Temporal and spatial variations of the thermal growing season in China during

Transcription

1 METERLGICAL APPLICATINS Meteorol. Appl. 25: (18) Published online 23 August 17 in Wiley nline Library (wileyonlinelibrary.com) DI: 10.2/met.1669 Temporal and spatial variations of the thermal growing season in China during Linli Cui, a * Jun Shi b and Yue Ma b a Shanghai Satellite Remote-Sensing and Application Centre, Shanghai Meteorological Bureau, China b Shanghai Climate Center, Shanghai Meteorological Bureau, China ABSTRACT: Based on the daily mean air temperature at 1863 meteorological stations over China from 1961 to 15, the temporal and spatial tendencies in the growing season parameters (the start date of the growing season, GSS; the end date of the growing season, GSE; and the length of the growing season, GSL) for 0, 3, 5 and 10 C temperature thresholds were investigated; the results showed that in the past 55 years GSL exhibited a significant extending trend at speeds of days per 10 years in most areas of northwest China (), north China (), northeast China () and western southwest China (). The extension of GSL was attributed to an earlier GSS at a rate of 1.5 days per 10 years in most areas of and and some parts of and, and a later GSE at speeds of days per 10 years in most of and northern and some parts of and northwestern. The extending trend of the GSL was prominent in northern and western China relative to southern China. The temporal variations in the regional average growing season parameters were different for four temperature thresholds and in six regions. In general, the regional GSL (GSS, GSE) was extended (advanced, delayed) continuously during or had no significant trend before about 19 and then extended (advanced, delayed) continuously. The temporal variations of the GSE were weaker than those of the GSS in most regions. KEY WRDS growing season length (GSL); growing season start (GSS); growing season end (GSE); trends; China Received 29 November 16; Revised 12 January 17; Accepted March Introduction The global-mean surface temperature shows a linear warming trend of 0.85 C during 18 12, and the increase in surface temperature is projected to be C globally for 81 2 relative to (IPCC, 13). In the context of climate change and global warming, a growing body of research based on satellite-derived vegetative indices, climatological data and phenological observation has shown the significant changes in the timing and length of the growing season associated with increases of air temperature in most parts of the northern hemisphere in the th Century (Chen et al., 05; Linderholm et al., 08; Barichivich et al., 13; Wang et al., 14; Xia et al., 15; Ci et al., 16). Variations in the growing season length may not only have profound influences on natural ecosystems and agricultural production but also may lead to changes of vegetation cover and carbon sequestration, which in turn may affect the global and regional climate system (Robeson, 02; Linderholm, 06; Piao et al., 07; Mozafari and Torki, 15). It has become increasingly evident that research into the change of the vegetation growing season is one of the most important scientific issues for global change and ecosystem adaptation (Chen et al., 00). An increasing number of studies suggest a lengthening of the growing season as the air temperature has risen over the past several decades, where an earlier start of the growing season is most obvious (Qian et al., 12; Barichivich et al., 13; Høgda et al., 13; Nagai et al., 15). For example, the potential * Correspondence: L. Cui, 166 Puxi Road, Xuhui District, Shanghai 00, China. cllcontact@163.com thermal growing season at high northern latitudes lengthened by about 10.5 days during (Barichivich et al., 13). In Fennoscandia, a warming climate led to an earlier onset of the growing season by 11.8 ± 2.0 days during (Høgda et al., 13). In northern China, the thermal growing season extended by 13.1 days during and the majority of this extension (9.7 days) was attributed to an earlier spring start of the growing season (Song et al., 10). The thermal growing season on the Tibetan Plateau also extended significantly by 16.5 days during and most of this change (9.1 days) was attributed to an earlier start of the growing season (Dong et al., 12). In Xinjiang, China, a prolonged thermal growing season of 12.5 days was found by Ci et al. (16), but most of this change came from the later end date of the growing season relative to the earlier start date (8.0 vs 3.5 days) during However, previous research has some deficiencies that need to be rectified. First, with parts or regions of China as study areas, it is impossible to contrast the differences in growing season variations between different regions of China under the same processing methods in most existing studies (Dong et al., 12; Ding et al., 15; Ci et al., 16). Moreover, based on historic meteorological data from limited stations (Song et al., 10), up-to-date and detailed dynamics of the growing season cannot be revealed from previous studies because of the lack of the latest observation data from a high density network of stations. Finally, most existing research related to variations of the growing season are based on a single temperature threshold of 5 C(Frichet al., 02; Dong et al., 12), and there is a lack of a comprehensive analysis of the growing season using multiple thresholds. The objectives of the present study were to analyse the spatial and temporal variations of the thermal growing season 17 Royal Meteorological Society

