The South China Botanical Garden is one of Guangzhou’s biggest draws.
Featuring rainforest, desert and alpine plants as well as rare palms, ferns and orchids, visitors flock here in summer to cool off under towering trees shading lush lawns. What these visitors might not realise is that the garden, part of the Chinese Academy of Sciences, is also a national centre for botanical education and research.
Liu Hui, a researcher at the garden, studies the mechanisms by which plants adapt and evolve, including how extreme heat affects them. She jokes that she studies how plants die of thirst.
In 2024, Liu and her team published two papers on how periods of extreme heat and drought were affecting the growth and survival of trees.
How trees dry up
The story starts in the summer of 2022, when China suffered a severe heatwave from June to late August.
The national average temperature that June was the highest since monthly records had begun in 1961. Daily temperatures that matched or exceeded all-time highs were recorded at 366 meteorological stations (15.1 per cent of the total). More than a thousand stations (about 43 per cent of the total) recorded temperatures meeting the definition of an “extreme” heat event, according to the China Meteorological Administration.
Liu quickly organised a survey of how plants were coping. She focused on five cities along the Yangtze River (Chengdu, Chongqing, Nanjing, Shanghai and Hangzhou), as 96 per cent of the weather stations that recorded 40 or more over-35°C days were located in its basin.
The team collected samples from 131 woody species and shipped them back to Guangzhou for testing and measurement. There, they determined the ability of around 60 of these species to resist heat and drought.
One of the team members, a post-doctoral student named Ning Qiurui, noted that the Japanese maple, which has thin leaves, suffered the most damage across all locations. Ning co-authored a paper produced by the team demonstrating that more intense heatwaves lead to significantly more leaf damage.
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Plants grow best under stable conditions. An uncertain climate means risks of individual plants dying and populations shrinking.
Liu Hui, researcher, South China Botanical Garden
Liu explains that, while a short heatwave will only damage leaves, sustained heat causes leaves to shrivel up completely as they dry. It is this dryness that kills trees.
“During a drought, the trees dry out and air gets into the xylem [the vessels that transport water and nutrients through the tree], forming bubbles,” says Liu. “When those bubbles pop, they make a sound, as if the tree is crying with thirst. If those bubbles block a vessel, the transportation of water is cut off.” And if enough vessels are blocked, the tree may die of thirst. “The overlapping heatwave and drought speed up the death of the tree.”
These findings tally with those of a glasshouse study published in 2021. Australian scientists had inflicted heat and drought stress on 20 broadleaf evergreen and shrub species.
They determined that the damage caused by dehydration is exacerbated by a concurrent heatwave. High temperatures increase “stomatal conductance” – the size of the pores that take up CO2 from the atmosphere and allow water vapour to evaporate. That means greater water loss, which may accelerate death.
But high temperatures can have drastic consequences even without a concurrent drought. For example, photosynthesis can only occur within a certain temperature range.
If leaves are too hot, their ability to photosynthesise plummets. Trees also rely on their leaves for cooling, via transpiration, when they release water vapour in the plant equivalent of sweating. If high temperatures kill off too many leaves, the tree may not survive.
According to a 2023 paper published in Nature, a tropical forest temperature increase of about 3.9°C could result in the loss of all leaves and a huge tree die-off.
Not just death and droughts
Other, less visible factors are also at work. Changes wrought by global heating will hamper the growth of trees and forests. For example, the spread of pests and plant diseases, animal relocations and more frequent wildfires.
Research published in November by North Arizona University studied aspens over a 20-year period. The prevailing hotter and drier conditions have been hitting younger aspens harder, it found. The study identified “a demographic bottleneck”, whereby older, more resilient aspens are not being joined in the canopy as readily by younger individuals.
Warmer winters also mean insects are breeding more and spreading out of their usual territory, putting huge pressure on forests. In the north-east and mid-west regions of the US, drought associated with climate change enabled millions of spongy moth caterpillars to hatch and devour tree leaves.
