Trees tell tales of climate change

As the climate crisis intensifies, forests are feeling the heat, as two studies just published in the journal Science highlight only too clearly.

Detailed modelling predicts that a vicious cycle of heatwaves may soon turn the Mongolian Plateau into a barren landscape, while in Europe researchers have found that unexpected and unwanted changes are happening in temperate forests.

In the first study, climate scientists led by Utah State University, US, warn that the past two decades have seen a serious increase in heatwaves and concurrent droughts, and that a feedback loop between heat and soil moisture may soon spiral irreversibly out of control.

They used tree-ring data to understand the Plateau’s climate before modern weather logs, piecing together heatwave and soil moisture records from the past 260 years. These show that recent consecutive years of record high temperatures and severe drought are unprecedented.

“Conifer trees respond strongly to anomalously high temperatures,” says co-author Hans Linderholm from Sweden’s University of Gothenburg. “By examining their growth, we can see their response to the recent heatwaves, and we can see that they do not appear to have experienced anything like this in their very long lives.”

The data show that as the temperatures soar, the soil dries out. This leads to even warmer temperatures, since evaporation from wet soil acts to cool air at the surface, while dry soil allows heat to transfer directly to the air.

“The result,” says co-author Deliang Chen also from the University of Gothenburg, “is more heatwaves, which means more soil water losses, which means more heatwaves – and where this might end, we cannot say.”

In their paper, the authors suggest that “the semi-arid climate of this region has entered a new regime in which soil moisture no longer mitigates anomalously high air temperature”.

Previous research has indicated how dramatically the area is drying out. A 2015 study, led by Chinese researchers, documented the loss of a quarter of lakes larger than one square kilometre in size on the Plateau, with even greater losses seen in the largest lakes.

“Now we are seeing that it isn’t just large bodies of water that are disappearing,” says a co-author of the new study, Jee-Hoon Jeong, from South Korea’s Chonnam National University. “The water in the soil is vanishing, too.”

This could have a severe impact on the region’s ecosystem, from the smallest plants to the large herbivores like wild sheep and antelope.

Even more troublingly, perhaps, Mongolia and surrounding countries have direct links to global atmospheric circulations – which means local climate shifts could have ramifications for the whole Northern Hemisphere.

In the second study, researchers led by ETH Zurich in Switzerland reveal how Europe’s falling autumn leaves may be signalling more than just a change of seasons.

Since falling leaves have historically been understood as a stress response to cold temperatures and lack of light, scientists have long thought that as the climate warms and summers lengthen, trees will hang onto their leaves for longer.

The new work suggests, however, that leaves are actually beginning to fall earlier, predicting a three-to-six-day advance of senescence (leaf-dropping) over the rest of the century under a business-as-usual climate model. Earlier models predicted a two-to-three-week delay.

It all comes back to the way a tree takes in carbon. Previous research has assumed that the longer leaves are on trees, the more carbon they will draw in and fix, not only boosting their own productivity but also potentially slowing the rate of climate change. However, this new research points to limits on how much carbon a tree can uptake in a single year.

To understand how senescence has changed over the years and build models for the future, the team combined experiments – designed to modify a tree’s carbon uptake – with long-term observations, taken from six dominant tree species across 3855 sites in Central Europe between 1948 and 2015.

Their results demonstrate that elevated carbon dioxide, light levels or temperature in the spring or summer can indeed boost a tree’s productivity, but this leads it to drop its leaves earlier rather than later.

The key is that a tree can only be so productive – it is limited by the ability of its tissues to take up carbon. At some point, its roots or wood will cease to use or store leaf-captured carbon, and then the leaves become costly to keep.

In a supporting commentary in Science, Christine Rollinson, a forest ecologist from Penn State University, US, notes that until recently, most research into this area concentrated solely the fact that atmospheric carbon is increasing.  

“Focusing only on carbon supply is a flawed line of investigation because it entails the assumption that photosynthesis can be sustained indefinitely so long as resources are available,” she says.

This research, she explains, is an important addition to growing body of literature that looks “at the demand side of the carbon cycle, in which growth and productivity are limited by the ability of different tissues, such as roots or stems, to use and store carbon.”

This comes as bad news to those who thought a warming climate may lengthen the growing season – but keep in mind this study only focused on Central Europe, while other forest types around the world may react differently.

This research also throws doubt on the use of widespread re-forestation as a major tool in the fight against climate change.

As Rollinson stresses, the productivity of trees is not infinite. “Forest carbon sequestration alone cannot be expected to keep pace with anthropogenic carbon emissions or climate change. A diverse portfolio of climate actions that include emissions reductions and tree conservation and planting is essential.”

Related reading: Climate change can alter plant genetics