气候变暖降低了霜冻发生的频率,但生长季延长使植被更容易暴露在霜冻中。同时,植被物候和光合初级生产力的变化导致植被变绿速率发生改变。大兴安岭是中国对气候变化最为敏感和最早做出响应的地区之一,该区域霜冻对不同植被变绿速率的影响还未得到充分研究。论文使用NOAA气候数据记录归一化植被指数数据,分析了1982—2019年大兴安岭地区植被变绿速率的时空变化特征。以霜冻天数和霜冻强度为指标,探讨从春季开始到植被生长季旺季开始的时间段内霜冻对植被变绿速率的影响。结果表明：研究时段内霜冻天数与强度均呈下降趋势,平均下降速率分别为-0.24 ± 0.07 d/a和-0.036 ± 0.015 ℃/(d·a)。霜冻天数的减少和霜冻强度的减弱加速了森林生态系统的植被变绿速率,却也同时减缓了草原生态系统的植被变绿速率。不同生态分区的植被变绿速率对霜冻响应不同,大兴安岭北段山地落叶针叶林区和北段西侧森林草原区的植被变绿速率与霜冻天数的偏相关系数分别为-0.42(P<0.05)和-0.48(P<0.01),与霜冻强度的偏相关系数分别为-0.49(P<0.01)和-0.36(P<0.05),霜冻对植被变绿速率的影响显著。研究突出了霜冻对植被变绿速率的影响,结果可为理解区域植被对气候变化的响应提供有用参考。

Although climate warming reduces the frequency of frost, the longer growing season increases the vulnerability of plants to frost. At the same time, changes of vegetation phenology and primary photosynthetic productivity lead to changes in the vegetation greenup rate. The Greater Khingan Mountains is one of the most sensitive and earliest responding regions to climate change in China. Further study of the impacts of frost on vegetation greenup rates in this region is insufficient. This study reconstructed the vegetation greenup rate in the Greater Khingan Mountains, following which the responses of vegetation greenup rate to frost were analyzed in detail. National Oceanic and Atmospheric Administration (NOAA) Climate Data Record (CDR) Normalized Difference Vegetation Index (NDVI) data were used to study the spatiotemporal variations in vegetation greenup rates in the Greater Khingan Mountains from 1982 to 2019. This study used frost days and intensity indices to analyze the effect of frost on the rates of vegetation greenup from the beginning of the spring to the beginning of the peak vegetation growth season. The results show that during the study period, the frost days and intensity showed a downward trend, and the average decline rates were -0.24 ± 0.07 d/a and -0.036 ± 0.015 ℃/(d·a), respectively. Although decreases in frost days and frost intensity resulted in the acceleration of the greenup of forest ecosystems, the grassland ecosystem greenup rate was reduced at the same time. The response of vegetation greenup rate to frost differed among the different ecological regions. The response of vegetation greenup rate to frost events is obvious in the western forest grassland area (IIB3) and mountain deciduous and coniferous forest area (IA1) in northern Greater Khingan Mountains, and the partial correlation coefficients between the vegetation greenup rate and frost days were -0.42 (P<0.05) and -0.48 (P<0.01), respectively; and the partial correlation coefficients between the vegetation greenup rate and frost intensity were -0.49 (P<0.01) and -0.36 (P<0.05), respectively. This study highlights the effect of frost on vegetation greenup rate, and the results can provide a useful reference for understanding the response of regional vegetation to climate change.

研究晚霜冻害危险性时空演变特征,对于优化区域农业生产布局和品种调优具有科学的指导意义。本研究利用西南茶区65个气象站点1971—2020年逐日气象数据,结合霜冻终日和茶芽萌发初日的变化特征及其相互关系,构建西南茶区灌木型茶树春梢晚霜冻害概率指数和冻害强度指数,分析西南茶区灌木型茶树晚霜冻害危险性时空演变特征。结果表明:1971—2020年,西南茶区霜冻终日和茶芽萌发初日均呈显著提前趋势,且霜冻终日的提早速率快于茶芽萌发初日的提早速率,萌发后的茶芽暴露于晚霜冻害的天数总体呈不显著下降趋势。西南茶区大部分区域灌木型茶树晚霜冻害危险性呈下降趋势,但贵州茶区呈不显著上升趋势。四川茶区西部边缘山区、贵州茶区西部与云南茶区东北部交界处等地灌木型茶树晚霜冻害危险性一直较高,四川盆地区、云南茶区南部和贵州茶区南部等地晚霜冻害危险性一直较低。云南茶区北部、中东部地区等区域晚霜冻害危险性呈明显下降趋势;而贵州茶区中部和东部区域灌木型茶树晚霜冻害危险性明显增加。

