Tree-ring evidence of ecological stress memory.

Plants experiencing stress could develop the ability to reshape their response toward present stress based on past stress experience, called 'ecological stress memory' (ESM), which is important for plant acclimation to repeated stresses. Although ESM has been largely reported, it remains unclear whether ESM could improve tree resistance to recurrent stress in subsequent decades. Here, we explore it from a tree-ring network of 1491 trees from 50 long-living juniper forests on the Tibetan Plateau. Through comparing performances of tree radial growth in past sequential growth stresses, we found that trees could obtain ESM under antecedent stresses and elevate resistance to subsequent stress after several years or even decades. Such positive effects of ESM are associated with post-stress recovery. Trees with slow recovery trajectories after antecedent stress show significantly improved resistance to subsequent stress, while trees with extremely fast post-stress recovery showed decreased resistance to subsequent stress. These results imply that temporary depressive tree radial growth after antecedent stress might be a trigger of long storage of ESM. Incorporating positive effects of ESM and relationship between ESM activation and post-stress recovery into future Earth system models could advance our capacity to predict forest dynamics and forest ecosystem stabilization under future stress conditions.

Non-linear modelling reveals a predominant moisture limit on juniper growth across the southern Tibetan Plateau.

BACKGROUND AND AIMS: Tree growth in plateau forests is critically limited by harsh climatic conditions. Many mathematical statistical methods have been used to identify the relationships between tree growth and climatic factors, but there is still uncertainty regarding the relative importance of these factors across different regions. We tested major climatic limits at 30 sites to provide insights into the main climatic limits for juniper trees (Juniperus tibetica Kom.) across the southern Tibetan Plateau. METHODS: We analysed the linear and non-linear relationships between tree growth and climatic factors using Pearson correlation statistics and a process-based forward Vaganov-Shashkin-Lite (VS-Lite) model, respectively. These relationships were used to identify the strength of the influence of different climatic factors throughout the species' growing season and to identify the main climatic factors limiting tree growth. KEY RESULTS: Growth of juniper trees began in April and ended in October in the study area. The radial growth of juniper trees was limited by soil moisture throughout the summer (June-August) of the current year at 24 sampling sites and was limited by temperature at the other six sites on the southern Tibetan Plateau. CONCLUSIONS: Soil moisture limited juniper growth at the majority of sites. Temperature in the current summer limited the growth of juniper trees at a few sampling sites in the western part of the study area. Local climate conditions may contribute to different limiting factors in the growth response of trees on the southern Tibetan Plateau. These findings may contribute to our understanding of divergent forest dynamics and to sustainable forest management under future climate scenarios.

Tree resilience to drought increases in the Tibetan Plateau.

Forests in the Tibetan Plateau are thought to be vulnerable to climate extremes, yet they also tend to exhibit resilience contributing to the maintenance of ecosystem services in and beyond the plateau. So far the spatiotemporal pattern in tree resilience in the Tibetan Plateau remains largely unquantified and the influence of specific factors on the resilience is poorly understood. Here, we study ring-width data from 849 trees at 28 sites in the Tibetan Plateau with the aim to quantify tree resilience and determine their diving forces. Three extreme drought events in years 1969, 1979, and 1995 are detected from metrological records. Regional tree resistance to the three extreme droughts shows a decreasing trend with the proportion of trees having high resistance ranging from 71.9%, 55.2%, to 39.7%. Regional tree recovery is increasing with the proportion of trees having high recovery ranging from 28.3%, 52.2%, to 64.2%. The area with high resistance is contracting and that of high recovery is expanding. The spatiotemporal resistance and recovery are associated with moisture availability and diurnal temperature range, respectively. In addition, they are both associated with forest internal factor represented by growth consistence among trees. We conclude that juniper trees in the Tibetan Plateau have increased resilience to extreme droughts in the study period. We highlight pervasive resilience in juniper trees. The results have implications for predicting tree resilience and identifying areas vulnerable to future climate extremes.

Nighttime warming alleviates the incidence of juniper forest growth decline on the Tibetan Plateau.

Recent warming over the Tibetan Plateau (TP) is approximately twice the global-mean surface temperature increase and poses a threat to the healthy growth of forests. Although many studies have focused on whether recent climate warming has caused forest growth decline on the TP, it remains unclear how asymmetric warming, that is faster increasing nighttime temperature than daytime, impacts forest growth decline. We explored this question by using a ring-width index series from 1489 juniper trees (Juniperus prezwalskii and J. tibetica) at 50 sites on the TP. We calculated the percentage of trees with growth decline (PTD) to reconstruct historical forest growth decline and employed a piecewise structural equation meta-model (pSEM) and linear mixed model (LMM) to explore influencing factors. We found that the PTD has decreased since the late 19th century, with an abrupt decreasing trend since the 1980s. Results of the pSEM show that winter minimum temperature has a stronger indirect negative effect on the variation in PTD (ß = -0.24, p < 0.05) compared to that of the weak indirect positive effect of summer maximum temperature (ß = 0.16, p < 0.05). The results of LMM show that the variation in PTD is directly negatively (p < 0.001) affected by both winter minimum temperature and summer total precipitation, but the former has a greater independent contribution than the latter (with 17.7% vs 2.5% of variances independently explained, respectively). These results suggest that increased winter minimum temperature substantially mitigates the growth decline in juniper forests on the TP. As the minimum temperature generally occurs at night, we conclude that the asymmetric increase in nighttime temperature has decreased the incidence of juniper forest growth decline on the TP under climate warming. This alleviating effect of nighttime warming is likely due to reduced low-temperature constraints and reduced damage to tree growth.