Droughts can actually benefit trees in certain circumstances

It’s generally assumed that drought spells disaster for trees. But in a surprising revelation, recent research shows that certain trees may indeed benefit from a record-breaking drought. This curious phenomenon, which defies expectations, is the subject of a study in the journal Global Change Biology.

A team of scientists led by Professor Joan Dudney from UC Santa Barbara delved into how whitebark pine, an endangered tree species, responded to droughts over the past century. This particular tree grows in harsh, chilly climates, often found at high altitudes and latitudes. 

Professor Dudney’s team discovered that in these hostile conditions, drought could, quite unexpectedly, provide an advantage to the trees by extending their growing season.

Important insights will help guide climate change initiatives

What does this mean for the future? This research offers crucial insight into the locations most vulnerable to the threats of extreme drought and how different species and ecosystems will likely react to climate change.

Trees encounter several obstacles to their growth. Cold temperatures, insufficient sunlight, and lack of water and nutrients can all hinder a tree’s growth. The distinction between energy-limited and water-limited systems, where trees struggle to grow due to either freezing conditions or drought, respectively, is of particular importance.

Over the years, numerous tree species have adapted to these harsh conditions, showing broadly similar responses. When faced with extreme weather, trees typically scale back on growth-related activities, such as photosynthesis and nutrient uptake, essentially going into survival mode until conditions improve.

“Intriguingly, the shift from energy-limited to water-limited growth can result in quite unexpected outcomes,” said Professor Dudney. “In severe, energy-limited climates, extreme drought can actually amplify growth and productivity, even in a place like California.”

How the study was conducted

Dudney’s team conducted thorough research by collecting 800 tree core samples from whitebark pine across the Sierra Nevada. They compared the tree rings to historical climate records from 1900 to 2018, a period encompassing three extreme droughts: 1959-61, 1976-77, and 2012-15. 

Their meticulous record-keeping highlighted where tree growth and temperature shared a positive relationship, and where it was negative.

What the researchers learned

The findings were startling. The experts observed a distinct change in growth during periods of drought when the average maximum temperature hovered around 8.4° Celsius (47.1° Fahrenheit) from October to May. Above this temperature threshold, extreme drought stunted growth and photosynthesis. But below it, drought seemed to spur the trees to grow more.

“The key question is, ‘how long is the growing season?'” said Dudney. Trees in colder climates with heavier snowpack usually face shorter growing seasons, thus limiting their growth. Even during intense droughts, many trees growing in these extreme environments did not exhibit severe water stress.

This discovery surprised the research team, several of whom had witnessed first-hand the unprecedented tree death that occurred at slightly lower elevations in the Sierra Nevada.

Professor Dudney wondered whether the impact of drought was confined to the main trunk or if it affected the entire tree. To answer this question, the team turned to chemistry. 

Uncertain future for the whitebark pine

By examining the different isotopes of carbon found in the tree’s tissues, such as leaves and needles, they could estimate the level of water stress the tree experienced during droughts. This proved especially useful, as the whitebark pine retains its needles for approximately eight years, providing a wealth of data across both drought and non-drought years.

What they found was that the entire tree, from trunk growth and needle growth to photosynthesis and nutrient cycling, was affected by the tipping point between water-limited and energy-limited systems.

The future of whitebark pine is fraught with uncertainty. The species, recently categorized as threatened under the Endangered Species Act, confronts numerous challenges, including disease, pine beetle infestation, and disturbances from altered fire regimes. 

This research underscores that drought and warming are likely to exacerbate these threats in water-limited regions. However, in energy-limited environments, warming could actually aid in their growth.

“This research can guide us towards more precise conservation strategies to help restore this historically widespread tree species,” Dudney stated, highlighting the critical relevance of their findings. The habitat of whitebark pine spans an impressive region, from California to British Columbia, and east to Wyoming.

Research has global implications

The implications of this research extend far beyond just the whitebark pine. Approximately 21% of the world’s forests are energy-limited, with an even higher percentage classified as water-limited. Therefore, it’s likely that transitions between these two climate regimes occur globally.

This transition also appears to influence nitrogen cycling. Trees in water-limited environments seemed to rely less on symbiotic fungi for nitrogen, which is indispensable for tree growth in harsh, energy-limited environments.

“Droughts are causing widespread tree mortality worldwide, which can accelerate global warming,” said Professor Dudney.

Understanding the multifaceted ways trees react to drought is a critical part of forecasting which ecosystems are most vulnerable to climate change. This knowledge will also guide us in crafting more focused strategies to protect our invaluable forests. After all, understanding how some trees grow despite adversity is a lesson in survival – not just for the trees, but for our planet too.

More about climate change and trees

Climate change has significant impacts on trees and forests around the world. With rising temperatures and shifting precipitation patterns, trees are being forced to adapt to new and challenging conditions, while forests face threats of altered habitats, biodiversity loss, and changes in carbon storage capacity.

Rising temperatures are one of the most direct effects of climate change. While it might seem that warmer weather would be beneficial for plant growth, it’s not that simple. 

Many trees are adapted to specific temperature ranges, and a sudden shift can result in stress and even death for those that can’t adapt quickly enough. This is particularly true for species in high altitudes and latitudes, which are adapted to cold environments and may struggle with even a slight increase in temperature.

Additionally, warmer temperatures can lead to more frequent and severe heatwaves, which can cause water stress and increase susceptibility to pests and diseases.

Changes in precipitation patterns are another major impact of climate change on trees and forests. Some areas are experiencing increased rainfall, leading to flooding and waterlogged soils, while others are becoming drier, leading to drought. 

Both situations can be detrimental to trees. Flooding can suffocate tree roots and promote disease, while drought can reduce growth and increase mortality. Drought can also make trees more susceptible to pests and wildfires.

Pests and diseases broaden their range

The impact of climate change on pests and diseases is a growing concern. Warmer temperatures can allow pests to expand their range and increase their population size. It can also accelerate the lifecycle of pests and diseases, allowing them to reproduce more quickly. This, coupled with trees weakened by heat and drought stress, can lead to outbreaks that can devastate entire forests.

Wildfires, too, are becoming more frequent and severe due to climate change. Warmer, drier conditions increase the risk of fire, and once a fire starts, it can spread more quickly and burn more intensely. 

Forests that are adapted to occasional fires may struggle to recover from these intense burns, leading to a loss of tree cover and biodiversity.

Lastly, climate change can also impact the role of forests as carbon sinks. Trees absorb carbon dioxide from the atmosphere as they grow, helping to mitigate climate change. 

But as forests are stressed, damaged, and destroyed by the impacts of climate change, their capacity to store carbon is reduced. This can create a feedback loop, where the loss of forests exacerbates climate change, leading to further forest loss.

In conclusion, climate change poses a major threat to trees and forests worldwide. However, understanding these impacts and how different tree species and forest ecosystems respond to them is crucial for developing strategies to conserve and manage our forests in a changing climate.

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