Altitudinal variation in drought resilience of Tianshan spruce (Picea schrenkiana) in the Middle Tianshan Mountains, China

] Dai [2011] (henceforth D11) reported that the PalmerDrought Severity Index (PDSI) is superior to other statisti-cally based drought indices including the StandardizedPrecipitation Index (SPI) and the Standardized PrecipitationEvapotranspiration Index (SPEI). D11 argued that given thephysical character of the PDSI water balance model, theindex provides robust estimates of drought severity becauseit takes the preceding conditions into account, in contrast toother drought indices that are based purely on past statisticsof particular climate variable(s). However, D11 has over-estimated the ability of the PDSI to realistically simulate thedistributed soil water balance at large spatial scales, andignored the inherent complexity and multiscalar character ofdrought phenomena, which are related to more than themoisture conditions of the soil. In this comment we discussthe complex characteristics of droughts and the limitationsof the PDSI to quantify drought conditions in a variety ofhydrological systems. We describe the advantages of statis-tically based drought indices including the SPI and the SPEI.ThefactthattheSPIandtheSPEIarenot(anddonotintendtobe) physically based indices is more liberating than con-straining, especially when the physical basis of PDSI can beseriously questioned.[

As global changes and human activities intensify, extreme drought events are becoming increasingly frequent. The entire process of drought disasters typically involves multiple stages, such as the risk assessment of drought occurrence, along with the response measures and influencing factors at various stages-before, during, and after the occurrence of drought disasters. However, existing assessments often focus on a single process of drought, and the definition of drought resilience remains unclear. Drought resilience is the result of the interplay between climatic, socio-economic, and hydraulic engineering factors, enabling a multi-process evaluation of drought conditions. This paper defined drought resilience based on the three components of resilience: "defensive capacity, recovery capacity, and adaptive capacity," and developed a comprehensive assessment framework for drought resilience from the perspective of the entire drought process, termed the "Climate-Drought Response" framework. This assessment framework employs the Standardized Precipitation Evapotranspiration Index (SPEI) to characterize regional climate features and assesses regional drought response capacity using a combination weighting method based on both subjective and objective factors through game theory. It integrates the characteristics of disaster-prone climates with drought response capacity to evaluate regional drought resilience comprehensively, analyzing the ability to defensive, recovery, and adaptive to drought disasters, as well as its alignment with regional climate features. This framework addresses the limitations of previous quantitative drought assessments that primarily focused on risk identification or mitigation measures, often neglecting the flexibility of the system to recover from drought to a normal state. It is applied to evaluate the drought resilience of three cities in the Jiaodong Peninsula of East China, aiming to provide insights for the development of economically viable drought management strategies. The results indicate a declining trend in the SPEI across the Jiaodong Peninsula, suggesting that future climate conditions may become increasingly arid. Most regions exhibit moderate to fairly strong drought resilience, effectively responding to slight drought events. However, their resilience is insufficient to cope with moderate to extreme droughts or prolonged drought events, particularly in Qingdao and Weihai. Although the overall capacities of "defensive capacity, recovery capacity, and adaptive capacity" show an upward trend, the resilience values are declining, indicating that the increases in some drought response components are insufficient to offset the negative effects of increasingly arid climate conditions. To effectively enhance drought resilience in the Jiaodong Peninsula, the primary task is to strengthen the supplementation of local conventional water sources with water transfers and unconventional water sources, while Qingdao and Weihai must further improve its water supply capacity to ensure water security during drought periods.

Differentiated growth of the most widely planted conifer in response to extreme droughts across semi-arid regions in Northern China

