Detection of early warning signals (EWSs) for forest declines is a critical yet challenging in forest ecology and management. Although the EWSs have been investigated from the perspective of external stresses, changes in tree resistance in stresses before forest declines have received much less attention. In this study, we utilized tree ring-width data from 346 juniper trees in the Qilian Mountains to calculate tree resistance in stresses during non-decline period of forest and to explore the temporal relationships between the occurrence of forest declines and changes in resistance of trees. The forest declines, characterized by sustained slow growth rates, were identified independently. The results showed that, there was a decrease in tree resistance during the 12 years before forest declines. This phenomenon was particularly common at the study sites. Resistance of trees before forest declines was positively correlated with tree growth during forest declines, which implying that lower pre-decline resistance was associated with more severe forest decline. These observations suggest that decreasing resistance can be used as an EWS of forest declines. Our results provide insights into the relationships between changes in ecological resilience and forest stability and are useful for monitoring or predicting changes in forest health under future climate change.