Asymmetric Warming Research Articles

Seasonal evolution differences of east Asian summer monsoon precipitation between Bølling-Allerød and younger Dryas periods

The rapid warming trend during the last deglaciation triggered significant global climate instabilities due to a complex non-linear response of the climate system to the gradual increase in insolation over the northern hemisphere. Although climate impacts can be detected globally, major regional imprints such as seasonal evolution and changes of the East Asian Summer Monsoon (EASM) during the last deglaciation are still poorly constrained due to a lack of comprehensive proxy data. In this study, we compare how the extreme climate shifts are linked to changes in EASM precipitation over China between the unusually warm Bolling-Allerod (BA) interstadial and the following strong cooling of the Younger Dryas (YD) stadial. Our analysis is based on the transient atmosphere-ocean simulations of TraCE-21ka, in addition to new results from high-resolution simulations of the CESM1 model for the BA and YD time slices. We find that the earlier onset and stronger intensity of the EASM in the BA interstadial lead to more precipitation in early summer (May–June) but drier conditions during mid-summer (July–August) over Southern China compared to a stadial climate during the YD episode. For Northern China, we find the opposite response. The insolation change in spring and the forced response of the atmospheric system are thought to be responsible for these differences. Relative to the YD episode, the hemispheric temperature gradient during the BA period is enhanced due to the asymmetric warming between the two hemispheres, leading to an intensified northward equatorial cross flow. Combined with a stronger sensible heating of the Tibetan Plateau in spring and the related earlier northward shift of the westerly jet, the early onset of the EASM is triggered. The latent heat release, which is accompanied by the onset of the EASM and the sudden increasing precipitation over Southern China in early summer, contributes to the westward shift of the Western Pacific Subtropical High (WPSH) and eastward movement of the South Asia High (SAH) in mid-summer. Under the above conditions, Southern China experiences a hot and dry climate, while Northern China receives more precipitation. Additionally, the La Nina-like pattern of the equatorial Pacific also partly contributes to the strong EASM in the warm period by influencing the WPSH location and Pacific-North American (PNA) teleconnection pattern.

Compensation Between Cloud Feedback and Aerosol‐Cloud Interaction in CMIP6 Models

AbstractThe most recent generation of climate models (the 6th Phase of the Coupled Model Intercomparison Project) yields estimates of effective climate sensitivity (ECS) that are much higher than past generations due to a stronger amplification from cloud feedback. If plausible, these models require substantially larger greenhouse gas reductions to meet global warming targets. We show that models with a more positive cloud feedback also have a stronger cooling effect from aerosol‐cloud interactions. These two effects offset each other during the historical period when both aerosols and greenhouse gases increase, allowing either more positive or neutral cloud feedback models to reproduce the observed global‐mean temperature change. Since anthropogenic aerosols primarily concentrate in the Northern Hemisphere, strong aerosol‐cloud interaction models produce an interhemispheric asymmetric warming. We show that the observed warming asymmetry during the mid to late 20th century is more consistent with low ECS (weak aerosol indirect effect) models.

Long‐term changes in layered soil temperature based on ground measurements in Jiangsu Province, China

AbstractWhile air temperature has been widely investigated to better understand the recent climate changes, the importance of soil temperature has been underestimated and rarely studied. Here, the long‐term soil temperature changes and the occurrence of extreme heats have been studied based on daily measurements in Jiangsu Province for 1970–2017. A general warming trend in soil temperature is observed at most stations, and these trends are more pronounced in the cold seasons than the warm seasons. Due to the asymmetric warming of maximum and minimum soil temperatures over the past 48 years, the diurnal range shows a long‐term decreasing trend with a rate of −0.34°C/10a. The occurrences of extreme soil temperatures have been analysed based on the probability density functions. It is found that the hot soil days have increased and cold soil days have decreased significantly, and these changes are particularly significant in the most recent decades.