Multi-scale assessments of droughts: A case study in Xinjiang, China

Bu çalışmada, Türkiye’de Standardize Yağış Evapotranspirasyon İndisine (SPEI) göre 3, 6 ve 12 aylık zaman ölçeklerinde kuraklıkların 1951–2022 dönemindeki zamansal değişimi incelenmiştir. Türkiye’yi temsil eden 102 grid verisinin ortalamasına göre yapılan analizler, su yılında ve tüm mevsimlerde incelenen dönemde SPEI değerlerinin azalma eğilimi gösterdiğini ortaya koymuştur. Bu durum Türkiye’de son 72 yılda SPEI değerlerinde "normal" sınıftan "orta ve şiddetli kurak" sınıflara doğru bir kayma olduğunu göstermiştir. Özellikle 1990’lı yılların sonlarından itibaren daha şiddetli ve uzun meteorolojik, tarımsal ve hidrolojik kuraklıklar gözlenmeye başlanmıştır. Türkiye ortalamasına göre 1951-2022 döneminde en şiddetli ve uzun kuraklıklar SPEI 3, 6 ve 12 zaman ölçeklerinde 2020-2021 su yılına aittir. SPEI-12 esas alındığında, 2020-2021 su yılında, Aralık 2020-Kasım 2021 tarihleri arasında birbirini izleyen 12 ay boyunca “şiddetli kurak” koşullar devam etmiş, aynı dönem içinde 4 ayda ise “ekstrem kurak” koşullar gözlenmiştir. Türkiye’de mevsimlik ortalamalara göre SPEI değerlerinin zamansal değişimi incelendiğinde, tüm mevsimlerde daha kurak koşullara kayma eğiliminin gözlendiği ancak bu eğilimin yaz ve sonbahar mevsimlerinde daha kuvvetli olduğu görülmektedir. Yaz mevsiminde SPEI-3 değerleri basit doğrusal regresyon analizine (Sen’in eğim değerine) göre son 72 yılda 0.001 düzeyinde istatistiki olarak anlamlı olmak üzere -1.02 (-1.03) azalma eğilimi göstermiştir. Bu durum son 72 yılda Türkiye’de yaz ve sonbahar mevsimlerinde meteorolojik ve tarımsal kuraklıkların arttığının göstergesidir. Yağış yanında evapotranspirasyon verilerinin de kullanıldığı SPEI yöntemi, Türkiye’de özellikle son 20 yılda daha belirgin olmak üzere indis değerlerindeki azalma eğiliminden sadece yüksek yağış değişkenliğinin sorumlu olmadığını, yükselen hava sıcaklıkları ve artan buharlaşma/terleme oranlarının da kuraklıkların şiddetlenmesi ve kurak dönemlerin uzamasına katkı sunduğunu göstermektedir.

Climate change has modified hydrometeorological patterns, influencing plague transmission risks in Inner Mongolia. Using 2013-2021 plague surveillance data from 12 regions in Inner Mongolia, we assessed drought and wet conditions' effects on flea parasitism in two key rodents: Mongolian gerbil (Meriones unguiculatus) and Daurian ground squirrel (Spermophilus dauricus). Extreme gradient boosting (XGBoost) modeling revealed the Standardized Precipitation Evapotranspiration Index (SPEI), maximum ground surface temperature, average relative humidity, average sunshine duration, and maximum wind speed collectively explained 52.60% of generalized flea index variation (SPEI contribution: 14.13%). Distributed lag non-linear model (DLNM) analysis revealed that drought conditions cumulatively increased generalized flea indices, particularly moderate drought (SPEI = -1.8), which showed a significant lagged effect on generalized flea indices after 3 months (RR = 2.76, 95% CI: 1.78-4.27). Conversely, the cumulative effects of wet conditions were detrimental to the increase in generalized flea indices. In addition to exhibiting the generalized flea index characteristics under drought conditions, the plague vectors Nosopsyllus laeviceps and Xenopsylla conformis parasitic on M. unguiculatus showed a facilitative effect in the 3rd month following severe wet conditions, with effect sizes of relative risk (RR) = 1.97 (95% CI: 1.13-3.45) and RR = 5.96 (95% CI: 3.25-10.94), respectively. With increasing drought severity, the flea index of M. unguiculatus rose significantly after a 3-month lag, with Z-test statistics of 2.16 (SPEI_3 = -1) and 2.63 (SPEI_3 = -1.5), both p < 0.05. Under severe drought (SPEI_3 = -1.5), the cumulative RR showed a significant difference in the two rodent species (Z = 2.27, p < 0.05). Therefore, it is essential to proactively monitor drought conditions in Inner Mongolia, particularly during the 3 months following a drought, and special attention should be paid to the increased abundance of Nosopsyllus laeviceps and Xenopsylla conformis in the 3rd month following severe wet conditions.

