Stable carbon and oxygen isotope ratios in Norway spruce (Picea abies (L.) Karst.) tree rings along an elevation gradient in the Rarau Mts. (Romania)

This study investigates the variations of δ¹³C and δ¹⁸O isotopes in alpha-cellulose extracted from Norway spruce ( Picea abies (L.) Karst.) across an altitudinal gradient in the Eastern Carpathians. Results reveal mean δ¹⁸O values decrease with elevation aligning with the altitude effect and indicating influences from source water composition, precipitation dynamics, and evaporative enrichment. In contrast, mean δ¹³C values do not exhibit consistent altitudinal trends, highlighting the impact of local environmental factors like water stress and light exposure. In addition, the isotopic composition was correlated with environmental variables such as temperature (maximum and minimum) and precipitation to explore physiological responses of Norway spruce to climate conditions. The correlation between δ¹⁸O values and summer precipitation is weakening, while growing season temperature signal is strengthening towards higher elevations suggesting that the role of available water is diminishing, and ambient temperature is enhancing as approaching the timberline. A weakening trend was found for the negative correlation between δ¹³C values and summer precipitation. The correlation between δ¹³C values and precipitation (negative) and temperature (positive) in the growing period for the low elevation stand points to strong drought stress explained by changes in stomatal conductance during dry and warm conditions. The faint negative correlation with temperature for higher stands can be an indirect response to other direct controlling factors, such as relative humidity which is inversely related to temperature.