My favorite trees are the large ones that you can easily climb and perhaps build a tree house in, but what if the tree died due to climate changes and drought? This is definitely a current concern, and recent research may help find answers. Dr. Brendan Choat gave a recent seminar on his research about how trees deal with hydraulic failure. This research can help us understand the resilience of trees to drought, and perhaps ways to help.
The Drought Problem
I grew up in Utah where drought is common along with dry and dead trees that sometimes lead to forest fires in summer. However, this is happening all over the world, not just in deserts. A NASA-led study used satellite images to look at severe drought which affected about 30 percent of the Amazon rainforest in 2005. Some of the older trees did not survive, and their decaying wood can lead to an excess release of carbon dioxide, which will affect our ecosystem. Another study did a global assessment of tree mortality due to drought and high temperatures. The study found that various types of forests and trees all over the world can be affected by drought.
Climate changes are definitely affecting tree mortality rates, and the problem is that trees can die a lot quicker than they can grow. As Choat presented, we need to understand more about the mechanisms involved in tree survival and mortality. This information may help us predict, and hopefully prevent, die-off.
How Trees Get Water
Naturally it’s best to start with the basics. Water is pulled up through a tree by negative pressure created from leaf tension. This is very similar to how you drink through a straw. The xylem are the straw-like structures that are dead at maturity (just like plastic straws) and are the vessels through which the water flows (see diagram to the right). However, these vessels have small pits where water can also flow between the xylem. This special hydraulic system can also get clogged through embolism, which is a gas bubble that blocks the water flow (see my diagram below).
Xylem structures vary a lot between the angiosperms (flowering trees) and the gymnosperms (“naked seeds” like pine cones) as well as between species. Whether they have many thin xylem, or fewer large ones, this variance can definitely affect tree vulnerability and susceptibility to embolism.
Can a tree get rid of embolism once it occurs? Choat and his collaborators were able to use x-ray tomography at a synchrotron light source to be able to see not only the occurrence of embolism, but embolism repair. Parenchyma cells near the xylem actually begin to refill the embolism and eventually allow water to continue up the tree. However, this repairing process may not be the same for all species and classes of trees as some, angiosperms in particular, have more parenchyma cells. So, trees may vary widely in their vulnerability to embolism as well as their ability to repair embolism.
Understanding more about embolism, and embolism repair over a wide variety of trees species can help to predict species variability during a drought. Additionally, this information may assist us in forming strategies to protect the trees that we can, particularly for droughts. Unfortunately it may be some time before Choat has had the opportunity to study a large sample of trees to make predictions, but that doesn’t mean we can’t work on saving the trees where we can. There are many ways to help save more trees such as recycling, using less paper, planting trees, or joining an organization.
Links to References
- Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function; Choat et al., 2008
- Global convergence in the vulnerability of forests to drought; Choat et al., 2012
- Study finds severe climate jeopardizing Amazon forest; NASA study article
- A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests; Allen et al., 2010
- Blog on Water Movement in Plants
- Global Trees Campaign