Frost-free season lengthening and its potential cause in the Tibetan Plateau from 1960 to 2010

Frost-free season was an important index for extreme temperature, which was widely discussed in agriculture and applied meteorology research. The frost-free season changed, which was associated with global warming in the past few decades. In this study, the changes in three indices (the last frost day in spring, the first frost day in autumn, and the frost-free season length) of the frost-free season were investigated at 73 meteorological stations in the Tibetan Plateau from 1960 to 2010. Results showed that the last frost day in spring occurred earlier, significantly in 39 % of the 73 stations. For the regional average, the last frost day in spring occurred earlier, significantly at the rate of 1.9 days/decade during the last 50 years. The first frost day in autumn occurred later, significantly in 31 % of the stations, and the regional average rate was 1.5 days/decade from 1960 to 2010. The changing rate of the first frost day in autumn below 3,000 m was 1.8 times larger than the changing rate above 3,000 m. In addition, the first frost day in autumn above 3,000 m fluctuated dramatically before the early 1990s and then it was later sharply after the early 1990s. The frost-free season length increased significantly at almost all stations in the Tibetan Plateau from 1960 to 2010. For the regional average, the frost-free season lengthened at the rate of 3.1 days/decade. The changing rate of the frost-free season length below 3,000 m was more significant than the changing rate above 3,000 m. Eight indices of large-scale atmospheric circulation were employed to investigate the potential cause of the frost-free season length change in the Tibetan Plateau during the past 50 years. There was a significant relationship between the frost-free season length and the Northern Hemisphere Polar Vortex indices. The weakening cold atmospheric circulation might be an essential factor to the Tibetan Plateau warming since 1960.     

浙南春茶早春霜冻的时空分布特征

利用1971—2015年浙南7县(市、区)早春(2—4月)逐日气温观测资料,根据茶叶早春霜冻的气象灾害指标,采用多尺度趋势方法分别从空间、时间分析浙南茶叶早春霜冻的气候变化特征,并利用Mann-Kendall检验验证浙南茶叶早春霜冻的变化趋势和突变转折。结果表明:浙南茶叶早春霜冻发生次数较高,年均发生天数为16.8天,其中2月发生早春霜冻的可能性最高,且以轻度和重度霜冻为主。早春霜冻的空间分布结构由东南面向西北面递增,青田县发生早春霜冻的可能性最低,缙云县发生的可能性最高。茶叶早春霜冻日数的长期变化趋势表现为显著减少,且极端最低气温上升趋势明显,即浙南茶叶早春出现低温霜冻的可能性减小。浙南茶叶早春霜冻减少发生的突变点在20世纪80—90年代,且在21世纪初减少趋势最为显著。     

Trends of spring time frost events and phenological dates in Central Europe

Summary ?Over large parts of the Northern Hemisphere’s continents temperature has been increasing during the last century. Particularly minimum temperatures show a more pronounced increase than maximum temperatures. Not only the phenological seasons, but also the potentially plant damaging late frost events are governed by the atmosphere. In case of a rise of minimum temperatures one would expect phenological phases and spring late frost events to occur earlier. In this work the question is elucidated whether plant phenology shifts at a higher or lower rate towards earlier occurrences than potential plant damaging events, like spring late frost events. Frost events based on the last occurrence of daily minimum temperatures below a certain threshold have been moving faster to earlier occurrence dates than phenological phases during the last decades at 50 climate stations in Central Europe. Trend values of frost time series range around −0.2 days/year and of phenological time series are between −0.2 and 0.0 days/year over the period from 1951–1997. ‘Corylus avellana beginning of pollination’ is the only one of the 13 phases considered here with a lower trend value of −0.28 days/year. Early phases are more adapted to below zero temperatures and therefore follow more closely the temperature variability. Later phases seem to have more reason to be concerned about possible late frost events and react more cautiously towards higher spring temperatures and earlier last frost dates. The risk of late frost damage for plants should have been lower during the last decade as compared to the previous decades. Received June 28, 2002; accepted July 18, 2002     

松山区农业气候资源的变化特征分析

统计分析了1959—2009年松山区4—9月农业气候条件的变化特征。结果表明:1997年以来气候趋向变暖。80年代后,积温增加,初霜偏晚,终霜偏早,无霜期延长约10天,热量条件有利。51年来降水量具有多—少—多—少的波动变化特点,干旱频发且有增强的趋势。     

A topoclimatic classification to map spring frost risk for six deciduous tree fruit varieties

Abstract

A topoclimatic classification was developed to map the frequency of damaging spring radiation frosts to six deciduous fruit varieties at a scale of 1:25,000. The methodology utilizes night‐time temperatures from climatological stations operated by the Atmospheric Environment Service (aes) ofEnvironment Canada, as well as night‐time temperatures from a temporary network of standard climate stations and mobile temperature surveys. Maps of the deviation of minimum temperature from the local aes climate station, and of spring frost risk for each fruit variety are produced.     

