Hubbard Fellowship Alumni Post – Chelsea Calls out Cedars

This post was written by recent Hubbard Fellow Chelsea Forehead, who just completed her Fellowship with us at the end of January. After hearing numerous discussions about the eastern redcedar invasion in Nebraska and many other places, Chelsea took a deep dive into the literature and talked to numerous biologists to better understand the issue. This blog post synthesizes much of what she learned – and should be helpful to others trying to deal with the cedar challenge.

One of the largest threats facing North American grasslands today is the encroachment of eastern red cedar (Juniperus virginiana). After attending numerous talks about how to combat cedar encroachment it seemed to me that there was a lack of discussion about the source of the species’ dispersal. Amid contemplation of reactive measures, I heard little about the small blue berries being dispersed. Prescribed fires at regular intervals are known to kill off younger cedars. The methods and effectiveness of chemical treatment to remove the juniper are discussed in painstaking detail in the literature. Mechanical removal with large machinery is known to be necessary for taking out larger trees. Grazing goats may help keep the already growing junipers at bay. But is there something that can be done (without the help of bulldozers, goats or burn crews) before eastern red cedar seeds germinate in prairie soils?

Eastern redcedar is invading grasslands across the Great Plains, including here in the Nebraska Sandhills.

Despite the crisis it has created for grasslands, the native evergreen deserves some credit. Its bushy form has helped producers shield homes, roads, crops, and livestock from the relentless wind and snow that is part and parcel of winters on the plains. The red cedar asks for little in return; a low maintenance choice for windbreaks, it persists in soils that other tree species would turn their noses up at. The eastern red cedar has, therefore, become a trusted choice among landowners for sheltering life on the Great Plains.

There is an average of 850,000 eastern red cedar trees cultivated for distribution in Nebraska each year- more than any other state in the region. The costs and risks of the tree’s invasion are not lost on landowners, however. The conversion of grazing lands to cedar forests can take place within one lifetime, and the loss of forage for livestock can be as high as 80% after just two decades without active control efforts. If eastern red cedar remains the top choice for landowners seeking shelterbelts, and if the presence of the species on their land threatens their livelihood (without persistent control), is there a way to keep cedar windbreaks but stop their seed dispersal?

Eastern red cedar is a dioecious species. This means that it has separate male and female trees. Female trees, therefore, are the only ones bearing fruit and producing seeds. Prioritizing the removal of female trees would therefore do more to stop dispersal than removing males. If removing all cedars of substantial size seems daunting, perhaps removing only those guilty of dispersal is more manageable.

Eastern redcedar male cones. (Flikr User CameliaTWU Omeka@CTL, accessed March 10, 2020,
Female eastern redcedar ‘berries’.

After planting a windbreak or shelterbelt it can take up to 10 years before a landowner might be able to identify whether a red cedar is male or female. With nearly a decade of growth before sexual maturation cutting down female trees could leave holes in a tree line, potentially reducing its effectiveness in sheltering home and farm. Eastern red cedars, however, are bushy enough to serve as a windbreak in single file. With thoughtful design the removal of females from a multi-row planting might result in reduced loss of windbreak effectiveness.

But how many trees might be females? The consensus is that eastern red cedar has a natural sex ratio of 1:1 male to female. Though debated, some studies have even found that ratio to be higher. Harsh conditions and the decreased ability of females to live long and prosper in them is cited as the cause for higher male proportions.

While there is no evidence of sex-change happening in eastern red cedar, the rumors that it might occur have persisted. It would, after all, be pointless to remove females if in doing so the remaining male trees began producing female cones. After braving the bowels of botanical literature, I found that while no evidence or argument exists for the ability of eastern red cedar to change sex, the true confirmation of this relies on the identification of a genetic marker for sex. If available, a test for genetic sex would likely be cost prohibitive on larger scales. Even if they were capable of changing sex, it is more costly to produce female parts than male parts. Sex changing in other species is often in response to harsh environmental conditions like shade and lack of soil nutrients. This means that while sex change in plants is not unidirectional, environmentally induced changes would likely go from female to male in stressful situations, rather than the other direction. Although plants often adapt to their surroundings in surprising ways, all current evidence points to genetically determined sex in eastern red cedars.

Cutting down trees is hard work. I can’t say that my days removing cedars from the preserve were without exertion. I can confidently say, however, that the logistics of cutting down cedars were much simpler than those of carrying out a prescribed burn. Mary and I were able to average 30 minutes per cedar with the help of one trusty chainsaw. The cost, equipment and labor of removing females from windbreaks is, by all accounts, much lower than the cost of reacting to their spread across whole pastures.

Former Hubbard Fellow Mary Parr cutting down an eastern redcedar.

