Climate change is extending the growing season in European forests, but this effect will not be sufficient to offset the negative impacts of increasing drought stress. This is a conclusion of an international research team led by Jan Tumajer from the Department of Physical Geography and Geoecology, Faculty of Science, Charles University, which analyzed more than 2,000 tree-ring width chronologies across Central, Eastern, and Southeastern Europe. The study, recently published in the prestigious journal Nature Communications, shows that while a longer growing season may promote tree growth until mid-century, the negative effects of summer drought will outweigh the benefits of the extended growing season in drier regions after the 2050s.
The researchers used an empirical model simulating wood formation based on temperature, precipitation, and photoperiod, and validated its outputs against empirical data, including tree-ring width measurements over the last sixty years, dendrometer records (instruments attached to tree stems monitoring hourly growth throughout the growing season), and satellite-derived vegetation indices. Model projections under four climate change scenarios revealed contrasting growth patterns towards the year 2100: while growth in cool and humid forests, particularly at higher elevations, is projected to increase due to extended growing seasons, tree-ring widths in drought-prone lowlands are expected to decline. The highest vulnerability was observed in low-elevation stands of both broadleaved and coniferous species, where the growing season will lengthen, but the growth will become seasonally interrupted by recurrent summer droughts.
“Until about the 2040s–2050s, the extension of the growing season may still compensate for growth losses caused by summer drought. In the second half of the century, however, the situation will worsen, especially under scenarios of high greenhouse gas emissions,” says Jan Tumajer, the study’s lead author. In the most pessimistic scenario, mean annual tree growth at dry sites is projected to decrease by up to one third under average climatic conditions expected for the end of the 21st century, and by as much as 70 % during exceptionally warm and dry years. In contrast, humid mountain forests may temporarily benefit from warming thanks to the earlier onset of growth in spring and prolonged cambial activity.
“These shifts may have major consequences not only for timber production, but also for the carbon cycle and hydrological functioning of forests and landscapes. Our results indicate that the ability of tree species to adjust their intra-annual growth dynamics to seasonal moisture availability will be critical for their survival in a warmer climate,” Tumajer summarizes. The study further suggests that maintaining non-declining growth in dry forests with the current species composition will only be possible under low-emission scenarios.
The findings are thus essential for forest management and conservation planning. If current trends in global climate change continue, a longer growing season alone will not compensate for growth loss induced by summer drought. Without substantial adaptation measures – including shifts in tree species composition toward taxa tolerant to seasonal drought – large areas of European forests might experience a decline in growth and in their ability to provide key ecological and economic functions.
Read the original article here
Photos by: Lucie Živsová, Jelena Lange, Jan Tumajer
„Geohazards“ (officially entitled “Natural and Anthropogenic Georisks”) is a project awarded within the Excellent Research call of the Program Johannes Amos Comenius (P JAC) funded by the Czech Ministry of Education, Youth and Sports and co-funded by the European Union (Reg.No.: CZ.02.01.01/00/22_008/0004605).
