Visualizing Plant Community Change

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Quick announcement: if you’re a graduate student doing research related to conservation in the Great Plains of North America, you should check out the J.E. Weaver Competitive Grant Program. We are offering up to five grants of $1,500 each to graduate students. Read more here.

About a week ago, I posted a bunch of nerdy graphs with data from some of our restored sites at the Platte River Prairies. I’ve been glad to hear from at least some of you that found those interesting. However, graphs showing how species richness or individual species occurrence changes over time are interesting, but they only show parts of the overall story. I’ve been playing with another way to better illustrate the full picture of a plant community’s species composition through time.

Photographs can help illustrate what prairies look like and how they change through time, but they only show the plants that are most visually apparent at the time of the photo. This is a 2009 photo of a restored prairie planted in the year 2000. It is one of the two sites I’m highlighting in today’s post.

What I really want to do is to step back and look at an entire plant community to examine how its composition changes through time. The concept I’m sharing today has been useful to me, but I’d love to hear feedback from others. I’d also like help naming the graphics I’ve constructed. When I started, I called them plant community signatures, but that’s doesn’t feel quite right. Help?

(For those of you who are more interested in pretty pictures than data, I did throw in a few recent photos of ice bubbles at the very end of today’s post, just for fun. If you want to see them, however, you’ll have to scroll past all my cool visual data representations first.)

The data used to create these illustrations are the same data I showed graphs of in my last data post. As a reminder, to collect the data, I plop down a 1x1m plot frame, list the plant species within it, and then repeat that over and over across a site (about 70 or more times per site). I can then calculate frequency of occurrence for each species, which is simply the percentage of those 70 or so samples each species was found in.

Once I have the data, I lay it out in a spreadsheet format to display the frequency of occurrence changes for each species through time. However, to make it easier to see patterns, I’ve colored each cell so that less frequent occurrence is light green and more frequent occurrence is dark green. I’ve also tried to group species together in a logical way. The result – at least to me – is a fascinating way to step back and study how plant composition varies over time.

I’m displaying results for two sites here. The first is a 1995 planting that I’ve collected data from annually since 2002. The second is a 2000 planting that I’ve collected data from every other year since 2003. I’m not displaying results for every species – just 60 or so of the species in each site that are abundant enough to be captured fairly regularly in 70 1x1m plots. If you want more details on these sites, you can get it from my last data-heavy post.

SITE 1 – Dahms 1995 Prairie Restoration. A 45 acre mesic site on sandy loam soils planted in spring of 1995 with approximately 150 plant species.

Here is the full graphic for the 1995 prairie restoration. It shows the frequency of occurrence between 2002 and 2023 for each featured species.

Remember to click on the title to open this post online if you’re reading it in an email. Doing that will then allow you to click on each image to see a larger, more clear version of it.

When I stare at this graphic, the first thing I notice is that there are very few examples of species that are on a distinct trajectory of increasing or decreasing frequency of occurrence. Most species simply fluctuate up and down through the years – some more dramatically than others. As expected, most of the most variable species are in the ‘annuals and biennials’ category. I think that’s great, and is evidence of ecological resilience.

Now, let’s look more closely at each of the four categories.

This graphic shows perennial grasses and sedges, including three invasive perennial grasses. Two species show a strong trend of increasing abundance. One is a native sedge (Carex brevior) and the other is Kentucky bluegrass.

Looking at the perennial grasses, it’s clear that big bluestem has been a dominant feature of this site for the duration. That’s fine, but not particularly surprising to me. What’s more interesting is the increasing abundance over time of both short-beaked sedge and Kentucky bluegrass. I’m happy about the sedge becoming more common. It’s a cool little plant. Kentucky bluegrass, of course, is less exciting because it has the potential to form monocultures and decrease plant diversity.

However, and this is really important, I’m not seeing any evidence – either here, in my other analyses, or on the ground, that Kentucky bluegrass is having a significant impact on plant diversity. Now, that could certainly change, but at least so far, it seems to be just hanging out (though it’s hanging out in a lot more places than it used to). It’s a great reminder that we should always measure the impact of invasive species, rather than just measuring the extent or abundance of the invaders themselves.

I’ll quickly highlight two other species that intrigue me. Little bluestem has become less common across the site over time, though it seems to have stabilized at about 10-20% occurrence in recent years. Simultaneously, prairie cordgrass has done the opposite, becoming more common.