Modeling monthly meteorological and agronomic frost days,based on minimum air temperature,in Center-Southern Brazil

Although Brazil is predominantly a tropical country, frosts are observed with relative high frequency in the Center-Southern states of the country, affecting mainly agriculture, forestry, and human activities. Therefore, information about the frost climatology is of high importance for planning of these activities. Based on that, the aims of the present study were to develop monthly meteorological (F _MET) and agronomic (F _AGR) frost day models, based on minimum shelter air temperature (T _MN), in order to characterize the temporal and spatial frost days variability in Center-Southern Brazil. Daily minimum air temperature data from 244 weather stations distributed across the study area were used, being 195 for developing the models and 49 for validating them. Multivariate regression models were obtained to estimate the monthly T _MN, once the frost day models were based on this variable. All T _MN regression models were statistically significant (p < 0.001), presenting adjusted R ² between 0.69 and 0.90. Center-Southern Brazil is mainly hit by frosts from mid-fall (April) to mid-spring (October). The period from November to March is considered as frost-free, being very rare a frost day within that period. Monthly F _MET and F _AGR presented significant sigmoidal relationships with T _MN (p < 0.0001), with adjusted R ² above of 0.82. The residuals of the frost day models were random, which means that the sigmoidal models performed quite well for interpreting the frost day variability throughout the study area. The highlands of Santa Catarina, Rio Grande do Sul, São Paulo, and Minas Gerais had in average more than 25 and 13 frosts per year, respectively, for F _MET and F _AGR. The F _MET and F _AGR maps developed in this study for Center-Southern Brazil is a useful tool for farmers, foresters, and researchers, since they contribute to reduce frost spatial and temporal uncertainty, helping in planning project for strategic purposes. Furthermore, the monthly F _MET and F _AGR maps for this Brazilian region are the first zoning of these variables for the country.     

Predicting the time of green up in temperate and boreal biomes

Direct observations as well as Normalized Difference Vegetation Index (NDVI) data from satellites have shown earlier leaf appearance in the northern hemisphere, which is believed to result from climate warming. The advance of leaf out to earlier times in the year could be limited or even reversed, however, as temperate and boreal trees require a certain amount of chilling in winter for rapid leaf out in spring. If this chilling requirement is not fulfilled, an increasing amount of warming is required. Implications of these chilling requirements at the biome level are not clear. One approach to estimate their importance is to generalize the exponential relationships between chilling and warming established for single species. Previous work using NDVI data suggests that this is indeed feasible but much has been limited to specific biomes or a very few years of data for the modelling. We find chilling requirements for northern temperate and boreal biomes by fitting various phenology models to green-up dates determined from NDVI using various methods and 12 years of data. The models predict that in northern middle and high latitudes the advance of green-up will be limited to a total of 4 to 5 days on average (but up to 15 days regionally) over the time period 2000–2060 as estimated using two contrasting climate simulations. This results from the exponentially increasing warming requirements for leaf out when winter chilling falls below a threshold as shown by a comparison with models that consider only spring warming. The model evaluation suggests an element of regional adaptation of the warming required for leaf out in large biomes.     