Climate change will continue to cause more frequent heatwaves and cold snaps. Combine this with the urban heat island effect, and it becomes very important to rethink which tree species we plant in our cities. Such action has been advocated by Zhang Deshun, a professor at Tongji University’s College of Architecture and Urban Planning in Shanghai.
The Japanese maple is one of the most common trees in Chinese parks. Today’s more frequent high temperatures mean this thin-leafed tree may no longer be suitable there.
Climate adaptation for trees?
Despite growing climatic challenges, trees can show their “smarts” and adapt in the face of higher temperatures and droughts.
“Plants have certain ways to deal with heat,” explains Ning Qiurui. “Trichomes on the back of leaves can reduce the absorption of solar radiation. Trees may drop or reorient their leaves to protect themselves. It also depends on the species of course.”
The thin leaves of the Japanese maple, gingko and white sandalwood make each of them more vulnerable to heat. Trees with thicker leaves, such as the crepe myrtle, osmanthus and loquat, all cope better, found the study by Liu Hui’s team.
Ning believes reasons for this include: the wax and cuticle layers of thick leaves can reflect part of the sunlight; the leaf cells are arranged more closely and have better heat insulation; they generally contain more water and so can more readily combat overheating via transpiration.
While cities can opt to plant heat-tolerant species, Liu points out that nobody manages tree planting in the wild: “Nature itself will retain heat and drought-tolerant species and eliminate those which fail to survive.” That process would usually happen over decades or centuries, she says, but “sudden and intense climate change could be too much for most plants, resulting in huge die-offs over a short period of time.”
And even heat-tolerant plants might not be able to cope with extremes. According to a 2022 paper in Nature Plants, 60-90 per cent of cactus species will be “negatively impacted” by climate change. Extinction will threaten 90 per cent by 2070.
Perhaps the biggest climate change challenge, though, is uncertainty.
“Plants grow best under stable conditions,” says Liu. “An uncertain climate means risks of individual plants dying and populations shrinking.”
Not just trees
Over 1,600 years ago, the Jin dynasty general Huan Wen asked: “If this is what becomes of trees, how can we bear our own fate?” That might be applicable today, as both trees and humans are forced to adapt to the onslaught of extreme weather events.
“The impacts of extreme weather events on plants are overwhelmingly negative,” says Liu. “Sustained high temperatures can disrupt plant phenology. For example, they might flower or grow leaves too early, before the risk of a frost has passed. Perhaps they fail to set fruit as they are out of sync with pollinators.
“Sustained drought could reduce timber output. Combined high temperatures and drought could lead to wildfires. The various types of extreme weather events mean only one thing for agriculture: reduced or failed harvests.”
The impacts of a more extreme climate on plant life ultimately affect us. Research looking at global data between 1964 and 2007 has found national grain production fell by around 9-10 per cent during years of drought or extreme heat.
Another global study found a 1°C increase in temperature causes wheat yields to fall by 2.9 per cent, rice by 5.6 per cent, maize by 7.1 per cent, and soy by 10.6 per cent. For China, every 1°C increase in temperature means a 2.6 per cent fall in yields of the country’s three main crops – maize, wheat and rice – found a 2019 meta-analysis.
Globally, the eight years from 2015 to 2022 were the hottest eight on record. Last year was the hottest since records began in 1850. Yet it may well prove to be the coolest we will see for many years to come.
Ambitious emission cuts driven by governments, businesses and individuals can still make a big impact on how warm the world gets. Beyond this, there are adaptation measures. China is already taking steps to adapt: in the National Climate Change Adaptation Strategy (2035), the Ministry of Ecology and Environment called for more disaster and climate resilience work in the agricultural sector, to ensure food security.
Liu tells Dialogue Earth that over the past two years, she has been in contact with over 180 researchers working on heat and cold tolerance in plants. She hopes to feed in data about typhoons, floods and extreme cold. Ultimately, this could create a national network for monitoring damage to plants from extreme weather, along with an early warning and reporting system.
But the researchers do not have the necessary time and resources. Without external support, little progress is likely. And so, all the trees can do is wait.
This article was originally published on Dialogue Earth under a Creative Commons licence.
