Lixin Lyu

Alpine forests are increasingly exposed to rising temperatures and intensified drought, potentially pushing species beyond their tolerance limits. However, the extent to which rising atmospheric CO₂ (C_a) can mitigate these stressors by enhancing tree intrinsic water-use efficiency (iWUE) remains unclear. We investigated the growth and physiological responses of Himalayan fir (Abies spectabilis) using basal area increment (BAI) and δ¹³C data to track ecophysiological processes over recent decades along an altitudinal gradient in regions with hydrologically distinct regions on the Tibetan Plateau. Significant growth increases were observed at all altitudes in wet regions, while negative growth trends were noted at lower altitudes in dry regions. Climate–growth correlation analysis revealed that growth is primarily constrained by growing season temperatures and spring moisture availability. Tree iWUE increased over time at all altitudes, with a stronger increase in wet regions. Tree growth at lower altitudes in dry stands was negatively related to iWUE, whereas BAI in wet regions was positively associated with iWUE during the post-1965 period. Structural equation modeling indicated that temperature was a key driver of BAI and iWUE at all altitudes in wet regions, while temperature had negative effects on BAI at lower altitudes in dry regions. These results suggest that elevated C_a and temperature can stimulate tree growth in high-altitude forests in wet regions, but the positive effects do not compensate for the negative impacts of reduced water availability at lower altitudes in dry regions. Warming-induced drought stress may thus emerge as a more significant driver of growth compared to increasing C_a levels in comparable alpine forests. Our findings provide critical insights for refining assumptions about CO₂ fertilization and climate change effects in ecophysiological models.

Alpine forests are increasingly exposed to rising temperatures and intensified drought, potentially pushing species beyond their tolerance limits. However, the extent to which rising atmospheric CO₂ (Cₐ) can mitigate these stressors by enhancing tree intrinsic water-use efficiency (iWUE) remains unclear. We investigated the growth and physiological responses of Himalayan fir (Abies spectabilis) using basal area increment (BAI) and δ¹³C data to track ecophysiological processes over recent decades along an elevational gradient in warming and drying sites on the Tibetan Plateau. Significant growth increases were observed at all elevations in wet regions, while negative growth trends were noted at lower elevations in dry regions. Climate–growth correlation analysis revealed that growth is primarily constrained by growing season temperatures and spring moisture availability. Tree iWUE increased over time at all elevations, with a stronger increase in wet regions. Tree growth at lower elevations in dry stands was negatively related to iWUE, whereas BAI in wet regions was positively associated with iWUE. Leaf intercellular CO₂ (Cᵢ) increased proportionally to Cₐ after 1965. Structural equation modeling indicated that temperature was a key driver of BAI and iWUE at all elevations in wet regions, while temperature had negative effects on BAI at lower elevations in dry regions. These results suggest that elevated Cₐ and temperature can stimulate tree growth in high-elevation forests in wet regions, but the positive effects do not compensate for the negative impacts of reduced water availability at lower elevations in dry regions. Warming-induced drought stress may thus emerge as a more significant driver of growth compared to increasing Cₐ levels in comparable alpine forests. Our findings provide critical insights for refining assumptions about CO₂ fertilization and climate change effects in ecophysiological models.

Alpine forests are increasingly exposed to rising temperatures and intensified drought, potentially pushing species beyond their tolerance limits. However, the extent to which rising atmospheric CO₂ (Cₐ) can mitigate these stressors by enhancing tree intrinsic water-use efficiency (iWUE) remains unclear. We investigated the growth and physiological responses of Himalayan fir (Abies spectabilis) using basal area increment (BAI) and δ¹³C data to track ecophysiological processes over recent decades along an elevational gradient in warming and drying sites on the Tibetan Plateau. Significant growth increases were observed at all elevations in wet regions, while negative growth trends were noted at lower elevations in dry regions. Climate–growth correlation analysis revealed that growth is primarily constrained by growing season temperatures and spring moisture availability. Tree iWUE increased over time at all elevations, with a stronger increase in wet regions. Tree growth at lower elevations in dry stands was negatively related to iWUE, whereas BAI in wet regions was positively associated with iWUE. Leaf intercellular CO₂ (Cᵢ) increased proportionally to Cₐ after 1965. Structural equation modeling indicated that temperature was a key driver of BAI and iWUE at all elevations in wet regions, while temperature had negative effects on BAI at lower elevations in dry regions. These results suggest that elevated Cₐ and temperature can stimulate tree growth in high-elevation forests in wet regions, but the positive effects do not compensate for the negative impacts of reduced water availability at lower elevations in dry regions. Warming-induced drought stress may thus emerge as a more significant driver of growth compared to increasing Cₐ levels in comparable alpine forests. Our findings provide critical insights for refining assumptions about CO₂ fertilization and climate change effects in ecophysiological models.

