Unexpected forest mortality following extreme weather events worries scientists worldwide

A new review study in the journal Annual Review of Plant Biology warns that forests that were not considered threatened by extreme climatic events are being strongly affected. The study was carried out by a team of international forestry experts including Jordi Martínez-Vilalta, professor at Universitat Autònoma de Barcelona and CREAF researcher. To do so, the team has analysed the main episodes of tree and forest death occurred around the world in the last two decades in response to extreme climate conditions.

The results have come as a surprise to the team and underline that it is likely that there will be a greater mortality of trees and forests in the coming years, that wasn’t taken into account with the available scientific knowledge. The experts stress that improved data infrastructures can contribute to better research in this field and early detection of these mortality events. Professor Craig Allen, researcher from the Univeristy of New Mexico (USA), lead author of the study and founding father of the global forest mortality map, assures that sudden and unexpected tree deaths are likely to continue to occur, "soon, in a forest near us".

In addition to rising temperatures, climate change also manifests itself in more frequent and more severe extreme weather events. This is the case with droughts, for example, the droughts of the summers of 2018 and 2019 that damaged German forests. However, these extreme events and their impacts are not new phenomena, nor are they limited to Central Europe. Reports of climate-induced tree mortality and large-scale forest damage due to climate extremes date back almost 50 years and have been occurring worldwide.

"Unlike what has been observed in central and northern Europe, where episodes of forest mortality have accelerated in recent years, in Catalonia and the Iberian Peninsula in general the most important episodes occurred in the 1990s; and the problem is that we don't understand why. In this sense, long-term forest condition monitoring data, such as those provided by the DeBosCat project and used in the article, are very useful"

JORDI MARTÍNEZ-VILALTA, CREAF researcher and UAB professor.

The analysis has also taken into account the knowledge of local forestry experts "What concerns us most is that forests that were not considered at risk have been strongly affected," says Henrik Hartmann, leader of the study at the Max Planck Institute for Biogeochemistry in Jena, Germany. The authors document in detail examples from the Amazon, Costa Rica and the United States, but also from Spain, Australia and Germany. "All these forest ecosystems were located in regions where climate extremes were previously considered unlikely," explains Martínez-Vilalta, "or else ecosystems and tree species that were considered drought-tolerant were affected.

The case of Scots pine in Germany

More specific research has also been carried out on the large-scale dieback of native Scots pine. In the study area in Thuringia, but also throughout Germany, this tree species has suffered greatly from the droughts of 2018 and 2019, as happened in Catalonia during the 1990s and the first decade of the 2000s. Mortality rates have soared in recent years. With its deep roots, Scots pine is known to cope well with water shortages. It can extract water from deeper soil layers and compensate for the lack of surface water in the soil, at least in the short and medium term. However, the climatic trend of recent decades has led to desiccation even in deep soil layers, especially in eastern and northern Germany. "This means that the pine tree's survival strategy no longer works, it suffers from drought stress and thus also becomes susceptible to insect infestation, which is often the last straw," Hartmann explains.

Difficulties in early detection

The authors also investigated methodologies for early detection of these mortality events caused by climatic extremes. The results are not very encouraging, remote sensing and vegetation models seem to have limitations. On the one hand, satellites detect changes in forest cover, tree crowns, but do not capture the real extent of forest damage. As for vegetation models, they have always had difficulties in predicting tree mortality. "We are in a situation where current expert knowledge is reaching its limits, because recent climatic events, as well as those expected in the future, are unprecedented. Therefore, we cannot predict the responses of forests," says Dr Ana Bastos, co-author of the study and also group leader at the Max Planck Institute for Biogeochemistry. "We do not yet have adequate tools for the rapid detection of forest damage, causality analysis and prediction".

 "We are in a situation where current expert knowledge is reaching its limits, because recent climate events, as well as those expected in the future, are unprecedented. Therefore, we cannot predict the responses of forests," according to Dr Ana Bastos, co-author of the study and also group leader at the Max Planck Institute for Biogeochemistry.

Mechanisms causing forest dieback could be identified by combining existing datasets and thus contribute to improved vegetation modelling. Interesting datasets exist, such as forest inventories or other monitoring activities, but their spatial and temporal resolution should be improved. Currently, dead trees and possible causes of mortality are often not recorded at all. In addition, the intervals between data collection are often too long and the spatial density of the plots is too low.

One possible solution to fill these data gaps is remote sensing. "But even in this case, the spatial resolution is often too low to detect the death of individual trees," according to Prof. Bastos. To better understand the death of trees, it may be crucial to relate site conditions to the behavior of species during extreme weather conditions. But there is still a long way to go before this happens. "Easy and open access to data and internationally coordinated improvement of forest surveys are key elements," says Hartmann, "and are actually technically easy goals to achieve in today's world". But administrative and legislative obstacles often stand in the way.

"Easy and open access to data and internationally coordinated improvement of forest surveys are key elements," says Hartmann, "and actually technically easy to achieve in today's world. But administrative and legislative obstacles often stand in the way. But administrative and legislative obstacles often stand in the way. 

Article

Climate Change Risks to Global Forest Health: Emergence of Unexpected Events of Elevated Tree Mortality Worldwide

Henrik Hartmann, Ana Bastos, Adrian J. Das, Adriane Esquivel-Muelbert, William M. Hammond, Jordi Martínez-Vilalta, Nate G. McDowell, Jennifer S. Powers, Thomas A.M. Pugh, Katinka X. Ruthrof, Craig D. Allen.