Little bluestem likes drier habitats and cordgrass likes wetter. Does that mean the site has gotten wetter? I don’t think so. Instead, I think what we’re seeing is a long-term self-sorting process of a planted prairie. When the site was planted, the seed mix included way more little bluestem seed than cordgrass seed. As a result, I think the early establishment of those two species wasn’t representative of their actual adaptation to the site. Over time, I think they’ve both shifted around until they’ve settled into the microsites (and abundances) they’re best suited to.

Frequency of occurrence for perennial forbs in the 1995 planting.

I don’t see much drama happening within the perennial forbs at this site. That’s pretty reassuring, actually, given the kinds of stresses that the prairie has experienced. It’s been managed with patch-burn grazing or open gate grazing since 2002, when this data collection started. As a result, the plant community has been exposed to fire, season-long intensive grazing, and multi-year rest/recovery periods. On top of that, of course, are the wet and dry years that can also significantly affect the growth and survival of plants. It’s good to see that none of these species seems to be disappearing (which also matches up with my other analyses of our restored prairies).

The only species I see that seems to show a significant trend toward lower abundance is tall boneset. That’s not a species known to be particularly sensitive to grazing pressure, as far as I know. In fact, I think of it more as a plant that does well with the kind of lower grass dominance that comes after a bout of grazing.

My best guess is that tall boneset is a species that started out very abundant because of both lots of seed in the mix and a slow, weedy early establishment period for this planting. Over time, it might just be dropping in abundance because it’s not as competitive in this soil type when competition from its neighbors is stronger. I’m just guessing here, though. The point is that the graphic helps me identify this pattern and ask questions!

Short-lived plants and their frequency of occurrence through time.

Within the annual and biennial plants I included, it’s fascinating to see how volatile their frequency of occurrence can be from year to year. I don’t see any species with a decided trend – the species seem to just bounce up and down. What’s most intriguing to me is that all the bouncing doesn’t seem to be synchronized across species. In other words, all these short-lived plants are responding to different stimuli as they increase and decrease in their abundance.

SITE 2 – Dahms 2000 Prairie Restoration. A 69 acre mesic prairie on sandy loam soils planted over the winter between 1999 and 2000.

Now, let’s look at the second site (the one shown in the photo at the beginning of this post).

The Dahms 2000 restoration was planted with 202 species, most of which have been found at the site. Here is the full graphic for this planting, showing frequency of occurrence data for about 60 of those plant species.

I’ve only collected data from the Dahms 2000 site every other year, so there are fewer columns. Also, the site was only in its fourth growing season when I started collecting data, so we’re seeing more of the ‘early establishment’ phase of this site than we did of the 1995 planting, which was in its 8th year of growth when I first collected data there.

Frequency of occurrence for perennial grasses and sedges, including two invasive grasses.

Interestingly, there is a little more directional change within the grasses at this site than in the 1995 planting. Is that because the data includes earlier periods? Maybe, but even so, the change seems to continue past the first 8-10 years for at least some species. Big bluestem, Indiangrass, short-beaked sedge, and switchgrass all seem to be on trends of increasing frequency. Canada wildrye seems to be going in the opposite direction so it’ll be interesting to see if/where it levels off.

Both Kentucky bluegrass and smooth brome have increased in percent frequency over time. However, as in the 1995 planting, I’m not seeing any negative impacts on species diversity (yet?).

Frequency of occurrence of perennial forbs in this 2002 planting.

Within the perennial forbs, a few species at the top (stiff sunflower, yarrow, and bergamot) seem to be on an upward trend, at least before the last sampling period. You might notice that there are a lot of species – across all the categories – that dipped in frequency in 2023. I’m pretty sure that’s a consequence of thatchiness caused by a recent lack of fire and grazing, which was intentional on the part of the Platte River Prairies preserve manager (a combination of a scheduled rest period and a response to a couple dry years). I’m confident the numbers will bounce back up again over the next few years as the site gets more fire and grazing again.

Apart from those species, I think the most interesting thing about the perennial forbs is the lack of many obvious trajectories. For the most part, species seem fairly stable in their abundance through time, though some ebb and flow in interesting ways. Some species that have been labeled ‘aggressive’ in some circumstances don’t seem to be acting that way here, including Canada goldenrod and Maximilian sunflower.

Frequency of occurrence of annual and biennial plants in a 2000 prairie planting.

Just as in the first site, short-lived plants bounce around a lot in their frequency of occurrence. The variation over time is evident even though the data was only collected every two years. Once again, I don’t see much synchrony within those bounces, which tells me each species responds individually to the myriad stresses applied to the site over time. Fascinating!

So, there’s my attempt at a visual display of plant community change in our restored prairies. Does the approach seem helpful? Suggestions for improvement? Ideas for what to name the graphics?