2 Growing season in China 57 parameters (the growing season start, GSS; the growing season end, GSE; and the length of the growing season, GSL) in China, based on daily mean air temperature data at 1863 stations during The outline of the paper is as follows. The data and processing method are described in Section 2. Section 3 is dedicated to analysing the changes of the thermal growing season parameters in different regions of China. In Section 4, the main results and findings of this study are discussed. Finally, the main conclusions of the study are given in Section Data and methods 2.1. Data Daily mean air temperature data during the period from 1951 to 15, obtained from the National Meteorological Information Center, China Meteorological Administration, were used to calculate the growing season parameters. In total there are 2474 surface stations, and data quality control was performed by the departure accumulating method before release (Fischer et al., 12). Given the number of stations and the integrity of the original data, the observation data before 1961 were excluded from the study because of the limited number of stations and the high missing rate in the early years. Those stations with a long-time average missing rate of more than 1% during were also removed. Finally, 1863 of 2474 available meteorological stations were selected, which had a lower missing rate (<1%) in spring (from March to May), autumn (from September to November) and throughout the year in the past 55 years, and the data series covered the period from 1961 to 15 (Figure 1). The 1863 stations are not evenly distributed, with more stations in central, eastern and southern China and fewer stations in western and northeastern China, where the main land types are high mountains, deserts or bare lands. The statistics of the missing data show that there are 1736 stations (93.2% of the total 1863 stations) which have a multi-year average missing rate of less than 0.1%, and 107 stations (5.7% of the total stations) which have a missing rate of %. For the higher missing rate of %, there are stations (1.1% of the total stations), which are mainly distributed in the north China () and Yangtze River valley () regions (Figure 1). The inter-annual distribution of the missing data is also uneven. During two sub-periods, and 04 15, the multi-year average missing data were concentrated and the rate was higher (Figure 2); the highest percentage of missing data was 0.56%, which appeared in In each year, the missing data of one single day were filled in by the average of the two adjacent days before and after, and the missing values of two or more consecutive days were completed using a simple linear regression method according to the observation data of neighbouring stations which had the best correlation with the former data and had no missing data during the same period Methods The definition of growing season parameters in China In the middle and high latitudes, temperature has long been considered to be the major controlling factor of vegetation phenology (Wang et al., 14). The thermal growing season is a period of time when plant growth can occur in theory; in other words, the constraints of temperature on plant growth are cancelled and if other environmental needs are met growth will take place (Carter, 1998). The growing season is generally expressed as the number N Percentage (%) E Figure 1. Spatial distribution of the annual average percentage of missing data in China during Percentage of missing data Figure 2. Annual average percentage of missing data in China during of days when the air temperature is higher than a predefined threshold (Shen et al., 12). For middle and higher latitudes, the 5 C daily mean temperature is generally accepted as the temperature threshold for it is generally recognized as the lowest temperature required for plant growth (Frich et al., 02; Liu et al., 10; Song et al., 10; Barichivich et al., 12, 13). wing to the vast territory and diverse and complex climate in China, one single temperature threshold is not sufficient for calculation of the growing season and cannot be applicable to all stations (Song et al., 10). Shen et al. (12) also suggested that the magnitude of the extension of the thermal growing season relied on the adopted temperature threshold to a great extent, and a small