Specification of thermal growing season in temperate China from 1960 to 2009

A number of studies have reported an extension of the thermal growing season in response to the warming climate during recent decades. However, the magnitude of extension depends heavily on the threshold temperature used: for a given area, a small change in the threshold temperature results in significant differences in the calculated thermal growing season. Here, we specified the threshold temperature for determining the thermal growing season of local vegetation across 326 meteorological stations in temperate China by using vegetation phenology based on satellite imagery. We examined changes in the start, end, and length of the thermal growing season from 1960 to 2009. The threshold temperatures for determining the start and end increased strongly with increasing mean annual temperature. Averaged across temperate China, the start of the thermal growing season advanced by 8.4?days and the end was delayed by 5.7?days, resulting in a 14.1-day extension from 1960 to 2009. The thermal growing season was intensively prolonged (by 0.59?day/year) since the mid-1980s owing to accelerated warming during this period. This extension was similar to that determined by a spatially fixed threshold temperature of 5?°C, but the spatial patterns differed, owing to differences in the threshold temperature and to intra-annual heterogeneity in climate warming. This study highlights the importance of specifying the temperature threshold for local vegetation when assessing the influences of climate change on thermal growing season, and provides a method for determining the threshold temperature from satellite-derived vegetation phenology.     

Phenology shift from 1989 to 2008 on the Tibetan Plateau: an analysis with a process-based soil physical model and remote sensing data

Phenology is critical to ecosystem carbon quantification, and yet has not been well modeled considering both aboveground and belowground environmental variables. This is especially true for alpine and pan-arctic regions where soil physical conditions play a significant role in determining the timing of phenology. Here we examine how the spatiotemporal pattern of satellite-derived phenology is related to soil physical conditions simulated with a soil physical model on the Tibetan Plateau for the period 1989–2008. Our results show that spatial patterns and temporal trends of phenology are parallel with the corresponding soil physical conditions for different study periods. On average, 1 °C increase in soil temperature advances the start of growing season (SOS) by 4.6 to 9.9 days among different vegetation types, and postpones the end of growing season (EOS) by 7.3 to 10.5 days. Soil wetting meditates such trends, especially in areas where warming effect is significant. Soil thermal thresholds for SOS and EOS, defined as the daily mean soil temperatures corresponding to the phenological metrics, are spatially clustered, and are closely correlated with mean seasonal temperatures in Spring and Autumn, respectively. This study highlights the importance and feasibility of incorporating spatially explicit soil temperature and moisture information, instead of air temperature and precipitation, into phenology models so as to improve carbon modeling. The method proposed and empirical relations established between phenology and soil physical conditions for Alpine ecosystems on the Tibetan plateau could also be applicable for other cold regions.     

气候变暖背景下修水县春茶气候品质等级变化趋势分析

基于1961—2019年修水县气象观测资料,运用气候倾向率方法分析了近59 a春茶采摘期间（3月1日—5月10日）主要气象要素、春霜冻灾害和基于气候品质指数的春茶气候品质等级的变化趋势。结果表明：在气候变暖背景下,1961—2019年修水县春茶采摘期间平均气温呈明显上升趋势,平均相对湿度呈减小趋势,日照时数呈显著增加趋势;春霜日数呈明显减少趋势,春霜冻发生概率在下降;近59 a来修水县春茶气候品质等级为特优、优和良的总日数呈增加趋势;春茶采摘期间,气候品质特优等级的概率随时间呈先增加后减小的趋势,等级为优和良的概率呈增加趋势。综合考虑平均气温、平均相对湿度、日照时数等气象条件,以及春霜冻和春茶气候品质等级变化特征,认为气候变暖有利于修水县春茶品质趋好。     

Combined Impact of Climate Change,Cultivar Shift,and Sowing Date on Spring Wheat Phenology in Northern China

Distinct climate changes since the end of the 1980 s have led to clear responses in crop phenology in many parts of the world. This study investigated the trends in the dates of spring wheat phenology in relation to mean temperature for different growth stages. It also analyzed the impacts of climate change, cultivar shift, and sowing date adjustments on phenological events/phases of spring wheat in northern China(NC).The results showed that significant changes have occurred in spring wheat phenology in NC due to climate warming in the past 30 years. Specifically, the dates of anthesis and maturity of spring wheat advanced on average by 1.8 and 1.7 day(10 yr)~(-1). Moreover, while the vegetative growth period(VGP) shortened at most stations, the reproductive growth period(RGP) prolonged slightly at half of the investigated stations. As a result, the whole growth period(WGP) of spring wheat shortened at most stations. The findings from the Agricultural Production Systems Simulator(APSIM)-Wheat model simulated results for six representative stations further suggested that temperature rise generally shortened the spring wheat growth period in NC.Although the warming trend shortened the lengths of VGP, RGP, and WGP, the shift of new cultivars with high accumulated temperature requirements, to some extent, mitigated and adapted to the ongoing climate change. Furthermore, shifts in sowing date exerted significant impacts on the phenology of spring wheat.Generally, an advanced sowing date was able to lower the rise in mean temperature during the different growth stages(i.e., VGP, RGP, and WGP) of spring wheat. As a result, the lengths of the growth stages should be prolonged. Both measures(cultivar shift and sowing date adjustments) could be vital adaptation strategies of spring wheat to a warming climate, with potentially beneficial effects in terms of productivity.     

