What makes forests brown in summer

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What makes forests brown in summer
Forests throughout Europe suffer increasingly from heat and drought in summer - in some years trees brown prematurely and sometimes even die completely. Researchers at ETH Zurich and WSL show how extreme weather conditions favor the browning of forests over several years.

As if it were already autumn in July. Anyone walking through Swiss or German forests in the summer of 2018 could literally see how the hot, dry weather in Central Europe was taking its toll on the trees. Spruces and beeches in particular wilted prematurely, leaves and needles turned brown, and entire stands were under constant stress. Since then, further summers have followed in which forests throughout Europe turned brown.

Researchers at ETH Zurich have now systematically investigated all events of such premature browning in Europe’s temperate and Mediterranean forests over the last 21 years (2002 to 2022). Their study, which they wrote together with colleagues from the WSL Swiss Federal Institute for Forest, Snow and Landscape Research, has just been published in the journal external page Biogeosciences call_made.

Browning record last summer

To study forest browning across Europe, researchers used high-resolution satellite data to identify events of widespread reduced forest green during the summer. Reduced green is a sign of reduced forest vitality and stress, and is also used as an indicator of forest dieback.

The results confirm previous observations: Summer browning has spread across Europe. Especially the central European (temperate) forests have browned strongly and over a wide area in the last five years. In the Mediterranean region, such large-scale phenomena have already occurred several times since 2003.

In their analysis, the researchers also quantified for the first time the record summer of 2022 and its consequences for European forests. Now it turns out that the hottest summer on record also saw the most extensive browning in Europe, affecting 37 percent of temperate and Mediterranean forest regions - "far more than any other event in the past two decades," says Mauro Hermann, a doctoral student in atmospheric dynamics with ETH professor Heini Wernli and lead author of the study.

Wanted: Common antecedents

However, the ETH-WSL team was actually pursuing a different goal. "We wanted to understand how weather affects forests over large areas over several seasons," explains ETH professor Heini Wernli, who led the study. That drought plays a central role in this is obvious. "However, the relationship between forests and weather is far more complex than it might appear at first glance," reveals the professor of atmospheric dynamics.

"Not every dry period - even if it is intense and prolonged - immediately turns forests brown," adds Hermann, referring to the so-called legacy effect that has been observable in our forests for a few years. How well trees survive heat and drought depends not only on the current weather conditions, but also on those in the months or years before.

For this reason, the researchers also analyzed the meteorological history of the browning events. The goal: to find characteristic weather patterns that preceded many events with premature browning.

Specific precursors identified

In fact, the researchers found characteristic weather signals long before the events - a kind of precursor to browning, with some specific features for central Europe and the Mediterranean region. "In general, we see that periods of low precipitation over two to three years prior to the events are unusually common," Hermann explains.

Increased dry periods with a clear precipitation deficit over at least two preceding years are the most conspicuous meteorological precursor in both zones - in the Mediterranean region, frequent dry periods even go back as far as three years. Other signals are elevated temperatures for at least two years in the temperate zone. "In the case of forest browning in central Europe, we thus usually observed at least two unusually dry and hot summers in a row," Hermann notes.

Burden of previous years confirmed

Examples from the 21 years studied illustrate this: The summer of 2003, which was very hot and dry in large parts of Europe, left hardly any large-scale traces in the color of the forests. Since 2018, however, large-scale drought and heat have occurred repeatedly, so that extensive browning even occurred several times.

"A particular finding of the study is that the weather signals identified are indicative of the consequences of previous years’ drought, thus supporting the legacy effect," Hermann notes. That is, a single hot and dry summer usually does not lead to browning immediately, but may contribute to future browning in a delayed manner.

No predictions, but indications

Do the precursor signals allow us to predict drought stress and browning in forests? Here, the researchers are cautious: "We analyzed events retrospectively - but we did not investigate predictability," Hermann says. The fact that drought stress also indirectly promotes summer browning of trees by encouraging bark beetle and fungal infestations and forest fires makes predictions generally difficult.

Thomas Wohlgemuth, head of the Forest Dynamics Research Unit at WSL and co-author of the study, also considers a prediction from weather data alone to be unrealistic. However, the forest ecologist thinks that the process understanding gained will lead to better forest models and thus be useful for "prevention through silvicultural measures." "Targeted monitoring of weather patterns over several seasons could provide valuable information on whether prematurely discolored forests are likely in the following summer," the forest ecologist says.
Michael Keller