Understanding the spatiotemporal evolution characteristics of the risk of late frost damage has scientific guiding significance for optimizing the regional agricultural production layout and varie-ty tuning. Based on the daily meteorological data of 65 weather stations in the southwest China tea region from 1971 to 2020, we analyzed variation characteristics of the last frost date (LFD), tea bud open date (BOD), and their relationships, constructed frost damage probability index and frost damage severity index of spring shoots of shrubby tea trees, and analyzed the spatiotemporal evolution chara-cteristics of the late frost damage risk of shrub tea trees in the southwest tea region. The results showed that both the BOD and LFD had a significant ahead of trend from 1971 to 2020 and the early rate of the LFD was relatively faster than that of the BOD in the southwest tea region. The number of days that the tea buds were exposed to late frost damage after germination showed an non-significant declining trend. The risk of late frost damage of shrubby tea trees in most parts of the southwest tea region showed a declining trend, but Guizhou tea planting region showed an insignificant increasing trend. The risk of late frost damage to shrubby tea trees was high in the western marginal mountai-nous areas of Sichuan tea region, and the junction of Guizhou and Yunnan tea region. The risk of late frost damage was at low level in Sichuan Basin, southern Yunnan tea region, and southern Guizhou tea region. The risk of late frost damage to shrubby tea trees in the northern and central-eastern parts of Yunnan tea region showed an obvious decreasing trend, but increased significantly in the central and eastern parts of Guizhou tea region.

Spring frost can be a limiting factor in sweet cherry (Prunus avium L.) production. Rising temperatures in spring force the development of buds, whereby their vulnerability to freezing temperatures continuously increases. With the beginning of blossom, flowers can resist only light frosts without any significant damage. In this study, we investigated the risk of spring frost damages during cherry blossom for historical and future climate conditions at two different sites in NE (Berlin) and SW Germany (Geisenheim). Two phenological models, developed on the basis of phenological observations at the experimental sweet cherry orchard in Berlin-Dahlem and validated for endodormancy release and for warmer climate conditions (already published), were used to calculate the beginning of cherry blossom in Geisenheim, 1951-2015 (external model validation). Afterwards, on the basis of a statistical regionalisation model WETTREG (RCP 8.5), the frequency of frost during cherry blossom was calculated at both sites for historical (1971-2000) and future climate conditions (2011-2100). From these data, we derived the final flower damage, defined as the percentage of frozen flowers due to single or multiple frost events during blossom. The results showed that rising temperatures in this century can premature the beginning of cherry blossom up to 17 days at both sites, independent of the used phenological model. The frequency and strength of frost was characterised by a high temporal and local variability. For both sites, no significant increase in frost frequency and frost damage during blossom was found. In Geisenheim, frost damages significantly decreased from the middle of the twenty-first century. This study additionally emphasises the importance of reliable phenological models which not only work for current but also for changed climate conditions and at different sites. The date of endodormancy release should always be a known parameter in chilling/forcing models.

In temperate zones, trees tend to unfold their leaves earlier due to climate warming. However, changes in the timing of the bud development also affect the dynamics of the cold-hardening process, which may increase frost injuries endured by trees because new leaves unfold at a period when frost events can still occur. This possible increase in frost damage in response to climate change is known as the “frost-damage hypothesis”. In this study, we have tested this hypothesis by forcing a process-based frost-injury model with process-based phenological models for 22 North American species with two Intergovernmental Panel on Climate Change storylines. Using a simplified parameterization of the frost-injury model, we found that risk of frost injury changed with climate change for all species. In fact, frost injury decreased for the vast majority of the species, but this trend varied across species and throughout each species’ distribution. We further explored the variability of response among species using their phenological and geographic characteristics. The interspecific trends depicted here show what could be the implications of climate change on the ecophysiology of boreal and temperate trees and highlight the importance of process-based models in studying the complexity of long-term impacts of climate change on species biology.