Seasonality in soil temperature may drive the seasonal dynamics of carbonate‐derived CO2 efflux in a calcareous soil

AbstractCarbonates in soils may act as a potential source of CO2 efflux from calcareous soils to the atmosphere. However, its seasonal dynamics and relationships with soil temperature or soil moisture are not well‐understood. A 5‐yr warming experiment was performed to evaluate the effects of symmetric (all season) and asymmetric (seasonal) warming on soil organic fraction (SOC)‐derived efflux and carbonate‐derived CO2 efflux (partitioned by δ13C signature) from a calcareous soil in the karst region of southwest China. In region where precipitation mostly falls during summer, an average of 2.0°C warming under both symmetric and asymmetric warming scenarios produced 6–34% and 11–14% increases in the SOC‐derived efflux and carbonate‐derived efflux, respectively. The SOC‐derived efflux in the symmetric warming scenario was 1.6 times that of the asymmetric warming scenarios, but no changes were found for the carbonate‐derived efflux. The SOC‐derived efflux was positively correlated with both soil temperature and soil moisture, suggesting an entailment with biotic processes. In contrast, the carbonate‐derived efflux was positively correlated with soil temperature, but was poorly correlated with soil moisture. Thus, the seasonal dynamics of the carbonate‐derived efflux in calcareous soils were likely driven by abiotic processes.

Differential response of rice evapotranspiration to varying patterns of warming

Rice is a staple food crop that provides more calories to the global population than any other crop. Rice production is also a major consumer of fresh-water resources. Hence, changes in rice evapotranspiration (ETc) due to projected warming patterns is becoming necessary in any management of water resources and food security assessments. Here, air temperature (Ta) measurements from 1003 meteorological stations covering the period from 1967 to 2016 in China, Japan and the Philippines are first used to assess warming trends. Energy fluxes were then assembled so as to evaluate the responses of rice ETc to various warming trends. A modified Priestley-Taylor formulation was used to interpret ETc under differing warming scenarios. Results showed that the average values of daily mean Ta from 1997–2016 increased by 4.6% relative to the period from 1967–1996, where 85% of all stations marked an increase of 0.5–1.5 °C. Greater increment in average daily minima in Ta (5.1%) was noted in the past 20 years compared to the average daily maximum in Ta (3.7%), showing asymmetric warming. The changed growth duration linearly decreased as ambient seasonal mean Ta increased, and higher temperature sensitivity of altered growth duration occurred at greater warming level. Overall, the proposed modified Priestley-Taylor model can be used for estimating ETc of rice for both half-hourly and daily scales provided the growth duration is a priori known. Changes in seasonal ETc of rice under varying types of warming patterns are largely explained by both ambient seasonal mean Ta and changes in growth duration.

The Spatiotemporal Patterns of Climate Asymmetric Warming and Vegetation Activities in an Arid and Semiarid Region

Asymmetric warming was bound to have a major impact on terrestrial ecosystems in arid regions during global warming. Further study was necessary to reveal the spatiotemporal patterns of asymmetric warming in Xinjiang; this study analyzed the climate and normalized difference vegetation index (NDVI) data (2000–2020). The change trends of the day and nighttime warming (DNW), seasonal warming, and the diurnal temperature range in northern Xinjiang (S1) and southern Xinjiang (S2) were determined. The findings indicated that the DNW rate showed a significant (p < 0.05) upward trend, especially in winter. The nighttime warming rate (0.65 °C (decade)−1) was faster than the daytime warming rate (0.4 °C (decade)−1), and the diurnal temperature range between daytime and nighttime exhibited a decreasing trend. The diurnal temperature range was the highest in spring and the lowest in winter. Extreme values of the diurnal temperature range appeared in autumn (48.6 °C) and winter (12.3 °C) and both in S1. The Tmin in S1 had an abruption trend in 2006–2017, the Tmax in S2 had an abruption trend in 2005–2011, and the probability of spatial abruption in S1 was higher than that in S2. The partial correlation between the NDVI and Tmin was significantly higher than that between the NDVI and Tmax in the area where the significance test passed; therefore, asymmetric nighttime warming had a greater impact on the NDVI than the asymmetric daytime warming.

