生态与农业气象研究进展

Progress in Ecological and Agricultural Meteorology Research

1 生态气象

1 Ecological meteorology

1.1 Critical leaf water content for maize photosynthesis under drought stress and its response to rewatering

Crop photosynthesis is closely related to leaf water content (LWC), and clarifying the LWC conditions at critical points in crop photosynthesis has great theoretical and practical value for accurately monitoring drought and providing early drought warnings. This experiment was conducted to study the response of LWC to drought and rewatering and to determine the LWC at which maize photosynthesis reaches a maximum and minimum and thus changes from a state of stomatal limitation (SL) to non-stomatal limitation (NSL). The effects of rehydration were different after different levels of drought stress intensity at different growth stages,and the maize LWC recovered after rewatering following different drought stresses at the jointing stage;however, the maize LWC recovered more slowly after rewatering following 43 days and 36 days of drought stress at the tasseling and silking stages, respectively. The LWC when maize photosynthesis changed from SL to NSL was 75.4% ± 0.38%, implying that the maize became rehydrated under physiologically impaired conditions. The LWCs at which the maize Vcmax25reached maximum values and zero differed between the drought and rewatering periods. After exposure to drought stress, the maize exhibited enhanced drought stress tolerance, an obviously reduced suitable water range, and significantly weakened photosynthetic capacity.These results provide profound insight into the turning points in maize photosynthesis and their responses to drought and rewatering. They may also help to improve crop water management, which will be useful in coping with the increased frequency of drought and extreme weather events expected under global climate change. (Zhou Guangsheng)

1.2 Quantitative response of maize Vcmax25 to persistent drought stress at different growth stages

Drought stress has adverse effects on crop growth and yield, and its identification and monitoring play vital roles in precision crop water management. Accurately evaluating the effect of drought stress on crop photosynthetic capacity can provide a basis for decisions related to crop drought stress identification and monitoring as well as drought stress resistance and avoidance. In this study, the effects of different degrees of persistent droughts in different growth stages (3rd leaf stage, 7th leaf stage and jointing stage) on the maximum carboxylation rate at a reference temperature of 25 (Vcmax25) of the first fully expanded leaf and its relationship to the leaf water content (LWC) were studied in a field experiment from 2013 to 2015. The results indicated that the LWC decreased continuously as drought stress continued and that the LWC decreased faster in the treatment with more irrigation. Vcmax25showed a decreasing trend as the drought progressed but had no clear relationship to the growth stage in which the persistent drought occurred. Vcmax25showed a significantly parabolic relationship (R2= 0.701, p < 0.001) with the LWC, but the different degrees of persistent drought stress occurring in different growth stages had no distinct effect on the LWC values when Vcmax25reached its maximum value or zero. The findings of this study also suggested that the LWC was 82.5 ± 0.5% when Vcmax25reached its maximum value (42.6 ± 3.6 μmol m−2s−1) and 67.6 ± 1.2% (extreme drought) when Vcmax25reached zero. These findings will help to improve crop drought management and will be an important reference for crop drought identification, classification and monitoring as well as for the development of drought monitoring and early warning systems for other crops or maize varieties. (Zhou Guangsheng)

1.3 The interrelationship between water use efficiency and radiation use efficiency under progressive soil drying in maize