We investigated how deciduous trees can adjust their freezing resistance in response to temperature during the progress of the ecodormancy phase, from midwinter to budburst. We regularly sampled twigs of four different temperate deciduous tree species from January to the leaf-out date. Using computer-controlled freezers and climate chambers, the freezing resistance of buds was measured directly after sampling and also after the application of artificial hardening and dehardening treatments, simulating cold and warm spells. The thermal time to budburst in forcing conditions (c. 20°C) was also quantified at each sampling as a proxy for dormancy depth. Earlier flushing species showed higher freezing resistance than late flushing species at either similar bud development stage or similar dormancy depth. Overall, freezing resistance and its hardening and dehardening potential dramatically decreased during the progress of ecodormancy and became almost nil during budburst. Our results suggest that extreme cold events in winter are not critical for trees, as freezing resistance can be largely enhanced during this period. By contrast, the timing of budburst is a critical component of tree fitness. Our results provide quantitative values of the freezing resistance dynamics during ecodormancy, particularly valuable in process-based species distribution models.© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.

Although positive effects on growth and reproduction of Antarctic vascular plants have been reported under warmer temperatures, it could also increase the vulnerability of these plants to freezing. Thus, we assessed in situ whether warming decreases the freezing resistance of Colobanthus quitensis and Deschampsia antarctica, and we compared the level and mechanism of freezing resistance of these species in the field with previous reports conducted in lab conditions. We assessed the freezing resistance of C. quitensis and D. antarctica by determining their low temperature damage (LT50), ice nucleation temperature (NT) and freezing point (FP) in three sites of the King George Island. Plants were exposed during two growing seasons to a passive increase in the air temperature (+W). +W increased by 1K the mean air temperatures, but had smaller effects on freezing temperatures. Leaf temperature of both species was on average 1.7K warmer inside +W. Overall, warming decreased the freezing resistance of Antarctic species. The LT50 increased on average 2K for C. quitensis and 2.8K for D. antarctica. In contrast, NT and FP decreased on average c. 1K in leaves of warmed plants of both species. Our results showed an averaged LT50 of 15.3 degrees C for C. quitensis, and of -22.8 degrees C for D. antarctica, with freezing tolerance being the freezing resistance mechanism for both species. These results were partially consistent with previous reports, and likely explanations for such discrepancies were related with methodological differences among studies. Our work is the first study reporting the level and mechanisms of freezing resistance of Antarctic vascular plants measured in situ, and we demonstrated that although both plant species exhibited a great ability to cope with freezing temperatures during the growing season, their vulnerability to suffer freezing damage under a warming scenario increase although the magnitude of this response varied across sites and species. Hence, freezing damage should be considered when predicting changes in plant responses of C. quitensis and D. antarctica under future climate conditions of the Antarctic Peninsula.

Frost events during the active growth period of plants can cause extensive frost damage with tremendous economic losses and dramatic ecological consequences. A common assumption is that climate warming may bring along a reduction in the frequency and severity of frost damage to vegetation. On the other hand, it has been argued that rising temperature in late winter and early spring might trigger the so called "false spring", that is, early onset of growth that is followed by cold spells, resulting in increased frost damage. By combining daily gridded climate data and 1,489 k in situ phenological observations of 27 tree species from 5,565 phenological observation sites in Europe, we show here that temporal changes in the risk of spring frost damage with recent warming vary largely depending on the species and geographical locations. Species whose phenology was especially sensitive to climate warming tended to have increased risk of frost damage. Geographically, compared with continental areas, maritime and coastal areas in Europe were more exposed to increasing occurrence of frost and these late spring frosts were getting more severe in the maritime and coastal areas. Our results suggest that even though temperatures will be elevated in the future, some phenologically responsive species and many populations of a given species will paradoxically experience more frost damage in the future warming climate. More attention should be paid to the increased frost damage in responsive species and populations in maritime areas when developing strategies to mitigate the potential negative impacts of climate change on ecosystems in the near future.© 2018 John Wiley & Sons Ltd.

Climate warming has advanced leaf unfolding of trees and shrubs, thus extending the growing period but potentially exposing plants to increased frost risk. The relative shifts in the timing of leaf unfolding vs. the timing and intensity of frost events determine whether frost risk changes under climate warming. Here we test whether the frost risk for unfolding leaves of 13 European tree and shrub species has changed over more than 60 years using dynamic state-space models and phenological observations from 264 sites located between 200 and 1900 m a.s.l. across Switzerland. Trees and shrubs currently feature sufficient safety margins regarding frost risk, which increase from early-to late-leafing species and tend to decrease with increasing elevation. Particularly after 1970 to 1990 and at higher elevations, leaf unfolding has advanced across all species. While the time between the last critical frost and leaf unfolding has shifted from predominantly positive trends in the late 1950s and 1960s to a trend reversal since the 2000s, the minimum temperature during leaf unfolding has mostly increased since the 1980s. These dynamic shifts in leaf unfolding and frost risk demonstrate species- and site-specific responses of trees and shrubs to climate cooling and warming.

