Drought and Tree Mortality: Science Reveals Harsh Future for World’s Forests
New research on the inner workings of trees and plants is providing clarity on just how vulnerable these living organisms are to the effects of climate change. In an interview, Craig Brodersen, a pioneer in this emerging field, describes how a warmer — and drier — climate threatens the world’s trees and forests.
By Kevin Dennehy
As a series of catastrophic droughts over the past few years has made clear, a future of increasingly extreme weather events will make life harder for the world’s trees. A growing body of research is illustrating exactly why that is.
A new paper in the journal Nature, co-authored by Yale’s Craig Brodersen, highlights an emerging scientific field that uses 3D imaging and other technologies to better understand the inner workings of plants and trees — and what its findings have revealed about the vulnerabilities of these living organisms.
Brodersen, an assistant professor of plant physiology ecology at the Yale School of Forestry & Environmental Studies, has been at the forefront of this field, developing techniques to describe the hydraulic systems of plants and predict how a warming planet will likely affect these functions.
In an interview, Brodersen describes some of the important insights this field has revealed over the past two decades and the potential consequences for the world’s forests.
In this paper you review the latest research on the link between plant physiology and drought. What have scientists learned?
Craig Brodersen: We tried to offer a synthesis of what the field has learned over the past 15 or 20 years and to put what we now know about drought-induced mortality within the context of the major drought events that have taken place over the past few years — particularly in places like Texas and California — where we’ve seen millions of trees die in a very short period of time.
We’ve been able to identify that the water transport system in plants and trees is the critical component that is under threat as we see more intense and long-lasting droughts, and that many of these trees are getting pushed beyond their physiological thresholds or tipping points. What we’re still trying to understand is how ‘legacy’ effects of previous droughts affect forests when they experience subsequent droughts. And how does that affect their mortality threshold?
Ideally we’ll be able to take information about what we know about what’s going on with the physiology and anatomy of individual trees and use that to scale up to larger landscape models. For instance, what are the long-term consequences for carbon storage by forests? And how will it affect the water flux patterns across landscapes? Because these drought events — and the drought-induced mortality that we’ve observed — are not exclusive to the United States. It’s a worldwide phenomenon.
How does the frequency of drought that we’ve seen in recent years compare with historic trends? And how is it affecting trees?
Brodersen: One thing that appears to be happening over the past five to 10 years that hasn’t necessarily happened in the past is that, as our climate is warming, we’re getting an additional combination of effects. Drought is certainly something that plants and trees have experienced before. But then when coupled with warming temperatures, it amplifies the effects of drought. Cool and dry is not nearly as bad for plants as when it’s hot and dry. That’s when the vascular system of plants and trees becomes really, really stressed, and that’s when things start to get bad for trees. And as soon as you have that combination of hot and dry, the vascular system starts to break down in trees that aren’t adapted for that kind of environment.
How does this combination of hot and dry conditions disrupt a tree’s hydraulic systems?
Brodersen: We’re finding that as a tree enters a drought, the first thing to happen is that it closes its stomata in order to conserve water. But that puts the tree in a dilemma; if it closes its stomata to conserve water that means it can’t pull CO2 out of the atmosphere in order to do photosynthesis. It then must rely on its internal storage to get by during times of drought. Ultimately, the tree’s ability to survive drought is a function of how much carbon it has currently stored and available to keep itself alive.
In terms of a tree’s ability to recover, we are now finding that if a tree loses a certain percentage of its total conductivity — how much water it can transport — there’s a tipping point after which it can’t come back. It needs to be able to have enough water stored in its trunk, and then have enough of its vascular system functioning in order to grow more xylem the next year, in order to replace any of the wood that was lost to the hydraulic disfunction. When trees get pushed beyond that tipping point of not having enough water and not having enough stored carbohydrates, they become very susceptible to pests and disease because their defense systems are significantly reduced. And if the tree gets pushed too far there’s the possibility that they go beyond that tipping point, if the insects don’t get it first.
How many trees have been lost due to drought in the past decade?
Brodersen: In the paper we cite a number of different studies that have tried to put a number on that. Between California and Texas, for instance, there have been more than 400 million trees in just the past decade. There have been huge losses in Australia, where a mangrove forest has lost 7,000 hectares due to drought. There have even been documented events in the Amazon. You might think that it’s wet all the time there, but they have experienced seasonal dry periods in which there have been huge mortality events.