The maximizing of water use efficiency (WUE) and radiation use efficiency (RUE) is vital to improving crop production in dryland farming systems. However, the fundamental question as to the association of WUE with RUE and its underlying mechanism under limited-water availability remains contentious. Here, a twoyear field trial for maize designed with five progressive soil drying regimes applied at two different growth stages (three-leaf stage and seven-leaf stage) was conducted during the 2013–2014 growing seasons. Both environmental variables and maize growth traits at the leaf and canopy levels were measured during the soil drying process. The results showed that leaf WUE increased with irrigation reduction at the early stage, while it decreased with irrigation reduction at the later stage. Leaf RUE thoroughly decreased with irrigation reduction during the progressive soil drying process. Aboveground biomass (AGB), leaf area index (LAI), a fraction of absorbed photosynthetically active radiation (fAPAR), and light extinction coefficient (k) of the maize canopy were significantly decreased by water deficits regardless of the growth stages when soil drying applied. The interrelationships between WUE and RUE were linear across the leaf and canopy scales under different soil drying patterns. Specifically, a positive linear relationship between WUE and RUE is unexpectedly found when soil drying was applied at the three-leaf stage, while it turned out to be negative when soil drying was applied at the seven-leaf stage. Moreover, the interaction between canopy WUE and RUE was more regulated by fAPAR than by LAI under soil drying. Our findings suggest that more attention must be paid to fAPAR in evaluating the effect of drought on crops and may bring new insights into the interrelationships of water and radiation use processes in dryland agricultural ecosystems. (Zhou Guangsheng)

1.4 Capability of leaf water content and its threshold values in reflection of soil-plant water status in maize during prolonged drought

Leaves play an important role in plant growth and development through photosynthesis and transpiration.Many studies have explored the effects of short-term drought stress on leaf water status; however, few studies have focused on the leaf water content capacity as an indicator of soil-plant water status during prolonged droughts. The results of a field experiment with various irrigation regimes that was conducted during two consecutive maize growing seasons from 2013 to 2014, indicated that the water content of the first fully expanded leaf (LWCtop1) was representative of the soil–plant water status with the development of drought.LWCtop1 was closely linked to the water condition of other leaves and different plant parts in response to progressive water stress. LWCtop1 shared a quadratic relationship with the photosynthetic rate (Pn), and Pnpeaked when LWCtop1 approached 84.11% and decreased to zero when LWCtop1 dropped to 68.26%.Moreover, three stages (slow-fast-slow) were observed as leaf water content responded to soil drying, and two important threshold values (minimum: 70.86 ± 0.80%, maximum: 84.58 ± 1.10%) of the leaf water content were determined. The results of this study may provide a physiological and ecological basis for the identification and monitoring of crop drought. (Zhou Guangsheng)

1.5 Increasing temperature shortened the carbon uptake period and decreased the cumulative net ecosystem productivity in a maize cropland in Northeast China

Phenology—mainly associated with climatic factors—is crucial for the accurate estimation of cumulative annual carbon exchange between terrestrial ecosystems and the atmosphere. However, the effects of changes in phenology on annual vegetation productivity and its regulatory mechanisms remain unclear, particularly in agricultural ecosystems. Therefore, in this study, we examined the associations among cumulative net ecosystem productivity (NEP), phenological metrics, and climatic factors based on long-term (2005–2014)eddy covariance flux and meteorological observations in a maize cropland in Northeast China. The results showed that carbon uptake period (CUP) was mainly determined by the end date of CUP (ECUP) in autumn.Cumulative NEP from May to September (NEP5-9), a period generally corresponding to the growing season,significantly increased with NEPmax(defined in this study as the 90th percentile of daily NEP during CUP) and CUP. NEPmaxexplained greater interannual variation in NEP5-9 than CUP. The start date of CUP (SCUP) and ECUP were both advanced with increasing winter temperature, but ECUP was more temperature-sensitive than SCUP. Thus, CUP tended to shorten with increasing temperature, ultimately decreasing cumulative NEP. In addition, NEPmaxdecreased with increasing precipitation in summer and autumn. The Greenup and MidGreendown dates from the MODIS Global Vegetation Phenology (MCD12Q2) product generally captured the interannual variation in the carbon flux-based SCUP and ECUP, respectively, well. The results of this study would be of great significance for predicting the response of ecosystem productivity to plant phenology shifts in agricultural ecosystems in future climate change scenarios. (Zhou Guangsheng)