The standardized precipitation evapotranspiration index (SPEI) was widely used in climatology and hydrology studies, because its combination the sensitivity of the PDSI to the changes in ET (caused by air temperature fluctuations and trends) with the simplicity of calculation, but also has the robustness of the multitemporal nature of the SPI. In this article, the temporal and spatial pattern of drought based on SPEI was explored in the Inner Mongolia during 1960–2013. The results showed that SPEI can effectively reveal the multiscalar feature of drought, and the effect of air temperature rising on drought severity and cumulative effect of drought itself. SPEI is suitable for those longer time scales such as 6-month or longer, and better in semi-humid and semi-arid areas than in arid area, due to much zero values or extreme maximum values at shorter time scale or in arid area. Generally, there is a drying trend in the whole Inner Mongolia, and a very severe drought was revealed at multi-time scales during the 2000s. Annual SPEI change at 6-month scale from APR to SEP indicated that drought in plant-growth season is very frequent and increasingly serious, and it should be given more attention. The severities of drought vary in different sub-regions. A significant abrupt change point of drought change trend in the whole Inner Mongolia at 12-month time scale was diagnosed in late 1990s, while this abrupt point in the middle-west subarea is a little earlier than those in the northeast subarea and the southeast subarea. This lasting severe drought should be the result of joint action of increasing air temperature and obviously decreasing precipitation since 2000s. At the same time, the most severe and frequent drought mainly occurred along the Sino-Mongolia border and the Horqin Sandy Land, while the continuous belt along the Da Hinggan Ling Mountains, the Yinshan Mountains, the Hetao Plain and the Mu Us Sandy Land is the area with relative slight drying trend during the last decades. Drought spatial pattern indicated by SPEI was supported by spatial distribution of temperature vegetation drought index based on remote sensing.

Climate and climate change affect plant species worldwide. This PhD dissertation aims to understand how climatic variation and species traits affect the growth of a wide range of 28 conifer specie. I studied the stem growth of conifer species planted in 1960’s in a common garden experiment in the Netherlands and assessed growth sensitivity to climate variation and especially drought resilience, and its two underlying components, i.e., drought resistance (reduction in stem growth during a dry year) and drought recovery (a measure of achieving pre-drought growth rate). To identify possible mechanisms that can explain species differences in growth, I also measured 43 plant traits that are important for carbon, water, and nutrient use.In chapter 2, a dendrochronological approach was used to assess the growth sensitivity of 19 conifer species to climatic variation. The growth of conifers was most negatively affected by summer drought (significantly for 89% of species), followed by spring frost (37%) and winter cold (32%). This implies that conifer species will lose productivity in a warmer and drier future climate during the growing season.In chapter 3, I related drought resilience in stem growth to multiple dimensions of drought (timing, duration and severity), and addressed the possible underlying hydraulic mechanisms. Droughts led to 22% reduction in stem growth for 90% of species, but most species (80%) were resilient due to high recovery. Drought resistance decreased when droughts occurred early (significant for 65% of species), lasted longer (60%) or were more intense (55%). Surprisingly, hydraulic traits could not explain drought resilience of conifer species, perhaps because species avoid drought through other traits such as deep roots, leaf shedding or early leaf and cambial activity before summer drought. This chapter highlights the importance of addressing multiple dimensions of drought, i.e., timing, duration and severity to predict species responses to climate change.The variation in growth and drought resistance might be determined by tracheids and pits since they could affect hydraulic safety and efficiency. In chapter 4, I assessed 1) the mechanisms underlying cavitation resistance and hydraulic conductivity, and 2) the phylogenetic signal of pits and tracheids across 28 conifer species. High cavitation resistance was determined by small pit size and strong pit sealing, which restrict air seeding, and are under strong phylogenetic control, whereas all hydraulic conductivity and tracheid traits were under weak phylogenetic control. Surprisingly, none of tracheid and pit traits could predict hydraulic conductivity, probably because species varied relatively little in hydraulic conductivity. Hydraulic conductivity only decreased with the cell wall thickness, probably due to the increased flow resistance between adjacent tracheids or reduced lumen area. In sum, conifer species differ largely in cavitation resistance, the underlying traits, and hydraulic conductivity. They may therefore differ strongly in their climatic distribution and drought responses to climate change.Relatively few studies have assessed how a comprehensive suite of traits affects the growth and drought resilience of conifer species. In chapter 5, I measured 43 functional traits for 28 conifer species and assessed how multiple leaf and stem traits were associated, and how these traits affected stem growth and drought resilience. Two trait spectra were found, reflecting a trade-off between hydraulic- and biomechanical safety versus hydraulic efficiency, and a trade-off between tough, long-lived tissues versus high carbon assimilation rate. Stem growth rate only increased with hydraulic efficiency (i.e., pit aperture and tracheid diameter), and drought resilience decreased with leaf lifespan. A longer leaf lifespan reduces drought recovery and resilience because of a reduced ability to replace drought-damaged tissues and track new climatic conditions with new, acclimated leaves. These insights may improve growth- and carbon cycling-related models and predict how trees respond to a drier future.In sum, this thesis shows that 1) most conifer species have low resistance to early, prolonged and intense droughts, but they have a high recovery and are, therefore,&nbsp; highly resilient to drought; 2) small pit size and strong pit sealing capacity facilitate cavitation resistance, whereas none of the anatomical traits (except wall thickness) can explain hydraulic conductivity; 3) stem growth rate only increased with hydraulic efficiency (i.e., pit aperture diameter) and 4) drought resilience decreased with leaf lifespan rather than hydraulic traits. Conifer tree species can adopt multiple strategies of water or carbon use, which allows them to grow fast or be highly resilient to climatic variation.&nbsp;

