New research shows that trees in wetter areas are more susceptible to the effects of drought, challenging previous beliefs about trees' ability to withstand drought. The study, which analyzed more than 6.6 million tree rings, showed that trees in drier regions are surprisingly resilient to drought. The findings highlight the widespread impacts of climate change on forests and suggest that genetic diversity in drylands is critical for adaptation to changing environments.

Scientists have surprising news for your Christmas tree this holiday season. They just found that, globally, trees growing in wetter areas are more sensitive to drought.

Scientists have long debated whether trees are more or less resistant to drought under drought conditions. Intuitively, trees living at their biological limits are most vulnerable to the effects of climate change, since even a little extra stress could push them to the brink of death. On the other hand, these populations have adapted to harsher environments, so they may be better able to withstand drought.

The trees in this lush temperate forest in Washington state's Cascade Mountains may not be as drought-resistant as those in arid areas to the south. Photo credit: Joan Dewdney

According to a new study published in the journal Science by researchers at UC Santa Barbara and UC Davis, greater water availability can "destroy" trees by reducing their ability to adapt to drought. "It's critical to understand this when we think about the vulnerability of global forest carbon stocks and forest health," said ecologist Joan Dudney, an assistant professor in the environmental studies program at the Bren School of Environmental Science and Management at the University of California, Berkeley. "You don't want to be the same tree that was 'spoiled' when faced with a severe drought."

Dewdney and her co-authors expected that trees growing in the driest areas would be more sensitive to drought because they already live on the edge of their limits. What's more, climate change models predict that these areas will experience drought more rapidly than wetter areas, and this change in climate could expose trees to conditions beyond their ability to adapt.

To measure sensitivity to drought, the authors analyzed 6.6 million tree ring samples from 122 species around the world. Based on the width of the growth rings, they measured whether the trees grew faster or slower than average each year. They linked these trends to historical climate data, including precipitation and temperature.

The team then compared drought responses in different regions. "As you move toward the drier edges of a species' range, trees become less and less sensitive to drought," said lead author Robert Heilmayr. "The trees are actually quite hardy."

Dewdney, Hellmeier and their co-author Frances Moore were inspired in part by the work of UC Berkeley professor Tamma Carleton on the impact of climate change on humans. "This paper highlights the value of interdisciplinary scientific work," added Moore, an associate professor at UC Davis. "We were able to adapt an economic approach originally developed to study how humans and businesses adapt to a changing climate and apply it to an ecological context, studying the sensitivity of forests to drought."

"Heat waves may kill more people in a cooler place like Seattle than in a hot city like Phoenix," Heilmeier said. "The Southwest is already quite hot, so heat waves there can be very scorching. But he noted that cities in the region have adapted to extreme climates. Now we know that forests show similar trends."

Unfortunately, warmer areas will become drier in the coming decades. "A significant proportion of species' ranges will face completely unfamiliar climates that are not currently seen anywhere in their ranges," Heilmeier explains. The authors found that in 2100, an average of 11 percent of species' ranges will be drier than the driest parts of their historical ranges. This proportion will increase to over 50% for some species.

"Overall, our research highlights that few forests are immune to the impacts of climate change. Even wetter forests are under greater threat than we thought," Dewdney said.

However, the coin also has its other side. Some species have a reserve of drought-resistant genes in drier parts of their ranges that can support forests in wetter areas. Previous UCSF research has shown that many species do have the ability to adapt to environmental changes. However, these researchers also found that trees migrate slowly from one generation to the next. This means that, in order to exploit this genetic diversity, human intervention, such as assisted migration, may be necessary.

Reference: Robert Heilmayr, Joan Dudney and Frances C. Moore published the article "Drought sensitivity of Mesozoic forests increases their vulnerability to climate change" published in "Science" on December 7, 2023.

DOI:10.1126/science.adi1071

Compiled source: ScitechDaily