Understanding Threats to Old Growth Ponderosa Pines

Larissa Yocom is an associate professor of Fire Ecology in the Department of Wildland Resources and the Ecology Center at Utah State University. Her research interests include the ecological role of fire, how climate, fire and vegetation influence each other over time and space, and how forest management can promote the beneficial aspects of fire and minimize the negative consequences. Her current research projects include assessing fuel treatment effectiveness in meeting management objectives in Utah, evaluating the climatic and environmental factors influencing tree regeneration after wildfire, and investigating to what degree forest species composition affects fire behavior and effects.

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Ray Mesa, near the La Sal mountains on the Colorado-Utah border is notable for its old-growth Ponderosa pines (estimated to be several hundred years old) that are growing in cool drainages amongst pinyon-juniper woodlands. Recent Ponderosa tree mortality raised concerns among land managers and Larissa Yocom, a fire ecologist at Utah State University, began exploring the potential causes of these deaths. 

Science Moab: Ray Mesa is east of the town of LaSal, near the Colorado-Utah border. The ponderosas you are looking at are surrounded by pinyon juniper forest. Why are ponderosas growing in that area? 

Yocom: It’s a really interesting place because they’re growing in these drainages. Most of the mesa is pinyon and juniper, but they’re growing in these cold air drainages, where the cold air provides them just enough of what they need climate wise for them to thrive.

So they’re actually lower in elevation than the mesa around them. 

Science Moab: How old do you estimate some of these stands are? 

Yocom: They are several hundred years old, probably three or 400 at least, and maybe even older. We weren’t able to get an age on the centers of the biggest ones, but at least a few hundred years old.

Science Moab: What brought you to want to study these? 

Yocom: It was through the Bureau of Land Management. They are managing those old ponderosa stands to try to keep them alive and healthy and they noticed that there was some mortality. So they were wondering if there was a way we could look into what was causing those old trees to die, because that’s not what they want to see happening out there. 

Science Moab: What are some of the reasons these trees are dying? 

Yocom: One of them is drought. So there has been a lot of drought in that area over the last couple of decades, and that’s been cited as a cause for mortality or death for some old trees in other places around in the Southwest. Insect attacks can also impact trees, and then we also thought about actually the lack of fire over the past 140 years.

Without fire, you can get an increase in competition from pinion and juniper moving into these drainages. Historically frequent fire would have kept out that competition, but it was one of our hypotheses that maybe with the absence of fire, more competition means less availability of water and other resources for these old trees.

We found plenty of evidence of insect attack and we saw pretty low growth in a lot of these areas and a good amount of competition, especially where those stands have not been thinned out recently. Then we also found that the managers have been doing prescribed fires to try to increase the resilience and resistance of these old trees to things like drought and insects. In a couple places, those prescribed fires got too hot and those were likely the actual direct cause of some mortality of a few of the trees that we documented.

Science Moab: If fire is good for these stands of ponderosas, how can a fire become too hot to where it actually damages the tree? 

Yocom: That’s also probably to do with how long it has been since there was a fire in that area. The last fires, frequent fires, were about 1880. So that’s a long time for fuel to build up around the base of these trees. in some places we could see more than a foot of pine needles had built up so then with the reintroduction of fire, that kind of material can burn for hours or even days and eventually the heat can get through even that really thick Ponderosa pine bark and kill the living tissues. 

Science Moab: Logistically, how did you go about studying these stands of trees?

Yocom: We looked for every old growth ponderosa pine tree in four areas, two treated and two untreated areas and we documented whether it was alive or dead and whether it had any sign of insect damage or any other notes.  We also took tree cores from them to measure their growth and that’s done with a sort of a drill, a hollow drill type machine. Instrument and you can take a little pencil sized core of wood out of the tree and look at how fast it’s been growing over the over as long as you can see those rings. We also took samples of where we identified trees that had recorded fires in their rings. We also looked at satellite imagery and because the trees’ canopies are so big, we could actually see going back through approximately when they died.

Science Moab:  How is the BLM going to proceed after knowing all this stuff? 

Yocom: Hopefully they will continue to do prescribed fire. I think it is really helpful. That’s shown by the trees that did survive. They’re doing better now after these treatments. It was a pretty small percentage that were negatively impacted by the prescribed fires, so I think it’s still worth doing the prescribed fires. They can try to remove some of the fuel around these old trees before they burn. I think they’re glad to know that prescribed fire is useful, but that care needs to be taken to try their best to protect these old trees.