As I travel across Nebraska this year, I keep seeing dead and dying eastern red cedar trees. Some are big, some are small, but they’re definitely dead. Since cedar trees are a major invader of grasslands across the state, I’m not complaining about all the dead ones, but I do wonder what’s killing them.
Interestingly, the trees seem to be dying in clusters, rather than as random or scattered individuals. To me, that indicates at least two possible causes of death. One possibility is that some kind of disease or insect is killing trees and then spreading to others nearby. The second is that trees are dying from last year’s drought conditions and that local variation in soil texture means that cedars in some places were more vulnerable to drought than others.
Since my experience with trees is mostly limited to attempts to keep them out of my prairies, I thought I’d contact someone who has a broader range of expertise on the subject. I emailed Scott Josiah, the state forester with the Nebraska Forest Service and asked him why cedar trees are dying. Scott said he thought the drought hypothesis made the most sense, and that trees growing in soils with coarse sands and low levels of organic matter, for example, would be stressed more than those in soils that hold more moisture.
It was good to hear from an expert on the subject, but I’ll admit to a little skepticism. I really like Scott, and as I said above, I’m no expert on trees, but cedar trees sure seem like they’d be tough enough to survive a one year drought… Heck, I’ve seen them growing out of ROCKS! I took Scott’s answer and filed it away, but continued to wonder about the possibility of a disease or insect outbreak that foresters and others just hadn’t yet identified.
And then last week, I found some pretty convincing evidence that I think has solved the mystery.
We were at our Kelly Tract on the North Platte River, working on some vegetation sampling and Canada thistle control when I noticed some dead cedar trees in some old shelterbelts on the property. As I got closer, I realized this was a perfect site to test Scott’s idea that drought was killing cedar trees. The Kelly Tract is a floodplain prairie with strong patterns of alluvial (river deposited) soils across the site. That means there are lots of different soil types all mixed together – a naturally-occurring experimental design.
While conducting my vegetation surveys, I noticed that last year’s drought had definitely affected the grasses and wildflowers much more severely in some places than others. Broad streaks of green and brown wound across the prairie, tracing the old channels and sandbars formed when the river had long ago flowed across the site. When we’ve done soil sampling elsewhere along the Platte, we’ve found that soils with coarse sand and low organic matter are quickest to dry up in drought conditions – I assume the same is true at the Kelly Tract. I figured that if drought, combined with soil texture, was killing cedar trees, I’d be able to see whether the dead trees were in the same “streaks” that contained dried up grasses and wildflowers.
They were. In fact, every brown tree I saw was located along a streak of brown grass, and every green tree was in a streak of green grass. It was as perfect a pattern as you could hope for.
While cedar trees are certainly tough, it sure looks as if Scott was right – last year’s drought was just too much for those trees to handle, at least in some soils. Mystery solved!
Way to go, Scott! Now I have a new problem… Do I hope for a wet year to help our prairies recover? Or a dry year to kill more cedars??