1.6 Climatic warming enhances soil respiration resilience in an arid ecosystem

Precipitation plays a vital role in maintaining desert ecosystems in which rain events after drought cause soil respiration (Rs) pulses. However, this process and its underlying mechanism remain ambiguous,particularly under climatic warming conditions. This study aims to determine the magnitude and drive Rsof Rsresilience to rewetting. We conducted a warming experiment in situ in a desert steppe with three climatic warming scenarios—ambient temperature as the control, long-term and moderate warming treatment, and short-term and acute warming treatment. Our findings showed that the average Rsover the measurement period in the control, moderate and acute warming plots were 0.51, 0.30 and 0.30 μm(CO2) m−2s−1, respectively, and significantly increased to 1.72, 1.41 and 1.72 μm(CO2) m−2s−1, respectively, after rewetting. Both microbial and root respiration substantially increased by rewetting; microbial respiration contributed more than root respiration to total Rs. The Rssignificantly increased with microbial biomass carbon and soil organic carbon(SOC) contents. The Rsincrease by rewetting might be due to the greater microbial respiration relying heavily on microbial biomass and the larger amount of available SOC after rewetting. A trackable pattern of Rsresilience changes occurred during the daytime. The resilience of Rsin acute warming plots was significantly higher than those in both moderate warming and no warming plots, indicating that Rsresilience might be enhanced with drought severity induced by climatic warming. These results suggest that climatic warming treatment would enhance the drought resilience of soil carbon effluxes following rewatering in arid ecosystems,consequently accelerating the positive feedback of climate change. Therefore, this information should be included in carbon cycle models to accurately assess ecosystem carbon budgets with future climate change scenarios in terrestrial ecosystems, particularly in arid areas. （Zhou Guangsheng）

1.7 Resistance, recovery, and resilience of desert steppe to precipitation alterations with nitrogen deposition

Precipitation fluctuations with high nitrogen (N) deposition severely impact terrestrial ecosystem functioning, particularly in arid areas. Here, with rainout shelter facility, a field experiment with a large variation in precipitation and N addition was conducted to disentangle the responses of the plant community to normal precipitation, less precipitation, and rewetting conditions in a desert steppe, Inner Mongolia, the northern China. The field experiment established a unique annual precipitation change cycle across normal precipitation, less precipitation, and rewetting processes to quantify drought resistance, recovery, and resilience by calculating functional differences among three years. Furthermore, the relationships between plant community functional traits and response indices (i.e., the resistance, recovery, and resilience) were tested to clarify the mechanisms driving their responses to precipitation regimes and N addition. The aboveground net primary production (ANPP) increased with annual precipitation and was enhanced by N addition. ANPP with reduced precipitation regimes was less resistant to drought but recovered significantly greater than that with increased precipitation regimes. The perennial species, C3 plants, and forbs mainly contributed to the variations in vegetation productivity in response to drought and wet status cycles. Drought resistance and the recovery of species’ functional diversity, evenness, and ANPP stability were closely associated with precipitation changes. The present findings suggested that altered precipitation patterns, community composition, and functional stability contribute to ecosystem stability during water change cycles and are mediated slightly by N deposition. These findings advance understanding of the mechanisms of ecosystem functioning underlying the responses to climatic change. （Zhou Guangsheng）

1.8 Vertical distribution of gas exchanges and their integration throughout the entire canopy in a maize field

Fluxes of carbon and water along a vertical profile within a canopy, particularly the associations between canopy and ecosystem levels, are not well studied. In this study, gas exchange along the vertical profile in a maize canopy was examined. The relationships between leaf- and ecosystem-level carbon and water fluxes were compared. The results from research conducted over two growing seasons showed that during vegetative growth, the top and middle leaf layers in the canopy contribute most to the carbon and water fluxes of the entire canopy. During the grain-filling stage, gas exchange processes were performed mostly in the middle leaves with and near the ears. Significant relationships were observed between the net ecosystem CO2exchange rate(NEE) plus soil respiration and the assumed canopy levels (Acanopy) and between evapotranspiration rates at the ecosystem (ET) and assumed canopy levels (Ecanopy). This highlights the close associations between these parameters by integrating the leaf gas exchange rates measured in a conventional leaf cuvette and those at the ecosystem level via the eddy covariance technique. These results improve our understanding of how carbon assimilation varies vertically within a canopy, highlighting the critical role of ear leaves. （Zhou Guangsheng）