Temperate climates are defined by distinct temperature seasonality with large and often unpredictable weather during any of the four seasons. To thrive in such climates, trees have to withstand a cold winter and the stochastic occurrence of freeze events during any time of the year. The physiological mechanisms trees adopt to escape, avoid, and tolerate freezing temperatures include a cold acclimation in autumn, a dormancy period during winter (leafless in deciduous trees), and the maintenance of a certain freezing tolerance during dehardening in early spring. The change from one phase to the next is mediated by complex interactions between temperature and photoperiod. This review aims at providing an overview of the interplay between phenology of leaves and species-specific freezing resistance. First, we address the long-term evolutionary responses that enabled temperate trees to tolerate certain low temperature extremes. We provide evidence that short term acclimation of freezing resistance plays a crucial role both in dormant and active buds, including re-acclimation to cold conditions following warm spells. This ability declines to almost zero during leaf emergence. Second, we show that the risk that native temperate trees encounter freeze injuries is low and is confined to spring and underline that this risk might be altered by climate warming depending on species-specific phenological responses to environmental cues.

<p>The temperate monsoon area of China is an important agricultural region but late spring frosts have frequently caused significant damage to plants there. Based on phenological data derived from the Chinese Phenological Observation Network (CPON), corresponding meteorological data from 12 study sites and phenological modeling, changes in flowering times of multiple woody plants and the frequency of frost occurrence were analyzed. Through these analyses, frost risk during the flowering period at each site was estimated. Results of these estimates suggested that first flowering dates (FFD) in the study area advanced significantly from 1963 to 2009 at an average rate of -1.52 days/decade in Northeast China (<em>P</em><0.01) and -2.22 days/decade (<em>P</em><0.01) in North China. Over the same period, the number of frost days in spring decreased and the last frost days advanced across the study area. Considering both flowering phenology and occurrence of frost, the frost risk index, which measures the percentage of species exposed to frost during the flowering period in spring, exhibited a decreasing trend of -0.37% per decade (insignificant) in Northeast China and -1.80% per decade (<em>P</em><0.01) in North China, implying that frost risk has reduced over the past half century. These conclusions provide important inform</p>

In recent decades, the final frost dates of winter have advanced throughout North America, and many angiosperm taxa have simultaneously advanced their flowering times as the climate has warmed. Phenological advancement may reduce plant fitness, as flowering prior to the final frost date of the winter/spring transition may damage flower buds or open flowers, limiting fruit and seed production. The risk of floral exposure to frost in the recent past and in the future, however, also depends on whether the last day of winter frost is advancing more rapidly, or less rapidly, than the date of onset of flowering in response to climate warming. This study presents the first continental-scale assessment of recent changes in frost risk to floral tissues, using digital records of 475,694 herbarium specimens representing 1,653 angiosperm species collected across North America from 1920 to 2015. For most species, among sites from which they have been collected, dates of last frost have advanced much more rapidly than flowering dates. As a result, frost risk has declined in 66% of sampled species. Moreover, exotic species consistently exhibit lower frost risk than native species, primarily because the former occupy warmer habitats where the annual frost-free period begins earlier. While reducing the probability of exposure to frost has clear benefits for the survival of flower buds and flowers, such phenological advancement may disrupt other ecological processes across North America, including pollination, herbivory, and disease transmission.© 2020 John Wiley & Sons Ltd.

厘清驱动植物物候变化的气候因子对评估全球变化对生态系统的影响具有重要意义。针对欧洲与北美地区木本植物的控制实验表明春季物候变化主要受冷激、驱动温度和光周期影响,但这3个气候因子是否对东亚地区木本植物的春季物候存在相似的作用仍需验证。论文选取了5种典型木本植物(迎春Jasminum nudiflorum、连翘Forsythia suspensa、荚蒾Viburnum dilatatum、玉兰Yulania denudata与东京樱花Cerasus yedoensis),利用生长箱对冬季休眠枝条的展叶过程开展了控制实验。实验设置了3种驱动温度(12 ℃、15 ℃、18 ℃)、3种冷激(3 d、38 d、59 d)以及2种光周期(10 h、14 h)处理,并利用分层贝叶斯模型建立了展叶始期与各影响因子间的关系。结果表明,驱动温度每升高1 ℃导致展叶始期提前2.6 ~ 9.0 d,但展叶始期的温度敏感度随温度和冷激天数的增加而减弱。冷激也是影响木本植物展叶始期的重要因子。与冷激处理3 d相比,冷激处理59 d使展叶始期提前20.3 ~ 66.6 d,但随冷激天数增加,展叶始期对冷激响应的敏感度减弱。光周期延长对展叶始期具有提前效应,但总体小于驱动温度与冷激的影响。光周期由10 h延长至14 h后,展叶始期提前0.3 ~ 12.9 d,且在低冷激处理时光周期的效应更明显。这些结果为理解近几十年来春季物候变化及其对增温的响应提供了科学依据。