One of the other things that is coming to the forefront is that a lot of the really big, older trees are also dying. We think it’s linked to hydraulic failure and the ability to deliver water to the canopy. Those findings are alarming because the large, old trees in the forest are also the ones that sequester the most carbon. And so a as soon as they die then a lot of that stored carbon starts being released back into the atmosphere faster than it would have otherwise.
Are any species or biomes particularly vulnerable?
Brodersen: A lot of really great work has come out of the Southwest, including by Craig Allen [of the U.S. Geological Survey], Nate McDowell and the other people at the Los Alamos National Laboratory, where they are seeing really intense droughts coupled with hot temperatures and the subsequent fires that are radically changing what the forests look like. In many cases the pinyon and juniper forests are dying from drought or because they get attacked by insects. And then they’re being replaced by other species, such as oaks and aspen. As a consequence the forests out there are changing significantly.
We’ve seen some regions that have been significantly impacted, like the Sierra mountains in California, where millions of trees have been lost. A lot of the changes, we think, are a result of the fact that these trees are now exposed to drought year after year after year. A tree might be able to survive one low-intensity drought just fine. But when they are exposed to chronic drought, the accumulation of the effects of each drought weakens trees over many years until ultimately they die. So places that are already hot and dry are problematic. In New England we had the third driest year on record in 2016 and the trees didn’t seem to be affected all that much. But we do have invasive or native insects that are having an impact. We’ve seen the emerald ash borer that has come through and damaged a lot of the ash trees in our area that were already weakened by drought. New invasive insects like southern pine beetle, for example, are moving into our forests because of our recent warmer winters, with the potential for serious problems on their own, and adding drought on top of that will amplify the stress for the trees in our forests.
How have 3D imaging technologies enabled you to better understand these plant functions?
Brodersen: Plants have a vascular system that, once developed, allows them to transport huge volumes of water with almost zero energy cost. There’s no metabolic pump to run the circulatory system like you find in animals. Instead, the xylem sap is under negative pressure: basically the evaporation of water out of the leaves pulls the water up the trunk through miniature hollow tubes, similar to household plumbing except the pipes are smaller than the diameter of a human hair, and the water is pulled instead of pushed. But one of the problems in studying how plants transport water is that that vascular system is hidden behind the bark. And even if you pull off the bark its still really hard to see what’s happening inside the xylem. You can’t cut into it without damaging it, and there hasn’t been a good way to study what’s going on in the inside plant and visualize what’s going on in there.
Over the past 10 years or so, our group — including Andrew McElrone at UC Davis and groups led by Brendan Choat and Tim Brodribb in Australia — has been using high-resolution X-ray imaging that allows us to see inside the plant and look at the functional status of plant vascular systems to understand whether they’re filled with water or air during a simulated drought.
By using these non-invasive imaging tools, which we’ve borrowed from the medical imaging fields, we’ve been able to see how the plumbing system of the plant starts to malfunction during drought, as well as the mechanisms that plants use to recover. As a result, we can now test longstanding hypotheses that we haven’t been able to answer because we haven’t been able to see inside plants.
Our group is looking at the influence of drought on the physiology of the trees that are common in New England, and trying to understand which ones are most at risk based on current climate change predictions. There are certainly other groups around the U.S. and the world that are doing similar projects, but the advantage we have is that we’ve reached the point where the instrumentation and image processing we’ve developed is sophisticated enough that we can do additional computer modeling exercises to simulate the types of drought that haven’t occurred yet here in New England.
What are some of the remaining gaps in knowledge that this community of scientists wants to address?
Brodersen: The big push right now is to figure out how to characterize the hydraulic status of an individual plant either through remote sensing or through the use of sensors on the ground. That would allow us to hand the data off to climate modelers. One of the major limitations is that we’re making assumptions about when plants are going to shut their stomata and stop pulling water from the soil and putting it into the atmosphere. Being able to predict when different forest types will stop transporting water based on easy to measure atmospheric data will ultimately help us better predict the amount of carbon being pulled out of the atmosphere, the amount of stored carbon being released, and the water fluxes. So we’re trying to work across scales by integrating cellular-level processes that have cascading effects at the whole tree level, and the whole forest or ecosystem.
It takes a collaborative effort by a group of people who are specialists in different fields coming together. That’s what we’re aiming for and this paper outlines some research proprieties and opportunities that will help us make better predictions.