Species-specific drought resilience in juniper and fir forests in the central Himalayas

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 cen-turies. 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 re-silience should be taken into account when developing forest management policies against the influence of extreme droughts in future

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 (beta = -0.24, p < 0.05) compared to that of the weak indirect positive effect of summer maximum temperature (beta = 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. (C) 2021 Elsevier B.V. All rights reserved

Long-term changes in the tree radial growth and intrinsic water-use efficiency of Chuanxi spruce (Picea likiangensis var. balfouriana) in southwestern China

Elevated CO2 level in the atmosphere is expected to improve the tree growth rates and intrinsic water-use efficiency (iWUE). Although current results inferring from tree rings found the tree growth decline in water-limited area, it is still unclear whether spruce trees in humid southwestern China benefit from the increasing CO2. In this study, tree-ring width data were used to investigate the tree radial growth rate of Chuanxi spruce (Picea likiangensis var. balfouriana). Moreover, combining with the tree-ring carbon isotope date, we analyzed the physiological responses of Chuanxi spruce to rising CO2 concentrations in the atmosphere (Ca) associated with climatic change in southwestern China. From 1851 to 2009, iWUE of Chuanxi spruce rose by approximately 30.4%, and the ratio of atmospheric CO2 to leaf intercellular CO2 concentration (Ci/Ca) showed no significant trend in the study area. The result suggested that Chuanxi spruce used an active response strategy when Ca was significantly increased. iWUE showed a significant increasing trend in parallel with tree radial growth, indicating that the increasing iWUE resulted in an increase in radial growth. These results suggest that spruce forests in southwestern China have not shown declining trends under increasing Ca and climate change scenarios, in contrast to trees growing in water-limited areas. Therefore, spruce forests benefit from the increasing CO2 in the atmosphere in the humid areas of southwestern China

Tree rings reveal a major episode of forest mortality in the late 18th century on the Tibetan Plateau

There is a growing research interest on studying forest mortality in relation to ongoing climate warming, but little is known about such events in past history. The study of past forest mortality provides valuable information for determining baselines that establish the normal parameters of functioning in forest ecosystems. Here we report a major episode of previously undocumented forest mortality in the late 18th century on the northern Tibetan Plateau, China. The event was not spatially uniform, in which a more severe mortality happened in dryer sites. We used dendrochronology to compare radial growth trajectories of individual trees from 11 sites in the region, and found that many trees showed positive growth trend, or growth release, during 1796-1800 CE. Growth releases are a proxy indicator of stand thinning caused by tree mortality. The growth release was preceded by an almost two-decade long growth reduction. Long-term drought related to weakened North Atlantic Oscillation and frequent El Nino events are the likely factors causing the tree mortality in a large area of the plateau. Our findings suggest that, besides the effect of drought in the late 18th century, large-scale forest mortality may be an additional factor that further deteriorated the environment and increased the intensity of dust storms

Recent tree growth decline unprecedented over the last four centuries in a Tibetan juniper forest

Forest structure and function are subject to risks of growth declines from intensified drought and frequent extreme events related to climate warming. Knowledge of tree growth declines will help anticipate future responses of forests to climate change. In this study, we investigated tree growth declines over the last four centuries in a juniper forest on the eastern Tibetan Plateau. By analyzing the radial growth trajectories of individual trees, we identified two events of intense growth decline, one in 1817-1830 and the other in 1969-1999 over the past four centuries. The intensity of the recent decline was unprecedented in the period under study. Ring-width chronology showed a positive correlation with self-calibrating Palmer Drought Severity Indices and a negative correlation with mean monthly temperatures in May and June. The recent intensified growth decline may have been due to temperature-induced frequent droughts in the study area. Our findings suggest that trees in this juniper forest may face a higher risk of growth decline and even mortality under continued climate warming

The frequency and severity of past droughts shape the drought sensitivity of juniper trees on the Tibetan plateau

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

Pervasive tree-growth reduction in Tibetan juniper forests

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

Recovery time of juniper trees is longer in wet than dry conditions on the Tibetan Plateau in the past two centuries

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