For years, researchers have blamed droughts, storms and insect infestations for the worsening condition of European forests. While those threats remain important, a new study suggests another, less obvious factor may be contributing to the problem: unusually favourable spring weather.
The research, led by scientists from France's Laboratoire des Sciences du Climat et de l’Environnement and the Swiss Federal Research Institute WSL, found that warm, wet springs can increase the likelihood of trees dying later in the same year.
The team analysed information from the French Forest Inventory collected between 2015 and 2023, examining the factors associated with increasing rates of premature tree mortality.
A growing problem across Europe
Many European forests have shown signs of decline over the past two decades. Beech trees have been losing their leaves unusually early, spruce forests have suffered severe bark beetle attacks and storms have caused greater damage to already weakened woodland.
Using machine-learning techniques and climate models, the researchers found that unusual shifts in seasonal weather patterns are increasingly influencing whether trees survive.
What surprised the scientists was that conditions traditionally associated with healthy growth sometimes appeared to increase the risk of later mortality.
When good growing conditions become a trap
Trees typically thrive during warm, wet springs. Such conditions encourage rapid growth and abundant foliage.
However, that growth comes at a cost.
According to the researchers, vigorous spring development increases a tree's water consumption early in the year. If rainfall then declines during summer, trees may enter drought conditions faster because both the trees and the soil have already used a significant portion of available water resources.
Tall species such as silver fir were particularly affected by this pattern.
Lead researcher Pascal Schneider explained that trees experiencing strong spring growth can become more vulnerable once dry conditions arrive, as their water requirements have already increased substantially.
At the same time, wet conditions can create favourable environments for fungal pathogens, adding another layer of stress.
As a result, a tree's decline may begin months before any visible symptoms appear.
More than just drought
The study suggests that tree deaths are rarely caused by a single event.
Instead, several climate-related pressures often combine over the course of a year.
A wet spring may be followed by drought. A mild winter can allow larger numbers of pests to survive. Earlier spring growth may expose young leaves to damaging late frosts.
Researchers found that these overlapping stresses can be more harmful than one isolated extreme event.
The findings also show that not all species respond in the same way. Some trees are more vulnerable to prolonged water shortages, while others are more susceptible to intense but short-lived droughts or disease outbreaks.
Even summer droughts do not affect every forest equally.
What it means for forest management
The study's authors argue that forest managers will need to rethink how they prepare for a warmer and less predictable climate.
Among the measures suggested are planting more drought-tolerant species, particularly varieties adapted to warmer southern conditions, and reducing competition for water by thinning forests more aggressively.
Removing some large, water-intensive trees earlier than usual could help remaining trees better withstand prolonged dry periods.
The researchers stress that future forestry strategies must account not only for extreme weather events but also for apparently favourable seasons that can unintentionally increase vulnerability later in the year.
Lessons from half a million trees
The conclusions are based on data from the French Forest Inventory, one of Europe's largest forest-monitoring programmes, covering around 500,000 trees from 52 species.
Because France encompasses a wide range of climates, from Mediterranean landscapes to Alpine environments, the researchers believe the results have implications for forests across much of Europe.
Their message is a simple but important one: a forest that looks healthy in spring may already be accumulating the stresses that threaten it by summer.
The study was published in Nature Communications.
Source: earth.com