山西近50 a初霜冻的气候变化特征及其周期分析

初霜冻日的变化对农业生产的布局和耕作模式具有一定的影响。利用山西62个气象观测站点1961-2010年的逐年初霜冻日及逐日最低地温资料,应用统计学方法分析了山西初霜冻日的变化特征,结果表明:1)山西近50 a平均初霜冻日的时间分布总体为推后趋势,且具有明显的年代际特征,20世纪70年代、90年代初霜冻日提早显著,20世纪60年代、80年代和21世纪初霜冻日明显推后。2)山西近50 a平均初霜冻日的空间分布为"5级阶梯"型分布,从9月中旬到11月上旬,自北向南相继出现初霜冻。3)M-K突变检验表明,山西近50 a平均初霜冻日在1989年发生了一次显著的气候突变。4)山西近50 a初霜冻日变化趋势的分布具有明显的区域特征,大部分地区初霜冻日有推后的趋势,但南部和中南部的部分地区初霜冻日有提前的趋势。5)Morlet小波分析结果表明,山西的初霜冻、轻微初霜冻、中度初霜冻和重度初霜冻发生的主周期分别为5 a、2 a、5 a、6 a;目前山西正处在初霜冻和轻微初霜冻的推后期、中度初霜冻和重度初霜冻的提前期。     

Long-term spatial and temporal trends in frost indices in Kansas, USA

Frost indices such as number of frost days (nFDs), number of frost-free days (nFFDs), last spring freeze (LSF), first fall freeze (FFF), and growing-season length (GSL) were calculated using daily minimum air temperature (T_min) from 23 centennial weather stations across Kansas during four time periods (through 1919, 1920–1949, 1950–1979, and 1980–2009). A frost day is defined as a day with T_min?<?0 °C. The long- and short-term trends in frost indices were analyzed at monthly, seasonal, and annual timescales. Probability of occurrence of the indices was analyzed at 5 %, 25 %, 50 %, 75 %, and 95 %. Results indicated a general increase in T_min from 1900 through 2009 causing a decrease in nFDs. LSF and FFF occurred earlier and later than normal in the year, respectively, thereby resulting in an increase in GSL. In general, northwest Kansas recorded the greatest nFD and lowest T_min, whereas southeast Kansas had the lowest nFD and highest T_min; however, the magnitude of the trends in these indices varied with location, time period, and time scales. Based on the long-term records in most stations, LSF occurred earlier by 0.1–1.9 days/decade, FFF occurred later by 0.2–0.9 day/decade, and GSL was longer by 0.1–2.5 day/decade. At the 50 % probability level, Independence in the south-eastern part of Kansas had the earliest LSF (6 April), latest FFF (29 October) and longest GSL (207 days). Oberlin (north-western Kansas) recorded the shortest GSL (156 days) and earliest FFF (7 October) had the latest LSF (2 May) at the 50 % probability level. A positive correlation was observed for combinations of indices (LSF and GSL) and elevation, whereas a negative correlation was found between FFF and elevation.     

21世纪气候变化情景下环北极地区植被生长季与活动积温变化

基于部门间影响模式比较计划（ISI-MIP, Inter-Sectoral Impact Model Intercomparison Project)对CMIP5中5个气候（地球）系统模式模拟结果的降尺度数据,利用多模式集合预估了气候变化情景下21世纪环北极地区植被生长季与活动积温变化。研究发现:1）多模式集合模拟能够基本再现观测的初、终霜日及无霜期长度与>10°C积温的空间分布特征以及1979～2004年各指标变化趋势的空间分布特征,但其对气候变化年际变率的模拟能力较弱;2）至21世纪末,终霜日最多将提前60 d,初霜日将推迟20～40 d,无霜期延长幅度最高可达100 d,积温将增加1000～1200°C。其中RCP8.5情景下,各指标变幅最大,RCP2.6情景下变幅最小;3）各指标变幅呈现出较大的空间差异,亚欧大陆中西部的变幅普遍较大,随着气候变暖,>10°C积温增加幅度表现出明显的纬度地带性,南部增幅较大,北部增幅较小。     

