Just When I Think I’ve Got Something Figured Out…

I pay close attention as I walk through prairies.  I watch for tracks to see what animals are around and I notice which flowers are blooming and which insects are feeding on them.  Often, I notice changes in prairie plant communities and try to attribute them to our management treatments, weather patterns, or other factors.  Observations such as these are an important part of how I learn more about prairies and adjust the way we manage and restore them.

Observations are a critical component of understanding natural systems and how management and restoration actions affect them.

Careful observation helps ecologists and land managers understand how natural systems work and how they are impacted by management actions.  However, observations are tricky because they provide an incomplete picture of what’s happening and are influenced by personal bias.

Unfortunately, observations are inherently biased.  When I start to notice a pattern through observation, I construct a theory to explain it.  That’s good science.  However, once I have a theory in mind, it influences the way I see things – and I tend to interpret my observations based on my theory.  That means it’s pretty easy to start telling myself a story that sounds good, but isn’t actually true.  Sometimes, I figure out that my story is wrong through repeated observations.  More often, however, what causes me to stop and reconsider is cold hard data.  Here’s a recent example of my data showing me that I need to reconsider a theory based on observations.

Canada wildrye (Elymus canadensis) is a native cool-season grass that establishes very quickly from seed in our prairie restoration (cropfield conversion) work.  It is often very abundant during the early years of a seeding before settling down into the plant community after a few years as other prairie plants become more dominant.  A common complaint from those working with Conservation Reserve Program (CRP) or similar prairie seeding projects is that wildrye tends to disappear a few years after planting, leading people to question the value of including the species in the seed mixture.  However, in our restored prairies, I’ve observed that while Canada wildrye declines in abundance after the initial establishment phase of a seeding, it seems to persist pretty well at a moderate abundance from then on.  I’ve attributed that to our grazing management, which is partially aimed at periodically decreasing the vigor of dominant grasses.  Weakening dominant grasses should help prevent less dominant species such as Canada wildrye from being pushed out of the community altogether.

Canada wildrye  is a native cool-season grass that is common in our restored prairies.

Canada wildrye is a native cool-season grass that is common in our restored prairies.

So, my theory (which I’ve shared with many people) has been that Canada wildrye is a grass species is tied to disturbance regimes.  If a prairie is managed with a mixture of intensive grazing periods and rest periods, wildrye can sustain a fairly steady population.  It was a good theory, and seemed to fit my observations.  Until last week, I was pretty happy with it.  Then I looked at some recent data.

One

These data were collected from about a hundred 1 meter square plots across a prairie.  Each time I laid down the plot frame, I listed the plant species found within it.  By looking at the data from all of the plots, I can calculate a frequency of occurrence for a plant species (the % of all plots it was found in).   The data in this graph were collected from a restored prairie seeded in 2002.

.

Two

These data were collected from a restored prairie that was seeded in 2000.

.

three

More data – this time from a restored prairie seeded in 2001.

From looking at the above three graphs, you might conclude that Canada wildrye populations stayed high for the first eight or nine years after planting a restored prairie and then started to taper off.  Another interpretation might be that our grazing was able to prop up those wildrye populations for a certain amount of time, but it is now starting to suffer the same fate my colleagues have complained about in CRP plantings.

Now, consider this next graph.

four

This graph shows data from the same 2002 planting as shown in the first graph (red lines in both graphs).  In this graph, however, the orange line is data from a large exclosure at the same site that has been burned but never grazed.  The two blue lines are from a different restored prairie seeded in 1995.  The dark blue has had fire and grazing management since 2002, while the light blue line is from a large exclosure at that site that has never had grazing within it.

By looking at the red and orange lines you might conclude that grazing management had a strong negative impact on Canada wildrye in the 2002 planting between 2011 and 2013.  The lines from the grazed portion of the site (red) and the exclosed portion (orange) are heading strongly in opposite directions.  However, the site was also grazed in 2008 and 2009, and there is less difference between grazed and exclosed in those years.  In addition, the blue lines on this graph show data from populations in a different nearby restored prairie (seeded in 1995) which don’t seem to be strongly affected by whether or not they are grazed.  There is less wildrye in the ungrazed exclosure of that 1995 planting but the changes in frequency seem pretty similar between the grazed and ungrazed areas.  Hmmm…

five

This graph shows data from two of the sites shown earlier (2000 and 2001 plantings) as well as another nearby restored prairie that was seeded in 1999.  Despite very similar management regimes, the 1999 restoration seems to be maintaining a high population of Canada wildrye.

