When most people think of western forests, they picture mountains covered in tall pine trees, or maybe groves of stately redwoods reaching into the fog. But there’s another kind of forest in the West that’s just as important: the piñon-juniper forest.
This forest, in which piñon and juniper trees grow together, is a unique natural community common throughout the arid West. It covers millions of acres in nine states, but is most abundant in Colorado, Arizona, New Mexico, Nevada and Utah.
Piñon and juniper trees, somewhat shrubby and short, are not particularly majestic compared to a ponderosa pine or a sequoia. But they’re vitally important to people, wildlife and water supplies. By providing shade in sunny, high-elevation landscapes, piñon-juniper forests help ensure snow and rain last long enough to reach rivers and groundwater before evaporating.
Unfortunately, piñon trees seem to be particularly vulnerable to climate change. They’ve been dying in great swathes in recent years due to heat and drought. What does this mean for water supplies?
A new study led by scientists at University of New Mexico begins to answer that question. At a 10-acre research site, the researchers intentionally girdled the piñon trees (removed a strip of bark from their circumference) to kill them, then compared results over several years to a nearby control site. They found a surprising result: Losing piñon trees did not make more water available in the soil or for surviving juniper trees, as common sense would dictate. Instead, the entire test plot lost moisture more rapidly.
This has potentially huge implications for western states faced with the prospect of losing more piñon trees. Water Deeply recently spoke with the study’s lead author, Laura Morillas, who was then a postdoctoral fellow at University of New Mexico and is now at the University of British Columbia.
Water Deeply: Why is this forest type important?
Laura Morillas: Piñon-juniper woodlands are the third-largest vegetation type in the U.S., even though they have not been the subject of very many detailed studies. They have been one of the least studied North American forest types. They are located throughout all of the western U.S. and they actually cover around 30 million hectares [74 million acres]. And the ecosystem, of course, plays an important role in providing ecosystem services like wildlife habitat and vegetative cover for watershed protection, which, I think, is especially important in these areas because they are only located in very dry areas.
Of course, there are other resources from the forest that are important for local communities, like piñon nuts and firewood. So they are useful and they are very common. But from my research point of view, these systems are especially interesting because this is almost the driest you can go for a forest.
Water Deeply: What surprised you most about your results in this study?
Morillas: We kind of knew, or imagined, that the total amount of water that was being released to the atmosphere from precipitation was not going to change, because these systems are very forced in terms of radiation and energy. We were actually not expecting, at all, that the remaining trees would do worse.
Our hypothesis was that if you lost almost half of the tree canopy, we were expecting the remaining trees will have more water available for them, so they will do better and they will transpire more and will have a better or more active physiology. This is actually known in the research community as the moisture release hypotheses. For example, if you have a town and half of the people moved away, you will expect you will have more water for the rest of the community, right? And this has been proven and observed in more temperate forests.
So that was what we were expecting. And when we found through sap flow measurements the remaining trees were doing worse, even a year after the mortality of piñon trees, we were actually worried. I spent a lot of time looking at the data to make sure I wasn’t making a mistake in my calculations, because it was exactly the opposite of what we were expecting. But we became absolutely sure that’s what was happening. That’s the most shocking result that we had.
Water Deeply: It seems you uncovered an interesting connection among the plants in these forests. Can you explain?
Morillas: What we observed is that the proportion of water that was released to the atmosphere didn’t change. So basically evaporation didn’t change. But what changed – and that was the important part – is the way evaporation was partitioned. Basically, the components of the forest that are using the water changed.
What happened is the non-canopy components of the system were returning to the atmosphere a larger portion of the water that was available than at the control site, where no tree mortality happened. So, processes like soil evaporation and transpiration by the understory vegetation (grasses and other annual plants) were enhanced.
Instead of that water the piñon trees weren’t using anymore being used by the juniper trees that remained, it was used by these other processes. You have a smaller canopy and that creates less shade over the soil, and so basically you have more energy reaching the soil and understory vegetation. These new components play a more important role in the system and use more water, and that basically results in drier soils – in less water being available.
The main reason is because the timing in which the water gets used by the soil evaporation or understory vegetation is a little different from the timing by which the water is used by the canopy of the trees. Water use timing was changed, and that changed the available soil moisture.
Water Deeply: Were juniper trees harmed without living piñon trees around them?
Morillas: I wouldn’t say the junipers are going to die. But what we saw is they struggled. The junipers are still there, and the experiment kept going and the junipers haven’t died. But let’s say, in those two very dry years of the study, they suffered and the transpiration and physiological activity of the junipers was affected negatively.
Water Deeply: Do your results suggest any new ways to manage these types of forests?
Morillas: I think the only thing you can do in terms of management is to be prepared for the fact that the water scarcity keeps happening in these systems. Most likely, the ecosystem is going to transition into a savanna, and we should be ready for that.
The last two years of the study were pretty dry. Even though we cannot be sure the same mortality would have happened without those two very dry years right after the treatment, we know that’s common in these ecosystems. And actually, predictions for the Southwest of the U.S. are that droughts are going to become more recurrent. So it’s not difficult to imagine this is not an isolated case. There’s not much you can do. It’s a natural process. You cannot really avoid it. The changes are almost inevitable.
Water Deeply: What are the potential long-term effects on habitat?
Morillas: We focused basically on vegetation, so I cannot tell you much about wildlife. What we can expect is, the system is going to turn into a drier system. So water scarcity is increasing at these sites. And even the piñon trees that remained after our mortality treatment, they died, actually, after the experiment period. It turned into a drier landscape and that had definite consequences for piñons. At least if the water scarcity continued, the system is not going to recover completely. It’s almost like the piñons are never going to grow back at these sites.
This is not something we have seen in more temperate forests. They normally behave very differently, and several years after mortality the ecosystem comes back to the pre-mortality structure. And I don’t think that will happen in these piñon ecosystems.
Another thing I think is important is that the system turns into a less resilient system, and that’s scary because predictions are that droughts are going to continue. It’s going to keep hitting these ecosystems. So we can imagine in the long term, most likely, these systems are going to turn into juniper savannas. But let me be cautious. This is my hypothesis. Our study was not long term. But as far as I know, the piñon trees are not growing back in these sites right now.