1.9 Effects of mosaic representation of land use/land cover on skin temperature and energy fluxes in Noah-MP land surface model over China

The representations of land use/land cover (LULC) play an important role in land surface models (LSMs)for the simulation of the energy flux partition, soil moisture redistribution, and runoff generation. This study was designed to investigate the regional effects of mosaic LULC representations on skin temperature (Ts) and energy fluxes over China at three horizontal resolutions and how these effects changed with climate regimes,using Noah with multi-parameterization (Noah-MP) LSM. The current officially released Noah-MP only considered the most abundant LULC type within one model grid. In this study, the mosaic method considering all the LULC types existing in one model grid was implemented into Noah-MP. Against the reference data(including MODIS land surface temperature products, FLUXCOM energy flux data and numerical terra dynamic simulation group evapotranspiration data), the mosaic method generally performed better than the default method and reduced the root-mean-squared-error of Tsand energy fluxes significantly over urban region. The mosaic method affected the Tsand energy fluxes by changing leaf area index and soil moisture,mainly by the former. The warm (monthly mean air temperature larger than 10 ) and relatively humid climate(annual total precipitation larger than 200 mm) could enlarge the effect of mosaic method on Tsand energy fluxes. The mosaic method reduced discrepancies of Tsand energy fluxes among three horizontal resolutions(0.0625º, 0.25º, and 0.50º), especially over the heterogeneous vegetated and urban region. （Zhou Guangsheng）

1.10 Climate warming-induced drought constrains vegetation productivity by weakening the temporal stability of the plant community in an arid grassland ecosystem

An investigation of the influences of climatic warming on ecosystem function and stability is crucial to project the impact of global climate change on terrestrial ecosystems. However, few studies have applied multiple warming treatments in arid ecosystems, which play a critical role in the global carbon cycle and are among the ecosystems most sensitive to future climatic change. To explore the effects of climatic warming on plant community function and stability, moderate warming and acute warming treatments were conducted in desert grassland, Inner Mongolia, China, using free-air temperature increase facilities. Aboveground net primary production (ANPP) of plant community significantly decreased with climatic warming, particularly in warmer years with drier conditions. The decrease in ANPP was mainly caused by decreased soil moisture induced by climatic warming. Climatic warming reduced the temporal stability of the plant community by weakening plant species asynchrony and shifting key functional groups, such as perennial vs annual grass and C3 vs C4 species. Our findings indicate that climatic warming could hamper plant community productivity via decreased soil moisture and constrain plant community functioning by weakening community stability. This result highlights that shifts in plant community composition and consequent functional changes can play a key role in predicting the responses of arid ecosystems to climatic change. （Zhou Guangsheng）

1.11 Photosynthetic resistance and resilience under drought, flooding and rewatering in maize plants

Abnormally altered precipitation patterns induced by climate change have profound global effects on crop production. However, the plant functional responses to various precipitation regimes remain unclear. Here,greenhouse and field experiments were conducted to determine how maize plant functional traits respond to drought, flooding, and rewatering. Drought and flooding hampered photosynthetic capacity, particularly when severe and/or prolonged. Most photosynthetic traits recovered after rewatering, with few compensatory responses. Rewatering often elicited high photosynthetic resilience in plants exposed to severe drought at the end of plant development, with the response strongly depending on the drought severity/duration. The associations of chlorophyll concentrations with photosynthetically functional activities were stronger during post-tasseling than during pre-tasseling, implying an involvement of leaf age/senescence in responses to episodic drought and subsequent rewatering. Coordinated changes in chlorophyll content, gas exchange,fluorescence parameters (PSII quantum efficiency and photochemical/non-photochemical radiative energy dissipation) possibly contributed to the enhanced drought resistance and resilience and suggested a possible regulative trade-off. These findings provide fundamental insights into how plants regulate their functional traits to deal with sporadic alterations in precipitation. Breeding and management of plants with high resistance and resilience traits could help crop production under future climate change. （Zhou Guangsheng）