difference in the threshold temperature could cause major differences in the calculation results for a given area. In China, when the daily mean air temperature in spring steadily passes through 3 C, crops such as winter wheat, highland barley, potato, sugarbeet and Chinese chives begin to turn green and spring wheat is sown, so the period during which the daily mean temperature is more than 3 C is called the growing season of cold-resisting crops. In addition, when the daily mean temperature steadily passes through 10 C, crops such as corn, millet, sorghum and soybean begin to grow; 10 Cisviewedas a critical temperature for thermophilic crops. Thus, in this study, the start date of the growing season (GSS), the end date of the growing season (GSE) and the resulting length of the growing season (GSL) were determined for four thresholds of daily mean temperature: 0, 3, 5 and 10 C; GSS is the last day in the first 6 day spell with daily mean air temperature remaining above 0, 3, 5 and 10 C respectively, and GSE is the first day in the first 6 day spell with daily mean temperature remaining below the corresponding thresholds. GSL is the number of days between 17 Royal Meteorological Society Meteorol. Appl. 25: (18)

3 58 L. Cui et al. Table 1. Linear trends (days per decade) in the regional average GSS, GSE and GSL for the four temperature thresholds during Regions Growing season start (GSS) Growing season end (GSE) Growing season length (GSL) 0 C 3 C 5 C 10 C 0 C 3 C 5 C 10 C 0 C 3 C 5 C 10 C 1.5 a 1.5 a 1.6 a 1.7 a a 1.2 a 1.3 a 2.1 a 2.5 a 2.8 a 2.9 a 2.4 a 2.5 a 2.3 a 1.9 a a 2.8 a 3.2 a a 2.7 a a a 2.1 a 2.9 a a 1.3 a 1.3 a 1.3 a 1.4 a 1.6 a 1.5 a 1.2 a 2.9 a 2.8 a 2.7 a 2.5 a 0.6 a 0.7 a 1.0 a 1.0 a 0.4 a 0.5 a 0.6 a 1.2 a 0.9 a 1.2 a 1.6 a 2.1 a a Statistically significant at the 5 level. Table 2. Linear trends (days per decade) in the regional average GSS, GSE and GSL for the four temperature thresholds during Regions Growing season start (GSS) Growing season end (GSE) Growing season length (GSL) 0 C 3 C 5 C 10 C 0 C 3 C 5 C 10 C 0 C 3 C 5 C 10 C a a 1.3 a a 2.7 a 3.5 a 3.4 a 2.5 a a 3.9 a 4.6 a 4.5 a 3.9 a a a 0.1 a 0.3 a a 2.9 a 2.7 a 2.6 a 2.3 a 2.4 a 2.5 a 2.1 a 5.0 a 5.3 a 5.1 a 4.7 a 0.8 a a 0.5 a 0.6 a 0.8 a 1.9 a 1.2 a 1.2 a 1.9 a 4.4 a a Statistically significant at the 5 level. GSS and GSE, where GSS and GSE are Julian (calendar) days The variation of growing season parameters in China According to the four thresholds determined above, the thermal growing season parameters (GSS, GSE and GSL) were extracted for each year and at each station. Given the natural geography and climate, the whole of China was further divided into six regions, i.e. northwest China (), north China (), northeast China (), the mid-lower Yangtze River valley (), south China () and southwest China (), which is in line with other work (Xu et al., 11; Shi et al., 16) and is shown in Figure 1. For analysing the temporal variations of growing season parameters in each region, the regional average GSS, GSE and GSL were calculated in each year using the simple average method based on the number of stations in the six regions of China. In terms of the regional average values of growing season parameters in the six regions of China, linear trend values were calculated for by the statistical method of ordinary least squares (LS) regression (Kruger and Sekele, 13; de Lima et al., 13) and statistical significances were tested at the 95% confidence level using a two-tailed Student s t test. LS regression is a generalized linear modelling technique; it is one of the major techniques used to analyse data and forms the basis of many other techniques (e.g. ANVA and the generalized linear models) (Hutcheson, 11). By minimizing the sum of square differences between the observed and predicted values, the LS regression method can closely fit a function with the data. Many studies from remote sensing monitoring or surface observation have shown that the growing season has extended significantly in China since the 19s (Zheng et al., 02; Song et al., 10; Shen et al., 12), and so the entire study period ( ) was further divided into two sub-periods (i.e and ), and the trends in GSS, GSE and GSL were further calculated and compared for the two sub-periods. Based on the annual values of the GSS, GSE and GSL at each station, the linear trend values were also calculated by the statistical method of LS regression for each station. In accordance with the latitude and longitude of each station, the Kriging interpolation technique was applied to generate the spatial trend of GSS, GSE and GSL, which reflected the changes of growing season parameters on a decade timescale; the results are displayed by Golden Software Surfer. Areas of significant change (P < 5) are coloured while those with insignificant changes (P > 5) are not coloured and are shown as white spaces. Given the sparse meteorological stations in some parts of northeastern China and most parts of western China, the interpolation results of the growing season parameters in these areas may not reflect the real variations correctly. 