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And now, for those of you hoping to see pretty pictures, here are three photos of ice bubbles from a restored wetland at the Platte River Prairies last week. Enjoy!

A Long-Awaited Fire

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This week, about 22 people from a variety of organizations gathered to conduct a prescribed fire at The Niobrara Valley Preserve. The 900 acre burn unit was on the north side of the Niobrara River. Before a big wildfire in the summer of 2012, the area was covered with mixed-grass and tallgrass prairie, bur oak savanna, and ponderosa pine/eastern redcedar savanna/woodland.

That wildfire swept across more then 70,000 acres, including more than 30,000 acres of the Preserve. North of the river, it wiped out the vast majority of the ponderosa pine and eastern redcedar trees and top-killed many of the bur oaks, though the oaks resprouted from their bases. Where the pines and cedars had been, smooth sumac quickly filled in, spreading quickly across much of the area and forming large clones that now cover hundreds of acres (and are still spreading).

Remember to click on photos to see bigger/sharper versions. If you’re reading this in your email, you’ll have to click on the post’s title to open it online so you can click photos.

Here’s a photo of part of the area we burned this week, taken back in September of this year. You can see the red sumac clones, the green bur oaks, and the white skeletons of pine and cedar trees. The Niobrara River is just out of frame to the right.

This area has been the subject of countless hours of discussion about how we should guide it into its next ecological phase. We could just let the sumac spread unimpeded, creating a huge shrubland. Unfortunately, eastern redcedars are reinvading that area too. Letting sumac spread ahead of that invasion would suppress grass growth, and without grass we’d be unable able to use prescribed fire to control the cedars. Since we know dense stands of eastern redcedar provide poor habitat for wildlife and suppress the diversity of plants and other species, we want to avoid that.

We’d be glad to have some ponderosa pines come back, but they are notoriously slow to re-seed themselves after a ‘stand-replacing fire’, especially when there aren’t mature trees around to drop pine cones. We can plant some (and the Nebraska Forest Service has already planted a fair number for us on the steeper slopes), but any fire hot enough to kill young eastern redcedars would kill pines too, unless we plant them in steep rocky areas where there’s insufficient grass to carry fire. Of course, those are the same places where it’s difficult for trees to establish because the ground, such as it is, is dry and hard.

The consensus reached among stewardship staff was that we needed to get a fire through the area, both to remove a lot of the downed wood (making it easier to access the area) and to kill the numerous invading cedar trees. It would be nice not to kill the small pines, but controlling the cedar invasion is a bigger priority right now. One fire won’t suppress the sumac much, but staff might be able to mow or otherwise treat those sumac stems in the year following the fire to amplify the suppression effect.

Deciding to burn the area was relatively easy compared to the task of figuring out the logistics and prepping the site. Firebreak routes had to be found on the steep slopes between the flat above the ridge and the river below. Huge numbers of dead pine and cedar skeletons – some standing, some on the ground – had to be cleared away from those firebreaks to give us a chance to control the fire along those boundaries. Chad Bladow, fire manager and burn boss for the fire, had to figure out how to get crews, water, and vehicles into places where the terrain made that difficult.

Eventually, though, everything came together and we assembled this week to make it happen. My role was pretty small. I showed up and did as I was told. The crew at the Niobrara Valley Preserve had already put in tremendous effort getting the site ready, coordinating the crew and their needs, working with local fire departments to get permission and support, and doing all the little things that go into making a big event happen.

We started the burn on the big flat above the ridge, where we could use trucks to lay down a wet line ahead of igniters.
Here, we’re widening the backing fire (on the downwind edge of the unit) to make it wide enough to stop any fire coming at the line later in the day.

We had help from some nearby ranchers, Nebraska Game and Parks Commission, Northern Prairies Land Trust, Central Platte Natural Resources District, the Bureau of Indian Affairs Pine Ridge Agency, and others. There was a lot of expertise and experience among crew members, as well as some people who were still relatively new to prescribed fire. Even for many of us with quite a bit of experience, this would be a more complex burn than we’re used to. Fortunately, Chad Bladow and several others on the crew were very capable leaders, and the fire was pulled off smoothly and effectively.

We started by burning a big flat area above the ridge, but the line I was on quickly started working down some very steep slopes, igniting as we went. Getting water to those slopes so we could control the fire took a lot of planning and equipment. A big tank was set up above the ridge and a very long hose was connected to it and stretched downslope. A pump charged up the hose with water so the crew could spray the edges of the firebreak and keep the fire contained.