1.12 Quantitative evaluation of the trade-off growth strategies of maize leaves under different drought severities

The leaf is one of the most drought-sensitive plant organs. Investigating how leaf traits change and their trade-off growth during a drought would contribute to developing targeted drought-resistance measures. We investigated changes in five key maize leaf traits (leaf area, dry mass, effective number, water content, and specific weight) and their trade-off growth based on a drought simulation experiment. We also developed an indicator (0, 1) to quantitatively evaluate drought severity. The results showed a trade-off growth between different leaf traits of maize plants under drought conditions. Maize maintained relatively high leaf water content to maintain high leaf metabolic activity until drought severity was greater than 0. When drought severity was (0, 0.48), maize tended to adopt rapid growth strategy by maintaining regular leafing intensity and investing more energy into leaf area rather than specific leaf weight so that more energy could be absorbed.When the drought severity exceeded 0.48, maize conserved its resources for survival by maintaining relatively lower metabolic activity and thicker leaves to minimize water loss. The results provide an insight into the acclimation strategies of maize under drought, and contribute to targeted drought prevention and relief measures to reduce drought-induced risks to food security. （Zhou Guangsheng）

1.13 Growth variations of dahurian larch plantations across Northeast China: Understanding the effects of temperature and precipitation

Climate change is affecting the growth and distribution of trees in the Chinese boreal forest. Such changes in China, the southern terminus of the extensive Eurasian boreal forests, reflect on the changes that could occur further north under a warming climate. Most studies have found that tree growth increases with increasing temperature and precipitation in boreal forests, but there is little observational evidence of the climate thresholds that might slow these growth rates at the more extreme temperatures which are predicted to occur under future global warming. Here, we examine growth responses of this dominant boreal tree species (Larix gmelinii) to climate based on the data from plantation sample plots across a broad region (40º51'–52º58' N,118º12'E−133º42' E) in Northeast China. From statistically significant fits to quadratic equations, temperature and precipitation are the important climatic factors determining tree growth in L. gmelinii plantations at two age classes (<10 year and 10–30 years-old stands). The maximum rates of tree height and diameter at breast height (DBH) were about 0.53 m year−1and 0.46 cm year−1at <10 year stands, and about 0.63 m/year and 0.60 cm/year at 10–30-year stands, respectively. For stands with the highest values of mean annual increment(MAI), the corresponding optimal mean annual temperature (MATopt) focused between 0.66 and 1.57 .The optimal mean annual precipitation (MAPopt) between 663 mm and 708 mm produced the maximal growth increments. With mean annual temperature of −2.4 and precipitation of 470 mm averaged over 1954–2005 in Chinese boreal forest region as baseline, we conservatively estimated that trees in Chinese boreal forest appear to have higher growth potentials with the maximum temperature increase of 3.6 and precipitation increase of 40%. （Zhou Guangsheng）