Abstract. Distinction between drought and aridity is crucial to understand water scarcity processes. Drought indices are used for drought identification and drought severity characterisation. The Standardised Precipitation Index (SPI) and the Palmer Drought Severity Index (PDSI) are the most known drought indices. In this study, they are compared with the modified PDSI for Mediterranean conditions (MedPDSI) and the Standardised Precipitation Evapotranspiration Index (SPEI). MedPDSI results from the soil water balance of an olive crop, thus real evapotranspiration is considered, while SPEI uses potential (climatic) evapotranspiration. Similarly to the SPI, SPEI can be computed at various time scales. Aiming at understanding possible impacts of climate change, prior to compare the drought indices, a trend analysis relative to precipitation and temperature in 27 weather stations of Portugal was performed for the period 1941 to 2006. A trend for temperature increase was observed for some weather stations and trends for decreasing precipitation in March and increasing in October were also observed for some locations. Comparisons of the SPI and SPEI at 9- and 12-month time scales, the PDSI and MedPDSI were performed for the same stations and period. SPI and SPEI produce similar results for the same time scales concerning drought occurrence and severity. PDSI and MedPDSI correlate well between them and the same happened for SPI and SPEI. PDSI and MedPDSI identify more severe droughts than SPI or SPEI and identify drought occurrence earlier than these indices. This behaviour is likely to be related with the fact that a water balance is performed with PDSI and MedPDSI, which better approaches the supply-demand balance.

Droughts represent the most complex and damaging type of natural disaster, and they have taken place with increased frequency in China in recent years. Values of the standardized precipitation evapotranspiration index (SPEI) calculated using station-based meteorological data collected from 1961 to 2013 in the middle and lower reaches of the Yangtze River Basin (MLRYRB) are used to monitor droughts. In addition, the SPEI is determined for different timescales (1, 3, 6, and 12 months) to characterize dry or wet conditions in this study area. Moreover, remote sensing methods can cover large areas, and multispectral and temporal observations are provided by satellite sensors. The temperature vegetation dryness index (TVDI) is selected to permit assessment of drought conditions. In addition, the correlation between the SPEI and TVDI values is calculated. The results show that the SPEI values over different timescales reflect complex variations in drought conditions and have been well applied in the MLRYRB. Droughts occurred on an annual basis in 1963, 1966, 1971, 1978, 1979, 1986, 2001, 2011, and 2013, particularly 2011. In addition, the regional average drought frequency in the study area during 1961–2013 is 30%, as determined using the SPEI. An analysis of the correlation between the monthly values of the TVDI and the SPEI-3 shows that a negative relationship exists between the SPEI-3 and the TVDI. That is, smaller TVDI values are associated with greater SPEI-3 values and reduced drought conditions, whereas larger TVDI values are associated with smaller SPEI-3 values and enhanced drought conditions. Therefore, this study of the relationship between the SPEI and the TVDI can provide a basis for government to mitigate the effects of drought.

