Hot tree summer: Measuring the effects of the 2017 heat wave on Europe’s forests

Article: Rita, A., Camarero, J.J., Nolè, A., Borghetti, M., Brunetti, M., Pergola, N., Serio, C., Vicente-Serrano, S.M., Tramutoli, V., & Ripullone, F. (2019). The impact of drought spells on forests depends on site conditions: the case of 2017 summer heat wave in southern Europe. Global change biology.

Anyone who has suffered through a summer heatwave knows that they can be miserable at best and dangerous at worst. While we often see the human impacts of heatwaves in the news, people are not the only ones affected. High temperatures and little rainfall can also have major impacts on plants. If you have tried to keep a garden or lawn alive over the summer, you have probably learned the importance of frequent watering to keep plants alive and healthy. While this might work for plants around your home, thirsty plants in natural environments are not so lucky.

It’s getting hot in here, but don’t transpire all your water
Under dry and hot conditions, plants become stressed and start to show signs like leaves that look yellow or even burned. Image Credit: Christie (CC BY-NC-SA 2.0)

Plants need water because of an inherent tradeoff in how they are structured: as a plant takes up carbon dioxide through tiny holes in its leaves to make food for itself, it also loses water through those very same openings in its leaves. This water loss is called transpiration. If plants don’t have enough water, or are losing too much of it, they have to shut those openings in their leaves to save water. But, that also means they aren’t getting any carbon dioxide to make food.

This tradeoff between taking up carbon dioxide and saving water can become a problem for plants when things get hot and dry, like during a heatwave. When temperatures get hotter, plants lose water faster through their leaves. It’s similar to how when you hop out of the pool on a hot day, you dry off a lot faster than if it’s a cooler day. If these high temperatures also coincide with a lack of rainfall, which is often the case, things can be even worse for the plant since they have less water available to take up. In most cases this water stress means that plants are not able to function as well, but if it is particularly severe, it can lead to plant death.

Death can occur for a couple different reasons. If a plant is forced to keep its leaves closed to save water, that means it also is not able to photosynthesize and make food for itself. If this continues on for long enough, the plant can essentially starve. Another way a plant can die is by losing its ability to take up water, leading to the plant dying of thirst, so to speak. Plants take up water through their stem in a sort of pulling action that acts as a sort of tug-of-war between the plant and the soil. Under normal conditions, the plant is able to do this easily. As things get drier, however, this game of tug-of-war gets harder for the plant and it has to pull a lot harder to pull up the dwindling supply of water from the soil. Under really dry conditions, there is so much pulling on this “rope” of water that it can break – causing air bubbles to form within the plant stem and preventing any further movement of water up the stem. Oftentimes, plants die because of some combination of both drought and thirst, and scientists are still working really hard to further study exactly what happens in these extreme cases.

Far out methods for looking at plant responses

Recognizing the severe impacts heatwaves can have on plants, a team of scientists led by Dr. Angelo Rita looked at what happened to forests across southern Europe during a heatwave that occurred during the summer of 2017. The research team was interested to see how various plant communities reacted and how those responses may have differed between locations across southern Europe. In order to look at how plants responded over such a large area, the researchers turned to data collected by satellites, specifically a satellite called MODIS. One thing MODIS measures is called the normalized difference vegetation index (NDVI), which is a measure of how healthy and active plants are. The researchers looked at how NDVI changed during the heatwave relative to previous times when temperatures were cooler and plants had plenty of water. For each study location the scientists also used weather data to assess how severe the heat and drought were, which also varied across the region.

NDVI works by differentiating between areas with healthy, green plants and areas that are bare or have plants that are less green. This shows an example of a view of a farm (normal picture on left) and how NDVI (on right) can distinguish where more plants are (in green) and where there is bare soil or dead plants (red). Image Credit: Chris Fastie (CC-BY-NC-SA 2.0)
Not all plants feel the heat equally
Extreme cases of drought and high temperatures can lead to massive changes in an ecosystem as is seen here in a pinyon pine forest near Los Alamos, New Mexico. On the left is an image from 2002 as trees were starting to turn brown from drought stress and on the right is the same location two years later where most of the trees are dead. Image Credit: Craig Allen, U.S. Geological Survey (CC BY-SA 2.0)

Plant responses to the heatwave varied quite a bit between different types of plants and location within a region. One overall pattern was that plants at high elevations were typically did not experience as much stress. This might be because temperatures in higher elevations are generally cooler.  This means that the same relative increase above the average temperature at a high elevation is still a lower overall temperature, and thus not as dangerous for plants. The areas where the impacts were the worst were along the Mediterranean coast. Mediterranean climates are characterized by cool, wet winters and warm, dry summers. Plants in Mediterranean climates are often capable of dealing with some water stress, but also don’t have resources to withstand particularly severe cases of heat and water stress, as happened in summer 2017. Further, repeated incidences of water and heat stress, as is common in this region, may have made the plants more vulnerable. When the plants are stressed, they are not able to take up the carbon they need to build up food reserves, and as a result, might not be able to hold up as well during the next incidence of stress.

Studies like this are important because they can help us to better understand which ecosystems might be most at risk with the impacts of climate change. Although everywhere on the globe will feel the impacts of climate change, the types and magnitude of the effects will differ from place to place. With climate change, it is likely we will see heat waves and droughts that occur more often and are more severe. A better understanding of the factors that determine the extent of these impacts is a critical step in helping us to be more prepared to deal with them.


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Jeannie Wilkening

Jeannie Wilkening

I am currently a PhD student in Environmental Engineering at UC Berkeley where my research focuses on ecohydrology, which means I look at interactions between ecosystems and the water cycle. Before coming to Berkeley, I did my undergraduate in Chemical Engineering at University of Arizona and an MPhil in Earth Sciences at University of Cambridge, where my research focused on biogeochemical cycling in salt marshes. When I'm not in the lab, I enjoy knitting, hiking, watching too much Netflix, and asking strangers if I can pet their dog. Twitter: @jvwilkening

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