1.14 Responses of plant biomass and yield component in rice, wheat, and maize to climatic warming: A meta-analysis

The responses of crop yields to climatic warming have been extensively reported from experimental results, historical yield collections, and modeling research. However, an integrative report on the responses of plant biomass and yield components of three major crops to experimental warming is lacking. Here, a metaanalysis based on the most recent warming experiments was conducted to quantify the climatic warming responses of the biomass, grain yield (GY), and yield components of three staple crops. The results showed that the wheat total aboveground biomass (TAGB) increased by 6.0% with general warming, while the wheat GY did not significantly respond to warming; however, the responses shifted with increases in the mean growing season temperature (MGST). Negative effects on wheat TAGB and GY appeared when the MGSTs were above 15 and 13 , respectively. The wheat GY and the number of grains per panicle decreased by 8.4% and 7.5%, respectively, per increase. Increases in temperature significantly reduced the rice TAGB and GY by 4.3% and 16.6%, respectively, but rice straw biomass increased with increasing temperature. However,the rice grain weight and the number of panicles decreased with continuous increasing temperature (ΔTa). The maize biomass, GY, and yield components all generally decreased with climatic warming. Finally, the crop responses to climatic warming were significantly influenced by warming time, warming treatment facility, and methods. Our findings can improve the assessment of crop responses to climatic warming and are useful for ensuring food security while combating future global climate change. （Zhou Guangsheng）

1.15 ChinaSpec: A network for long-term ground-based measurements of solar-induced fluorescence in China

Remotely sensed solar-induced fluorescence (SIF) has emerged as a novel and powerful approach for terrestrial vegetation monitoring. Continuous measurements of SIF in synergy with concurrent eddy covariance(EC) flux measurements can provide a new opportunity to advance terrestrial ecosystem science. Here, we introduce a network of ground-based continuous SIF observations at flux tower sites across the mainland China referred to as ChinaSpec. The network consists of 16 tower sites until 2019 including six cropland sites, four grassland sites, four forest sites, and two wetland sites. An automated SIF system was deployed at each of these sites to collect continuous high-resolution spectra for high-frequency SIF retrievals in synergy with EC flux measurements. The goal of ChinaSpec is to provide long-term ground-based SIF measurements and promote the collaborations between optical remote sensing and EC flux observation communities in China. We present here the details of instrument specifications, data collection and processing procedures, data sharing and utilization protocols, and future plans. Furthermore, we show the examples how ground-based SIF observations can be used to track vegetation photosynthesis from diurnal to seasonal scales, and to assist in the validation of fluorescence models and satellite SIF products (e.g., from OCO-2 and TROPOMI) with the measurements from these sites since 2016. This network of SIF observations could improve our understanding of the controls on the biosphere-atmosphere carbon exchange and enable the improvement of carbon flux predictions. It will also help integrate ground-based SIF measurements with EC flux networks which will advance ecosystem and carbon cycle researches globally. （Zhou Guangsheng）

1.16 Grated remote sensing and model approach for impact assessment of future climate change on the carbon budget of global forest ecosystems

At present, global warming is an indisputable fact, and more and more attention has been paid to the impacts of climate warming on global ecological environments. Forests play increasing significant roles in regulating global carbon balance and mitigating climate change. Therefore, to understand the response mechanisms of the carbon budget of global forest ecosystems to future climate change, an improved version of the FORest ecosystem Carbon budget model for CHiNa (FORCCHN) and future Representative Concentration Pathway (RCP) scenario RCP4.5 and RCP8.5 were applied in this study. The results demonstrated that the global forest ecosystems will play a major role in the carbon sink under the future two climate change scenarios. In particular, the average carbon budget of global forest ecosystems under RCP4.5 scenario was estimated to be 0.017 kg(C) m−2yr−1from 2007 to 2100. The future carbon sink areas of global forest ecosystems will increase significantly. Under RCP4.5 and RCP8.5 climate scenarios, the carbon sink areas of global forest ecosystems during 2026‒2100 would be significantly been expanded than those in 2007‒2025,with increases of 83.16%‒87.26% and 23.53%‒29.70%, respectively. The impacts of future climate change on carbon budget of global forest ecosystems will significantly vary between different regions. The carbon budget of forests will be enhanced in the Northern Hemisphere and significantly weakened in the Southern Hemisphere under the future two climate change scenarios. The carbon sink regions of global forests will be mainly distributed in the middle and high latitudes of the Northern Hemisphere. In particular, the forests’carbon budget in the northeastern and central Asia, northern Europe and western North America will increase by 40% to 80%. However, the carbon budget of forests will decrease by 20% to 40% in the most regions of the Southern Hemisphere. In northern South America and central Africa, the forests’ carbon budget will be reduced by more than 40%. In the future, in some areas of Southern Hemisphere, where the forests’ carbon budget was predicted to be reduced, some measures for improving forest carbon sink, such as strengthening forest tending,enforcing prohibiting deforestation laws and scientific forest management, and so on, should be implemented to ensure immediate mitigation and adaptation to climate change. (Zhao Junfang)