Against the background of global warming, forests across environmental gradients show distinct responses to climate change, necessitating research on tree growth patterns under specific conditions. Climate and competition are critical factors affecting tree growth, yet their combined effects across altitudinal gradients remain unclear, especially in arid regions such as Central Asia. This study investigated how climate and competition influence radial growth of Picea schrenkiana Fisch. et C.A.Mey. across altitudinal gradients (1500–2670 m) in the Middle Tianshan Mountains. Using dendroclimatology, competition indices, multivariate statistical analyses, and nonlinear models across 12 plots, we examined spatial variability in growth responses. Results revealed significant altitudinal differences in growth responses to climate and competition across altitudes. At low elevations, growth is primarily limited by water availability; drought indices and spring precipitation exert positive effects, while high temperatures inhibit growth. At mid-elevations, climate becomes the dominant driver, particularly spring temperature and precipitation playing key roles, while competition has no significant effect. At high elevations, temperature becomes the primary driver of growth; however, the overall sensitivity to climate is reduced compared to lower elevations. Multiple regression analyses confirm that water-related factors drive growth at lower and middle elevations, whereas temperature is the primary driver at higher elevations. Further model comparison indicates that while nonlinear models performed slightly better at mid-elevations, linear approaches similarly provided interpretable climate–growth relationships. This study demonstrates significant spatial variation in growth determinants, with water-driven controls dominating at lower elevations and competition effects ranging from significant to non-significant as altitude increases. Future warming may further intensify drought stress at lower elevations, and whether or not the weak positive responses currently observed at higher elevations will persist remains uncertain. These findings provide a scientific basis for sustainable management of arid mountain forests under climate change.

Based on long-term climatological data from Ny-Ålesund, Svalbard Airport—Longyearbyen and the Polish Polar Station at Hornsund, we undertook an analysis of drought indices on Spitsbergen Island, Svalbard, for the period 1979–2019. The features and causes of spatiotemporal variability of atmospheric drought in Svalbard were identified, as expressed by the standardized precipitation evapotranspiration index (SPEI). There were several-year periods with SPEI indicating the dominance of drought or wet conditions. The long-term variability in the annual and half-year (May–October) SPEI values showed a prevalence of droughts in the 1980s and the first decade of the twenty-first century, while wet seasons were frequent in the 1990s and in the second decade of the twenty-first century. The seasonal SPEIs were characteristic of interannual variability. In MAM and JJA, droughts were more frequent after 2000; during SON and DJF of the same period, the frequency of wet seasons increased. The most remarkable changes in the scale of the entire research period occurred in autumn when negative values of SPEI occurred more often in the first part of the period, and positive values dominated in the last 20 years. The long-term pattern of the variables in consecutive seasons between 1979 and 2019 indicates relationships between the SPEI and anomalies of precipitable water and somewhat weaker relationships with anomalies of sea level pressure. The three stations are located at distances of more than 200 km from each other in the northern (Ny-Ålesund), central (Longyearbyen) and southern parts of Svalbard (Hornsund), and the most extreme values of drought conditions depended on the atmospheric circulation which could have been modified by local conditions thus droughts developed under various circulation types depending on the station. However, some similarities were identified in the atmospheric circulation patterns favouring drought conditions at Ny-Ålesund and Hornsund, both having more maritime climates than Longyearbyen. Extremely dry seasons were favoured by anticyclonic conditions, particularly a high-pressure ridge (type Ka) centred over Svalbard, air advection from the eastern sector under an influence of cyclone and negative precipitable water anomalies. During wet seasons anomalies of precipitable water were positive and cyclonic conditions dominated. These results were corroborated by the frequency of regional circulation types during JJA and DJF with the lowest and highest values of SPEI.