Asymmetry of Daytime and Nighttime Warming in Typical Climatic Zones along the Eastern Coast of China and Its Influence on Vegetation Activities

In this dissertation, the author adopted the normalized difference vegetation index (NDVI) and meteorological data from 1982 to 2016 of the typical climate zones in coastal areas of China to analyze the influence of daytime and nighttime warming asymmetric changes in different seasons on vegetation activities during the growing season period according to the copula function theory optimized based on Markov chain Monte Carlo (MCMC). The main conclusions are as follows: (1) The seasonal daytime and nighttime warming trends of Guangdong, Jiangsu and Liaoning over the past 35 years were significant, and the daytime and nighttime warming rates were asymmetric. In spring and summer of Guangdong province, the warming rate in the daytime was higher than that at night, while, in autumn, the opposite law was observed. However, the warming rate in the daytime was lower than that at night in Jiangsu and Liaoning provinces. There were latitude differences in diurnal and nocturnal warming rate. (2) The daytime and nighttime warming influences on vegetation showed significant seasonal differences in these three regions. In Guangdong, the influence of nighttime warming on vegetation growth in spring is greater than that in summer, and the influences of daytime warming on vegetation growth from strong to weak were spring, summer and autumn. In Jiangsu, both the influences of daytime and nighttime warming on vegetation growth in summer were less than that in autumn. In Liaoning, both the influences of daytime and nighttime warming on vegetation growth from strong to weak were autumn, spring and summer. (3) In Guangdong, Jiangsu and Liaoning provinces, their maximum temperature (Tmax) and minimum temperature (Tmin) and the joint probability distribution functions of NDVI, all had little effect on NDVI when Tmax and Tmin respectively reached their minimum values, but their influences on NDVI were obvious when Tmax and Tmin respectively reached their maximum values. (4) The smaller the return period, the larger the range of climate factor and NDVI, which has indicated that when the climate factor is certain, the NDVI is more likely to have a smaller return period, and the frequency of NDVI over a certain period is higher. In addition, the larger the climate factor, the greater the return period is and NDVI is less frequent over a certain period of time. This research can help with deep understanding of the dynamic influence of seasonal daytime and nighttime asymmetric warming on the vegetation in typical coastal temperature zones of China under the background of global climate change.

Uniforming spring phenology under non-uniform climate warming across latitude in China

Temperature is the primary factor controlling plant phenology. As temperature changes with latitude, leaf phenology in spring always shows a significant latitudinal pattern. However, under asymmetric warming at the low and high latitudes, the variability of the spring leaf phenology with latitude is becoming unclear. Based on the 23,094 observations of the leaf unfolding date (LUD) for woody species located in eastern China within latitudes 23–49°N, we analyzed the variability of LUD and its latitudinal sensitivity (Slat, days °N−1, expressed in delayed days per degree in latitude) during 1963–2008. The results showed an earlier LUD at the mid- (−2.2 days decade−1) and high (−2.5 days decade−1) latitude regions, while a stable LUD at the low-latitude regions during 1963–2008. However, the temperature sensitivity of LUD (ST, days °C−1, expressed in advanced days per degree in temperature) remained stable across the latitudes although a slight decreasing trend from 1963 to 2008. As a result, the non-uniform optimal preseason warming with latitude (Tlat, °C °N−1, expressed in the increase of temperature per degree in latitude) decreased Slat from 2.38 (days °N−1) in 1963 to 1.55 (days °N−1) in 2008. Further analyses indicated that the Growing Degree Hours (GDH) played a critical role in these processes, although the Chilling Hours (CH) showed significant variability after 1991. Our results provide evidence that the change in the balance of CH and GDH across latitude induced declining Slat over the last 40 years in eastern China. Furthermore, it may continue under the future climate warming scenarios and ultimately has important consequences on the structure and function of ecosystems.

Asymmetric Expansion of Summer Season on May and September in Korea

Global warming and its associated changes in the timing of seasonal progression may produce substantial ripple effects on the regional climate and ecosystem. This study analyzes the surface air temperature recorded during the period 1919–2017 at seven stations in the Republic of Korea to investigate the long-term changes at the beginning and ending of the summer season and their relationship with the warming trends of spring and autumn. The temperatures at the starting (June 1) and ending (August 31) dates of the past period (1919–1948) advanced by 13 days and delayed by 4 days, respectively, for the recent period (1988–2017). This asymmetric change was caused by continuous warming in May for the entire period of analysis and an abrupt warming in September in the recent decades. Different amplitudes of the expansion of the western North Pacific subtropical high in May and September are responsible for the asymmetric expansion of the summer season. The projections of surface warming for spring and autumn in Korea used the downscaled grid data of a regional climate model, which were obtained by the Representative Concentration Pathway 8.5 scenario of a general circulation model, and indicated a continuous positive trend until 2100. Larger interannual variability of blooming timing of early autumn flowers than that of late spring flowers may represent the response of the ecosystem to the seasonally asymmetric surface warming. Results suggest that the shift of seasons and associated warming trend have a disturbing effect on an ecosystem, and this trend will intensify in the future.