1.17 Analysis of wheat yield losses at the county level in mainland China

There have been few pieces of research focused on quantifying wheat yield loss risk based on highresolution long-term historical data. What is more, the existence of the area scale effect reduces the certainty and spatial comparability of results. In this study, long-term wheat yield and planting area data at the county level from 1981 to 2010 were used. The spatial distribution of wheat yield loss risks was analyzed in the mainland of the People’s Republic of China (China for short). An improved comprehensive risk index of yield loss risk was established by integrating the reduction rate, coefficient of variation, and the probability of occurrence for different yield reduction rates after removing the effect of area scale. The main wheat-growing areas of 874 counties in the mainland of China were divided into lowest, lower, moderate, higher, and highest risk areas based on it. The high-risk areas are located in the Yellow-Huai-Hai Plain, including Shandong,Henan, northern Anhui, and parts of Jiangsu Province. （Fang Shibo）

1.18 Using long-term earth observation data to reveal the factors contributing to the early 2020 desert locust upsurge and the resulting vegetation loss

Massive desert locust swarms have been threatening and devouring natural vegetation and agricultural crops in East Africa and West Asia since 2019, and the event developed into a rare and globally concerning locust upsurge in early 2020. The breeding, maturation, concentration and migration of locusts rely on appropriate environmental factors, mainly precipitation, temperature, vegetation coverage and land-surface soil moisture. Remotely sensed images and long-term meteorological observations across the desert locust invasion area were analyzed to explore the complex drivers, vegetation losses and growing trends during the locust upsurge in this study. The results revealed that (1) the intense precipitation events in the Arabian Peninsula during 2018 provided suitable soil moisture and lush vegetation, thus promoting locust breeding, multiplication and gregarization; (2) the regions affected by the heavy rainfall in 2019 shifted from the Arabian Peninsula to West Asia and Northeast Africa, thus driving the vast locust swarms migrating into those regions and causing enormous vegetation loss; (3) the soil moisture and NDVI anomalies corresponded well with the locust swarm movements; and (4) there was a low chance the eastwardly migrating locust swarms would fly into the Indochina Peninsula and Southwest China. （Fang Shibo）

1.19 Analyzing the probability of acquiring cloud-free imagery in China with AVHRR cloud mask data

Optical remote sensing data are used widely in many fields (such as agriculture, resource management and the environment), especially for the vast territory of China; however, the application of these data is usually limited by clouds. Although it is valuable to analyze the probability of acquiring cloud-free imagery (PACI),PACI using different sensors at the pixel level across China has not been reported. In this study, the PACI of China was calculated with daily advanced very high resolution radiometer (AVHRR) cloud mask data from 1990 to 2019. The results showed that (1) PACI varies dramatically in different regions and months in China.The value was larger in autumn and winter, and the largest figure reached 49.55% in October in Inner Mongolia(NM). In contrast, relatively small values occurred in summer, and the minimum value (5.26%) occurred in June in South China (SC). (2) As the climate changes, the PACI has increased significantly throughout the country, especially in North China (NC), with a growth rate of 1.9% per decade. The results can be used as a reference for selecting appropriate optical sensors and observation times in areas of interest. （Fang Shibo）

