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Forests around the world are turning brown during summer

Escalating summer heat and droughts are impacting forests around the world, with trees browning early and some even experiencing dieback. Scientists from ETH Zurich and the WSL reveal how extraordinary weather conditions spanning multiple years are causing forest browning.

It seemed as if autumn had come in July. During the summer of 2018, hikers traversing Swiss or German forests could visibly observe the effects of hot weather on trees in central Europe. Spruces and beech trees, especially, wilted ahead of time, their leaves and needles turning brown, and entire sections of forests were consistently stressed. In the Mediterranean region, such widespread events have recurred several times since 2003.

The research team has now conducted a systematic analysis of all low-greenness occurrences in temperate and Mediterranean European forests over a 21-year period (2002-2022). The study is published in the journal Biogeoscience.

To investigate forest browning throughout Europe, the experts utilized high-resolution satellite data to pinpoint large-scale instances of diminished forest greenness during summer months. Decreased greenness signals reduced vigor and stress in forests, and serves as an indicator of forest dieback.

The results corroborate earlier observations that summer browning has become more widespread across Europe. Central European (temperate) forests have experienced particularly significant browning in recent years, while the Mediterranean region encountered major events as early as the 2000s.

The researchers also measured the impact of the record-breaking summer of 2022 on European forests for the first time. During the hottest summer ever recorded, Europe witnessed its most extensive browning to date, encompassing 37 percent of temperate and Mediterranean forest areas. 

According to study lead author Mauro Hermann, a doctoral student in Atmospheric Dynamics under ETH Professor Heini Wernli, this level of browning was “far greater than any other event in the past two decades.”

The team’s original objective was not what they ultimately discovered. “Our aim was to comprehend how weather impacts forests over vast regions across multiple seasons,” explained Professor Wernli. “The relationship between forests and weather is much more intricate than it seems at first.”

Hermann noted that not every dry spell, even if intense and prolonged, immediately causes forests to brown, referring to the “legacy effect” observed in forests for several years. The ability of trees to withstand heat and drought relies not only on current weather conditions but also on those from previous months or years.

This particular aspect motivated researchers to delve deeper into the meteorological history of low-greenness events, with the goal of identifying specific weather patterns that preceded multiple investigated occurrences.

The researchers indeed identified distinct weather patterns that emerged long before the events, acting as precursors to browning with specific characteristics for both central Europe and the Mediterranean region. “Generally, we observe that periods of low precipitation occur unusually frequently two or three years before the events,”explained  Hermann.

Noticeable meteorological precursors in both regions include increased dry spells with a significant precipitation deficit for at least two years before the events. In the Mediterranean region, such dry periods may even extend back three years. 

Additional indicators involve recurring periods of higher temperatures for a minimum of two years in the temperate zone. “Before observing low forest greenness in central Europe, we typically witnessed two consecutive dry, hot summers,” said Hermann.

The findings are supported by examples from the 21 years of data. The summer of 2003, which saw intense heat and drought across much of Europe, left little large-scale impact on forest coloration. However, since 2018, Europe has faced recurring widespread droughts and elevated temperatures, resulting in several episodes of extensive browning.

Hermann highlights a key discovery of the study: “The identified weather signals indicate the effects of drought in previous years, thereby reinforcing the legacy effect.” This implies that a single hot and dry summer typically does not immediately cause a decline in forest greenness but can contribute to future low-greenness events with a delayed response.

Can these precursor signals enable scientists to foresee drought stress and forest browning in the future? The researchers approach this question with caution: “We have analyzed events in hindsight, but we haven’t investigated their predictability,” explained Hermann. The challenge of forecasting is further complicated by the fact that drought stress indirectly promotes low forest greenness through the facilitation of bark beetle and fungal infestations as well as forest fires.

Thomas Wohlgemuth, leader of the Forest Dynamics research unit at WSL and co-author of the study, asserts that predicting based solely on weather data is implausible. However, the forest ecologist does believe that the newly gained understanding of the process will contribute to improved forest models and aid in preventative measures by forest management. “Closely monitoring weather conditions across multiple seasons could offer valuable insights into the likelihood of early leaf discoloration occurring the following summer.”

Escalating summer heat will continue to have several significant impacts on forests, affecting their health, structure, and overall functioning:

  1. Drought stress: Rising temperatures can cause increased evaporation, leading to more frequent and severe droughts. Drought stress impairs the ability of trees to absorb water and nutrients from the soil, resulting in reduced growth and increased vulnerability to pests and diseases.
  2. Browning and dieback: Prolonged periods of heat and drought can cause trees to wilt prematurely, with leaves and needles turning brown. In extreme cases, this can lead to forest dieback, where large areas of forest experience decline or death.
  3. Shifts in species distribution: As temperatures rise, some tree species may become less suited to their current environments, leading to shifts in their distribution. Tree species that are more adapted to warmer climates may replace those that are less tolerant of heat.
  4. Pest and disease outbreaks: Warmer temperatures can provide more favorable conditions for pests and diseases, such as bark beetles and fungal infections, leading to more frequent outbreaks that can further weaken trees and contribute to forest decline.
  5. Increased risk of wildfires: Escalating summer heat can lead to drier forest conditions, increasing the risk of wildfires. These fires not only destroy vast areas of forest but also release large amounts of carbon dioxide, contributing to climate change.
  6. Changes in forest productivity: Higher temperatures can lead to longer growing seasons in some regions, potentially increasing forest productivity. However, this may be counteracted by the negative impacts of drought, pests, and diseases.
  7. Alteration of ecosystem services: Forests provide essential ecosystem services such as carbon sequestration, water regulation, and habitat provision. The impacts of escalating summer heat on forest health and structure can lead to changes in these services, with potential consequences for biodiversity and human well-being.

Adaptive forest management strategies, such as planting more heat-tolerant tree species, monitoring pest and disease outbreaks, and implementing fire prevention measures, will be crucial for minimizing the impacts of escalating summer heat on forests.


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