This last graph just muddies the water even more.  While populations in the 2000 and 2001 restoration seem to be declining over time, wildrye frequency in the 1999 restoration has been holding steady for 15 years.  All of these sites are within a mile or two of each other, were planted with the same kinds of seed mixtures and have been managed with very similar mixtures of periodic fire, intensive grazing, and rest (modified patch-burn grazing).

It’s possible that soils play a role in the differences between sites, but all the prairies above are on similar alluvial (river deposited) soils.  None of them seem all that different from each other (I need some data on that!).  Weather could also be a factor, though most of the declines seemed to start around 2008 or 2009 – a few years after a long drought period – and have continued through both wet and dry years since.  I’m not sure what weather factors might have popped up since 2009 that would cause a decline in wildrye abundance (and only at some sites).

Surprise!  Reality is more complicated than a simple cause and effect relationship between a management regime and a plant species…

I take two major lessons from this.  First, I need to be more careful in my assumptions about how our management is impacting prairies.  That’s nothing new – I fall into that trap all the time, and frequently have to remind myself not to overgeneralize.  In this case, I had constructed a logical story explaining why Canada wildrye was abundant in our well-established (old) restored prairies but rare in ungrazed plantings such as CRP fields.  There are, of course, many possible explanations for that phenomenon (differences in soil types, plant diversity, seeding rates – particularly of warm-season grasses, fire management, etc.) but I grabbed one simple explanation without adequately considering all those other factors.

The second lesson is that it’s dangerous to rely solely on observations when trying to figure out natural systems.  This is not a new lesson either, and it’s why I try to collect as much data as I can.  Observations are really important, but are easily biased by what we think is – or should be – happening.  It’s natural to see what you expect to see.

Collecting some unbiased data, even just a little, is well worth the effort – especially if you target the data collection to your objectives.  It’s not useful to just collect data for the sake of data, but if you have a specific question (“are we maintaining diverse plant communities in our restored prairies?”) data can help answer it.  I consider plant community data collection to be a very important part of my job, but it doesn’t actually take that much time.  It takes about a day per year for each site I sample, including data collection, data entry, and analysis.  I strongly encourage every land manager to collect some kind of data from their sites.  It doesn’t have to be complicated or time consuming – just something that can help evaluate how management treatments are working.  Observations are great, but unbiased data is a good way to make sure you’re telling yourself (and others) the right story.

I’m not sure what to think about Canada wildrye now.  It’s a little disappointing to find out that my story was wrong – or at least incomplete.  On the other hand, the complexity of interactions that apparently drives Canada wildrye populations are why I love prairies and prairie management.  If it were simple, it’d be boring.

And no one likes a boring story.

About these ads

About Chris Helzer

Chris Helzer is an ecologist and Eastern Nebraska Program Director for The Nature Conservancy. He supervises the management and restoration of approximately 4,000 acres of land in central and eastern Nebraska - primarily along the central Platte River. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press.
This entry was posted in Prairie Management, Prairie Natural History, Prairie Plants and tagged , , , , , , , , , , , . Bookmark the permalink.

16 Responses to Just When I Think I’ve Got Something Figured Out…

  1. I apologize, if this duplicates. Something weird happened with me signing on.

    Keep in mind that frequency is much less sensitive to change than stem counts or cover, because it requires colonization or extirpation at the sampling scale in order to change. A pattern that appears to “hold steady” might mask a decrease in plant size or number within sampling units, and what appears to be a sudden drop off may just be the culmination of such changes in abundance.

    • Chris Helzer says:

      I’m not sure I agree about sensitivity. I think frequency is actually a lot more accurate than cover estimates, especially in grazed sites, because cover estimates change between grazed and ungrazed plants just because of their size – not because of an actual change in abundance. Getting the right plot size to measure frequency is important, however, in order to get the needed sensitivity. That said, I do agree that there is some fuzziness in the data that makes it hard to say when a decline is real vs. just due to sampling issues.

      • That is true cover about cases where grazing is not uniform. In any case spatial scale of sample units is necessary for understanding presence/absence data, because there is a huge difference between even a 0.25m2 and 1m2 quadrat (the former, within adequate repetition, giving much more resolution for something with the footprint of caespitose grass than the latter). Either plant or stem densities are probably needed to really interpret population trends.