Rapid Local Urbanization around Most Meteorological Stations Explains the Observed Daily Asymmetric Warming Rates across China from 1985 to 2017

AbstractThe increasing rate of the observed daily minimum temperatureTminhas been much higher than that of the observed daily maximum temperatureTmaxduring the past six decades across China. In this study, the local urbanization impact on these observed asymmetric warming rates was investigated. The latest released land-cover data with a 30-m spatial resolution and annual temporal resolution from 1985 to 2017 were used to quantify the urbanization ratios around weather stations. Although urbanized areas occupied only 2.25% of the landmass in China, the percentage of stations with an urbanization ratio over 20% increased from 22.1% to 68.2% during the period 1985–2017. Significant asymmetric warming rates at urban stations were identified, which were approximately 3 times larger compared to the average asymmetry observed at all 2454 stations in China. However, this asymmetry disappeared at rural stations. These differences are mainly due to the rapid local urbanization around most meteorological stations in China since 1985, which affected the spatial representation of observations and led to the observed asymmetry warming rates. The results reported here indicate that the observed asymmetric warming rate over China from 1985 to 2017 is an observational bias due to local urbanization around most stations rather than large-scale climate change. The results also explain the phenomenon that the observed warming rate ofTminremains higher than that ofTmaxafter 1990 when the surface solar radiation stops decreasing in China.

Spring phenophases of larch are more sensitive to spring warming than to year-round warming: Results of a seasonally asymmetric warming experiment

Plant phenology is a highly sensitive indicator of climate change. The advancement in time of spring phenology due to climate warming is well documented. However, although the progressive warming process is asymmetrical across seasons in temperate and boreal regions, the consistency of spring phenological response to non-uniform seasonal warming has not been well studied. Furthermore, the different responses of budburst and leaf unfolding to seasonally uneven warming have not been fully explored. We investigated the responses of budburst and leaf unfolding in two-year-old larch (Larix principis-rupprechtii) seedlings to seasonally asymmetric warming treatments (with passive warming applied in open-top chambers) in northern China. We found that spring warming significantly advanced budburst and leaf unfolding by 7 and 2 days, respectively; year-round warming advanced budburst by 4 days but had no impact on the timing of leaf unfolding. Summer and winter warming had no effects on the timing and duration of spring phenophases. Notably, spring leaf unfolding was significantly delayed, by 2 days, following an autumnal warming, i.e., there was a carry-over effect (a 1-day delay in leaf senescence delayed leaf unfolding by 0.3 day in the next spring). The temperature sensitivities of budburst and leaf unfolding in the spring warming treatment (9.56 days/°C and 2.66 days/°C, respectively) were significantly greater than those in the year-round warming treatment (6.43 days/°C and 0.66 days/°C, respectively). We also found a trade-off relationship between the durations of budburst and leaf unfolding in the spring and year-round warming treatments. Our demonstration of (i) differences in the magnitude of responses between budburst and leaf unfolding, and (ii) the carry-over effect under non-uniform seasonal warming, should be incorporated into modeling simulations of spring phenological responses to future warming scenarios. Overall, our findings would have some critical implications for resistance and performance (e.g., frost damage risk and carbon uptake) in the seedling stage of plantation establishment under ongoing climate warming scenarios.

