Vegetation phenology is one of the most direct and sensitive indicators of terrestrial ecosystem in response to climate change. Based on daily mean air temperature at 877 meteorological stations over northern China from 1961 to 2015, the correlations and differences for different definitions of the growing season parameters (start, end, and length of the growing season) were investigated, and results show that higher correlations of 0.81–0.93 are found when indices which do not consider frost are compared with those of the same length which include the frost criteria, and lower correlations of 0.63–0.79 are observed when the length of indices is different and one of the indices includes the frost criteria or EI 3 (10 d < 5°C) is included. Lower correlations and larger differences are generally observed in the eastern and northwestern parts while higher correlation and smaller difference appeared in the northeastern and southwestern parts of northern China; thus the applicability comparison and selection of different definitions have important influence on the identifying and counting of the timing and length of the growing season in the eastern and northwestern regions of northern China.
The globalmean surface temperature has increased by a linear trend of 0.85 [0.65–1.06]°C from 1880 to 2012, and the increase in surface temperature during 2081–2100 is projected to likely be 0.3–4.8°C relative to 1986–2005 [
The thermal growing season is a span of time within which plants can theoretically grow; in other words, the constraints of air temperature on plant growth are released during this time and consequently growth can take place if other environmental needs are satisfied [
However, few studies have conducted a comparative analysis of the homogeneity and difference of different definitions used to calculate the thermal growing season in one or more specific areas. Walther and Linderholm [
This work is aimed at comparing the different definitions of growing season parameters over northern China, based on daily mean air temperature data at 877 stations during 1961–2015. The paper is organized as follows. Section
Daily mean air temperature data from the National Meteorological Information Center, China Meteorological Administration were utilized to calculate each type of growing season parameters (start, end, and length of the growing season). There are totally 1218 observation stations over northern China with the span of time from 1951 to 2015, and quality control of data was conducted before release by the departure accumulating method. Before 1961, the observation data had higher missing rate and the number of stations was less, so only those stations with longtime average missing rate of less than 1% in the whole year, spring, and autumn, respectively, during 1961–2015 were included in our study. Finally, 877 out of 1218 stations were selected (Figure
The administrative division (a) and spatial distribution of selected meteorological stations (b) in northern China.
Based on daily mean temperature data, fifteen indices of the growing season parameters, including four indices of the start of growing season (SI), five indices of the end of growing season (EI), and six indices of the length of growing season (LI), were calculated according to the definitions in Table
Definition of the start, end, and length of the thermal growing season indices used in this study.
Parameters  Indices  Definition 

Start of the growing season  =5 d > 5°C (SI 1)  5day spell with 
>5 d > 5°C (SI 2)  6day spell with 

=5 d > 5°C Fr (SI 3)  5day spell after the last 

>5 d > 5°C Fr (SI 4)  6day spell after the last 



End of the growing season  =5 d < 5°C (EI 1)  5day spell with 
>5 d < 5°C (EI 2)  6day spell with 

10 d < 5°C (EI 3)  10day running mean of 

Fr OR = 5 d < 5°C (EI 4)  First autumn/winter 

Fr OR > 5 d < 5°C (EI 5)  First autumn/winter 



Length of the growing season  =5 d > 5°C  =5 d < 5°C (LI 1)  Start (SI 1), end (EI 1) 
>5 d > 5°C  >5 d < 5°C (LI 2)  Start (SI 2), end (EI 2)  
=5 d > 5°C Fr  10 d < 5°C (LI 3)  Start (SI 3), end (EI 3)  
>5 d > 5°C Fr  10 d < 5°C (LI 4)  Start (SI 4), end (EI 3)  
=5 d > 5°C Fr  Fr OR = 5 d < 5°C (LI 5)  Start (SI 3), end (EI 4)  
>5 d > 5°C Fr  Fr OR > 5 d < 5°C (LI 6)  Start (SI 4), end (EI 5) 
For indices of growing season start, SI 1 (=5 d > 5°C) and SI 2 (>5 d > 5°C) were defined as the first 5day and 6day spells, respectively, with daily mean temperature remaining above 5°C, according to Carter [
The definitions of the end of growing season were corresponding to those of the start to some extent. EI 1 (=5 d < 5°C) and EI 2 (>5 d < 5°C) were defined as the first 5day and 6day spells, respectively, with daily mean temperature remaining below 5°C [
Combining the start and end indices, six different indices of the growing season length were obtained. LI 1 (=5 d > 5°C  =5 d < 5°C) was the combinations of SI 1 and EI 1 [
To compare the homogeneity and the difference of different definitions, correlation analysis was performed for indices in each type of growing season parameters using the Pearson correlation coefficient (
The correlation coefficients for the indices of the growing season start in the whole study are listed in Table
Pearson’s correlation coefficient (
Start of GS  >5 d > 5°C (SI 2)  =5 d > 5°C Fr (SI 3)  >5 d > 5°C Fr (SI 4)  

=5 d > 5°C (SI 1) 