1.20 Analysis of variation in reference evapotranspiration and its driving factors in mainland China from 1960 to 2016

Understanding the variation in reference evapotranspiration (ET0) is vital for hydrological cycles, drought monitoring, and water resource management. With 1507 meteorological stations and 130 radiation-measured stations, the annual and seasonal ET0were calculated at each site from 1960 to 2016 in mainland China. The phenomenon of coefficient a being less than 0.25 and coefficient b being greater than 0.50 in the Angstrom–Prescott model occurred in almost the whole country, except for a small area of western and northeastern China. Moreover, the Xiao’s method was more applicable to calculate the net longwave radiation (Rnl) and then improve the estimation accuracy of ET0. The annual ET0varied from 538.8 to 1559.8 mm and had a high-value center located in the plateau and desert of the northwestern China and a low-value center located in Northeast China and near the Sichuan Basin. The spatial distribution of seasonal ET0was roughly similar to that of annual ET0, except for that in winter when ET0was high in the south and low in the north. In mainland China,the annual ET0decreased by 21.2 mm per decade because of the declining sunshine duration before 1993 and increased by 21.1 mm per decade due to the decreased relative humidity (RH) after 1993. Generally, the abrupt change of ET0mainly occurred in the southern China rather than northern China (except for Qinghai Tibet Plateau). Basically, the dominant driving factors of annual and seasonal ET0were RH and/or Tmaxafter the abrupt change in most parts of China. （Fang Shibo）

1.21 New aricultural drought index for monitoring the water stress of winter wheat

Timely and effectively monitoring agricultural droughts for winter wheat production is crucial for water resource management, drought mitigation and even national food security. With soil moisture and actual evapotranspiration (ET) products from 2001 to 2018 supplied by the European Centre for Medium-Range Weather Forecasts (ECMWF) and moderate resolution imaging spectroradiometer (MODIS) data, respectively,two agricultural drought indices, i.e., the univariate soil moisture and evapotranspiration index (USMEI) and bivariate soil moisture and evapotranspiration index (BSMEI), were developed to reflect water stress for winter wheat. Our case study on the North China Plain (NCP) indicated that the USMEI could effectively monitor agricultural drought, especially in autumn and winter from October to January. Furthermore, compared with the evaporative stress index (ESI) and soil moisture anomaly percentage index (SMAPI), the correlations between the USMEI and climatic yields were acceptable at the county level or site scale. However, for the rest of the winter wheat growing season, the ESI and SMAPI performed better than the USMEI. In addition,the BSMEI was not suitable for monitoring droughts for winter wheat because this index overestimated the drought intensity. （Fang Shibo）

1.22 生态气象：起源、概念和展望

生态气象是应人类面临的生存环境危机而兴起的地球系统科学新兴学科，已经成为大气科学的二级学科。本文阐述了生态气象的学科起源、概念、主要研究内容与特征，指出生态气象是研究生态系统与气象条件之间相互关系的科学，是地球系统多圈层相互作用的核心，服务于人与自然和谐发展；探讨了生态气象观测的主要指标与可能的业务服务产品；阐释了生态气象研究与生态文明气象保障、气象防灾减灾和应对气候变化的关系。当前生态气象迫切需要开展的重点研究任务如下：（1）生态气象长期观测联网研究；（2） 基于大数据与人工智能的生态气象信息提取与分析技术； （3） 生态系统对气候变化的适应性及其变化归因； （4） 生态系统主要气象灾变机制及其致灾临界气象条件； （5） 陆地生态系统关键物候期对多环境要素响应的生理生态机制与模拟模型研究； （6） 耦合生物—物理—化学—管理过程的生态气象数值模式研发； （7） 陆地生态系统变化对气候系统的反馈作用与可持续发展对策研究。（周广胜）

1.23 河水生态承载力的流域整体性和时空连通性

黄河是中华民族的母亲河，黄河流域是中华文明的重要发育地。在中国5000多年的历史长河中，黄河流域作为全国政治、经济和文化中心占据了3000多年，孕育了河洛文化、河湟文化、关中文化等，分布有西安、郑州、洛阳、开封等古都，诞生了四大发明和《诗经》《老子》《史记》等经典著作。……