Missing rings of Qilian juniper associated with drought on the Northeastern Tibetan Plateau, China

A missing ring occurs when the radial growth of a tree is disturbed when the plant is under stress (e.g., extreme drought, fire, insect attack, frost) as a result of inhibition of cambial activities. Missing rings have been detected in several tree species in different parts of the world, and this phenomenon may be an indicator of imminent tree death. However, we still lack an in-depth understanding of the characteristics and causes of missing rings and their relationship with tree mortality. In this study, we examined missing ring occurrence in Qilian juniper (Juniperus przewalskii Kom.), one of the most long-lived tree species in China, growing in an extremely harsh environment on the Northeastern Tibetan Plateau. The results showed that the missing ring rate was sensitive to drought events during the period 1500–2000 AD, with extreme values of missing ring rate occurring in certain drought years, such as 1665, 1687, 1715, 1824, 1918, etc. In addition, climate warming in recent decades did not lead to an increase in the missing ring rate of Qilian juniper, which may be related to the improving regional water conditions on the plateau. The missing ring rate of Qilian juniper was related to the age of the tree, i.e., it increased first in younger trees and then decreased, with the highest missing ring rate occurring in trees about 800-years-old. Furthermore, trees that eventually died had a higher rate of missing rings before their death than those that survived, suggesting that missing rings have the potential to predict tree mortality. However, the relationship between tree mortality and missing ring rate is a very complex issue and requires further study.