Nighttime warming enhances ecosystem carbon‐use efficiency in a temperate steppe

Abstract Reductions in the diurnal temperature range (DTR), for example, greater increases in daily minimum than maximum temperatures, have occurred for several decades and are projected to continue over this century, which could affect terrestrial carbon (C) cycling. Carbon‐use efficiency of plants (CUEp) and ecosystems (CUEe) represents the capacity of plants to capture C and ecosystems to store C fixed from the atmosphere, respectively. Few studies have examined how grassland CUE responds to asymmetric warming. As part of a long‐term (2006–2016) field experiment that simulated daytime and nighttime warming in a semi‐arid temperate steppe on the Mongolian Plateau, this study was conducted to examine the effects of daytime and nighttime warming on CUEp and CUEe. Nighttime warming, unlike daytime warming, increased CUEe by 54.2%, whereas neither nighttime nor daytime warming affected CUEp. Ecosystem‐level CUE linearly increased with soil moisture but declined with soil temperature variation (CVST). The suppressed ER, which might have been driven by the reduced CVST, primarily contributed to the higher CUEe under nighttime warming. These observations indicate that a reduced DTR, associated with a warmer night under asymmetric diurnal climate warming, could increase the capacity of the temperate steppe transferring C into ecosystems by elevating CUEe, resulting more C sequestration. A free Plain Language Summary can be found within the Supporting Information of this article.

Season Affects Yield and Metabolic Profiles of Rice (Oryza sativa) under High Night Temperature Stress in the Field.

Rice (Oryza sativa) is the main food source for more than 3.5 billion people in the world. Global climate change is having a strong negative effect on rice production. One of the climatic factors impacting rice yield is asymmetric warming, i.e., the stronger increase in nighttime as compared to daytime temperatures. Little is known of the metabolic responses of rice to high night temperature (HNT) in the field. Eight rice cultivars with contrasting HNT sensitivity were grown in the field during the wet (WS) and dry season (DS) in the Philippines. Plant height, 1000-grain weight and harvest index were influenced by HNT in both seasons, while total grain yield was only consistently reduced in the WS. Metabolite composition was analysed by gas chromatography-mass spectrometry (GC-MS). HNT effects were more pronounced in panicles than in flag leaves. A decreased abundance of sugar phosphates and sucrose, and a higher abundance of monosaccharides in panicles indicated impaired glycolysis and higher respiration-driven carbon losses in response to HNT in the WS. Higher amounts of alanine and cyano-alanine in panicles grown in the DS compared to in those grown in the WS point to an improved N-assimilation and more effective detoxification of cyanide, contributing to the smaller impact of HNT on grain yield in the DS.

Vegetation Change and Its Response to Climate Change between 2000 and 2016 in Marshes of the Songnen Plain, Northeast China

Songnen Plain is a representative semi-arid marshland in China. The Songnen Plain marshes have undergone obvious loss during the past decades. In order to protect and restore wetland vegetation, it is urgent to investigate the vegetation change and its response to climate change in the Songnen Plain marshes. Based on the normalized difference vegetation index (NDVI) and climate data, we investigated the spatiotemporal change of vegetation and its relationship with temperature and precipitation in the Songnen Plain marshes. During 2000–2016, the growing season mean NDVI of the Songnen Plain marshes significantly (p < 0.01) increased at a rate of 0.06/decade. For the climate change effects on vegetation, the growing season precipitation had a significant positive effect on the growing season NDVI of marshes. In addition, this study first found asymmetric effects of daytime maximum temperature (Tmax) and nighttime minimum temperature (Tmin) on NDVI of the Songnen Plain marshes: The growing season NDVI correlated negatively with Tmax but positively with Tmin. Considering the global asymmetric warming of Tmax and Tmin, more attention should be paid to these asymmetric effects of Tmax and Tmin on the vegetation of marshes.

Effects of winter chilling vs. spring forcing on the spring phenology of trees in a cold region and a warmer reference region

Regions at high latitudes and high altitudes are undergoing a more pronounced winter warming than spring warming, and such asymmetric warming will affect chilling and forcing processes and thus the spring phenology of plants. We analyzed winter chilling and spring forcing accumulation in relation to the spring phenology of three tree species (Ulmus pumila, Populus simonii, and Syringa oblata) growing in a cold region (CR) compared with trees in a warmer reference region (WR), using the Dynamic Model and the Growing Degree Hour (GDH) model. We tested that forcing rather than chilling affects the spring phenology of trees in CR (hypothesis I), and that trees in CR have both lower mean chilling and forcing temperature and thus longer accumulation periods than trees in WR (hypothesis II). The modeling results confirmed that chilling and forcing occur simultaneously during the early spring when temperature gradually increases. In line with our hypotheses, forcing played a crucial role in spring phenology in CR, but chilling and forcing combined to determine spring phenology in WR. The temperature during the chilling and forcing periods was lower and the accumulation period started earlier and ended later in CR than in WR. Moreover, the chilling accumulation was broken into two periods by the low deep winter temperature in CR, and that interruption will be removed by future strong winter warming. Future asymmetric warming, with a stronger temperature increase in winter than in spring, could decrease the forcing accumulation effects and increase the chilling effects on the spring phenology of plants in CR. This change in the balance between chilling and forcing will lead to a shift in plant phenology, which will further have major impacts on biogeochemical cycles and on ecosystem functions and services.