Clarifying the climatic factors driving plant phenological changes is vital for assessing the effects of global change on the ecosystem. The previous controlled experiments on European and North American woody plants demonstrated that spring phenological changes were mainly affected by chilling, forcing temperature, and photoperiod. However, it was unclear whether these three climatic factors have similar effects on the spring phenology of East Asian woody plants. In this study, we selected five typical woody plants (Jasminum nudiflorum, Forsythia suspensa, Viburnum dilatatum, Yulania denudata and Cerasus yedoensis) to investigate the leaf-out date of their dormant twigs in growth chambers with controlled environments. We set three forcing temperature treatments (12 ℃, 15 ℃, and 18 ℃), three chilling treatments (3 d, 38 d, and 59 d), and two photoperiod treatments (10 h and 14 h) and monitored the leaf-out date in each treatment. Finally, we established the relationship between the leaf-out dates and the climatic factors using the hierarchical Bayesian model. The results show that 1 ℃ warming in forcing temperature advanced the leaf-out dates by 2.6-9.0 d, but the temperature sensitivity of the leaf-out dates decreased with the increase in forcing temperature and chilling days. Chilling is also an important factor affecting the leaf-out dates of woody plants. Compared with 3 d chilling treatment, 59 chilling days advanced the leaf-out dates by 20.3-66.6 d. However, the sensitivity of leaf-out dates on chilling decreased with the increase in chilling days. Longer photoperiod caused earlier leaf-out dates, but the effect of photoperiod was smaller than that of forcing temperature and chilling. When the photoperiod was extended from 10 h to 14 h, the leaf-out dates were advanced by 0.3-12.9 d, and the effect of photoperiod was more significant in lower chilling treatment. These results provide a scientific basis for understanding spring phenological changes in responses to climate warming over the past decades.

The change in the phenology of plants or animals reflects the response of living systems to climate change. Numerous studies have reported a consistent earlier spring phenophases in many parts of middle and high latitudes reflecting increasing temperatures with the exception of China. A systematic analysis of Chinese phenological response could complement the assessment of climate change impact for the whole Northern Hemisphere. Here, we analyze 1263 phenological time series (1960-2011, with 20+ years data) of 112 species extracted from 48 studies across 145 sites in China. Taxonomic groups include trees, shrubs, herbs, birds, amphibians and insects. Results demonstrate that 90.8% of the spring/summer phenophases time series show earlier trends and 69.0% of the autumn phenophases records show later trends. For spring/summer phenophases, the mean advance across all the taxonomic groups was 2.75 days decade(-1) ranging between 2.11 and 6.11 days decade(-1) for insects and amphibians, respectively. Herbs and amphibians show significantly stronger advancement than trees, shrubs and insect. The response of phenophases of different taxonomic groups in autumn is more complex: trees, shrubs, herbs and insects show a delay between 1.93 and 4.84 days decade(-1), while other groups reveal an advancement ranging from 1.10 to 2.11 days decade(-1). For woody plants (including trees and shrubs), the stronger shifts toward earlier spring/summer were detected from the data series starting from more recent decades (1980s-2000s). The geographic factors (latitude, longitude and altitude) could only explain 9% and 3% of the overall variance in spring/summer and autumn phenological trends, respectively. The rate of change in spring/summer phenophase of woody plants (1960s-2000s) generally matches measured local warming across 49 sites in China (R=-0.33, P<0.05).© 2014 John Wiley & Sons Ltd.

While climate warming reduces the occurrence of frost events, the warming-induced lengthening of the growing season of plants in the Northern Hemisphere may actually induce more frequent frost days during the growing season (GSFDs, days with minimum temperature < 0 degrees C). Direct evidence of this hypothesis, however, is limited. Here we investigate the change in the number of GSFDs at latitudes greater than 30 degrees N using remotely-sensed and in situ phenological records and three minimum temperature (T-min) data sets from 1982 to 2012. While decreased GSFDs are found in northern Siberia, the Tibetan Plateau, and northwestern North America (mainly in autumn), similar to 43% of the hemisphere, especially in Europe, experienced a significant increase in GSFDs between 1982 and 2012 (mainly during spring). Overall, regions with larger increases in growing season length exhibit larger increases in GSFDs. Climate warming thus reduces the total number of frost days per year, but GSFDs nonetheless increase in many areas.