Combined impact of climate change,cultivar shift,and sowing date on spring wheat phenology in Northern China

Distinct climate changes since the end of the 1980s have led to clear responses in crop phenology in many parts of the world. This study investigated the trends in the dates of spring wheat phenology in relation to mean temperature for different growth stages. It also analyzed the impacts of climate change, cultivar shift, and sowing date adjustments on phenological events/phases of spring wheat in northern China (NC). The results showed that significant changes have occurred in spring wheat phenology in NC due to climate warming in the past 30 years. Specifically, the dates of anthesis and maturity of spring wheat advanced on average by 1.8 and 1.7 day (10 yr)^?1. Moreover, while the vegetative growth period (VGP) shortened at most stations, the reproductive growth period (RGP) prolonged slightly at half of the investigated stations. As a result, the whole growth period (WGP) of spring wheat shortened at most stations. The findings from the Agricultural Production Systems Simulator (APSIM)-Wheat model simulated results for six representative stations further suggested that temperature rise generally shortened the spring wheat growth period in NC. Although the warming trend shortened the lengths of VGP, RGP, and WGP, the shift of new cultivars with high accumulated temperature requirements, to some extent, mitigated and adapted to the ongoing climate change. Furthermore, shifts in sowing date exerted significant impacts on the phenology of spring wheat. Generally, an advanced sowing date was able to lower the rise in mean temperature during the different growth stages (i.e., VGP, RGP, and WGP) of spring wheat. As a result, the lengths of the growth stages should be prolonged. Both measures (cultivar shift and sowing date adjustments) could be vital adaptation strategies of spring wheat to a warming climate, with potentially beneficial effects in terms of productivity.     

Evaluating the vegetation growing season changes in the arid region of northwestern China

Temperature has long been accepted as the major controlling factor in determining vegetation phenology in the middle and higher latitudes. The influence of water availability is often overlooked even in arid and semi-arid environments. We compared vegetation phenology metrics derived from both in situ temperature and satellite-based normalized difference vegetation index (NDVI) observations from 1982 to 2006 by an example of the arid region of northwestern China. From the satellite-based results, it was found the start of the growing season (SOS) advanced by 0.37 days year^?1 and the end of the growing season (EOS) delayed by 0.61 days year^?1 in Southern Xinjiang over 25 years. In the Tianshan Mountains, the SOS advanced by 0.35 days year^?1 and the EOS delayed by 0.31 days year^?1. There were almost no changes in Northern Xinjiang. Compared with satellite-based results, those estimates based on temperature contain less details of spatial variability of vegetation phenology. Interestingly, they show different and at times reversed spatial patterns from the satellite results arising from water limitation. Phenology metrics derived from temperature and NDVI conclude that water limitation of onset of the growing season is more severe than the cessation. Phenology spatial patterns of four oases in Southern Xingjiang show that, on average, there is a delay of the SOS of 1.6 days/10 km of distance from the mountain outlet stations. Our results underline the importance of water availability in determining the vegetation phenology in arid regions and can lead to important consequences in interpreting the possible change of vegetation phenology with climate.     

ENSEMBLES-based assessment of regional climate effects in Luxembourg and their impact on vegetation

Projected future regional climate changes in Luxembourg are assessed based on a six-member ensemble of regional climate models (RCM) from the ENSEMBLES project. The key aspects are projected changes in air temperature and their impacts on vegetation. Up to now, there have been only few assessments of future climate conditions for Luxembourg. As agriculture is the dominant land use in Luxembourg, possible effects on crops and vegetation in general are highly relevant. Different RCMs at 25 km spatial and a daily temporal resolution, ranging from 1961 to 2100 based on the SRES A1B emission scenario are used. To reduce systematic biases in the RCM-derived time series, a bias correction is applied. Multi-model annual mean temperatures are projected to increase by 3.1 °C between the reference time span (1961 to 1990) and the far future (2069 to 2098). Clear change signals are found in seasonal bivariate frequency distributions of air temperature and precipitation. Derived impacts are an elongation of the thermal vegetation period by 6.2 days per decade due to an earlier onset in spring; growing degree day sums show a substantial increase leading to potentially better growth conditions; the earlier onset of the vegetation period causes an increase in late frost risk, especially in the near future (2021 to 2050) projections compared to the reference period.     