Alpine forests are increasingly exposed to rising temperatures and intensified drought, potentially pushing species beyond their tolerance limits. However, the extent to which rising atmospheric CO₂ (Cₐ) can mitigate these stressors by enhancing tree intrinsic water-use efficiency (iWUE) remains unclear. We investigated the growth and physiological responses of Himalayan fir (Abies spectabilis) using basal area increment (BAI) and δ¹³C data to track ecophysiological processes over recent decades along an elevational gradient in warming and drying sites on the Tibetan Plateau. Significant growth increases were observed at all elevations in wet regions, while negative growth trends were noted at lower elevations in dry regions. Climate–growth correlation analysis revealed that growth is primarily constrained by growing season temperatures and spring moisture availability. Tree iWUE increased over time at all elevations, with a stronger increase in wet regions. Tree growth at lower elevations in dry stands was negatively related to iWUE, whereas BAI in wet regions was positively associated with iWUE. Leaf intercellular CO₂ (Cᵢ) increased proportionally to Cₐ after 1965. Structural equation modeling indicated that temperature was a key driver of BAI and iWUE at all elevations in wet regions, while temperature had negative effects on BAI at lower elevations in dry regions. These results suggest that elevated Cₐ and temperature can stimulate tree growth in high-elevation forests in wet regions, but the positive effects do not compensate for the negative impacts of reduced water availability at lower elevations in dry regions. Warming-induced drought stress may thus emerge as a more significant driver of growth compared to increasing Cₐ levels in comparable alpine forests. Our findings provide critical insights for refining assumptions about CO₂ fertilization and climate change effects in ecophysiological models.

Alpine forests are increasingly exposed to rising temperatures and intensified drought, potentially pushing species beyond their tolerance limits. However, the extent to which rising atmospheric CO₂ (Cₐ) can mitigate these stressors by enhancing tree intrinsic water-use efficiency (iWUE) remains unclear. We investigated the growth and physiological responses of Himalayan fir (Abies spectabilis) using basal area increment (BAI) and δ¹³C data to track ecophysiological processes over recent decades along an elevational gradient in warming and drying sites on the Tibetan Plateau. Significant growth increases were observed at all elevations in wet regions, while negative growth trends were noted at lower elevations in dry regions. Climate–growth correlation analysis revealed that growth is primarily constrained by growing season temperatures and spring moisture availability. Tree iWUE increased over time at all elevations, with a stronger increase in wet regions. Tree growth at lower elevations in dry stands was negatively related to iWUE, whereas BAI in wet regions was positively associated with iWUE. Leaf intercellular CO₂ (Cᵢ) increased proportionally to Cₐ after 1965. Structural equation modeling indicated that temperature was a key driver of BAI and iWUE at all elevations in wet regions, while temperature had negative effects on BAI at lower elevations in dry regions. These results suggest that elevated Cₐ and temperature can stimulate tree growth in high-elevation forests in wet regions, but the positive effects do not compensate for the negative impacts of reduced water availability at lower elevations in dry regions. Warming-induced drought stress may thus emerge as a more significant driver of growth compared to increasing Cₐ levels in comparable alpine forests. Our findings provide critical insights for refining assumptions about CO₂ fertilization and climate change effects in ecophysiological models.

Although neighbourhood interactions are important for the establishment and survival of tree line trees, it remains poorly understood how they affect growth rates under changing climatic conditions. Here, we combine census data and dendrochronological measurements from 18 tree line plots between 4300 and 4520 m asl on the eastern Tibetan Plateau to develop a retrospective Neighbourhood Index. We then investigate whether tree interaction is facilitative or competitive and whether it changes over time. Our results show that competition for water not only dominates neighbourhood interactions on the eastern Tibetan Plateau, but also increased significantly from 1960 to 2009 (p < 0.001). Contrary to the common belief of facilitative neighbourhood interactions at high elevation, we demonstrate that competition among mature trees is common in tree line ecotones of the eastern Tibetan Plateau, and likely to increase under projected global warming. Read the free Plain Language Summary for this article on the Journal blog.

