A combined effect of heat and drought limits the growth of Central European silver fir

Abstract

Prolonging drought periods and more extreme heatwaves limit tree species adaptation to environmental changes. Silver fir (Abies alba, Mill.) has been considered one of the species suitable for forest ecosystem adaptation and mitigation strategies in future climate conditions. Here, we present a dendroecological study of the joint effects of droughts and heatwaves on the radial growth of silver fir. We sampled tree-ring width series of 105 dominant individuals collected between 2016 and 2019 across the region of Western Carpathians on the elevational gradient of 200 – 1480 m a.s.l. The results suggest a rapid growth decline (∼1.00 cm².year^-1) of dominant individuals since the mid-1990s compared to the growth release during the 1980′s. A combined approach of absolute and relative temperature thresholds for heatwave quantification revealed a more frequent combination of severe heatwaves and droughts after 2000, especially in the lower and middle parts of the elevation gradient. Furthermore, we found an increasingly negative joint effect of drought and heatwaves on the growth of silver fir. We revealed statistically significant (p < 0.05) growth reduction (>25 %) during years with the joint effect of severe droughts and severe heatwaves on both the upper and lower edges of silver fir’s vertical distribution. Individuals growing at higher elevations experienced a significant growth reduction at a lower absolute temperature threshold (∼30 °C) compared to individuals growing in the lower parts of the Western Carpathians, where this reduction occurred at a higher absolute temperature threshold (∼35 °C). Our findings reveal heatwaves as an important ecological factor that should be considered in future dendroecological studies.