0.56  0.99  0.37  1.00  0.34  0.97  
>5 d > 5°C (SI 2)  — 



0.39  0.97  0.57  1.00  
=5 d > 5°C Fr (SI 3)  —  — 


0.61  1.00 
Correlation coefficients for combinations of the end indices of growing season. Single bold values show the mean coefficients over northern China, and the paired values below show the minimum (left) and maximum (right) coefficients.
End of GS  >5 d < 5°C (EI 2)  10 d < 5°C (EI 3)  Fr OR = 5 d < 5°C (EI 4)  Fr OR > 5 d < 5°C (EI 5)  

=5 d < 5°C (EI 1) 





0.39  0.99  0.36  0.96  0.37  1.00  0.23  0.99  
>5 d < 5°C (EI 2)  — 




0.25  0.94  0.25  0.99  0.31  1.00  
10 d < 5°C (EI 3)  —  — 



0.23  0.96  0.21  0.94  
Fr OR = 5 d < 5°C (EI 4)  —  —  — 


0.55  1.00 
Correlation coefficients for combinations of indices of growing season length. Single bold values show the mean coefficient over northern China, and the paired values below show the minimum (left) and maximum (right) coefficients.
Length of GS  SI 2  EI 2 (LI 2)  SI 3  EI 3 (LI 3)  SI 4  EI 3 (LI 4)  SI 3  EI 4 (LI 5)  SI 4  EI 5 (LI 6)  

SI 1  EI 1 (LI 1) 






