Tag Archives: prairie

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

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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…

Why A Warming Climate Is Making This Spring So Cold (… and Last Spring So Warm)

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Melting sea ice might not seem important to those of us living in the middle of a continent.  It is.

Weather and climate have always been complicated and difficult to understand, so it’s no wonder that climate change is a topic that confuses most of us.  The fact that most climate change discourse is more political than scientific these days makes things worse.  It’s hard to have reasonable discussions because most people’s opinions tend to be linked to whichever loud voices they listen to, and few of us understand climate science well enough to draw our own independent conclusions.

The poor groundhog has been a popular scapegoat for this year's cold spring temperatures. In reality, both this year's cold spring and last year's warm spring are much more strongly tied to global warming and melting arctic ice.

The poor groundhog has been a popular scapegoat for this year’s cold spring temperatures. In reality, both this year’s cold spring and last year’s warm spring are much more strongly tied to global warming and melting arctic ice.

I’m certainly not going to wade into the politics of climate change, and I’m not qualified to get very far into climate science.  However, I did read something recently that clarified some things for me, so I’m hoping it will help you as well.  Thanks to Joel Jorgensen for passing along the article that spawned this post.

One of the most difficult things to understand about global warming is that it can make local temperatures get colder as well as warmer.  Here in Nebraska, we’re experiencing a very cold spring – if you can call it spring – this year, but had a very warm spring in 2012.  How, you might ask, is it possible that both the warm spring of 2012 and the cold spring of 2013 are a result of global warming?

Last year at this time, pussytoes was starting to bloom in our Platte River Prairies. This year, there's no indication that they're anywhere close to that stage.

Last year at this time, pussytoes was starting to bloom in our Platte River Prairies. This year, there’s no indication that they’re anywhere close to that stage.

Scientists have long suggested that more extreme weather patterns (including warm and cold, wet and dry) are a consequence of global warming, but I’ve never had more than a vague understanding of why.  Apparently climate scientists are still figuring it out too, but new research published by Jennifer Francis and Stephen Vavrus in Geophysical Research Letters seems to help.  After reading a summary of the work in the Omaha World Herald and stumbling through the actual scientific journal article, here is my best shot at explaining the results.

First two pieces of background information you need to understand.  This is based on my own rudimentary understanding of this topic, so please take it as such.

1.  The warming of the Arctic and the subsequent loss of sea ice is reducing the contrast in temperature between the cold Arctic region and the warmer center of the globe.

2. The contrast between warm and cold areas of the globe is a major driver of weather patterns because it creates an imbalance in atmospheric pressure.  The jet stream is the major current of air that tends to run along the boundary between those cold and warm areas (there is actually more than one jet stream, but let’s not get into that). When the jet stream is strong, it moves strongly in a relatively straight west to east direction.  However, when it is weak, it makes large north-south loops as it ambles slowly to the east.

Ok, armed with that background knowledge, here’s what’s happening with global warming.  Arctic air to the north of us is less cold than it used to be, so there is less contrast between that air and the warm air to our south.  That weakens the jet stream, causing it to make large loops as it moves from west to east.  Equally importantly, those loops tend to stay in the same place for a long time.

When Nebraska is inside a southward loop of the jet stream, the jet stream’s current allows lots of cold arctic air to come down from the north.  That’s what is making our 2013 spring so cold.  The opposite is true when we’re inside a northward loop – our weather is dominated by warm air coming up from the south, creating a weather pattern such as the one we saw in 2012.  Because a weak jet stream causes those loops to not only be greater in size, but also to stick around longer weather patterns persist for longer periods than they otherwise would.  If the weather extra warm for a long time, we tend to have drought, but extended weather periods can just as easily lead to flooding, extended cold temperatures, etc. – depending upon whether we’re north or south of the jet stream current.

When we are inside a southward loop of the jet stream (top picture) cold air from the north dominates our weather. When we are inside a northward loop of the jet stream, warm air moves in from the south.

When we are inside a southward loop of the jet stream (top picture) cold air from the north dominates our weather. When we are inside a northward loop of the jet stream (bottom picture) warm air moves in from the south.

Of course, there is much more to weather and climate than just jet stream loops, so a slower, more wandering jet stream is only part of the story.  In addition, understanding why we’re getting more extreme and extended weather patterns doesn’t change the situation – it just explains it.  I’ve written in the past about some climate change adaptation strategies for those interested in prairie management, restoration, and conservation.  A big part of our responsibility is to make prairies as ecologically resilient as possible.  

Since creating and sustaining resilience in prairies is largely dependent upon factors we’ve been working on for a long time anyway – species diversity, habitat size and redundancy, etc. – not much changes when we add climate change into the mix, except perhaps that we should feel a little more urgency.

Again, I’m no climate scientist, so I’m trying to explain things I barely understand myself.  Please correct me if I’ve mis-stated something or explained things poorly.