CAM plant expansion favored indirectly by asymmetric climate warming and increased rainfall variability.

Recent observational evidence suggests that nighttime temperatures are increasing faster than daytime temperatures, while in some regions precipitation events are becoming less frequent and more intense. The combined ecological impacts of these climatic changes on crassulacean acid metabolism (CAM) plants and their interactions with other functional groups (i.e., grass communities) remain poorly understood. Here we developed a growth chamber experiment to investigate how two CAM-grass communities in desert ecosystems of the southwestern United States and northern Mexico respond to asymmetric warming and increasing rainfall variability. Grasses generally showed competitive advantages over CAM plants with increasing rainfall variability under ambient temperature conditions. In contrast, asymmetric warming caused mortality of both grass species (Bouteloua eriopoda and Bouteloua curtipendula) in both rainfall treatments due to enhanced drought stress. Grass mortality indirectly favored CAM plants even though the biomass of both CAM species Cylindropuntia imbricata and Opuntia phaeacantha significantly decreased. The stem's volume-to-surface ratio of C. imbricata was significantly higher in mixture than in monoculture under ambient temperature (both P < 0.0014); however, the difference became insignificant under asymmetric warming (both P > 0.1625), suggesting that warming weakens the negative effects of interspecific competition on CAM plant growth. Our findings suggest that while the increase in intra-annual rainfall variability enhances grass productivity, asymmetric warming may lead to grass mortality, thereby indirectly favoring the expansion of co-existing CAM plants. This study provides novel experimental evidence showing how the ongoing changes in global warming and rainfall variability affect CAM-grass growth and interactions in dryland ecosystems.

Impacts of nighttime warming on rice growth, physiological properties and yield under water saving irrigation

Asymmetric diurnal warming is one of the main features of global climate warming. More attention has been paid to the impacts of nighttime warming on rice production. However, how rice responds to nighttime warming under water saving irrigation is still unclear. A field experiment was conducted to investigate the effects of nighttime warming on rice growth, physiological properties, and grain yield under water saving irrigation. The results indicated that, nighttime warming significantly increased leaf area index (LAI) of rice at tillering stage but reduced it at jointing-maturity stage. Nighttime warming decreased grain yield by 17.2% and 12.98% under flooding (traditional irrigation) and wetting (water saving irrigation) conditions, respectively. The above findings indicate that wetting treatment is feasible in rice production instead of flooding treatment under future climate warming.

Recent asymmetric warming trends of daytime versus nighttime and their linkages with vegetation greenness in temperate China.

Asymmetric warming has been increasingly discussed recently, yet knowledge of this difference in warming between daytime and nighttime is still limited. Most studies of how climate warming influences the terrestrial ecosystem often ignore this asymmetric effect. We investigated the change in temperature between daytime and nighttime and analyzed the relationships between normalized difference vegetation index and the temperature in the daytime (Tmax) and the nighttime (Tmin) from 1982 to 2015 in temperate China. Results showed a faster increase in Tmin (0.46°Cdec-1, p < 0.01) during the nighttime than in Tmax (0.42°Cdec-1, p < 0.01) during the daytime, which indicated an asymmetric warming rate. The asymmetric warming during the daytime and nighttime was closely related to variations in precipitation and solar radiation. The increasing Tmin and Tmax were most pronounced over a large portion of the entire temperate China, and their warming trends displayed a non-uniform spatial distribution. The area with daytime warming was larger than that with nighttime warming, approximately accounting for 99.53% and 96.22% of temperate China, respectively. The area with warming enhancing vegetation greenness was larger during the day (71.16% of temperate China, p < 0.05) than at night (61.60% of temperate China, p < 0.05), and vice versa, which presented asymmetric warming effects on China's temperate vegetation. We also found clear differences in the responses of the normalized difference vegetation index among different vegetation biomes to this asymmetric warming. Averagely, Tmax was significantly related to the NDVI of shrub, desert, broadleaf forest, needleleaf forest, and swamp (p < 0.01). However, this similar relationship appeared only between Tmin and desert vegetation (p < 0.01). Our findings emphasized the crucial role of asymmetric warming between the daytime maxima and nighttime minima in climate change research.