3. Results 3.1. The date of the growing season start (GSS) Regional trends in the GSS For the four thresholds of daily mean temperature, i.e. 0, 3, 5 and 10 C, the regional average date of the GSS was advanced significantly in the,, and regions, with rates of , , and days per decade respectively during (Table 1). In the and regions, the regional average date of the GSS for the four temperature thresholds advanced rapidly during , with rates of and days per 10 years respectively (Table 2). The date of the GSS advanced significantly only at the 10 C temperature threshold in the region in the past 55 years, with an advancing rate of 2.3 days per 10 years. In the region, the average date of the GSS varied insignificantly for the four temperature thresholds during , but during it advanced significantly at the 3 and 5 C temperature thresholds, with rates of 0.1 and 0.3 days per decade respectively. The regional average date of the GSS also advanced significantly for the 10 C temperature threshold in the, and regions in the past 35 years, with rates of 1.7, 6.1 and 2.6 days per decade respectively (Table 2). 17 Royal Meteorological Society Meteorol. Appl. 25: (18)

4 Growing season in China 59 N N 0 C 3 C E E N N 5 C 10 C E E Figure 3. Linear trends (days per decade) in the date of the growing season start for China during using the following temperature thresholds: 0 C, 3 C, 5 C, 10 C. Regions with significant trends are shown as a grey-scale map while those with insignificant trends are shown as white spaces Spatial trends in the GSS For the four thresholds of daily mean temperature, the date of the GSS advanced significantly at speeds of days per 10 years in most parts of the and regions and some areas of and during , with advancing rates of 1.5 days per 10 years accounting for the largest areas (Figure 3). There was only one station with a significant delay of the GSS for the 3 C temperature threshold in the region and three stations with a significant delay of the GSS for the 5 C temperature threshold in the region; at the 0 and 10 C temperature thresholds, there was no station with a significant delay of the GSS. Statistics show that there were 511, 652, 758 and 886 stations with significant advancing speeds of days per 10 years at the 0, 3, 5 and 10 C temperature thresholds, accounting for 27.4, 35.0,.7 and 47.6% respectively of the total stations across China, and there were 285, 390, 473 and 595 stations with significant advancing speeds of 1.5 days per 10 years at the above four temperature thresholds, accounting for 15.3,.9, 25.4 and 31.9% respectively of the total stations. In several very small areas in the southeastern, eastern, and regions, the date of the GSS was advanced significantly at speeds of days per 10 years. In the vast majority of the and regions, the changes in the GSS were not significant for the 0, 3, 5 and 10 C temperature thresholds. With increase of the temperature threshold, areas with a significant advance of the GSS increased, especially in the eastern and southeastern regions Temporal variations in the GSS The temporal variations in the date of the GSS were different in the six regions. For the four temperature thresholds, the GSS either advanced continuously during or varied with no significant trend before about 19 and then advanced continuously during (Figure 4). In the region, the date of the GSS for 0 and 3 C temperature thresholds advanced continuously during and then varied with no significant trends, but for the 10 C temperature threshold the GSS varied with no significant trends before about 19 and then advanced continuously. The date of the GSS for the 5 C temperature threshold advanced continuously in the region during In the region, the date of the GSS for 0, 3 and 5 C temperature thresholds advanced continuously during , and that for the 10 C temperature threshold varied with no significant trend before about 19 and then advanced continuously. In the region, the date of the GSS advanced significantly only at the 10 C temperature threshold during , with a delay before the 19s and then a continuous shift to an earlier time. In the region, the dates of the GSS for the 0, 3, 5 and 10 C temperature thresholds were all delayed slightly before 19 and then advanced continuously. In the region, the dates of the GSS for the 0, 3 and 5 C temperature thresholds all advanced continuously, and that for the 10 C temperature threshold was delayed slightly before 19 and then advanced continuously during (Figure 4) The date of the growing season end (GSE) Regional trends in the GSE The regional average date of the GSE was delayed significantly for the four temperature thresholds in the and regions in both periods, with delaying rates of and days per decade respectively during and and days per decade respectively during (Tables 1 and 2). Except for the 0 C temperature threshold, the date of the GSE in the region was delayed 17 Royal Meteorological Society Meteorol. Appl. 25: (18)

