December 10, 2019

The Tree That Survives the Desert

Tu B’Shevat, a Jewish holiday commemorating the importance and sanctity of trees, could not be more fitting in this day and age. Undoubtedly, it’s not easy to be a tree these days. Extended drought periods induced by climate change make it more and more difficult for those that need water to survive. Add to this the constant increase in pests and invasive species, one gets an idea of the grim reality in which many tree species are forced to survive the rapid and extreme changes.

However, there are trees that remain strong and durable and even thrive in the most hostile conditions. In a new Israeli study, scientists found that the acacia tree in the Arava desert in Israel is the world’s largest tree growing in such a hot and dry climate. The tree has an even higher growth rate during the hot, rain-free desert summers than during the slightly wetter winters, and research into this remarkable resilience might provide valuable information about how to deal with climate change.         

One of the expected effects of climate change is the northward movement of the global desert strip and the transformation of semi-arid regions into desert-like regions. This process negatively impacts natural vegetation and agricultural produce, and as a result, jeopardizes the nutritional security of millions of people. Due to Israel’s unique geographical location at the transitional zone from the Mediterranean to the desert climate, many local studies look into the resilience of natural vegetation and drought-tolerant crops. 

Some of the latest research into acacia trees was conducted by Tamir Klein of the Department of Plant and Environmental Sciences at the Weizmann Institute of Science, in collaboration with Gidon Winters of the Dead Sea and Arava Science Center, and Shabtai Cohen of the Volcani Institute. For three consecutive years, the researchers tracked 10 acacia trees from two species that are distributed in the Arava desert — a strip of desert stretching about 41 miles between the Gulf of Eilat and the southern shore of the Dead Sea in Israel. The researchers looked at two species: Acacia tortilis (the flat-top tree known to us from African landscapes) Acacia raddiana. Tracking was done using stem thickness sensors (to determine if the trees grow or merely survive and remain the same size), water-flow sensors in the stem, and a camera that documents the state of the leaves. 

The area in which the trees are being examined, the Sheizaf Stream in the Arava, is hot and dry, with annual precipitation of only 20 millimeters (less than an inch). Therefore, the researchers hypothesized that the trees in the area grow for about two to three weeks a year, when the stream enjoys floods and rains, and are dormant for the rest of the year. “It was a naive thought,” Klein said. “We got exactly the opposite of what we expected. We found that the acacia trees grow very actively in summer, in the dry season.” In the winter, however, the growth of the trees actually stopped. 

These results are also significant on a global scale, Klein said. “Although there are very hot and dry places in the Sahara Desert, for example, they do not enable tree growth, or the few trees that exist in them do not grow on days when temperatures and dryness peaks. Understanding the mechanisms that help a tree survive in dry conditions is of supreme interest to science and decision-makers.” 

“In a new Israeli study, scientists found that the acacia tree in the Arava desert in Israel is the world’s largest tree growing in such a hot and dry climate.”

The researchers speculate that the survival secret of the acacia is a vast underground water source, which is available to the roots also in summer. In the Arava, there are a number of aquifers (underground water reservoirs), and the acacia trees have long roots, which can reach tens of meters of depth so that they may have access to one of these aquifers. The researchers’ hypothesis stems from the fact that water was found flowing through the trunks of the acacia tree all year round (as opposed to most trees in Israel) and also that the trees are green almost throughout the entire year. Additionally, the researchers found that the trees grow larger in summers that follow rainy winters when underground water sources fill up more.

“The tree has its origin in tropical Africa, from where it spread millions of years ago,” Klein said. “As a tropical tree, it is genetically programmed to grow in maximum light and heat conditions.”

In the next phase of the study, whose results have not yet been published, the researchers sought to understand in greater detail the source of the growth. They found that the trees perform photosynthesis even in summer and they can use it to grow during the hot season.

The researchers currently are trying to get a better idea of the unusual growth pattern several ways. On the one hand, they survey the water that flows through the tree in order to learn about the depth of its source. On the other hand, a collaboration between doctoral student Daphna Uni of Tamir Klein’s lab and Winters as well as Efrat Sheffer of Hebrew University is to test how much of the tree’s growth happens through photosynthesis products in summer and how much by using photosynthesis reservoirs. This year, another test is to compare the status of Israeli acacia trees to those growing in South Africa, where the climate is less extreme.

“In view of the climatic changes that are taking place right now, the research is of crucial importance,” Klein concluded.

A longer version of this article appeared on ZAVIT 

Racheli Wachs is a staff writer at Zavit environmental news agency in Tel Aviv.