0.64  0.97  0.48  0.97  0.37  0.96  0.34  1.00  0.32  0.95  
SI 2  EI 2 (LI 2)  — 





0.38  0.93  0.51  0.96  0.31  0.95  0.40  1.00  
SI 3  EI 3 (LI 3)  —  — 




0.74  1.00  0.54  0.98  0.40  0.97  
SI 4  EI 3 (LI 4)  —  —  — 



0.49  0.97  0.56  0.98  
SI 3  EI 4 (LI 5)  —  —  —  — 


0.65  0.99 
The correlations of the growing season start were spatially differentiated and were statistically significant at the 0.01 level in all regions of northern China. Stations with higher correlation coefficients among combinations of indices were consistently located in the northeastern part of the study area, whereas in the western parts, there was no constant and strong relationship between different indices (Figure
Correlation coefficients for combinations of the start indices of growing season in northern China.
SI 1 versus SI 2
SI 1 versus SI 3
SI 2 versus SI 3
SI 3 versus SI 4
The differences in the start of the growing season were also spatially differentiated, with smaller mean and maximum differences in the northeastern and western parts and larger values in the central and eastern parts of the study area (Figure
Differences (days) in the start of the growing season between selected indices in northern China. Shown are the mean values ((a), (c), (e), and (g)) and maximum values ((b), (d), (f), and (h)).
Mean SI 3SI 1
Maximum SI 3SI 1
Mean SI 4SI 2
Maximum SI 4SI 2
Mean SI 2SI 1
Maximum SI 2SI 1
Mean SI 4SI 3
Maximum SI 4SI 3
For combinations of SI 1 versus SI 2 and SI 3 versus SI 4, both the mean and the maximum differences were larger than those of SI 1 versus SI 3 and SI 2 versus SI 4 as a whole. The mean differences for SI 1 versus SI 2 mainly ranged from 2.0 to 5.0 days, accounting for 94.5% of the total stations across northern China, and larger differences were mainly distributed in the southeastern part of the study area, with the mean of 3.5–6.5 days (Figure
The mean correlation coefficients among combinations of indices of the growing season end ranged from 0.63 to 0.88 (Table
The correlations between indices of the growing season end were different from those of the start indices and had large spatial difference, but they were also significant at the 0.01 level in almost all areas of northern China. Stations with higher correlation coefficients among combinations of indices were mostly located in the western part of the study area, whereas in the eastern and northeastern part, there was no constant and strong relationship between different indices (Figure
Correlation coefficients for combinations of the end indices of growing season in northern China.
EI 1 versus EI 2
EI 1 versus EI 3
EI 1 versus EI 4
EI 2 versus EI 3
EI 3 versus EI 4
EI 4 versus EI 5
The correlation coefficients for EI 1 versus EI 4 were higher as a whole, and in most areas they ranged from 0.65 to 1.00, accounting for 96.0% of the total stations across northern China; especially in central and southern Xinjiang, northern Tibet, western Qinghai, and some areas of Gansu, Shaanxi, Hebei, Inner Mongolia, and other provinces, they were over 0.95 at most stations (Figure
For the combination of EI 3 and EI 4, correlation coefficients mainly ranged from 0.50 to 0.80 in northern Xinjiang, western Inner Mongolia, eastern Qinghai, and some areas in the northeastern and eastern parts of the study area, accounting for 64.5% of the total stations, but in central and southern Xinjiang, northern Tibet, western Qinghai, and some areas of Gansu, Shaanxi, Hebei, Inner Mongolia, and Heilongjiang, they were over 0.80 in most areas, accounting for 30.8% of the total stations (Figure
The differences in the end of the growing season were also spatially differentiated in northern China, with smaller mean and maximum differences in the western and northeastern parts and larger values in the central and eastern parts of the study area (Figure
Differences (days) in the end of the growing season between selected indices in northern China. Shown are the mean values ((a), (c), (e), (g), (i), and (k)) and maximum values ((b), (d), (f), (h), (j), and (l)).
Mean EI 1EI 4
Maximum EI 1EI 4
Mean EI 2EI 5
Maximum EI 2EI 5
Mean EI 1EI 3
Maximum EI 1EI 3
Mean EI 2EI 3
Maximum EI 2EI 3
Mean EI 2EI 1
Maximum EI 2EI 1
Mean EI 5EI 4
Maximum EI 5EI 4
Compared with above indices, the end of the growing season produced by EI 3 was systematically earlier (Figures
For the combination of EI 1 and EI 2, larger mean differences were mainly distributed in the eastern, northeastern, and some northwestern parts of the study area, with the mean of 1.6–3.2 days at most stations, and the differences were statistically insignificant at all stations except for several stations (Figure
The correlations among six indices of the growing season length are shown in Table
The correlations between indices of the growing season length were generally higher and statistically significant in the whole study area; especially in the majority of the western and northeastern parts, they were mainly over 0.75, and the spatial differences were less than those of the start and end indices (Figure
Correlation coefficients for combinations of indices of growing season length in northern China.
LI 1 versus LI 2
LI 1 versus LI 3
LI 1 versus LI 4
LI 1 versus LI 5
LI 3 versus LI 4
LI 3 versus LI 5
LI 4 versus LI 5
LI 5 versus LI 6
For combination of LI 1 and LI 5, there were 93.5% of the total stations with correlation coefficients of over 0.75 (Figure
The differences in the length of the growing season are displayed in Figure
Differences (days) in the length of the growing season between selected indices in northern China. Shown are the mean values ((a), (c), (e), and (g)) and maximum values ((b), (d), (f), and (h)).
Mean LI 1LI 3
Maximum LI 1LI 3
Mean LI 2LI 4
Maximum LI 2LI 4
Mean LI 1LI 5
Maximum LI 1LI 5
Mean LI 2LI 6
Maximum LI 2LI 6
For combinations of LI 1 and LI 5 and LI 2 and LI 6, smaller differences were observed in most of the southwestern and northeastern parts of the study area, with the mean values of 0–5.0 days and the maximum differences of 0–24 days at most stations (Figures
Plant phenology is a sensitive and observable indicator of terrestrial ecosystem response to climate change and has attracted increasing attention in the past few decades in the context of global change [
In the central and eastern parts of the study area, there were larger differences in the timing and length of the growing season generally (Figures
In the northeastern part of the study area, the correlations between indices were higher and the differences were lower (Figures
Over northern China, the occurrence of the last spring frost (daily mean temperature less than 0°C) generally preceded the first 5 or 6day spell with daily mean temperature remaining above the 5°C threshold (Figure
Average differences (days) in the start of the growing season between SI 1, SI 2, and the last spring frost (LSF) date in northern China during 1961–2015.
SI 1LSF
SI 2LSF
Average differences (days) in the end of the growing season between EI 1, EI 2, EI 3, and the first autumn/winter frost (FAF) date in northern China during 1961–2015.
EI 1FAF
EI 2FAF
EI 3FAF
However, some studies indicated that including the frost criterion or not in the definition of the start of the growing season could cause large differences. This is inconsistent with our results. For example, Walther and Linderholm [
Biological and environmental factors are the main factors influencing the plant growth, especially air temperature, photoperiod, and water [
Over northern China, higher correlation coefficients were found when indices without regard for frost were compared with those including the frost criteria and both had the same length of the spells exceeding the 5°C temperature threshold, and lower correlations were observed when the length of indices was different and one of the indices included the frost criteria or EI 3 (10 d < 5°C) was included in the combinations. Including the frost criterion or not in the definitions of the start, end, and the length of the growing season could provide smaller mean differences than the differences from the length of the spells exceeding 5°C temperature threshold, but the maximum differences were large. So the consideration of spring/autumn frost is important for determining the annual timing and length of the growing season regarding the maximum differences.
The correlations and differences of the growing season indices were spatially differentiated. Larger differences and lower correlations were generally found in the eastern and northwestern parts and smaller differences and higher correlations were observed in the northeastern and southwestern parts of the study area, so the northern China can be divided into an eastern and northwestern part and a northeastern and southwestern part according to the growing season variability. In the eastern and northwestern parts of the study area, growing season was more sensitive to the changes in index definitions, and it is important to compare the local applicability of different definitions according to the phenological observation and remote sensing monitoring. In the northeastern and southwestern parts, less difference in the growing season indices was observed, so the choice of definitions had less influences on the growing season characteristics.
The authors declare that they have no conflicts of interest.
This work was supported by National Natural Science Foundation of China (nos. 41571044 and 41001283), Climate Change Special Fund of the China Meteorological Administration (CCSF201716), and China Clean Development Mechanism (CDM) Fund Project (no. 2012043). The authors thank Bowen Zhang and Peipei Wei, Ecological Technique and Engineering College, Shanghai Institute of Technology, Shanghai, China, for their contributions.