  2. Orvin Bontrager says:

    Data……….important to you, me and about 25% of the human population that are analyticals. The rest make decisions based on controlling, enthusiastic, or amiable personality styles. That’s the way we are made.

  3. Ed May says:

    Good Morning Chris, Enjoyed your thoughts on Canada wildrye because I’ve noticed an increased population at my place since the last burn two years ago. This is also in an area that was cleared of cedars and other trees 3 years ago. You’ve got my curiosity up about the plant so I’m going to keep an eye on the population. Good suggestion on keeping a journal of observations. I need to do a better job of that. Have a great day! Ed

    • Patrick says:

      I’ve noticed the same thing Ed. In my case, I had cedars mechanically sheared in 2008, and a burn last year. I think it might represent new recruitment, because I have not seeded the cleared areas, and the area where it is most abundant is nearest to a badly degraded untreated remnant that held some wild rye on it. However, it is not present on adjacent slopes which had more native cover and from which I removed the cedars manually (as they were smaller and less dense). These observations would seem to support James’ hypothesis regarding the level of competition at the site.

  4. Bill Zales says:

    Good post. BUT a theory is as close to scientific “fact ” as we can come. Don’t you mean hypothesis? (Us college Prof types are so picky)

    • Chris Helzer says:

      Bill, you’re absolutely right. Thanks for correcting me. I was speaking colloquially but should still use the correct terminology. I actually had the word “narrative” instead of theory in an earlier draft but changed it at the last minute – serves me right. Good to hear from you! Keep up the good fight.

  5. George Shurr says:

    Hi, Chris~
    Thanks for another good post. It articulates very well the dynamic between qualitative “observations” and quantitative “data”. The observations are needed to guide the data collection. It’s the same thing in the Earth Sciences. And, it is a model that stands in contrast with the experimental approaches of physics and chemistry.

  6. James C. Trager says:

    I was going to mention the theory/hypothesis thing, too. It seems especially important in the modern climate of “only a theory” Anti-science-Speak.

    Also, regarding Orvin’s comment; While those differences are real, I suspect (my hypothesis is) they are at least as much about nurture as about the nature of people.

    And finally getting to this quite interesting Canada wild rye post…
    We see a similar pattern of decline over the years in other Elymus species that we use in woodland restoration here. My pet idea about this has been that their seedling survival is better in disturbed soil, low competition environments. Since they are short-lived, they ultimately get displaced by species that are longer lived and perhaps too, better at establishing in more competitive situations. Also, it seems surprising to me that any of them would be favored by grazing, since they make such nice, tasty-looking greenery in spring, when the C4 grasses are barely out of the ground.

    • Chris Helzer says:

      James – thanks for the comments. It’s great to hear your experience in Missouri, especially in woodland restorations. I would think they’d be very likely to do well in situations that Ed and Patrick mention as well – quick colonization of bare soil areas. They must have some ability to sustain themselves in a seed bank? In terms of grazing, Elymus doesn’t seem to be a favorite of cattle, at least in our sites. Because of that, it seems to benefit under moderate grazing because its main competitors (big bluestem and indiangrass, among others) get grazed more intensively than it does and that grazing also creates a better light environment (I assume) for seed germination.

  7. Prospective Grad Student says:

    ==========================================================================
    It’s possible that soils play a role in the differences between sites, but all the prairies above are on similar alluvial (river deposited) soils. None of them seem all that different from each other (I need some data on that!).
    ===========================================================================
    Hi Chris,
    Would you ever consider extracting soil cores from each of the prairies and conducting a soil texture analysis?

  8. Rex Peterson says:

    Chris,
    You don’t have enough data to analyze relative to events. Canadian Rye is a cool season grass and will vary in vigor with events in the spring and the fall and sometimes events that are stacked from year to year. A dry fall with burning or grazing the following spring would stress the rye but not the warm season grasses. Or a dry July could stress the warm season grasses and the fall could be wet which would give the rye a boost. Amazing how fearfully and wonderfully the prairie and everything else has been created.

  9. Pingback: Is Natural History Still Relevant for Conservation Science? | Gaia Gazette

PLEASE COMMENT ON THIS POST!

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s