Alpine forests are increasingly exposed to rising temperatures and intensified drought, potentially pushing species beyond their tolerance limits. However, the extent to which rising atmospheric CO₂ (Cₐ) can mitigate these stressors by enhancing tree intrinsic water-use efficiency (iWUE) remains unclear. We investigated the growth and physiological responses of Himalayan fir (Abies spectabilis) using basal area increment (BAI) and δ¹³C data to track ecophysiological processes over recent decades along an elevational gradient in warming and drying sites on the Tibetan Plateau. Significant growth increases were observed at all elevations in wet regions, while negative growth trends were noted at lower elevations in dry regions. Climate–growth correlation analysis revealed that growth is primarily constrained by growing season temperatures and spring moisture availability. Tree iWUE increased over time at all elevations, with a stronger increase in wet regions. Tree growth at lower elevations in dry stands was negatively related to iWUE, whereas BAI in wet regions was positively associated with iWUE. Leaf intercellular CO₂ (Cᵢ) increased proportionally to Cₐ after 1965. Structural equation modeling indicated that temperature was a key driver of BAI and iWUE at all elevations in wet regions, while temperature had negative effects on BAI at lower elevations in dry regions. These results suggest that elevated Cₐ and temperature can stimulate tree growth in high-elevation forests in wet regions, but the positive effects do not compensate for the negative impacts of reduced water availability at lower elevations in dry regions. Warming-induced drought stress may thus emerge as a more significant driver of growth compared to increasing Cₐ levels in comparable alpine forests. Our findings provide critical insights for refining assumptions about CO₂ fertilization and climate change effects in ecophysiological models.

Tree‐growth stability is crucial in upholding forest ecosystem services. Despite extensive research on the correlation between tree growth and climate, the influence of forest understorey on tree‐growth stability remains understudied. We surveyed forest plots and collected tree‐ring samples along two elevational gradients, ranging from 3600 to 4400 m a.s.l., in the southeastern Tibetan Plateau. An index was developed to quantify individualistic growth stability of trees, and its linkage with the complexity of forest understorey structure was examined. We found that tree‐growth stability is more pronounced in complex forest communities. In forests with higher understorey complexity, the proportion of trees experiencing growth release increased during wet years, while those with growth suppression augmented during dry years. Interestingly, a subset of trees exhibits abnormal growth increases even during the driest years, whereas another subset shows abnormal growth reductions during the wettest years. Furthermore, forests with more complex understorey structures exhibited a higher proportion of trees showcasing these two types of ‘anti‐phase’ growth statuses. Synthesis. Our study underscores the linkage between individualistic growth stability of trees and the complexity of understorey structures. The growth stability of the overstorey layer is conveyed at the expense of individual‐level growth stability and synchrony in forests with complex understorey structures. These findings emphasize the necessity for attention to the interplay between tree growth and understorey community structures when assessing the impacts of future climate change on forest dynamics.

The climate in the source region of the Yangtze River, Yellow River, and Mekong River is of great research interest because of its sensitivity to global change and its importance in regulating water resources to densely populated and vast areas downstream. A five-century long record of spring (May–June) for the Palmer Drought Severity Index (PDSI) was reconstructed for this region using tree-ring width chronologies of Qilian juniper (Juniperus przewalskii Kom.) from five high-elevation sites. The reconstruction explained 46% variance in the PDSI during the instrumental period 1955–2005. The reconstructed PDSI showed that the occurrence of dry extremes became frequent during the last century relative to the previous four centuries. The standard deviation of the reconstructed PDSI in the 100-year window showed that the recent century held apparent high values of standard deviation in the long-term context. Sustained droughts occurred in periods 1582–1631, 1737–1757, 1772–1791, 1869–1891, 1916–1939, and 1952–1982, whereas relatively wet intervals were observed in 1505–1527, 1543–1564, 1712–1736, 1792–1816, 1852–1868, 1892–1915, and 1983–2008. Notably, in the context of the past five centuries, the study region showed an increased inter-annual variability in the recent century, suggesting an intensified hydroclimatic activity possibly associated with global warming. Moreover, through diagnostic analysis of atmospheric circulation, we found that the negative phase East Asian–Pacific teleconnection pattern may be likely to trigger drought in the study region.