Responses of vegetation activity to the daytime and nighttime warming in Northwest China.

Though temperature over the past three decades has shown an asynchronous warming trend between daytime and nighttime, the response of vegetation activity to such non-uniform warming is still not very clear. In this study, the least squares linear trend analysis and geographic information system spatial analysis were conducted to analyze the spatiotemporal patterns of the daytime and nighttime warming based on the daily temperature data from 1982 to 2015 in Northwest China. The normalized difference vegetation index (NDVI) from Global Inventory Monitoring and Modeling System and vegetation type data were used to investigate the responses of vegetation activity to the daytime and nighttime warming using the partial correlation analysis. Our results suggested that (1) there was a very significant increasing trend in both daytime and nighttime temperatures in Northwest China from 1982 to 2015; night temperatures increased about 1.2 times faster than daytime temperatures, showing diurnal asymmetric warming; (2) the responses of vegetation activity to daytime and nighttime warming in Northwest China showed a distinct spatial pattern; the change in night temperatures had a more significant (positive in most regions) effect on vegetation; (3) various types of vegetation responded differently to asymmetric daytime and nighttime warming. Grassland NDVI, broad-leaved, and coniferous forest NDVI significantly responded to daytime warming. Shrub NDVI and desert NDVI significantly responded to night warming. These findings can deepen the understanding of the effects of the daytime and nighttime warming on vegetation activities in arid regions in the context of the current asymmetric warming.

Stem cycle analyses help decipher the nonlinear response of trees to concurrent warming and drought

High-resolution analysis of stem radius variation can quantify the impact of warming and drought on stem water balance and stem growth in black spruce [ Picea mariana (Mill.) B.S.P.)]. Drought affected plant water status and stem growth. However, warming affects the components of the circadian stem cycle differently if the impacts occur in the daytime or nighttime. The interactive effect of abiotic stresses had less impact on the circadian stem cycle than when the stresses occurred independently. Warming and recent droughts in boreal regions reflect the multiple dimensions of climate change. How these climate-related stresses will affect the stem growth of trees remains to be described. Plant water relations can detect the dynamics of stem depletion and replenishment under conditions of climate-forced stress. This study aimed to verify the impacts of a combination of asynchronous warming (nighttime versus daytime warming) and drought on stem water balance and stem growth in black spruce [Picea mariana (Mill.) B.S.P.)]. We investigated the water status and variations in stem radius of black spruce saplings growing in a controlled environment from May through August. We grew four-year-old saplings in warmer conditions either during the day (DW) or night (NW) at temperatures ca. 6 °C warmer than the ambient air temperature (CT). We then simulated a one-month drought in June. Automatic point dendrometers provided a high-resolution analysis of variations in stem radius, and we also monitored leaf water potentials and volumetric soil water content during the entire experimental period. We detected significant reductions in stem radius variation under water deficit conditions. In the daytime warming scenario, we observed a significant increase in the duration of contraction and a decrease in expansion of the stems. The amplitude of this contraction and expansion was reduced under the nighttime warming conditions. The main effect of warming was to enhance drought stress by accelerating soil water depletion. Changes in predawn water potential drove the duration of stem circadian cycles under conditions of daytime warming, whereas irreversible growth dynamics drove these cycles under nighttime warming conditions due to the midday water potential. The interaction of night/daytime asynchronous warming and drought reduced the amplitude rather than the duration of stem contraction and expansion. Water deficit decreased stem growth during the growing season. Asymmetric warming (as a single independent treatment) affected the timing and magnitude of stem circadian cycles. Under daytime warming scenarios, the duration of contraction and expansion were regulated mainly by predawn water potential, inducing longer (shorter) durations of contraction (expansion). Under nighttime warming, the smaller amplitudes of stem contraction and expansion were associated with midday water potential. Therefore, the interaction of abiotic stresses had less of an impact on the circadian stem cycle components than when these stresses were applied independently.