The shape of you
What is this?
Because climate varies between regions, seedlings from a given species might have different resistance to drought depending on where they are born. What makes some populations of seedlings better fit than others? Is there an indicator that predicts mortality risk across populations despite their differences?
Why does this matter?
- Because we don't know how much stress plants can tolerate before dying and how much that varies within a species.
- Because finding indicators that predict mortality regardless of population us to predict forest mortality risk under drought across larger areas.
How did you study this?
We planted ponderosa pine seeds from a wet and a dry area in our greenhouse and grew them for a year under the same climate conditions. We stopped watering the plants and let them die. Then, we measured their levels of stress weekly and simultaneously re-watered a subset of them to see how many were dead at each level of stress. Finally, we related the stress levels to the number of plants dead at each point in time to see which seedlings were the most resistant to drought.
We also measured the differences in shape, size, and the amount of roots, shoots, and leaves between populations to see if some populations die with less drought because of how their bodies are shaped.
We measured physiological processes like the capacity of plants to move and supply water to organs, the strain -or tension- that drought imposes in their vascular system, the water pools inside their organs, and the amount of sugars stored in them. If you want to know why we measured their sugars, you should check this other post!
So what did you find?
We found that seedlings whose mothers lived in different places and climates (dry and wet) looked very different. Seedlings originally from a wet area were taller, grew faster and bigger, and grew more roots than seedlings from the dry area. Because our seedlings were grown from seed in the same greenhouse under the same climate, we know that the differences we observed between populations are genetic!
When we put the plants under drought, seedlings from the wet area became stressed, lost sugars, stopped transporting water, and dried out faster than seedlings from the dry area. Because of this, seedlings from wet places died weeks earlier than seedlings from the dry area.
The reason why plants from the wet area died earlier was that they had invested a lot into growing leaves and roots which consume water. In doing so, they quickly used up all the water left in their pots during the drought to maintain these organs. On the other hand, seedlings from the dry area were able to hold on for longer because they didn’t have that many leaves or roots, so they didn’t need that much water nor lost so much water through their leaves. Of course, everyone died at some point anyway because that’s what I do for a living, and I like a job well done.
So, we found ourselves with a bunch of plants that looked different, behaved differently in terms of their physiology, and died at different times. This means that a bunch of different characteristics determines when a plant dies and that predicting when a plant will die would require knowing all these characteristics. This is why predicting tree death is SO difficult: every plant is different, and we cannot have all this information from every plant to tell when it will die. However, there is one thing that most trees have in common: trees die when they dry. As such, the amount of water in a tree can tell us about how likely a tree is to die and -as I explained in this post- plant water content integrates the different processes that kill trees. Based on this, we thought that maybe all plants died after their organs lost similar amounts of water and that water content could therefore be used as a broad indicator of mortality risk even when plants looked or behaved differently. Was that the case?
YES! Seedlings from both wet and dry areas had similar chances of dying at a given level of water content. One important reason that explains why both populations showed similar lethal water contents is that the variable we used considers the maximum amount of water a plant can hold. That is, this measures how dehydrated the tree is relative to its maximum hydration state. Hence, the differences in times to death we observed occurred because seedlings from the wet area reached lethal water content levels earlier in time.
Importantly, the water content level at which seedlings started dying -what we call the incipient mortality threshold (green lines)- was nearly the same in both populations. These findings have big implications for forest managers!
And the big point is...?
Trees need to stay hydrated to survive and whether they succeed or not depends on how they build their bodies. Trees can vary a lot on how they build themselves up, and they take advantage of that to adapt or acclimate to dry conditions typical from where they live. However, the levels of dehydration that kill trees are less variable. We can take advantage of this common trigger of death to predict mortality among trees that vary in shape and size. Additionally, because plant water content is a feature that can be measured using airplanes and satellites, these results give us hope that we may be able to monitor mortality risk at large scales even if trees vary in shape and form across the landscape.
Of course, there is still much work to do. We studied two populations of the same species, so some similarities are to be expected. Would we also find a common lethal dehydration point in trees from different species? What about plants that are not trees? We don’t know the answers to these questions yet. I guess I get to kill some more plants!