The magnitude of cedar invasion across Nebraska and surrounding states is daunting. Saving remaining grasslands from forestation will require the concerted efforts of landowners and conservation organizations alike. For those efforts to be successful, stakeholders will have to take a multi-faceted approach and consider all possible measures of combating cedar spread. Conversations about strategies to combat cedars have become focused on targeting areas where cedar invasion has just begun. Invasion begins with dispersal and dispersal begins with female trees. Removing female cedars from windbreaks would be a way for individual landowners to put some effort, a little time, and a bit of chainsaw fuel towards stopping the coming invasion.

Landowners who take out cone-bearing females will have certainly reduced the number of seeds spread from their land. When a single female can produce around 2 million seeds in a year, cutting down the right trees can magnify the effects of an individual’s efforts. What’s more, eastern red cedar seeds are only viable for around 14 months in the soil. This means a proactive sawyer won’t have to battle their victim’s dropped seeds for more than a year and some change. An all-male cedar windbreak could represent a nice compromise between the conditions of life on the plains and those who live there; a provision of protection for life on the prairie without a threat to the livelihoods earned there.

Letting nature take its course

The phrase “let nature take its course” is so widely used and accepted, it has gained idiom status.  The idea that nature is self-perpetuating and self-correcting is an attractive one.  The supporting evidence is strong too – I mean, look at how long nature thrived before humans were even a species! 

Unfortunately, the romantic notion that we should just back off and allow natural areas to manage themselves just doesn’t work in today’s world.  Or, to be more accurate, most of us would be uncomfortable with the results of that strategy – especially at individual sites.  Whether you like it or not, the earth today is heavily shaped and manipulated by human activity.  Within that context, deciding to suddenly back away and allow nature handle things on its own comes with some serious repercussions.  It’s akin to taking a bunch of athletic kids, training them for years to be elite volleyball players, and then entering them in a soccer tournament.  They might be great athletes, but they aren’t likely to fare well at a tournament for which they don’t know the rules and don’t have the appropriate skills to succeed.

Prairies like this one depend upon human management for their survival.

This isn’t a post about what would happen to the earth if humans suddenly disappeared.  That story has been told by others, and you can go explore and argue about that story with them.  This is a post about what happens when we walk away from natural areas – prairies, in particular – within the context of the world we inhabit today.  I also want to be clear that this post is not a criticism of the way humans have altered the earth.  There’s plenty to talk about on that topic, but today’s post is about how we manage (or not) natural areas in the contemporary world.

Let’s start by considering some of the ways in which humans have altered the playing field for species and natural communities.  We’ll focus on the grassland landscapes of central North America because that’s the setting most familiar to me.  First and foremost, we’ve converted much of the landscape to intensive agriculture and other human developments.  As a result, once expansive swaths of prairie are now divided into small isolated fragments, limiting the ability of animals and plants to migrate or otherwise move across the landscape.  We’ve brought plants and animals from the opposite side of the globe and released them into this fragmented landscape.  Many of those have become dominant competitors, with the ability to eliminate other species from their territories and reduce biological diversity. 

The fragmented nature of today’s landscape facilitates invasions, which most often occur along boundaries between prairies and nearby land uses, such as roads, crop fields, suburban areas, or other areas where invaders are established.  Introduced species are not the only invaders in this context, however.  Native trees and shrubs, which have battled prairie plants for dominance since the last Ice Age, have been given a huge advantage.  Instead of trying to spread into prairies from a few stream valleys or other fire-resistant sites, they now invade from countless locations – like an army that has dropped millions of paratroopers behind enemy lines.

Invasive grasses have become dominant in this grassland, which is not being actively managed for plant diversity. In addition, the numerous tree lines in the background provide a constant seed source for potential woody plant invasion.

Within our fragmented landscapes, our agricultural and industrial activities have increased the levels of nutrients such as nitrogen and phosphorus – and those chemicals enter prairies through both the water and the air.  We are essentially fertilizing prairies, which might sound positive, but usually favors invasive plants (e.g., reed canarygrass) or makes a few native plants exhibit the same aggressive diversity-reducing traits as invasives.  The inadvertent fertilization of prairies is most intense in areas near crop fields or factories, but the impacts are measurable even at great distances from those sources.

Adding a rapidly-changing climate to all those other stressors just seems unfair, doesn’t it?  We’ve introduced new enemies, provided them (and old foes) with access points and increased competitive advantages, and carved up the landscape to block escape routes and re-supply lines.  Now, we’re turning up the heat and quickly changing the basic growing conditions and living environment within the prairies that have managed to survive to this point. 

But, hey, prairies should be able to handle all that without our help, right?  (Good luck in the soccer tournament, kids!)