5 60 L. Cui et al y = 0.17 x R 2 = y = 0.19 x R 2 = y = 0.23 x R 2 = y = 7 x R 2 = 4 (e) y = 0.13 x R 2 = 0.22 (f) y = 0.10 x R 2 = Figure 4. Time series of the date of the growing season start (GSS) for six regions of China using 10 C as the temperature threshold: northeast China; north China; the Yangtse River valley; south China; (e) northwest China; (f) southwest China. The inter-annual variations are shown, and their corresponding quadratic polynomial regressions are the straight lines. significantly at speeds of days per 10 years during , and it was delayed significantly for the 5 and 10 C temperature thresholds during , with rates of 1.5 and 1.3 days per decade respectively. In the region, the regional average date of the GSE was delayed significantly only at the 10 C temperature threshold during and , with a rate of 0.8 and 1.3 days per 10 years respectively (Tables 1 and 2). The date of the GSE in the and regions changed insignificantly during and for the four temperature thresholds Spatial trends in the GSE The date of the GSE for the four temperature thresholds was delayed significantly at speeds of 0 days per 10 years in most parts of and northern regions and some areas of northwestern and regions during , with delaying speeds of days per 10 years accounting for the largest areas (Figure 5). There were only 2, 1, 1 and 5 stations with a significant advance of the GSE for the 0, 3, 5 and 10 C temperature thresholds respectively. The statistics show that there were 176, 3, 349 and 381 stations with significant delaying speeds of 0 days per 10 years for the 0, 3, 5 and 10 C temperature thresholds, accounting for 9.4, 16.3, 18.7 and.5% respectively of the total stations across China, and there were 118, 225, 247 and 277 stations with significant delaying speeds of days per 10 years at the four temperature thresholds, accounting for 6.3, 12.1, 13.3 and 14.9% respectively of the total stations. In some small areas of the and regions, the date of the GSE was delayed significantly at over days per 10 years. In the vast majority of the,, central and southern and southeastern regions, the changes of the GSE were not significant at the 0, 3, 5 and 10 C temperature thresholds. With an increase of temperature threshold, areas with a significant delay of GSE increased, especially in the and northwestern regions Temporal variations in the GSE The temporal variations in the date of the GSE were weaker than those of the GSS for a variety of temperature thresholds and in most regions. In general, the date for the GSE was either delayed continuously during or varied with no significant trends before about 19 and then was delayed continuously (Figure 6). The date of the GSE for the 0, 3 and 5 C temperature thresholds varied with no significant trends before about 19 and then was delayed continuously and that for the 10 C threshold was delayed continuously before about 00 and then varied with no significant trends in the region. In the region, the date of the GSE for the 0, 3, 5 and 10 C 17 Royal Meteorological Society Meteorol. Appl. 25: (18)

6 Growing season in China 61 N N 0 C 3 C N N 5 C 10 C E E E E Figure 5. Linear trends (days per decade) in the date of the growing season end for China during using the following temperature thresholds: 0 C, 3 C, 5 C, 10 C. Regions with significant trends are shown as a grey-scale map while those with insignificant trends are shown as white spaces. temperature thresholds varied with no significant trends before about 1985 and then was delayed slightly over the last 55 years. In the and regions, the date of the GSE for the 0, 3 and 5 C temperature thresholds was delayed continuously during The date of the GSE for the 10 C temperature threshold varied with no significant trends before about 1985 and then was delayed continuously in the region, but in the region it advanced slightly before about 19 and then was delayed continuously The length of the growing season (GSL) Regional trends in the GSL For the four thresholds of daily mean temperature, the regional average GSL extended significantly in the,, and regions, with rates of extension of , , and days per decade respectively during (Table 1). In the region, except for the 0 C temperature threshold, the regional average GSL again extended significantly, at days per 10 years in the past 55 years. In the region, the regional average GSL varied insignificantly for the four temperature thresholds during During the period from 1981 to 15, the regional average GSL in the, and regions extended rapidly for the four temperature thresholds, with rates of , and days per decade respectively (Table2). In the and regions, the regional average GSL extended significantly only for the 10 C temperature threshold, with extension rates of 