安徽省冬小麦春霜冻害气象指标的研究

本文根据安徽省12个农业气象观测站冬小麦春霜冻害观测调查数据,针对不同品种(春性和半冬性)冬小麦,全面分析了拔节期前15 d至拔节后20 d的最低气温变化规律,以日最低气温为指标,将春霜冻害等级划分为轻度和重度2个级别,并分时段确定等级。验证结果表明,虽然春霜冻害的发生受地形、土壤等多种因素的影响,但最低气温指标基本能够反映出冬小麦拔节前后不同品种春霜冻害的发生规律,可以在霜冻监测预警业务中应用。用该指标进行霜冻发生风险分析的结果表明,安徽省冬小麦主产区为轻度霜冻频发区、重度霜冻基本不发区,冬小麦春性品种春霜冻高发区比半冬性品种明显偏南,可用于指导冬小麦品种合理布局,减轻春霜冻危害风险。     

Inter-Annual Variability and Decadal Trends in Alpine Spring Phenology: A Multivariate Analysis Approach

Plant phenological observations are of increasing value as indicators of climate change and variability. We developed a robust multispecies estimate for Swiss Alpine spring phenology for the period 1965–2002 by applying empirical orthogonal function (EOF) analysis on a combination of 15 spring phases. The impact of climate parameters such as temperature and precipitation on the phenological development was investigated using a multivariate statistical model. This multispecies estimate proved to be a good approach to assess the pattern of spring appearance during the last 40 years. It revealed an earlier onset of spring in recent years, mainly since 1988 when a clear shift in spring appearance occurred. The mean overall trend of 1.5 days per decade was clearly driven by winter and spring temperatures whereas precipitation showed no significant influence. The dominant EOF patterns suggested a general climate forcing for the observed inter-annual variability independent of single plant phases. A more regional phenology signal was found in the second EOF mode, indicating slightly weaker phenological trends in southern Switzerland as well as in higher altitudes. Both, temperature and precipitation contributed to this pattern significantly. Analysis of single phases confirmed the pattern of the multispecies estimate. All species showed trends towards earlier appearance ranging from −1 to −2.8 days per decade and the appearance dates had a very high covariance with temperature.     

Impacts of climate change on the risk of snow-induced forest damage in Finland

This work studied the temporal and spatial variability of the risk of snow-induced forest damage in Finland under current and changing climatic conditions until the end of this century. The study was based on a snow accumulation model in which cumulative precipitation, air temperature and wind speed were used as input variables. The risk was analyzed in terms of the number of days per year when the accumulated amount of snow exceeded 20 kg m^???2. Based on the risk, the forest area and mean carbon stock of seedling, young thinning and advanced thinning stands at risk were calculated. Furthermore, the number of 5-day periods, when the accumulated amount of snow exceeded a risk limit, was calculated for the current and changing climatic conditions in order to study the frequency of damaging snowfalls. Compared to the baseline period 1961–1990, the risk of snow-induced forest damage and the amount of damaging snowfalls were predicted to decrease from the first 30-year period (1991–2020) onwards. Over the whole country, the mean annual number of risk days decreased by 11%, 23% and 56% in the first, second and third 30-year period, respectively, compared to the baseline period. In the most hazardous areas in north-western and north-eastern Finland, the number of risk days decreased from the baseline period of over 30 days to about 8 days per year at the end of the century. Correspondingly, the shares of the forest area at risk were 1.9%, 2.0% and 1.0% in the first, second and third 30-year period, respectively. The highest mean annual carbon stocks of young stands at risk were found in central, north-eastern and north-western Finland in the first and second 30-year period, varying between 0.6 and 1.2 Mg C ha^???1 year^???1, meaning at highest 3% of the mean carbon stock (Mg C stem wood ha^???1) of those areas. This study showed that although the risk of snow-induced forest damage was mainly affected by changes in critical weather events, the development of growing stock under the changing climatic conditions also had an effect on the risk assessment. However, timely management of forest stands in the areas with a high risk of snow-induced damage contributes to the trees’ increased resistance to the damage.