm say it also occurs in prairies with rather conservative species. Given this evidence, it seems that C=1 would be a nice value for it regardless of political or geographical area. But, as Brad pointed out, such subtle differences are averaged out during analysis. However, if the numbers differ from 2 to 10, as with Rotala ramosior, then someone is misinterpreting the behavior of said species.

Brad said...

On Rotala ramosior, I just did a quick check of our database and found that only 5 of 48 tracked occurrences are found in anthropogenic habitats (typically excavated ponds or wet sandy furrows). Based on the definition of C given above, in Michigan, a C-value of 10 is not far off for this species. The raw numbers give it a 9, or an estimated probability of 90% that an occurrence in Michigan was found in a natural habitat. Granted, there are additional untracked or undocumented occurrences (the species is being delisted), and heritage biologists are more likely to document the species in high quality habitats than in anthropogenically disturbed areas, but the species by and large is characteristic of natural areas in the state. Regardless of the reasons for its scarcity, the definition of "C" requires it be given a high value in Michigan considering the current data.

I can think of numerous species that behave differently, or less conservatively, in one region vs. another. Most of our boreal flora exhibits different behavior in northern Michigan vs. southern Michigan. The classic example is northern white-cedar, which is highly restricted to sloping peat deposits with subsurface flow of oxygenated groundwater in southern Lower Michigan. In northern Lower Michigan and the Upper Peninsula, the species colonizes old fields whereever limestone is near the surface, and elsewhere, too. The species becomes ubiquitous due to favorable climate and geology in the north vs. the south. Many ground layer species exhibit the same properties- the aforementioned Linnaea borealis is a good example.

Certainly, nobody is suggesting that all species behave differently in different ecoregions, but there are many, many examples of species that do exhibit variable conservatism related to differences in climate and habitat characteristics.

ellery said...

I'll throw my two cents in here...even though they're not worth that much nowadays...

I think Dana is on to something. It's easy for me to get the rarity of a species, the species' ability to withstand disturbance (conservatism), and "high quality" all mixed up. Even in my own head, sometimes I think of high C values as rarity, while at other times they mean that the species is not tolerant of anthropogenic disturbance. (With regard to Scott's earlier comments regarding low C values for all prairie species since the ecosystem requires disturbance...I would think that fire would be considered a part or process of that ecosystem, so it really wouldn't be considered something external to that system.)

Throughout this discussion, I keep thinking about Acer negundo. The reason is because it seems analogous to our conversation. The wetland status of Acer negundo in Region 3 is FACW-. I do not agree with this classification. It is not a wetland indicator species. It is a disturbance indicator species, at least from what I've seen. The reason I bring this up is because not all species fit nicely into our different ranking classifications. The people who put together the list had to give it a category though, and I assume provided that category based on the "natural habitat" of the species.

I'm tending to lean more towards Justin's side of things, because the C values are supposed to indicate a species general tolerance of disturbance. Brad's example of northern white cedar appears to me to be an example of a species behaving differently due to ecological or geological factors, not disturbance.

So, we may have a species that has only a few environmental factors that it requires to survive in an area, and it may not be affected at all by anthropogenic disturbance. Therefore, it may be a rare species due to the rarity of it's required environmental factors, and still survive in fairly highly degraded areas.

Sabatia angularis is an example that I can think of here in Michigan. The places that I have seen it are highly degraded, though it is still rare in the state. (I think this is probably an edge of range issue, but nevertheless...)...

I like this discussion...Should I bring up the question about whether or not humans should be considered part of the ecosystem, or an external influence on it...

Brad said...

Northern white-cedar is tolerant of disturbance in northern Michigan and colonizes disturbed areas, whereas it is almost exclusively limited to relatively undisturbed, hydrologically intact wetlands in southern Lower Michigan, at least away from Lake Michigan. We need to define what tolerance to disturbance is, because a species' tolerance to disturbance may be related to its saturation on the landscape and ability to compete in a variety of habitats. NWC, then, may be more likely to establish in degraded areas in northern Michigan because it is more ubiquitous in the landscape. Tolerance to disturbance may indeed be conflated with abundance.

Scott said...

To clarify my earlier question regarding Sporobolus asper... would anyone consider it a conservative species? I thought that it was somewhat conservative based on where I saw it in Missouri, but I didn't see all that many heavily degraded glades, and it's been a while since I've worked there. My question wasn't what its C-value should be if it is non-native in the Chicago Region, but rather I was pointing out that this may be a species that behaves differently in different regions.

In the November 1997 Erigenia, Taft, Wilhelm, Ladd, and Masters state "The native species most successful in badly damaged habitats were given C values of 0. At the other end of the spectrum, species virtually restricted to natural areas in Illinois received C values of 10." Maybe we need to consider that a "natural area" in Illinois is not the same as a "natural area" in Missouri, or in Colorado, or Oregon. They later state, "On occasion, during the coefficient assessment phase of this project, we needed to evaluate taxa that demonstrate regional behavioral differences in Illinois, such as Asclepias tuberosa and Oxalis violacea. These species are occaisional to common in degraded habitats in far southern Illinois, but in central and northern Illinois they are more restricted to remnant areas."

Regarding Ellery's thoughts on my prairie species comment, I agree that fire was a natural (sometimes)part of the ecosystem... however, there are anthropogenic processes that in a way mimic fire, yet we still treat these as disturbances. For example, look how common Asclepias verticillata has become on mowed roadsides. This is similar to scrapes in the soil mimicking a natural disturbance like a mastodon footprint.

