• Extreme heatwaves in the U.S. and California are causing many trees to die, even native ones that are accustomed to local climates.
  • Since these dead, dry conditions increase the fire risk, fire fighters have been closely watching these areas.
  • During droughts, trees cannot undergo vascular water transport (getting water from their roots to their leaves); a process that is essential for their survival.
  • We need to find ways to reduce greenhouse gas emissions, or this problem is likely to worsen, write two experts.

Like humans, trees need water to survive on hot, dry days, and they can survive for only short times under extreme heat and dry conditions.

During prolonged droughts and extreme heat waves like the Western U.S. is experiencing, even native trees that are accustomed to the local climate can start to die.

Central and northern Arizona have been witnessing this in recent months. A long-running drought and resulting water stress have contributed to the die-off of as many as 30% of the junipers there, according to the U.S. Forest Service. In California, over 129 million trees died as a consequence of a severe drought in the last decade, leaving highly flammable dry wood that can fuel future wildfires.

Firefighters are now closely watching these and other areas with dead or dying trees as another extremely dry year heightens the fire risk.

this chart shows that more than 90% of the U.S. West was in moderate drought or worse by late June 2021. More than a quarter was in exceptional drought, the highest level, indicating widespread risk of crop loss, fire and water shortages.
Dangerous droughts are on the rise.
Image: Chart: The Conversation/CC-BY-ND Source: Drought Monitor

For the full interactive chart and more data information, click here.

What happens to trees during droughts?

Trees survive by moving water from their roots to their leaves, a process known as vascular water transport.

Water moves through small cylindrical conduits, called tracheids or vessels, that are all connected. Drought disrupts the water transport by reducing the amount of water available for the tree. As moisture in the air and soil decline, air bubbles can form in the vascular system of plants, creating embolisms that block the water’s flow.

The less water that is available for trees during dry and hot periods, the higher the chances of embolisms forming in those water conduits. If a tree can’t get water to its leaves, it can’t survive.

this is a labelled diagram of a dyed cross section of a ponderosa pine sapling
A dyed cross section of a ponderosa pine sapling shows the water transport tissue and conduits.
Image: Raquel Partelli Feltrin

Some species are more resistant to embolisms than others. This is why more pinon pines died in the Southwest during the drought in the early 2000s than juniper – juniper are much more resistant.

Drought stress also weakens trees, leaving them susceptible to bark beetle infestations. During the 2012-2015 drought in the Sierra Nevada, nearly 90% of the ponderosa pines died, primarily due to infestations of western pine beetles.

Fire damage + drought also weakens trees

Although fire is beneficial for fire-prone forests to control their density and maintain their health, our research shows that trees under drought stress are more likely to die from fires. During droughts, trees have less water for insulation and cooling against fires. They may also reduce their production of carbohydrates – tree food – during droughts, which leaves them weaker, making it harder for them to recover from fire damage.

Trees that suffer trunk damage in a fire are also less likely to survive in the following years if drought follows. When trees have fire scars, their vascular conduits tend to be less functional for water transport around those scars. Traumatic damage to the vascular tissue can also decrease their resistance to embolisms.

So, burned trees are more likely to die from drought; and trees in drought are more likely to die from fire.

this graoh shows the amount of US wildland area burned each year
The burning of wildland areas creates a number of long term issues.
Image: The Conversation/CC-BY-ND Source: National Interagency Coordination Center

What does this mean for future forests?

Trees in Western forests have been dying at an alarming rate over the past two decades due to droughts, high temperatures, pests and fires. As continuing greenhouse gas emissions warm the planet and drive moisture loss, increasing the frequency, duration and intensity of droughts, research shows the U.S. and much of the world will likely witness more widespread tree deaths.

The impact that changing drought and fire regimes will have on forests farther in the future is still somewhat unclear, but several observations may offer some insight.

There is evidence of a transition from forests to shrublands or grasslands in parts of the Western U.S. Frequent burning in the same area can reinforce this transition. When drought or fire alone kills some of the trees, the forests often regenerate, but how long it will take for forests to recover to a pre-fire or pre-drought condition after a large-scale die-off or severe fire is unknown.

What’s the World Economic Forum doing about climate change?

Climate change poses an urgent threat demanding decisive action. Communities around the world are already experiencing increased climate impacts, from droughts to floods to rising seas. The World Economic Forum's Global Risks Report continues to rank these environmental threats at the top of the list.

To limit global temperature rise to well below 2°C and as close as possible to 1.5°C above pre-industrial levels, it is essential that businesses, policy-makers, and civil society advance comprehensive near- and long-term climate actions in line with the goals of the Paris Agreement on climate change.

The World Economic Forum's Climate Initiative supports the scaling and acceleration of global climate action through public and private-sector collaboration. The Initiative works across several workstreams to develop and implement inclusive and ambitious solutions.

This includes the Alliance of CEO Climate Leaders, a global network of business leaders from various industries developing cost-effective solutions to transitioning to a low-carbon, climate-resilient economy. CEOs use their position and influence with policy-makers and corporate partners to accelerate the transition and realize the economic benefits of delivering a safer climate.

Contact us to get involved.

In the past decade, the Western U.S. has witnessed its most severe droughts in over 1,000 years, including in the Southwest and California. A recent study found subalpine forests in the central Rockies are more fire-prone now than they have been in at least 2,000 years.

If there is no change in greenhouse gas emissions, temperatures will continue to increase, and severe drought stress and fire danger days will rise as a result.