积温需求是决定北半球温带地区木本植物开花时间的主要因子。全球变暖使植物在冬季受到的冷激量减少,可能会改变植物开花的积温需求。气候变化导致的中国木本植物开花始期积温需求的时空变化仍不清楚。有鉴于此,本文基于“中国物候观测网”1963—2018年垂柳（Salix babylonica）和榆树（Ulmus pumila）开花始期数据,利用3种积温算法（GDD、GDDS和GDH）系统分析了两种植物开花积温需求的空间格局和在代表性站点的年际变化,构建了基于冷激日数模拟积温需求的模型。主要结论为：垂柳和榆树开花始期的积温需求在低纬度地区大于中纬度地区。站点多年平均积温需求与冷激日数呈显著的负指数关系,即随冷激日数增加,积温需求降低。时间上,3个典型站点（贵阳、西安和牡丹江）垂柳开花积温需求的变化趋势分别达到1.28~1.41 °C·d/a（P &lt; 0.01）、1.63~1.89 °C·d/a（P &lt; 0.01）和0.12~0.58 °C·d/a（仅GDD算法P &lt; 0.05）,榆树开花的积温需求在贵阳和西安同样显著增加,但在牡丹江变化不显著。冷激日数随时间减少是两个站点积温需求显著增加的主要原因。因牡丹江冬季气温低,冷激日数多且年际变化小,冷激对积温需求变化的影响不显著。基于时空耦合样本建立的冷激日数—积温需求模型对垂柳开花积温需求的模拟效果较好,R²达0.54~0.66。对榆树开花积温需求的模拟效果稍差（R²为0.33~0.64）。就不同算法而言,冷激日数对GDD算法得到的积温需求模拟效果更好。本文为量化植物开花积温需求的时空变化及在气候变化背景下的花期预测提供了重要的科学依据。

The forcing temperature in spring is the main factor that determines the flowering time of woody plants in the Northern Hemisphere. Global warming has reduced the number of chilling days in winter, which probably alters the thermal requirement of flowering. In the past 50 years, the spatio-temporal changes in the thermal requirement of spring phenology in China remain unclear. Based on the first flowering date (FFD) data of Salix babylonica and Ulmus pumila derived from China Phenological Observation Network during 1963-2018, we used three methods to calculate the thermal requirements of FFD and systematically analyzed their spatial and temporal patterns at representative sites. We also developed chilling days-thermal requirement models to quantitatively simulate the thermal requirement at each site in different years. The results showed that the thermal requirement of FFD exhibited a large spatial difference, with a relatively high value at low latitudes than that at middle latitudes. There was a significant negative exponential relationship between the average thermal requirement and chilling days across sites. The thermal requirements of FFD also changed over time. The trend in thermal requirement of Salix babylonica FFD in Guiyang, Xi'an and Mudanjiang reached 1.28-1.41 °C·d/a (P<0.01), 1.63-1.89 °C·d/a (P<0.01) and 0.12-0.58 °C·d/a (P<0.05) for the growing degree day method, respectively. The thermal requirement of Ulmus pumila FFD also increased significantly in Guiyang and Xi'an, but the trend in Mudanjiang was not significant. The decrease in the number of chilling days was the main reason for the increase in the thermal requirements. Due to the low winter temperature in Mudanjiang, the number of chilling days was large and had a small interannual variation, thus chilling days exerted no significant impact on the thermal requirement. The chilling days-thermal requirement model performed better in simulating the thermal requirement of Salix babylonica FFD, with R 2 of 0.54-0.66. In comparison with Salix babylonica, the model showed a relatively low precision in simulating the thermal requirement of Ulmus pumila FFD, with R 2 ranging from 0.33 to 0.64. Among the three methods, the thermal requirement could be better simulated by the growing degree days method compared with the growing degree days-sigmoid and growing degree hours methods. This study provides an important scientific basis for quantifying the spatio-temporal variation of the thermal requirement of flowering and for predicting the future flowering date of woody plants under the background of climate change.