3.1 and 7.5 days per decade respectively in the past 35 years Spatial trends in the GSL For the four temperature thresholds, the annual GSL extended significantly at days per 10 years in most parts of the,, and western regions during , with rates of extension of days per 10 years accounting for the largest areas (Figure 7). There were only one and two stations with significant shortening trends of GSL for the 3 and 5 Ctemperature thresholds respectively, and for the 0 and 10 C temperature thresholds there were no stations with significant shortening of the GSL. The statistics show that there were 525, 759, 888 and 3 stations with significant extension speeds of days per 10 years at the 0, 3, 5 and 10 C temperature thresholds, accounting for 28.2,.7, 47.7 and 53.8% respectively of the total stations across China, and there were 443, 668, 7 and 900 stations with significant extension speeds of days per 10 years for the four temperature thresholds, accounting for 23.8, 35.9, 41.9 and 48.3% respectively of the total stations. In some small areas of the southern, eastern, and regions, the annual GSL extended significantly at speeds of days per 10 years. In the vast majority of the, and southeastern regions, the changes in GSL were not significant at the 0, 3, 5 and 10 C temperature thresholds. With an increase in temperature threshold, areas with significant extension of the GSL increased, especially in the northeastern, southern and and southeastern regions Temporal variations in the GSL n the whole, the GSL in the six regions either extended continuously during or varied with no significant trends before about 19 and then extended continuously. In the region, the GSL for the 0, 3, 5 and 10 C temperature thresholds extended continuously during (Figure 8). In the region, the GSL for the 0 and 3 C temperature thresholds extended continuously, but that for the 5 and 10 C thresholds varied with no significant trends before about 19 and then extended continuously. In the region, the GSL for the 17 Royal Meteorological Society Meteorol. Appl. 25: (18)

7 62 L. Cui et al. 294 y = 0.13 x R 2 = y = 8 x R 2 = (e) y = 6 x R 2 = y = 0.12 x R 2 = (f) y = 2 x R 2 = y = 0.12 x R 2 = Figure 6. Time series of the date of the growing season end (GSE) for six regions of China using 10 C as the temperature threshold: northeast China; north China; the Yangtse River valley; south China; (e) northwest China; (f) southwest China. The inter-annual variations are shown, and their corresponding quadratic polynomial regressions are the straight lines. 3 and 5 C temperature thresholds varied with no significant trends before about 19 and then extended continuously, and that for the 10 C threshold shortened slightly before 1985 and then extended continuously. The GSL for the 0, 3, 5 and 10 C temperature thresholds varied with no significant trends before about 1985 and then extended continuously in the region. In the region, the GSL for the 0, 3 and 5 C temperature thresholds extended continuously and that for the 10 C threshold varied with no significant trends before about 1985 and then extended continuously. 4. Discussion The GSL is important for local agriculture, carbon sequestration potentials, the ecological environment and human activities (Brown et al., 10). An earlier onset and longer length of the growing season are generally related to an increase of ecosystem carbon sequestration because there are more days available for biomass growth and carbon uptake (Richardson et al., 10). A reduction of the frost days and an increase in the duration of heat waves and the length of the growing season were revealed by both surface observations and modern model simulations in the later th Century (IPCC, 13). Barichivich et al. (13) studied the variations in the growing season at northern high latitudes during with combined satellite and ground observations and found that during the potential thermal growing season had extended by 10.5 days and an overall lengthening of the growing season was prominent in Eurasia relative to North America (12.6 vs 6.2 days). Variations in the length of the growing season in China have important scientific and practical significance for global climate change monitoring and the assessment of the regional ecological environment. In China, the average GSL extended by days per 10 years in most areas of,, and western during (Figure 7), which is in accordance with the results from Song et al. (10) and Liu et al. (10). Song et al. (10) analysed the changes in thermal growing season in China and showed that the growing season had extended at a rate of 1.3 days per 10 years in southern China and 2.3 days per 10 years in northern China during Liu et al. (10) analysed the climatic growing season in China and found that during the GSL had extended at speeds of days per decade. Yang et al. (13) investigated variations in the thermal growing season parameters in east China and their results also showed that over the period the regional average GSL had significantly extended at rates of 5 and 2.61 days per 10 years using threshold temperatures of 5 and 10 C, respectively. Dong et al. (12) analysed the variations in the thermal growing season throughout the Tibetan Plateau and their results showed that the regional average GSL had prolonged significantly at a rate of 3.3 days per decade during Royal Meteorological Society Meteorol. Appl. 25: (18)

8 63 Growing season in China N 0 C N 3 C N E 1 5 C N E 1 10 C E E Figure 7. Linear trends (days per decade) in the growing season length for China during using the following temperature thresholds: 0 C, 3 C, 5 C, 10 C. Regions with significant trends are shown as a grey-scale map while those with insignificant trends are shown as white spaces. The extension of the GSL is attributed to an earlier GSS at speeds of 1.5 days per 10 years in most parts of the and regions and some areas of the and regions, and a later GSE at speeds of days per 10 years in most areas of the and northern regions and some areas of the and northwestern regions (Figures 3 and 5), which is also in agreement with existing results. For example, Liu et al. (10) reported that the average GSS had been advanced days while the average GSE had been delayed days in China during In temperate China, Shen et al. (12) found that on average the start date of the thermal growing season had shifted 8.4 days earlier although the end date had moved 5.7 days later from 1960 to 09. Song et al. (10) also showed that the changes in the GSL in northern China (2.3 days per decade) were mostly attributed to an earlier spring start (1.7 days per decade), but in southern China the earlier spring start of the growing season (0.6 days per decade) was almost equal to the later end in autumn (0.7 days per decade). For a large geographical area, changes in the thermal growing season over a period have different amplitudes in different parts (Walther and Linderholm, 06). In terms of spatial trends, the lengthening of the growing season was prominent in northern China and western China relative to southern China in the past 55 years; in the, and the southeastern parts of the region in particular the trend was not significant (Figure 7). This is consistent with some previous results. Song et al. (10) also showed that there were great spatial differences in the growing season parameters over China during , and the changes in growing season were stronger and more prominent in northern China than southern China. Based on phenological observation data at 26 stations, Zheng et al. (02) found that plant phenophases had advanced in northern China, northeastern China and the lower Yangtze region in spring but were delayed 17 Royal Meteorological Society in areas in the middle reaches of the Yangtze River and the eastern part of southwestern China since the 19s. Using daily temperature data at 642 stations in China, Xu and Ren (04) analysed the change trend in the temperature-defined growing season during , and their results also showed that the growing season had extended by 10.2 and 4.2 days in northern China and southern China respectively, with the largest extension of 18.2 days occurring on the Qinghai-Tibet Plateau. In middle and higher latitudes, air temperature and water availability are two major factors controlling plant growth, and plant growth is largely restricted by air temperature, so the linear trends in the regional average GSS, GSE and GSL for the four temperature thresholds were mostly significant in the,, and regions with a rising temperature during (Table 1). However, in lower latitudes, such as in the and regions of China, temperature and precipitation are no longer the main factors limiting plant growth, and other factors such as continuous rain, evapotranspiration and soil parameters become restrictive factors so the linear trends of the growing season parameters were basically insignificant in the and regions during and (Tables 1 and 2). The relationships between climate and growing season are regionally different. Song et al. (10) also found that the GSL in southern China was less influenced by climate warming in comparison with that in northern China. In the past 55 years, the regional average GSL (GSS, GSE) at the four temperature thresholds extended (advanced, delayed) continuously or had no significant trend before about 19 and then extended (advanced, delayed) continuously in China (Figures 4, 6 and 8). These findings have also been reported in other studies. For example, Liu et al. (10) showed that the variations in the length of the climatic growing season in China could be divided into two distinct phases during , nationally Meteorol. Appl. 25: (18)

9 64 L. Cui et al y = 0.29 x R 2 = y = 0.27 x R 2 = y = 0.29 x R 2 = y = 0.10 x R 2 = (e) y = 0.25 x R 2 = 0.41 (f) 275 y = 0.21 x R 2 = Figure 8. Time series of the date of the growing season length (GSL) for six regions of China using 10 C as the temperature threshold: northeast China; north China; the Yangtse River valley; south China; (e) northwest China; (f) southwest China. The inter-annual variations are shown, and their corresponding quadratic polynomial regressions are the straight lines. an initial period where growing season indicators fluctuate near a base period average and a second period of rapid increase of the GSL. Qian et al. (12) analysed the changes of the growing season in Canada and also found an earlier GSS along with a lengthened growth season during compared with the previous periods of and Using normalized difference vegetation index values obtained from the Global Inventory Modeling and Mapping Studies (GIMMS) and SPT VEGETATIN, Ding et al. (15) investigated the GSS over the Tibetan Plateau and found the GSS values presented an advancing trend between 1982 and 1998, but there was no continuously advancing trend of the GSS during In the and regions, the onset date of the growing season had no continuously advancing trend for most temperature thresholds after 00 (Figure 4). Existing studies have shown that a sizeable difference in growing season values could result from a small change in the temperature threshold (Walther and Linderholm, 06; Shen et al., 12). Walther and Linderholm (06) also showed large differences in GSL trends in the Greater Baltic Area using many growing season definitions. In this study, the thermal growing season parameters (GSS, GSE and GSL) estimated from the four temperature thresholds show different spatial trends along latitudinal and altitudinal gradients (Figures 3, 5 and 7). Therefore, it is important to determine the temperature threshold correctly in assessing the impact of climate warming on the growing season. The statistics of the thermal growing season with a single temperature threshold can sometimes be incorrect. Integrated satellite remote sensing monitoring and plant phenological observations, comparing the applicability of different indices with a variety of temperature thresholds in the calculation of local thermal growing season parameters, and further analysing the spatial and temporal variations and simulating the evolution process of the growing season are the key points of future studies. The spatial-temporal information of the thermal growing season is applicable to the analysis of vegetation distribution, agricultural management and climatic simulation, and the policy making for climate change mitigation and adaptation, ecosystem management, healthcare and tourism (Shen et al., 12). Lengthening of the growing season has potential for earlier sowing, multiple cropping, ensuring maturation and increasing agricultural production (Linderholm, 06; Walther and Linderholm, 06; Ci et al., 16); in particular, it is beneficial for the seasonal yields of perennial crops such as alfalfa and other hay crops. However, the negative impacts of global climate change, including changing rainfall pattern, increasing transpiration and evaporation, increasing weeds, insects and diseases and favouring invasive species, should also be paid sufficient attention in the planning and management of sustainable agriculture and natural ecosystems (Santos et al., 15; Ci et al., 16). 17 Royal Meteorological Society Meteorol. Appl. 25: (18)

10 Growing season in China Conclusions The temporal and spatial changes in the thermal growing season in China for 0, 3, 5 and 10 C temperature thresholds were analysed and the results indicated that the start date of the growing season had shifted 1.5 days per decade earlier in China while the end date had moved days per decade later, extending the growing season length by days per decade depending on the temperature threshold chosen. However, there are a lot of space and time differences in the change of the growing season in China, where the greatest changes happened in western and northern parts of the study area. In the northeast China, north China, northwest China and southwest China regions, average growing season length was extended significantly at rates of , , and days per decade respectively for the four temperature thresholds, but in the south China region the variations were not significant during The regional growing season length also had different inter-annual variations during Some growing season lengths were extended continuously in the past 55 years, while others showed no significant trend before about 19 and then extended continuously. Acknowledgements This work was supported by the National Natural Science Foundation of China (No , and 41283) and China Clean Development Mechanism (CDM) Fund Project (No. 143). Peipei Wei and Bowen Zhang, Ecological Technique and Engineering College, Shanghai Institute of Technology, Shanghai, China, are thanked for their contributions. References Barichivich J, Briffa KR, Myneni RB, sborn TJ, Melvin TM, Ciais P, et al. 13. 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