Trees Respond to Drought Stress

Climate change is expected to bring more frequent and severe drought conditions to the Pacific Northwest, which is bad news for our region’s forests.

Drought conditions stress trees, making them more susceptible to insect attacks and wildfires. But, not surprisingly, the impacts of drought are not spread evenly from one tree species to another or even from young trees to old trees of the same species.

These are some of the findings in a recent study published in Agricultural and Forest Meteorology. The study was conducted by a team of Oregon State University researchers led by Hyojung Kwon.

The researchers set out to study how individual trees respond to drought conditions. Are young trees more vulnerable than old trees to drought stress? Are some tree species more sensitive than others? In both cases the researchers found that the answer was yes.

The researchers found that young trees were more vulnerable to drought stress than older trees. The researchers also found a marked difference in response to stress between ponderosa pines and Douglas firs, the two species covered in the study.

To figure out how a given tree was likely to respond to drought, Kwon and colleagues employed two study sites: the first in Central Oregon, where summers are particularly dry and dominated by ponderosa pines, and the second site in western Oregon, where summers are not quite so dry and where the researchers measured Douglas firs. On a tree-to-tree basis, the researchers then set out to measure drought stress by examining how much water each tree uses to grow. For their findings to make sense, let’s review a little basic science.

Trees are made up of carbon, which they capture from the atmosphere as the gas carbon dioxide (CO2). The trees convert CO2 into carbon via photosynthesis, that profound ability of plants to turn sunlight into energy. This captured and converted carbon makes up the bulk of a tree’s leaves, trunk, and roots. And the whole process hums along without a hitch provided conditions are right. Herein lies the catch, and it tells a lot about how trees respond to drought.

To capture CO2 from the atmosphere, trees use little holes on the undersides of their leaves called stomata. (Picture an octopus’s tentacles, but really, really tiny). When a tree opens its stomata to capture carbon, a little bit of water evaporates from inside the plant into the atmosphere—this is called transpiration. This relationship means that the flow of water and carbon through the tree is very tightly linked.

This relationship between water and carbon was used by Kwon and colleagues to examine how their study’s Douglas firs and ponderosa pines responded to drought stress. Specifically, the authors examined how much water the trees used (transpired) to grow over a given period of time. This measurement is called the tree’s water use efficiency.

Basically, water use efficiency is the ratio of the total amount of carbon photosynthesized to the amount of water transpired. Looking at this ratio can tell us how stressed a given tree is from drought. The relationship works like this:

If a tree has plenty of water available, more water will be transpired as the tree grows and its water use efficiency will be low. However, under drought conditions, trees are able to adapt and close their stomata a little bit in order to transpire less water. This makes their water use efficiency very high. Because trees only employ this adaptive strategy once drought conditions have reached a certain level, this means high water use efficiency signals potential drought stress while low water use efficiency signals business as usual.

In their study, Kwon and colleagues found that during periods of drought the younger ponderosa pines had higher water use efficiency levels than mature ponderosa pines. The reason was that the younger trees only transpired about half as much water as mature trees per unit of growth. This means that the same drought conditions stress younger trees more than mature trees. The authors attribute younger trees’ water use efficiency levels primarily to the trees’ shallow rooting depths, which limit their ability to access water stored deep in the soil.

Okay, that’s young and mature trees, now let’s talk about those species differences mentioned at the top of this post.

Kwon and colleagues found a notable difference in the water use efficiency between ponderosa pine and Douglas firs. In fact, the water use efficiency measures of the study’s ponderosa pines were nearly four times greater than those observed in the study’s Douglas firs.

When the authors looked closely at the reason for the difference in water use between species they found that ponderosa pines were much more sensitive to changes in soil moisture than Douglas fir trees. As soil moisture is depleted in the summer months, the pines close their stomata, which reduces the amount of water they lose through transpiration. This adaptation is what makes ponderosa pines more drought tolerant than Douglas fir, note the authors.

Douglas firs, on the other hand, are more sensitive to warming air temperatures. As air temperatures warm, the water in the tree’s stomata evaporates more quickly, thus more water is transpired.

Those are the findings. What they mean moving forward is less certain. What is certain is that our climate has been warming and we can anticipate warmer, drier summers. These warm and dry conditions are highly likely to enhance the stress on our already drought-prone ecosystems.

That is why studies like this one are so important. Knowing how and why drought affects different trees in different ways will help our region create targeted management and adaptation efforts that could help our forests stay productive and healthy as our climate changes.

Study: Agricultural and Forest Meteorology

Citation: Kwon, Hyojung, Beverly E. Law, Christoph K. Thomas, and Brittany G. Johnson. “The influence of hydrological variability on inherent water use efficiency in forests of contrasting composition, age, and precipitation regimes in the Pacific Northwest.” Agricultural and Forest Meteorology (2017).

Photo: Oregon Douglas firs. Taken on June 17, 2013. (Photo credit: Noël, some rights reserved.)

Linnia Hawkins is a Ph.D. candidate studying atmospheric science at Oregon State University. Working with the Oregon Climate Change Research Institute since 2014, Linnia’s research interests include, regional climate modeling and the impacts of climate change on forests in the western US. She is a regular contributor to The Climate CIRCulator. Other Posts by this Author

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