Who Will Survive the Drought?

D-02B: Conceptual Frames and Research Strategies for Integrated Studies of Adaptations to Drought

 

Ogden Archive, USDA Forest Service, Bugwood.org
Ogden Archive, USDA Forest Service, Bugwood.org

Large-scale tree mortality events are prevalent worldwide. Insect outbreaks, fire, and drought are just a few factors that could be contributing to these events. However, there remains much uncertainty as to how plants respond to these stresses and the physiological mechanisms that enable their survival or result in mortality. The research in this session employed a variety of methods to focus specifically on the role of tree adaptations to drought.

Understanding the causes of tree mortality is a priority from both industrial and ecological perspectives. For the timber industry, it is important to know for which species drought tolerance and number of trees per hectare can be maximized. From an ecological perspective, it is important to understand how species’ ranges may shift, how forest communities may change, and the effects these changes will have on biodiversity and forest productivity.

A number of studies focused on Eucalyptus species, grown in plantations or in their natural environments. Jean-Paul Laclau conducted a series of experimental trials in a Eucalyptus grandis plantation to investigate the interaction between nutrition and adjustment to water stress. He and his colleagues found that additions of potassium chloride and sodium chloride improved stomatal regulation, but did not influence water use efficiency and that transpiration was about 70 percent higher in the potassium fertilized treatments than the controls. He concluded that the addition of potassium increased the trees’ susceptibility to drought.

Gregor Sanders studied naturally occurring populations of Eucalypts across an aridity gradient in Australia to investigate variability in physiological adaptations to drought between species and across seasons. He found that along an aridity gradient, functional leaf adaptations and the degree of stomatal regulation varied. Additionally, the physiological differences observed between species were maintained regardless of growing conditions, but were able to vary between the wet and dry seasons.

A recurring theme in the session was that there are always trade-offs. Co-occurring species within the same community often utilize much different strategies in dealing with drought stress. Rick Meinzer discussed the fact that variability in hydraulic safety within a vegetation type is greater than or equal to the variability seen across vegetation types. He pointed to niche partitioning and the trade-off between embolism resistance versus the ability to recover from embolism to explain some of this variability. Meinzer stressed that there are many plant traits and trade-offs that need to be considered when modeling species specific hydraulic risk or resilience to drought. Similarly, David Montwé showed that there was a trade-off between growth and drought safety in populations of Pinus contorta across a gradient in temperature and latitude.

In light of current prolonged droughts and predictions of more extreme drought events in the future, understanding how trees respond and adapt to these conditions is crucial. Diverse research efforts across the globe have been addressing this issue with a variety of approaches, from large-scale manipulations to lab experiments. While drought presents a major risk to plant communities, it is important to recognize that there are other agents of change as well. Trees may often be faced with multiple stresses, such as insect outbreaks coupled with droughts or wildfires. Discerning how trees allocate resources in these types of situations will be an important step moving forward.

 

Written by: Allison Chan

 

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