All of this said, I think that for the majority of species, as Justin has shown, C values should not have regional variations. I do think that there are a handful of species for which variations are acceptable.

Brad said...

I think we have to keep in mind how the C values are intended to be used. A thought experiment will illustrate this point. In Michigan, Rotala ramosior occurs in natural habitats (yes, often disturbed, but recognizable as natural communities) at a roughly 9:1 frequency (hence C=10). In Missouri, the species apparently occurs most often at the margins of degraded farm ponds and in agricultural fields (hence C=2). Therefore, if somebody mentions they have this species on their property in Missouri, it appears unwise to get excited. However, in Michigan, this species is often associated with numerous coastal plain disjuncts, and may be worth getting excited about for that reason. The different C values serve to illuminate the relatively high fidelity of this species to natural areas in one state versus the other. If we assign the species a rangewide C=2, its value as an indicator of a natural area in Michigan is lost, and I become less able to identify a site lead.

I can see developing ecoregional C values, but do not see much value in assigning a single rangewide C value for each species given how the FQA is intended to be used. When trying to identify whether a site is of natural quality in Michigan, why should the lack of fidelity to natural communities of the component species elsewhere in their ranges affect the FQA score?

On the other hand, natural areas are by and large defined the same way by most states, so that should not affect the C values to a significant degree. The abundance and concentration of natural vs. anthropogenic habitats, however, could have an enormous impact on the C values among regions.

Justin said...

I see Brad's point, as it has been illustrated very well. However, I think something else is at play here. If I only looked a the occurances of Ambrosia artemisiifolia in the Flint Hills Ecoregion of Kansas (largely intact expanses of native grassland) I would see that it almost always associates with intact natural communities. Yet, if I looked at Ambrosia artemisiifolia in the Kansas City region I would see that it is much more common in areas of extensive anthropogenic disturbance. This is a matter of the landscape quality, not the species' tolerance. Ambrosia is behaving the same way in both locations, but has more opportunity to be weedy in the more disturbed region. I would argue that such may be the case with Rotala ramosior in the Chicago region verses the more rural parts of SW Michigan. I would guess that the Rotala of the SW Michigan sites are probably found in disturbed areas within larger intact natural communities rather than in clearly intact portions of the system. It seems to like open wet areas throughout it's range whether it be a ditch, mudflat, excavation site or coastal plain marsh. The Rotala of the coastal plain marsh sites shouldn't get a higher C simply because they are in a coastal plain marsh.

Secondly, in terms of species on the edge of their range (such as NWC), perhaps there are too few populations that are too fragmented to accurately judge their C value based solely on where they are found. This is where a review of a species across it range and its habitat tolerances would provide a more accurate number. I like this better than trying to interpret what a species is responding to in a small part of its range then trying to adapt a number to it.

Perhaps what we are arguing is not a matter of species behavior, but rather how C values are defined. Brad and Scott have both demonstrated that the creators of the C value as a tool define it as the probability out of ten that a taxa was found in an intact natural community. Perhaps this definition is deficient. Perhaps it should be a rank based on how tolerant a taxa is to anthropogenic disturbance (which is implied in their defintion but not explicit). It is becoming increasingly clear to me that the two are not the same.

As for Scott's S. asper: sorry if I was too dismissive. In Missouri S. asper is found on glades, in prairies, old fields and, like in the Chicago region, along road ditches. A C-value of 4 is defendable, though 3 seems more reasonable. I think this is a case, like ASCVER, where a tolerant native species is beginning to adapt to fairly stable anthropogenic community types (like roads and old fields). Actually, come to think of it, this phenomenon adds credence to the notion that C values should not be based on the probability that plant came from a natural area.

Nibb High Football Rules!!

ben said...

I am very glad we are having this fruitful discussion. Thanks to everyone for participating.

Brad said...

Justin, well-written, and I find more agreement with you now. My main point here is that the definition of C values I provided does not directly correlate to a species' tolerance to disturbance, but is merely a probability of a species being found in a clearly natural vs. clearly anthropogenic habitat. It is only when we begin to try to understand "conservatism" that we have significant problems with what "C" really is.

As for the Rotala, it is equally happy in an ORV track on bare soil or on a drawn-down marsh or pond shore on bare substrate. It's difficult to define "intact" in coastal plain marsh and intermittent wetland systems when tire tracks and other disturbances merely add microtopography to the wetland that mimics natural conditions.

By making C more complex and more "educated" in the sense of taking into consideration each species' autoecology, we may be making these assignments a difficult academic decision, which may end up compromising the utility of the FQA.

I like my FQA simple and easy, just like my...OK, enough!

Justin said...

I don't know if anyone is still following this story, but just in case.....

...I was just looking at the new C values assigned to Missouri's flora. Sporobolus asper has been dropped to a 3 and S. vaginiflorus has been dropped to a 1. Seems like the numbers may be normalizing on their own.

Tom Rooney said...

I just came across this thread and very much enjoyed reading it, and agreed with many of the points made here. I'm going to refrain from my own rant about C-values, other than to make three points:

1. I think the idea that C-values can range among regions should be questioned, because only through a rigorous analysis can we come to some conclusion.

2. The application of C-scores in assessments of the conservation significance of an area is a tool, one among many. I think it often works well, but should not be used in isolation.

3. Perhaps you should organize this thread and send it off to the Natural Areas Journal for consideration as a manuscript.

One more point I cannot help but make--and it is a shameless one. Check out my new book "The vanishing present: wisconsin's changing lands waters and wildlife." It was put together for people who like to get their botany on.

God That last point sure made me feel dirty.