Before we address that question, here’s one more consideration.  The last glaciers retreated from central North America thousands of years ago, and tundra and spruce forests gradually gave way to grasslands.  During and after that transition, people have been present and active managers of those grasslands.  Human hunters influenced the composition and behavior of animal communities, and arguably helped eliminate a number of important animal species.  Perhaps most importantly, humans were actively using fire as a management tool (to attract grazing animals, for example) as well as for warfare and other purposes.  Those fires were an essential factor that helped perpetuate grasslands and prevent them from being taken over by encroaching woody vegetation.  As a result, today’s prairies have never been separated from people and human management.    

People have been actively burning prairies in central North America since those prairies first constituted themselves after the last ice age.

So, what would happen to a prairie today if we decided to just leave it alone?  It’s not a hypothetical question – any experienced prairie manager can tell you stories based on their own prairies, or on prairies they’ve watched suffer from insufficient or no management.  The only fires that occur in today’s fragmented landscapes are those set by people.  In the absence of those prescribed fires (or a substitute such as haying or grazing), prairies begin accumulating thatch – the dead stems and leaves from successive years of annual growth of grasses and wildflowers.  Within a few years, that thatch begins to smother many of the plants trying to grow through it.  It also creates inhospitable habitat for most prairie animals.  Biological diversity, an essential component to the resilience and survival of prairie communities, decreases dramatically.

As thatch builds up, so does the competitive pressure from invasive plants and trees.  In my area, for example, smooth brome, reed canarygrass and tall fescue tend to flourish in the absence of management.  They do particularly well in high nitrogen environments, but they also get by just fine without inadvertent fertilization.  In addition, there are numerous tree and shrub species that can quickly take advantage of a lack of fire, including eastern red cedar, white mulberry, honey locust, Siberian elm, smooth sumac, and rough-leaved dogwood. Especially in highly-fragmented landscapes, those and other trees are often growing right on the edges of prairies, and if not, their seeds can easily reach prairies by bird, wind, or other conveyance.  Complete conversion of a diverse prairie to a woodland with a brome understory can happen within a couple of decades or less.

By the end of a growing season, prairies can produce a tremendous amount of vegetative growth, which can accumulate after multiple years (if not burned, grazed, or mowed off) to the point where it greatly inhibits the growth of plants beneath it and provides poor habitat for many animal species.

You might be thinking, “well, sure, I can see how that might happen in a tiny prairie surrounded by cornfields, woodlands, and suburban sprawl, but what about some of the big prairie landscapes of Kansas, Nebraska, or the Dakotas?   Surely those prairies can take care of themselves, right?“

We can argue about whether lightning fires alone would be sufficient to prevent tree encroachment in a huge expanse of prairie.  I feel confident they wouldn’t, but it’s an argument that can be had.  However, invasive species, spurred on by nutrient pollution and climate change, are still going to be a killer threat to biological diversity and the subsequent vitality of those prairies in the absence of human management.  Depending upon location, already-present invasive plant species such as leafy spurge, sericea lespedeza, spotted knapweed, cheatgrass, Kentucky bluegrass, and many others would expand their reach and power to the point where they would dominate large swaths of land, if not entire prairies.  That invasion and dominance would trigger a cascade of other impacts, leading to reduced plant diversity, decreased habitat quality for animals, eventual extirpation of many plant and animal species, and an ecosystem that would be unrecognizable – and undesirable to most of us.

Invasive species, such as crown vetch (Securigera varia), are already present throughout many prairies, but are held in check by constant and thoughtful management. In the absence of that management, prairies would be overrun by these species, losing plant diversity and ecological function as a result.

Anticipating protests, here are couple more quick points.  If you’re an advocate of the broad idea that we should get out of the way and let nature take its course, you might say prairies are an unfair example because they are a transitional ecosystem that relies heavily on disturbances such as fire to avoid becoming a woodland.  That’s fine, but if you look around the world, there are lots of other fire-dependent ecosystems, including many (most?) forests and woodlands.  Most of those also have very long histories of human fire management. 

In addition, woodlands and other ecological communities suffer from invasive species, habitat fragmentation, nutrient overload, and climate change, just as prairies do.  We’ve created a world that puts those natural systems at a disadvantage, and whether we like it or not, they now rely on us to help mitigate those threats.  Arguing about whether human management is natural or not is a moot argument that distracts from the great challenges we face in conservation.  Let’s focus on the important discussions about how best to manage ecosystems, not the settled issue of whether we should be managing in the first place. 

People are an intrinsic part of nature and the world we live in.  That shouldn’t make nature seem any less fascinating or inspiring – in fact, recognizing our interconnection with nature should inspire us even more. We are part of an incredibly complex and beautiful web of interacting species and communities across the glove. As such, it’s up to us to play our roles responsibly.  Just as we don’t exist outside of nature, we also can’t survive without it.