The China Meteorological Forcing Dataset (CMFD) is the first high spatial-temporal resolution gridded near-surface meteorological dataset developed specifically for studies of land surface processes in China. The dataset was made through fusion of remote sensing products, reanalysis datasets and in-situ station data. Its record begins in January 1979 and is ongoing (currently up to December 2018) with a temporal resolution of three hours and a spatial resolution of 0.1°. Seven near-surface meteorological elements are provided in the CMFD, including 2-meter air temperature, surface pressure, and specific humidity, 10-meter wind speed, downward shortwave radiation, downward longwave radiation and precipitation rate. Validations against observations measured at independent stations show that the CMFD is of superior quality than the GLDAS (Global Land Data Assimilation System); this is because a larger number of stations are used to generate the CMFD than are utilised in the GLDAS. Due to its continuous temporal coverage and consistent quality, the CMFD is one of the most widely-used climate datasets for China.

Recent shifts in phenology reflect the biological response to current climate change. Aiming to enhance our understanding of phenological responses to climate change, we developed, calibrated and validated spatio-temporal models of first leaf date (FLD) for 20 broadleaved deciduous plants in China. Using daily meteorological data from the Chinese Meteorological Administration and the Community Climate System Model, version 3 (CCSM3) created using three IPCC scenarios (A2, A1B and B1), we described the FLD time series of each species over the past 50 years, extrapolating from these results to simulate estimated FLD changes for each species during the twenty-first century. Model validation suggests that our spatio-temporal models can simulate FLD accurately with R² (explained variance) >0.60. Model simulations show that, from 1952 to 2007, the FLD in China advanced at a rate of -1.14 days decade⁻¹) on average. Furthermore, changes in FLD showed noticeable variation between regions, with clearer advances observed in the north than in the south of the country. The model indicates that the advances in FLD observed from 1952-2007 in China will continue over the twenty-first century, although significant differences among species and different climate scenarios are expected. The average trend of FLD advance in China during the twenty-first century is modeled as being -1.92 days decade⁻¹ under the A2 scenario, -1.10 days decade⁻¹ under the A1B scenario and -0.74 days decade⁻¹ under the B2 scenario. The spatial pattern of FLD change for the period 2011-2099 is modeled as being similar but showing some difference from patterns in the 1952-2007 period. At the interspecific level, early-leafing species were found to show a greater advance in FLD, while species with larger distributions tended to show a weaker advance in FLD. These simulated changes in phenology may have significant implications for plant distribution as well as ecosystem structure and function.

In most temperate tree species, phenological events such as flowering and autumnal cessation of growth are not primarily controlled by temperature.

Minimum temperature is assumed to be an important driver of tree species range limits. We investigated during which period of the year trees are most vulnerable to freezing damage and whether the pressure of freezing events increases with increasing elevation. We assessed the course of freezing resistance of buds and leaves from winter to summer at the upper elevational limits of eight deciduous tree species in the Swiss Alps. By reconstructing the spring phenology of these species over the last eight decades using a thermal time model, we linked freezing resistance with long-term minimum temperature data along elevational gradients. Counter-intuitively, the pressure of freeze events does not increase with elevation, but deciduous temperate tree species exhibit a constant safety margin (5-8.5 K) against damage by spring freeze events along elevational gradients, as a result of the later flushing at higher elevation. Absolute minimum temperatures in winter and summer are unlikely to critically injure trees. Our study shows that freezing temperatures in spring are the main selective pressure controlling the timing of flushing, leading to a shorter growing season at higher elevation and potentially driving species distribution limits. Such mechanistic knowledge is important to improve predictions of tree species range limits.© 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.

Climate change, with both warmer spring temperatures and greater temperature fluctuations, has altered phenologies, possibly leading to greater risk of spring frost damage to temperate deciduous woody plants. Phenological observations of 20 woody species from 1993 to 2012 in Trelease Woods, Champaign County, Illinois, USA, were used to identify years with frost damage to vegetative and reproductive phases. Local temperature records were used in combination with the phenological observations to determine what combinations of the two were associated with damage. Finally, a long-term temperature record (1889-1992) was evaluated to determine if the frequency of frost damage has risen in recent decades. Frost < or = -1.7 degrees C occurred after bud-break in 14 of the 20 years of observation. Frost damage occurred in five years in the interior and in three additional years at only the forest edge. The degree of damage varied with species, life stage, tissue (vegetative or reproductive), and phenological phase. Common features associated with the occurrence of damage to interior plants were (1) a period of unusual warm temperatures in March, followed by (2) a frost event in April with a minimum temperature < or = -6.1 degrees C with (3) a period of 16-33 days between the extremes. In the long-term record, 10 of 124 years met these conditions, but the yearly probability of frost damage increased significantly, from 0.03 during 1889-1979 to 0.21 during 1980-2012. When the criteria were "softened" to < or = -1.7 degrees C in April and an interval of 16-37 days, 31 of 124 years met the conditions, and the yearly damage probability increased significantly to 0.19 for 1889-1979 and 0.42 for 1980-2012. In this forest, the combination of warming trends and temperature variability (extremes) associated with climate change is having ecologically important effects, making previously rare frost damage events more common.

