Why will we be climbing the trees in the dark?

We will have to climb our study trees in the dark and it's not just because we're crazy. In order to be able to make measurements on the trees when they are at their most relaxed (i.e., not transpiring), we have to climb them in the before the sun illuminates the trees. We will make a water potential measurement on each tree a few hours before sunrise (pre-dawn) and then we will make a second measurement later in the day when the sun is at its highest and the trees are most active (mid-day).

 

Anthony making a pre-dawn water potential measurement

The reason for making measurements at the most relaxed and most active time of the day is so that we can assess the underlying water status of the trees and how they are regulating their water use. At night, when the sun is down, the trees are less active and the measurements we make then are more reflective of the water availability of the soil. During the day, when the sun is out, the leaves start to photosynthesize and lose water, which decreases their water potential. Water potential measurements made at mid-day are a reflection of how a tree is regulating its' water use, with more negative values indicating more water stress (see Lab Note #2 for more information about water potential measurements).

The water potential of the leaves changes on a daily basis and throughout the season, with more negative water potentials during the hotter, drier parts of the day and year. To get a real sense of how stressed the trees are, we want to make these measurements at the height of the drought, when they will be balancing the need to grow with the need to conserve water.

There is also a big difference in the water status of leaves at the top of the tree compared to leaves at the base of the crown. As you can imagine, it is harder for the tree to get water to the top than to the base because water has to overcome gravity and friction as it moves from the roots to the treetop. To get a real sense of the overall water status of the tree, it is important to measure the leaves from both locations. This is why we will be collecting leaves from the very top and the base of the crown at four in the morning.

These water potential measurements are just spot measurements, and not sufficient on their own to fully assess the water status of the trees. That is why we are also collecting leaf samples for chemical analysis, which will give us a more integrated signal of the overall water status of the trees and their physiological responses to the environment. We'll discuss the chemical analyses we hope to do in our next Lab Note.

Stay tuned!