Dead or alive?
What is this?
It's the 21st century and we can send spacecrafts to Pluto. However, we can't tell the moment when a plant dies nor tell why did it die! Can we find out a way to assess whether plants are dead or not and what killed them?
Why does this matter?
- Because we don't know what fails within a drought-stressed plant so that it dies.
- Because finding out what causes plants to die allows us to predict forest mortality risk under drought and gives us tools to prevent it.
- Because when the Aliens find out that we got space travel before understanding how a lettuce works it will get really awkward...
How did you study this?
We put 250 ponderosa pine seedlings in our greenhouse and we slowly killed them all with drought. Then, we took weekly measurements of several physiological processes that could potentially explain their deaths.
For instance, plants lose the ability to move water through their bodies when they experience drought stress and air bubbles clog their pipes. We call this process Hydraulic Failure. During periods of drought, plants also stop making sugars through photosynthesis because doing so costs much needed water. To counter this, they 'eat' stored sugars that they save for stressful situations like drought. We call this process Carbon Depletion. However, plants also use these stored sugars to retain water within their tissues through a process called osmosis. Thus, consuming stored sugars can make plants lose Water Content more readily, desiccate, and die faster under drought.
Simultaneously, we re-watered a subset of the seedlings every week to see how many would remain green and how many would die after so many weeks of drought. We did it this way because recently dead plants look just like live plants. So this was the only way to accurately quantify mortality and find which physiological process (not me) was responsible for the deaths of so many innocent lives. Here's an example:
If we trusted our eyes, we wouldn't have called wolf until week 6 when, in fact, it died somewhere before week 5 when we watered it again. We would have thought that the plant died at the levels of hydraulic failure, carbon depletion, and water content observed at week 6 when it actually died at the levels seen at week 5 or even earlier. How do when know when exactly? We look at plants re-watered the previous week, count how many died and how many survived, and calculate a probability of mortality with those numbers.
So what did you find?
We found that both hydraulic failure and carbon depletion make plants lose water and experience drought stress. In the case of hydraulic failure, air bubbles clogged the pipes of the plant and water couldn't reach the living tissues where it was needed. In the case of carbon depletion, plants that quickly ate their stored sugars had less of those to retain water within their tissues and water left their bodies faster. The opposite was true too. Plants that saved their sugars instead of eating them took longer to dry out!
The coolest thing of these results is that they imply that plant water content integrates both hydraulic failure and carbon depletion. Therefore, we might be able to predict forest mortality risk with just measuring water content and avoid the pain of measuring the other two processes. Will this be true?
YES! No matter how much water we gave them after the drought, plants couldn't recover once they had lost certain amount of water. You might be thinking ''Wait, you are telling me that plants die under drought because they dry? This is your discovery??'' Yes, this is our discovery. It might sound stupid and obvious at first but this is pretty important. We obviously knew that plants would die as they dried out. What we didn't know until now is how much water they could lose before they died. Now, we do. In fact, now we know the risk of death for every value of plant water content.
We also found that there's a point beyond which the risk of death rises above zero and increases sharply. This is an important discovery because forest managers can use it as a reference point to classify trees as healthy or at risk and decide whether they need to take action to save them.
And the big point is...?
While both hydraulic failure and carbon depletion were responsible for the loss of water content and ultimately the death of our plants, they are extremely difficult to measure. Thus, it is virtually impossible to measure these processes across the forest in order to predict mortality risk. However, we can measure plant water content in an instant by using new technologies that detect how light is reflected or absorbed by tissues, chemical compounds, and water within plants. These technologies are called hyperspectral techniques and can distinguish healthy forests from drought-stressed ones using several wavelengths:
If we put these technologies in drones, planes, or satellites we could generate maps of mortality risk across the landscape in no time!! I guess it's time to learn some of this!
Of course, it is never this simple. There is still a lot to do. As always, we need to make sure that what we found in these seedlings is also true in other species, adult trees, regions, etc. But this could be a first step toward saving our forests. Or so I tell myself to feel less guilty about torturing and killing 250 baby trees...
The actual paper for the nerds: