Integration of the Vegetation Phenology Module Improves Ecohydrological Simulation by the SWAT-Carbon Model

Vegetation phenology and hydrological cycles are closely interacted from leaf and species levels to watershed and global scales. As one of the most sensitive biological indicators of climate change, plant phenology is essential to be simulated accurately in hydrological models. Despite the Soil and Water Assessment Tool (SWAT) has been widely used for estimating hydrological cycles, its lack of integration with the phenology module has led to substantial uncertainties. In this study, we developed a process-based vegetation phenology module and coupled it with the SWAT-Carbon model to investigate the effects of vegetation dynamics on runoff in the upper reaches of Jinsha River watershed in China. The modified SWAT-Carbon model showed reasonable performance in phenology simulation, with root mean square error (RMSE) of 9.89 days for the start-of-season (SOS) and 7.51 days for the end-of-season (EOS). Simulations of both vegetation dynamics and runoff were also substantially improved compared to the original model. Specifically, the simulation of leaf area index significantly improved with the coefficient of determination (R2) increased by 0.62, the Nash–Sutcliffe efficiency (NSE) increased by 2.45, and the absolute percent bias (PBIAS) decreased by 69.0 % on average. Additionally, daily runoff simulation also showed notably improvement, particularly noticeable in June and October, with R2 rising by 0.22 and NSE rising by 0.43 on average. Our findings highlight the importance of integrating vegetation phenology into hydrological models to enhance modeling performance

Nutrient availability alters the correlation between spring leaf-out and autumn leaf senescence dates

Leaf senescence (LS) affects tree fitness, species distribution and ecosystem structure and functioning. The drivers of LS and the processes underlying it have been studied, but the studies have mainly focused on environmental cues and have mainly been based on statistical analyses using in situ data sets. Experimental investigation and field verification of the processes and drivers are thus urgently needed. We conducted a nutrient-addition experiment after a spring-warming experiment in which an ~40-day range of leaf-out (LO) dates was induced in horse chestnut (Aesculus hippocastanum) and beech (Fagus sylvatica) saplings. We found that both increased nutrient supply and advanced LO date significantly affected the timing of LS, but their effects were opposite, as the former delayed and the latter advanced the senescence. The effects of nutrient supply and LO interacted species specifically. In chestnut, the delay of senescence caused by fertilization increased with the delay of LO and was thus stronger for individuals that flushed late in the spring. On the contrary, in beech the delay of senescence caused by fertilization decreased with the delay of LO and was insignificant for individuals with the latest LO. The experimental findings for beech were confirmed with mature trees at a regional scale. The interactive effect between nutrients and LO on senescence may be associated with variable sensitivity to photoperiod, growth sink limitation and/or direct effect of foliar nutrition on the timing of senescence. Our novel results show that the interactive effects of LO and nutrient supply on the timing of LS should be further addressed experimentally in forthcoming studies. It would also be interesting to consider our results in the further development of phenological models used in assessing the effects of climatic change. The differences found in the present study between horse chestnut and beech suggest that the results found for one species cannot necessarily be generalized to other species, so studies with different temperate tree species are called fo

Daylength helps temperate deciduous trees to leaf-out at the optimal time

Global warming has led to substantially earlier spring leaf-out in temperate-zone deciduous trees. The interactive effects of temperature and daylength underlying this warming response remain unclear. However, they need to be accurately represented by earth system models to improve projections of the carbon and energy balances of temperate forests and the associated feedbacks to the Earth's climate system. We studied the control of leaf-out by daylength and temperature using data from six tree species across 2,377 European phenological network (www.pep725.eu), each with at least 30 years of observations. We found that, in addition to and independent of the known effect of chilling, daylength correlates negatively with the heat requirement for leaf-out in all studied species. In warm springs when leaf-out is early, days are short and the heat requirement is higher than in an average spring, which mitigates the warming-induced advancement of leaf-out and protects the tree against precocious leaf-out and the associated risks of late frosts. In contrast, longer-than-average daylength (in cold springs when leaf-out is late) reduces the heat requirement for leaf-out, ensuring that trees do not leaf-out too late and miss out on large amounts of solar energy. These results provide the first large-scale empirical evidence of a widespread daylength effect on the temperature sensitivity of leaf-out phenology in temperate deciduous trees

Nutrient availability alters the correlation between spring leaf-out and autumn leaf senescence dates

Leaf senescence (LS) affects tree fitness, species distribution and ecosystem structure and functioning. The drivers of LS and the processes underlying it have been studied, but the studies have mainly focused on environmental cues and have mainly been based on statistical analyses using in situ data sets. Experimental investigation and field verification of the processes and drivers are thus urgently needed. We conducted a nutrient-addition experiment after a spring-warming experiment in which an ~40-day range of leaf-out (LO) dates was induced in horse chestnut (Aesculus hippocastanum) and beech (Fagus sylvatica) saplings. We found that both increased nutrient supply and advanced LO date significantly affected the timing of LS, but their effects were opposite, as the former delayed and the latter advanced the senescence. The effects of nutrient supply and LO interacted species specifically. In chestnut, the delay of senescence caused by fertilization increased with the delay of LO and was thus stronger for individuals that flushed late in the spring. On the contrary, in beech the delay of senescence caused by fertilization decreased with the delay of LO and was insignificant for individuals with the latest LO. The experimental findings for beech were confirmed with mature trees at a regional scale. The interactive effect between nutrients and LO on senescence may be associated with variable sensitivity to photoperiod, growth sink limitation and/or direct effect of foliar nutrition on the timing of senescence. Our novel results show that the interactive effects of LO and nutrient supply on the timing of LS should be further addressed experimentally in forthcoming studies. It would also be interesting to consider our results in the further development of phenological models used in assessing the effects of climatic change. The differences found in the present study between horse chestnut and beech suggest that the results found for one species cannot necessarily be generalized to other species, so studies with different temperate tree species are called fo

Compound Extremes in Hydroclimatology: A Review

Extreme events, such as drought, heat wave, cold wave, flood, and extreme rainfall, have received increasing attention in recent decades due to their wide impacts on society and ecosystems. Meanwhile, the compound extremes (i.e., the simultaneous or sequential occurrence of multiple extremes at single or multiple locations) may exert even larger impacts on society or the environment. Thus, the past decade has witnessed an increasing interest in compound extremes. In this study, we review different approaches for the statistical characterization and modeling of compound extremes in hydroclimatology, including the empirical approach, multivariate distribution, the indicator approach, quantile regression, and the Markov Chain model. The limitation in the data availability to represent extremes and lack of flexibility in modeling asymmetric/tail dependences of multiple variables/events are among the challenges in the statistical characterization and modeling of compound extremes. Major future research endeavors include probing compound extremes through both observations with improved data availability (and statistical model development) and model simulations with improved representation of the physical processes to mitigate the impacts of compound extremes

A Supply-Chain Analysis Framework for Assessing Densified Biomass Solid Fuel Utilization Policies in China

Densified Biomass Solid Fuel (DBSF) is a typical solid form of biomass, using agricultural and forestry residues as raw materials. DBSF utilization is considered to be an alternative to fossil energy, like coal in China, associated with a reduction of environmental pollution. China has abundant biomass resources and is suitable to develop DBSF. Until now, a number of policies aimed at fostering DBSF industry have been proliferated by policy makers in China. However, considering the seasonality and instability of biomass resources, these inefficiencies could trigger future scarcities of biomass feedstocks, baffling the resilience of biomass supply chains. Therefore, this review paper focuses on DBSF policies and strategies in China, based on the supply chain framework. We analyzed the current developing situation of DBSF industry in China and developed a framework for policy instruments based on the supply chain steps, which can be used to identify and assess the deficiencies of current DBSF industry policies, and we proposed some suggestions. These findings may inform policy development and identify synergies at different steps in the supply chain to enhance the development of DBSF industry

Changes in the severity of compound drought and hot extremes over global land areas

Global warming has been shown to affect weather and climate extremes, such as droughts, floods, windstorms, cold waves, and heat waves. A number of studies have focused on the variability of different characteristics of these extremes, including their frequency, spatial extent, and severity. Recently, the study of compound extremes, defined by the co-occurrence of multiple events with extreme impacts, has attracted much attention. The compound dry and hot extreme is one type of compound extreme and may lead to detrimental impacts on the society and ecosystem. Most previous studies have focused on changes in the frequency or spatial extent of compound dry and hot extremes, while assessments of changes in the severity of compound extremes are lacking. This study evaluated changes in the severity of compound dry and hot extremes at the global scale, based on the Standardized Dry and Hot Index (SDHI). A significant increase in the severity of compound dry and hot extremes (or decrease of the SDHI value) during the warm season was found in western US, northern South America, western Europe, Africa, western Asia, southeastern Asia, southern India, northeastern China and eastern Australia. Moreover, a significant temporal increase in the average severity of the hottest month over global land areas was also observed. Results from this study highlight the increased severity of compound dry and hot extremes over global land areas and call for improved efforts on assessing the impact of compound extremes under global warming

Spatial and Temporal Variations of Compound Droughts and Hot Extremes in China

Droughts and hot extremes may lead to tremendous impacts on the ecosystem and different sectors of the society. A variety of studies have been conducted on the variability of the individual drought or hot extreme in China. However, the evaluation of compound droughts and hot extremes, which may induce even larger impacts than the individual drought or hot extreme, is still lacking. The aim of this study is to investigate changes in the frequency and spatial extent of compound droughts and hot extremes during summer in China using monthly precipitation and daily temperature data from 1953 to 2012. Results show that a high frequency of compound droughts and hot extremes mostly occur in the regions stretching from northeast to southwest of China. There is an overall increase in the frequency of co-occurrence of droughts and hot extremes across most parts of China with distinct regional patterns. In addition, an increasing trend in the areas covered by compound extremes has been observed, especially after the 1990s. At regional scales, the increase of the frequency and spatial extent of compound extremes has been shown to be most profound in North China (NC), South China (SC), and Southwest China (SWC), while the decrease of compound extremes was found in Central China (CC). These results show the variability of compound droughts and hot extremes and could provide useful insights into the mitigation efforts of extreme events in China

Dry-hot magnitude index: a joint indicator for compound event analysis

Weather and climate extremes, such as droughts and heat waves, have been commonly characterized by different properties, including frequency, duration, and magnitude. The magnitude is among the most important properties that determine the impact of extremes. Compound dry and hot events may cause detrimental impacts on water resources, energy security, crop production and food security, and have been receiving increasing attention in recent years. Although extensive studies have been conducted to investigate the magnitude of individual droughts or hot extremes, evaluation of the magnitude of compound dry and hot events has received limited attention. In this study, we develop a dry-hot magnitude index (DHMI) to characterize the magnitude of compound dry and hot events, using monthly precipitation and daily maximum temperature, which takes into account both dry and hot conditions. The DHMI is used to analyze the spatial and temporal patterns of the magnitude of compound dry and hot events in China during summer (June, July, and August) for the period of 1961–2013. Results show that high magnitudes of compound dry and hot events mainly occur in northeastern and southwestern China, with higher magnitudes mostly observed in recent decades since the 1990s. The proposed magnitude index has potential to be a useful tool for analyzing compound dry and hot events and their impacts