Timing of bud burst and frost damage risk for leaves of Betula spp. in response to climatic warming in Finland was examined with two models. In the first model, ontogenetic development in spring was triggered by an accumulation of chilling temperatures. The second model assumed an additional signal from the light climate. The two models gave radically different estimates of frost damage risk in response to climate warming. The chilling-triggered model forecast a significant and increasing risk with increased warming, whereas the light-climate-triggered model predicted little or no risk. The chilling-triggered model is widely applied in phenological research; however, there is increasing experimental evidence that light conditions play a role in the timing of spring phenology. Although it is not clear if the light response mechanisms are appropriately represented in our model, the results imply that reliance on a light signal for spring development would afford a degree of protection against possible frost damage under climate warming that would not be present if chilling were the sole determinant. Further experimental tests are required to ascertain the light-related mechanisms controlling phenological timing, so that credible model extrapolations can be undertaken.

The ongoing changes in vegetation spring phenology in temperate/cold regions are widely attributed to temperature. However, in arid/semiarid ecosystems, the correlation between spring temperature and phenology is much less clear. We test the hypothesis that precipitation plays an important role in the temperature dependency of phenology in arid/semiarid regions. We therefore investigated the influence of preseason precipitation on satellite-derived estimates of starting date of vegetation growing season (SOS) across the Tibetan Plateau (TP). We observed two clear patterns linking precipitation to SOS. First, SOS is more sensitive to interannual variations in preseason precipitation in more arid than in wetter areas. Spatially, an increase in long-term averaged preseason precipitation of 10 mm corresponds to a decrease in the precipitation sensitivity of SOS by about 0.01 day mm(-1). Second, SOS is more sensitive to variations in preseason temperature in wetter than in dryer areas of the plateau. A spatial increase in precipitation of 10 mm corresponds to an increase in temperature sensitivity of SOS of 0.25 day °C(-1) (0.25 day SOS advance per 1 °C temperature increase). Those two patterns indicate both direct and indirect impacts of precipitation on SOS on TP. This study suggests a balance between maximizing benefit from the limiting climatic resource and minimizing the risk imposed by other factors. In wetter areas, the lower risk of drought allows greater temperature sensitivity of SOS to maximize the thermal benefit, which is further supported by the weaker interannual partial correlation between growing degree days and preseason precipitation. In more arid areas, maximizing the benefit of water requires greater sensitivity of SOS to precipitation, with reduced sensitivity to temperature. This study highlights the impacts of precipitation on SOS in a large cold and arid/semiarid region and suggests that influences of water should be included in SOS module of terrestrial ecosystem models for drylands. © 2015 John Wiley & Sons Ltd.

Freezing temperatures and summer droughts shape plant life in Mediterranean high-elevation habitats. Thus, the impacts of climate change on plant survival for these species could be quite different to those from mesic mountains. We exposed 12 alpine species to experimental irrigation and warming in the Central Chilean Andes to assess whether irrigation decreases freezing resistance, irrigation influences freezing resistance when plants are exposed to warming, and to assess the relative importance of irrigation and temperature in controlling plant freezing resistance. Freezing resistance was determined as the freezing temperature that produced 50 % photoinactivation [lethal temperature (LT50)] and the freezing point (FP). In seven out of 12 high-Andean species, LT50 of drought-exposed plants was on average 3.5 K lower than that of irrigated plants. In contrast, most species did not show differences in FP. Warming changed the effect of irrigation on LT50. Depending on species, warming was found to have (1) no effect, (2) to increase, or (3) to decrease the irrigation effect on LT50. However, the effect size of irrigation on LT50 was greater than that of warming for almost all species. The effect of irrigation on FP was slightly changed by warming and was sometimes in disagreement with LT50 responses. Our data show that drought increases the freezing resistance of high-Andean plant species as a general plant response. Although freezing resistance increases depended on species-specific traits, our results show that warmer and moister growing seasons due to climate change will seriously threaten plant survival and persistence of these and other alpine species in dry mountains.