The east boundary had some very steep areas, where we couldn’t use any vehicles. The crew set up a ‘hose lay’ – a long stretch of fire hose stretching down the slope (right/dark yellow) with multiple smaller hoses (left/pale yellow) that fed off the main. A gas-powered pump pulled water from a big tank and into the hoses, which crews used to control the fire as we ignited down the slope.
Former Hubbard Fellow (and current Nebraska Game and Parks Commission employee) Chelsea Forehead was part of the crew. Here she’s igniting down one of the steep slopes.

Once our crew made it off the steeper slopes and down to the relatively flat areas at the bottom, everything moved much more quickly. We ignited all the way down to the river, which then acted as the south boundary for the fire. Meanwhile, the other half of the crew worked west along the top and then came down the slopes on the west side until they also hooked into the river. From there, it was a matter of lighting off everything in the interior before it got dark and temperatures dropped and relative humidity rose.

Here’s an aerial view (via drone) at about 2pm, after the perimeter was blacked-out on the east half of the unit and much of the interior had been ignited to allow fire to spread.

That interior ignition was partially accomplished by people with drop torches – directed by others who stood on vantage points and guided them down the slopes via radio directions. Additional ignition, especially in really steep areas, was done with a specialized flare gun. Even with all that effort, the difficult terrain and the short period of daylight at this time of year meant that the sun went down before everything could be ignited. (A subset of the crew went out the following day and lit off the remainder.)

Here’s Chelsea again, lighting a flat area near the river.
This drone photo shows the crew moving west (into the wind) and igniting the lower parts of the slopes and the area between the river (center) and the road (out of frame to the right).
The fire should top-kill these sumac stems, but they’ll grow back quickly unless we follow up with mowing or some other treatment next season.
Here’s a drone shot showing the east half of the unit shortly after a line of fire (to the left) was ignited and starting spreading to the northeast, pushed by the wind.

As the sun went down, many of us had little to do besides watch the fire spread across the interior of the site and patrol the outer edges. With permission, I grabbed my camera and took advantage of the late day light and smoke to get some ‘mood shots’.

A smoky scene with a tree in the foreground and sumac and other shrubs behind it.
I thought these three looked like they were walking away from the final scene of an action movie.
As sunset approached, the fire started to spread more slowly and some of the crew gathered along the road.
Crew silhouettes in the smoke.
Waiting for everything to burn.
More sunset smoke.

I also did some (relatively) close-up photography of small patches of fire. I love the way flames look when captured in still photos.

Close-up fire portrait.
Another fire portrait.
and another.
Bur oak leaves burning.

As darkness settled in, the remnants of the fire kept burning. We knew the bigger logs and standing dead trees would burn through the night, but we had wide bands of blackened ground around the perimeter to contain the fire. The overnight forecast was also favorable, with high humidity and temperatures well below freezing, which would keep the fire from spreading. After assuring the perimeter was secure, the crew retreated to the Preserve headquarters just across the river to watch from there while putting away equipment, grabbing some food, and reviewing the day’s events.

As darkness set in, I got this drone photo of the river and part of the fire. For those of you who know the area, the Preserve Headquarters are just to the right of the scene and the Norden Chute is just out of the bottom of the frame. (Click to see a better version of the image.)
I took this photo from across the river well after dark. You can just see a few stars above the big tree.

Ecologically speaking, the 900 acres we burned this week is a very small portion of the 56,000 acre Niobrara Valley Preserve, and neighboring properties contain similar habitat that stretches in all directions for miles. We’re hoping to learn what we can from this fire and any follow-up treatments so we can apply that to other areas of the Preserve. More importantly, we hope our experiences will be helpful to our neighbors, who are dealing with very similar issues in terms of sumac expansion, eastern redcedar invasion, and slow recruitment of ponderosa pine.

If you’re interested, you can look back at some of my old blog posts to see what this site looked like immediately after the wildfire and during the next couple of years. The grasslands on Conservancy land and neighboring properties bounced back from the summer wildfire quickly, as grasslands do. The woodland and savanna areas, though, were a bigger question mark for us because there is less collective wisdom about how those kind of sites respond to a big fire like that.

Since summer wildfires in sites invaded by eastern redcedar are becoming more common, we all need to work together to figure out what kind of land stewardship is most helpful after an area burns. Hopefully, this week’s fire and its impacts will be educational for us and others. In the meantime, it sure felt good to be able to pull off a burn we’ve been talking about for a very long time.

On behalf of all our staff at the Niobrara Valley Preserve – thank you to everyone who helped with the fire this week!