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.


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.



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



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.


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…


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.

Is Poison Hemlock Repelled By Plant Diversity? Early Results Say Yes

How important is plant diversity?  Most ecologists think it’s a critical component of resilient ecosystems.  Last week I collected some data that lends support to that view.  In some experimental prairie plantings we’ve established in our Platte River Prairies, plant diversity appears to be suppressing the invasion of poison hemlock (Conium maculatum).

A floristically rich restored prairie, in which prescribed fire and grazing are being used to maintain high plant diversity.  The Nature Conservancy's Platte River Prairies, Nebraska.

A floristically rich restored prairie, in which prescribed fire and grazing are being used to maintain high plant diversity. The Nature Conservancy’s Platte River Prairies, Nebraska.

Back in 2006, I established some research plots in our Platte River Prairies so we could take a more experimental approach to our work to understant how plant diversity affects prairie ecosystems.  Those research plots consist of 24 squares, each of which is 3/4 acre in size.  Half of those plots were planted with a high diversity seed mixture of about 100 plant species.  The other half was planted with a lower diversity mixture of 8 grass and 7 wildflower species.  Since then, several university researchers have helped us collect data on the differences between those high and low diversity plantings.  We’ve looked at a number of variables, including soils, drought response, insect populations, insect herbivory rates, and resistance to invasive species.

An aerial photo of our 2006 diversity research plots.  Each plot is 3/4 ac (1/3 ha) in size and is planted with either a high diverisity (100 species) or low diversity (15 species) seed mixture.

An aerial photo of our 2006 diversity research plots. Each plot is 3/4 ac (1/3 ha) in size and is planted with either a high diverisity (100 species) or low diversity (15 species) seed mixture.

Kristine Nemec, a recent PhD from the University of Nebraska-Lincoln, has done the bulk of the data collection and analysis from those experimental plots.  A soon-to-be-published research paper from that work will report that plant diversity appears to be suppressing the spread of two invasive species: bull thistle (Cirsium vulgare) and smooth brome (Bromus inermis).  Poison hemlock wasn’t included in that project because the methods we chose for measuring vegetation weren’t well suited to capture its presence and abundance.  However, from a purely observational standpoint, it’s always appeared that a lot less hemlock grows in the high diversity plots than in the low diversity plots.  Last week, I decided to test that observation by collecting some data.

Poison hemlock (Conium maculatum) has invaded portions of our research plots, sometimes forming large colonies that are near monocultures.

Poison hemlock (Conium maculatum) has invaded portions of our research plots, sometimes forming large colonies that are near monocultures.

Since hemlock is abundant mainly in the southern half of our 24 plots, I only collected data from those 12 plots for this pilot effort.  Half of those 12 plots had been seeded with a high diversity mixture and the other half with a low diversity mixture.  I walked three transects across each of those plots, and counted the number of last season’s hemlock stems that were within a meter of me on either side.  I only counted stems that still had seed heads to help ensure that I wasn’t counting stems from multiple years’ production.  You can see the results of my counts in the graph below.

The number of poison hemlock flowering stems found by transect in low diversity and high diversity plots.  Platte River Prairies - Diversity Research Plots.  April 2013

The number of poison hemlock flowering stems found by transect in low diversity and high diversity plots. Platte River Prairies – Diversity Research Plots. April 2013

Although I haven’t yet run any statistics on these data, there is a striking difference in the number of poison hemlock plants between the two treatments.  Hemlock was rare in the high-diversity plots, but was found in large numbers in many of the transects through the low-diversity plots.  This was just a quick and dirty pilot effort to see if there was enough difference to warrant a full-fledged research project, but I feel pretty comfortable that plant diversity is having an impact on hemlock abundance.

I plan to collect some more comprehensive data on poison hemlock this summer.  I’d also like to collect the same kind of data from an adjacent set of plots we established in 2010.  Those newer plots are the same size as those from 2006, but include three different seed mixtures: high diversity, low diversity, and a monoculture of big bluestem.  If I see a similar pattern of hemlock abundance there, that will go a long way to confirm what I think I’m seeing in the 2006 plots.

I’ve never considered poison hemlock to be a particularly dangerous invasive species in our Platte River Prairies.  It seems to be most abundant in old woodlots, and doesn’t often show up in our native or restored prairies.  On the other hand, the plant’s toxicity can cause big problems, especially from an agricultural perspective.  In fact, we’d considered haying our research plots last summer but couldn’t find anyone to harvest them because hay containing poison hemlock can’t be fed to livestock.  If prairie plantings with a high diversity of plant species resist invasion from hemlock, that could have important ramifications for farmers who want to establish new grasslands for hay or grazing production.

Poison hemlock is most often found in old woodlots along the Platte River.  It's unusual for us to find it in our diverse prairies.

Poison hemlock is most often found in old woodlots along the Platte River. We don’t usually see it in our diverse prairies.

My little pilot study is a small addition to a growing list of other research projects demonstrating the value(s) of plant diversity.  Unfortunately, high diversity prairie plantings are more expensive than lower diversity plantings, so it’s important for landowners and conservation organizations to know exactly what they get for that higher cost.  High plant diversity provides nectar and pollen resources for pollinators, improves total vegetative production, and has other benefits, including quality wildlife habitat.  However, one of the most intriguing aspects of plant diversity is its potential to help suppress invasive species.  If we continue to find that more diverse plantings help repel species such as bull thistle and poison hemlock, that will have important implications for both agricultural producers and wildlife/prairie managers.

Stay tuned as we keep learning…