Solar-induced chlorophyll fluorescence (SIF), derived from satellite remote-sensing observations, is closely related to photosynthetic activity in plants. Here, to evaluate the effectiveness of SIF as an indicator for drought stress monitoring in agricultural crops, specifically in maize, we analyzed the relationship between SIF and the standardized precipitation evapotranspiration index (SPEI) during the rain-fed maize growing season (May–September) in Liaoning Province, China, in 2001–2020. Comparing monthly averages of SIF anomalies (relatively to the 20-year average) with the SPEI, we determined proportions of positive correlations of 77% between negative SIF anomalies and negative SPEI (drought events) and of 67% between positive SIF anomalies and positive SPEI (no-drought periods), indicating a higher SIF sensitivity to SPEI under drought conditions. Sorting years into four categories related to drought properties (length, severity) and crop yield loss, we also analyzed SIF sensitivity to drought for representative years. SIF was sensitive to both concurrent and earlier drought events; its response, affected by a temporal lag of 8–64 days explained by drought length, severity, and previous drought conditions, was more rapid for increasingly severer droughts. Moreover, correlations were highly significant between SIF anomalies and SPEI values from earlier (8–24 days) severe drought events. Finally, maize SIF recovery from drought stress was more complicated when drought events occurred during critical reproductive growth stages (e.g., silking and milk stages), causing substantial yield loss. Over the study period, correlations between average SIF anomalies and maize yield were highest in August, demonstrating the usefulness of SIF for early drought warning and yield loss forecast.

In this study, the Standardized Precipitation Evaporation Index (SPEI) was applied to characterize the drought conditions in Southwest China from 1982–2012. The SPEI was calculated by precipitation and temperature data for various accumulation periods. Based on the SPEI, the multi-scale patterns, the trend, and the spatio-temporal extent of drought were evaluated, respectively. The results explicitly showed a drying trend of Southwest China. The mean SPEI values at five time scales all decreased significantly. Some moderate and severe droughts were captured after 2005 and the droughts were even getting aggravated. By examining the spatio-temporal extent, the aggravating condition of drought was further revealed. To investigate the performance of SPEI, correlation analysis was conducted between SPEI and two remotely sensed drought indices: Soil Moisture Condition Index (SMCI) and Vegetation Condition Index (VCI). The comparison was also conducted with the Standardized Precipitation Index (SPI). The results showed that for both SMCI and VCI, the SPI and SPEI had approximate correlations with them. The SPEI could better monitor the soil moisture than the SPI in months with significant increase of temperature. The correlations between the VCI and SPI/SPEI were lower; nevertheless, the SPEI was slightly superior to the SPI.

This study provides a comprehensive assessment of drought distribution and severity across the Middle East and North Africa (MENA) region from 1981 to 2021, utilizing two widely recognized drought indices: the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). We employed the CRU TS v. 4.07 datasets to assess changes in drought characteristics (e.g., frequency, intensity, duration, and severity) at the 12-months timescale for the main Koppen-Geiger climate zones in the region. Results indicate a significant shift from wetter conditions before 1998 to more frequent drought occurrences post-1998, particularly in northern MENA, including North Africa, the Levant, and western parts of the Arabian Peninsula, northern Iraq, and western Iran. Both the SPI and SPEI indicate significant positive changes in drought frequency across 19.4% of the MENA region during the period 1998–2021, compared to 12.9% in the earlier period of 1981–1997. The SPEI, which incorporates the effects of evapotranspiration, demonstrated a more pronounced increase in drought frequency and severity than the SPI, reflecting the growing impact of rising temperatures in the most recent decades. The region experienced more frequent and prolonged, but less intense, droughts in the past two decades. Spatially, the SPEI highlighted significant drought intensification in temperate and cold desert (CWb, BWk) zones across the region, primarily driven by increasing air temperatures. On the other hand, SPI tended to capture well the mild and moderate drought events, while it underestimated the severity of the most anomalous dry events (i.e., severe, extreme, and very extreme). For example, in comparison to the SPI, the SPEI indicated an increase of 15% in the areas affected by severe to very extreme droughts in arid regions (Aw). The study also noted discrepancies between the SPI and SPEI in hyper-arid regions like the Sahara and Arabian Peninsula (BWh), emphasizing the importance of using temperature-inclusive indices like SPEI for more accurate drought assessments in warmer and water-limited climates in the region. Our findings indicate that the SPEI can be seen as a more reliable indicator for understanding drought severity in the region in the context of global warming. Overall, our findings emphasize the urgency of adopting adaptation and mitigation strategies to address the increasing frequency and severity of droughts due to global warming.