Timothy Perez: extreme leaf temperature and heat tolerance

Timothy Perez, postdoc at the University of British Columbia, shows us his last work ‘Photosynthetic heat tolerances and extreme leaf temperatures’, explains the importance of plant tolerances under climate change and the challenge that is to do experimentation in hurricane season.

About the paper

What’s your paper about?

Our paper is about using extreme leaf temperature to explain variation in photosynthetic heat tolerance (i.e. the heat tolerance of photosystem II photochemistry). Our paper proposes that heat tolerances should be due to adaptations to extreme leaf temperatures, not air temperature, and that the use of leaf temperature changes how plant susceptibility to heat damage is interpreted.

What is the background behind your paper?

Plants from hot climates typically have high heat tolerances, which has led to the assumption that heat tolerance benefits plants by promoting growth, survival, or reproduction in hot field conditions. Provided this assumption is true, heat tolerance would potentially be a valuable trait for predicting species’ geographic distributions and the species that will be negatively impacted by extreme heat events due to climate change.

Physiologists have elucidated several mechanisms that contribute to plant heat tolerance at the sub-cellular level and ecologists know that heat tolerance gradually decreases from hot to cold climates. We also know that the range of species’ heat tolerances within a community can exceed the range of the community mean heat tolerance across coarse climate gradients –  there’s a lot of variation in heat tolerance within a plant community. However, there is surprisingly little information that explains why species experiencing potentially identical environments within a community exhibit vastly different heat tolerances; this is where leaf temperature comes in. Our study proposes leaf temperature, specifically extreme leaf temperature, should explain this variation in heat tolerance.

How did you come up with the idea for it?

When I started my PhD in Ken Feeley’s Lab, I gathered some preliminary heat tolerance data and presented it during a lab meeting. During the meeting our lab discussed what heat tolerance “meant” for plants. Beyond the physiological answer (that heat tolerance measured the temperature that reduces photosystem II photochemistry), I had a difficult time explaining what heat tolerance meant for a plant’s ecology. The literature was (and still is) full of assumptions that suggest heat tolerance influences various aspects of plant ecology, ranging from growth to biogeography, but there is little data to back up these assumptions. Around that time, I was also reading “The Emerald Planet”, by David Beerling, which got me thinking about coordination between leaf size, stomatal conductance, and how extreme leaf temperatures may have led to extinction in ancient plants. That’s when it became apparent to me that if heat tolerances were to be useful for predicting patterns in plant ecology – particularly the species most vulnerable to climate change – we need to understand them in terms of leaf temperature.

I also followed a paper trail of references from “The Emerald Planet” to a body of literature that used thermodynamic principles to estimate leaf temperature. Once I learned that leaf temperature could be estimated, I realized leaf temperature could be estimated across a species’ distribution and provide an independent way to test if extreme leaf temperature drives variation in plant heat tolerance.

What are the key messages of your article?

The key message from our article is that extreme leaf temperature should be considered a major driver of variation in heat tolerance. Secondarily, species with the highest heat tolerances may be at the greatest risk of thermal damage because of their high leaf temperatures.

How is your paper new or different from other work in this area?

Our paper is unique because it addresses two assumptions about plant heat tolerance and highlights a promising method for incorporating plant physiology into plant biogeography. The first assumption we address is that heat tolerance varies in response to air temperature. The second assumption we address is that species with higher heat tolerances are less susceptible to heat damage. The method we use for incorporating plant physiology into plant biogeography combines leaf energy balance theory and species distribution modelling.

Who should read your paper (people that work in a particular field, policy makers, etc.)?

I think that anyone that has used climate to study species responses to climate change or predict species occurrences would find this paper interesting. Although plants were the focus of our study, energy balance theory provides a mechanistic framework for understanding thermal ecology and can be applied to endothermic and ectothermic animals.

About the research

Did you have any problems setting up the experiment or gathering your data?

Before and after the hurricane in the FTBG’s Arboretum

One goal of the study was to observe leaf temperatures when they were likely to achieve their highest temperatures. This meant I had to conduct my study during the hurricane season in Florida, USA, During the study, it was announced that hurricane Irma was expected to make landfall in Miami. This was an issue because the data gathering aspect of this study consisted of two stages. The first stage required monitoring leaf temperatures while measuring environmental variables and stomatal conductance. The second stage of the study involved harvesting the leaves I was monitoring, measuring their thermoregulatory traits, and harvesting additional leaves for heat tolerance assays. So, I had the hard decision of terminating the study early, or pausing it and hope that the hurricane-force winds didn’t strip the leaves off of the plants I was monitoring. Ultimately, I chose to terminate the study early to avoid the potential of a hurricane destroying my study plants and leaving me with only the data from stage 1. My decision to terminate the study early meant that I had to complete the second stage of the project more rapidly than I anticipated, and I felt as though I was racing against a hurricane to finish data collection!

Where you surprised by anything when working on it?

I was really surprised to observe that leaves reached temperatures several degrees above ambient air temperature. The leaves of some species in the study got quite hot! Since this study, I often stop to touch leaves growing in full sun with the back of my hand to get an idea of how hot they are! I encourage everyone to touch leaves and observe for themselves if leaves are tracking air temperature or not!

What are the big questions still to answer?

Heat tolerances were first documented by Julius von Sachs in 1864, but plant biologists still don’t know if they influence plant growth, survival or reproduction. A long-overdue and basic question that our study highlights is: How does heat tolerance influence plant fitness or crop yield?

About the Author

How did you get involved in ecology?

I’ve always been interested in plants, but I started to become interested in ecological research only after I completed my undergraduate degree in botany and started a job conducting plant surveys for the United States Department of Agriculture. After a few years of field work, I decided I wanted to pursue a graduate degree in ecology. 

What are you currently working on?

I’m working on several projects, but all are related to plant temperatures and how they can be used to understand species responses to climate change.

What’s your current position?

I completed my PhD at the University of Miami with Dr. Kenneth Feeley in December, and recently moved across the North American continent to begin postdoctoral research with Dr. Sean Michaletz at the University of British Columbia.

What is the best thing about being an ecologist?

The best thing about being an ecologist is conducting research. Since the research I conduct typically takes place in a garden, arboretum or forest, I get to spend a lot of time simply observing plants and thinking about how they work. For me, being outside and around plants always inspires new ideas and research questions.

Read the paper here!

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