When it comes to arid climates, junipers are some of the toughest trees around. The past two years, however, have seen the death of huge swathes of junipers in southeastern Utah. In this interview, Steven Kannenberg, a postdoctoral researcher at the University of Utah, explains why so many junipers are now struggling to survive.
Science Moab: Can you explain the ecology of pinyon-juniper woodlands?
Kannenberg: Pinyon-juniper woodlands are one of the most extensive forest types in the western United States; they cover around 10 states and spill into northern Mexico. There are a couple of different pinyon and juniper species but, in general, this ecosystem really is characterized by these two dominant tree species. The species down here in southeastern Utah are the two-needle pinyon and the Utah juniper, and these species have contrasting drought responses. If you're looking for a simplified ecosystem to understand the effects of drought, it's nice to go to these pinyon-juniper forests and find pinyon pine, which is a little bit more drought-sensitive, and juniper, which is probably one of the most drought-tolerant trees in the West.
Science Moab: It’s interesting that these plants have different responses to drought. When you say drought, are you talking about the normal drought fluctuations that we see in the Southwest?
Kannenberg: Drought or dry conditions aren’t anything new. But in the past few decades, we've seen a new type of drought, sometimes termed “global change drought” or “hotter drought.” It's these periods of anomalously high temperatures that co-occur with a normal drought so there's not a lot of water in the soil. In addition, in the American Southwest, we’ve been in this 20-year-long “megadrought,” which has turned out to be the second driest multi-decadal period in the last 1,200 years. So things are a little bit bad around here.
The drought over the last 20 or so years has been followed by pinyon pines dying back, which makes sense, given what we know about their physiology and how they’re a lot more drought-sensitive than junipers. But in 2018, we had the most severe drought in the past 40 years, and now we’re seeing hotspots of juniper mortality while the pinyon is not dying back as much.
Science Moab: What are the differences in these droughts? Why would one drought kill pinyons, and the next drought kill junipers?
Kannenberg: We've been working on this problem for two years: why juniper, and why now? As the first step for this research, you map the spatial severity and extent of this mortality and then figure out what is actually killing these junipers. Was it the drought? Was it some other insect or pathogen? It turns out that it was pretty much entirely the effects of this drought that’s killing off the juniper. But this tree species has, for the most part, survived every other severe drought in the last 20 years. So the question of “why juniper, and why now” is such an important one, because maybe they've reached this kind of tipping point where you've had this 20-year megadrought and long-term drying trend that's been amplified by anthropogenic climate change. So maybe they've just been pushed to the point past which they can't survive in some of these regions.
Science Moab: If it's such an extreme event, why don't you see both kinds of trees die?
Kannenberg: The areas where you've seen a ton of juniper mortality tend to be these hotter, lower elevation sites so the idea is that not many pinyons could occur there in the first place, just because it is a very inhospitable environment. My idea is the pinyons that can persist in those sites are there because they have really deep water sources, so maybe they have a little tiny root that's wiggling its way down into a crack in the bedrock. It can access water sources that most of the other trees in that area can't.
This interview has been edited for clarity. To learn more and listen to the rest of Steven Kannenberg’s interview, visit www.soundcloud.com/user-495802209/why-are-the-junipers-dying.