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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. [ABSTRACT FROM AUTHOR]

• Juniper trees recover faster in dry than in wet conditions on the Tibetan Plateau. • Growth differences in individual trees are crucial for assessment of recovery time. • Long-term adaption of trees to dry conditions may improve the capacity of recovery. The recovery time of trees after extreme events may vary over space and among individuals, affecting the accuracy of predicting forest vulnerability to climate change. However, it remains unclear how different regional moisture conditions modulate the recovery time of trees in the long term. Here, we address this problem by examining 747 juniper trees at 24 sites on the Tibetan Plateau (TP). Tree-growth recovery time after extreme events was defined when reduced tree-ring indices returned to one, and site-level recovery time was assessed by calculating the time required to reach two levels of recovery, that is, 50% and 90% of trees recovered. We found that, in general, 50% of trees have recovered within three years following extreme events in the past two centuries; it took approximately 16 years for 90% of trees to recover. The recovery time for juniper trees increased from the northeastern TP to the southern TP and showed a significant positive correlation with the local latitudinal gradient of moisture conditions, particularly for the timespan at which 90% of trees recovered. Our study demonstrates that within certain moisture conditions, juniper trees on the TP recover faster after extreme events in dryer rather than wetter conditions, suggesting that juniper trees long established in dry conditions may have developed a greater ability to adapt to extremes than those in wet conditions. These findings highlight the importance of considering different performances of individuals in evaluating recovery time. In addition, the long-term influence of moisture conditions on tree recovery should be considered in the future when assessing the vulnerability of this high-elevation forest ecosystem. [ABSTRACT FROM AUTHOR]

• Trees at mid-latitudes of their distribution show greater sensitivity to drought. • Water deficit mainly controls tree resistance and spatial sensitivity to drought. • At wetter sites, higher drought frequency trains tree memory response to drought. • At drier sites, higher drought frequency causes substantial damage to junipers. The resistance of forests to extreme climatic events such as drought shapes their sensitivity to future extreme events in space and time. To a large extent, the ability of trees to learn from prior droughts explains how trees adjust their sensitivity to water deficit. We use tree-ring width data collected from 1565 juniper trees (Juniperus prezwalskii and Juniperus tibetica) across 57 sites on the Tibetan Plateau to model tree resistance to water deficit and to map drought sensitivity across the species' distribution. We test the effect of both the frequency and severity of drought on the drought memory of trees. We find that trees at mid-latitudes and in the northwestern part of the juniper distribution range exhibit higher drought sensitivity. Water deficit is the main factor controlling tree resistance and thus affects spatial sensitivity to drought. At wetter sites, higher drought frequency enhances tree adaptability through ecological memory, thereby promoting tree resistance and decreasing tree sensitivity to extreme drought events. At drier sites, higher drought frequency causes junipers' growth decline that is not beneficial for tolerance to extreme drought events. Regional drought conditions and the frequency of pre-droughts affect tree resistance and sensitivity to extreme drought events. This explains the spatial pattern of drought risk for juniper forests on the Tibetan Plateau, and helps us to better understand the vulnerability of this high-elevation forest ecosystem. Such information is important for maintaining forest health and informing the sustainable development of the Tibetan Plateau under a changing climate. [ABSTRACT FROM AUTHOR]

• Past tree-growth reductions were identified from sustained narrow ring widths. • Growth reduction occurred intermittently in Tibetan juniper forests in the past. • Synchronized tree-growth reduction reflected episodes of decreased forest health. Recent forest declines have been reported at many sites across the world and attributed largely to global-warming induced droughts. Occurrence of forest declines before the global warming is not well understood, especially with regard to its long-term characteristics. Here, we use tree-ring data to detect periods of growth reduction in 1429 juniper trees from 48 sites on the Tibetan Plateau (TP). We find that tree-growth reduction has occurred intermittently and dispersedly over the TP in the past 350 years. The growth reductions among trees could be synchronized at site level in different time, reflecting episodes of decreased forest health. Tree-growth reduction has been alleviated since the 1980s; sites that do show growth reduction events in recent decades are located at higher elevations and are more sensitive to winter mean temperature. We conclude that the pervasive tree-growth reductions may be related to irregular natural disturbances than climatic anomalies. The state of forest health can be estimated by identifying episodes of synchronous tree-growth reduction. Our findings help account for historical growth reduction in the assessment of forest health risks under future climate change. [ABSTRACT FROM AUTHOR]

• Juniperus tibetica and Abies spectabilis trees showed different resilience to drought extremes in the past two hundred years. • Trees' iso/anisohydric strategies of stomatal regulation might play a role in their resilience against droughts. • Abies spectabilis showed higher resistance than Juniperus tibetica in the twentieth century. With increased frequency and intensity of drought occurrence in the changing climate, the drought resilience of forest trees is of widespread interest. Particularly, it is not clear as to how the resilience differs between tree species and whether or not such resilience changes over time. Understanding tree resilience to drought requires observations not only from recent events but also from the historical past, information of which is usually hardly available. Here we defined historical drought based on isotope data and compared drought resilience in Juniperus tibetica and Abies spectabilis forest in the central Himalayas in five extreme droughts during the past two centuries. We found that juniper trees had a stronger resistance than fir trees in the three extreme droughts in the nineteenth century but this pattern reversed in the two drought events in the twentieth century. The length of response time to droughts and recovery time to pre-drought state were shorter in juniper trees than in fir trees. The proportion of declining trees showed a decreasing trend in fir trees but not in juniper trees. Our results indicate that the species-specific resilience might be related to the anisohydric (junipers) and isohydric (firs) strategies of stomatal regulation in response to droughts plants. The differences in species-specific drought resilience should be taken into account when developing forest management policies against the influence of extreme droughts in future. [ABSTRACT FROM AUTHOR]