Remembering Drought
As photosynthesis happens, trees take in carbon from the atmosphere and store it in the form of sugars or carbohydrates. These sugars can be stored for many years in trees as a sort of “memory” and used to support their growth and metabolism during times of stress, like a drought. We talk with Drew Peltier, ecophysiologist and professor at the University of Nevada, Las Vegas. Drew’s interests and research center on understanding how climate influences tree growth in our changing environment.
Meet the Scientist: Drew Peltier
Drew Peltier is an Assistant Professor at the University of Nevada-Las Vegas where he studies dendroecology and tree physiology. His research seeks to understand how tree growth responds to climate and climate extremes over long timescales, or “memory.” He works in forests across the western US, from the two-needle piñon (Pinus edulis) to very tall coast redwoods (Sequoia sempervirens), to understand how trees and forests will respond to future climate change.
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Interview Excerpt - Memory in Trees
We are talking with Drew Peltier, an assistant professor at the University of Nevada, Las Vegas. Drew is a tree ecophysiologist interested in how climate influences tree growth in our changing world. He does this by studying the memory in trees.
Science Moab: What are we talking about when we refer to a “memory” in trees?
Peltier: When I say trees have memory, it’s just an analogy. Animals obviously have memory, they have neurons and nervous systems that serve to kind of produce the memories that you and I are familiar with. But trees are such long lived complex organisms that it really is a useful word to use to describe this slow, progressive way in which trees respond to their environment over years and decades and centuries.
Science Moab: What are some of the issues you are looking out in terms of tree growth?
Peltier: What most people are familiar with is what we might call dendrochronology, the science of using tree rings to study past climates and environments. Tree rings can be used to study all sorts of things about the past: rain, droughts, heatwaves, volcanoes, fires, salmon runs in the Pacific Northwest. A new branch of tree science is dendroecology. This is using tree rings to study ecosystems and how the trees work themselves. This is the work that I do. One interesting thing to come out of this study is the finding of so-called drought legacies. These are examples of that memory phenomenon that I’m talking about. Every time we have a really extreme drought in the southwestern US, trees show that for multiple years. If you look 2, 3, 4 or five years after that drought event, oftentimes trees will be growing less than normal. So they “remember” that drought event for multiple years and that drought impacts those trees’ physiology for multiple years after the drought event has been released.
What we do see, and it’s very clear, is that those drought events are happening more often. So the concern here is that if we have all these trees on the landscape that are recovering from a drought that happened one or two years ago, and we have another drought in quick succession, how does that influence trees’ ability to be resilient to that drought event? Are they more likely to experience mortality?
Science Moab: And what methodologies are you using to look at this?
Peltier: Using tree rings to kind of understand how trees are processing and responding to their environment is what we do and what I’m really interested in. In the southwest, we have all sorts of tree ring records from a century of collecting tree rings all over and those are really useful because they capture all this environmental variation. We can look at correlations between tree ring width and climate extremes. We use those large datasets to produce complicated statistical models that quantify this memory.
The other thing that we do is go out and measure things about trees as they’re experiencing these drought events. There’s lots of different examples of that, but probably the most interesting one is this concept of carbon reserves. Essentially, these are products of past photosynthesis. Just like animals store fat, plants and trees store sugars for a very long time. One mechanism for how this memory might work is that trees use photosynthetic products, things like sugars that they took in from the atmosphere from many years ago to support their growth and metabolism during times of stress, like a drought.
Science Moab: Have you applied some of these methods in the SW?
Peltier: We’ve applied this all over the Colorado Plateau, specifically in Trembling Aspen and Two Needle Pinyon Pine. Aspen trees in southern Utah had on the order of 20 year old carbon reserves in their deepest tree rings. Pinyon pine similarly, in central New Mexico had on the order of 20 year old carbon reserves, we can also see that those are used in certain circumstances that are relevant to the things that we’re thinking about in terms of climate change.
Science Moab: what are some of the next or current things you’re working on in terms of the drought and southwest?
Peltier: one thing that we would like to do is work in some of these sky island ecosystems across Nevada to understand how these trees are experiencing and responding to climate change, and to have some predictions about how they will be able to respond in the future. The key to that is not just having tree ring data, but also collecting lots of other physiological data about these trees over long time periods. One of the things that we’re interested in is understanding how much water each tree is able to move from the roots to the atmosphere, if that’s changing, and how hard it is for those trees to do that. We call this water potential and it’s a measure of how hard those trees are pulling on the soil. Another goal that we have is trying to understand what conditions lead to really good tree growth and what doesn’t, when those happen and how those interact with phonology or the seasons. By putting all those things together, we hope to have a really holistic view of how tree growth works in our dry southwestern forest ecosystems.
