3.1. Annual Variations and Trends of the Major Climate Factors Affecting Apple Quality
shows the annual variations in the six major climate factors that may affect apple quality. The multi-year averages of the six climate factors were all within the optimal ranges for high apple quality. Among the annual variation trends of all of the factors, average annual temperature and average summer temperature were both significantly increased; annual sunshine duration was significantly decreased; the summer diurnal temperature range and summer air relative humidity were both decreased, but these variations were flat. In particular, the average annual temperature and average summer temperature were significantly increased after the 1990s, and the corresponding temporal trend reached 0.25 °C/10a−1
and 0.17 °C/10a−1
, whereas the variation in the annual sunshine duration was significantly reduced after the 1980s, reaching 43 h/10a−1
. The variation in the summer diurnal temperature range was reduced after the 1980s with a rate of −0.16 °C/10a−1
. Moreover, the inter-annual variation of precipitation included large magnitudes, but the decreasing trend was not significant with a temporal trend of only −5.5 mm/10a−1
. In addition, the summer air humidity varied with precipitation and started to slightly decrease around the 2000s.
The annual variations in the entire major production regions of China show that all of the average summer temperatures after the 1990s exceeded the upper limit of the optimal range for producing high-quality apples because of the effect of climate change. After the 2000s, however, the sunshine hours of some years were even lower than the lower limit of the optimal range. Thus, increased summer temperatures and reduced sunshine hours on the country scale might have affected the physicochemical indices including the fruit shape index, hardness, titratable acid content, sugar-acid ratio, and peel anthocyanin.
and Figure 3
show the distributions of the average values, change trend ratios, and annual variations of the six climate factors within the optimal index ranges in the apple-producing sub-regions. Temperature was the most important environmental factor in determining apple quality, and it was therefore significantly correlated with fruit hardness, soluble sugar content, and fruit coloring [12
]. In regions with relatively high annual average temperatures, the sugar-acid ratio was usually high, but the fruit hardness was low. Therefore, these fruits could not be stored for a long time. In contrast, in regions with low annual average temperatures, the titratable acid content was too high, which may affect fruit taste. The average annual temperatures in the different producing regions differ: The average annual temperature in the old course of the Yellow River was the highest, whereas that in Xinjiang was the lowest at 14.8 °C and 7.3 °C, respectively. Moreover, the average annual temperatures in Bohai Bay and the Loess Plateau were both within the optimal index range for good apple quality. The average annual temperatures in the old course of the Yellow River and Southwest Highlands were higher than the upper limit of the optimal range, whereas the titratable acid content, fruit hardness, VC content, and peel anthocyanin were lower than those of the other apple-producing regions. The average annual temperatures in all of the producing regions were increased. In particular, the temperature variation in the Loess Plateau was the most significant, 0.32 °C/10a−1
, and that in the Southwest Highlands was the smallest, only 0.18 °C/10a−1
. As Figure 3
shows, the increasing temperature trend had likely positive effects in the Loess Plateau. Since the late 1980s, the average annual temperature has always been within the optimal range. In contrast, the increased temperatures might negatively affected apple quality in the old course of the Yellow River and the Southwest Highlands, which had already exceeded the upper limit of the optimal range, especially with regard to the fruit shape index, titratable acid content, VC content, and peel anthocyanin.
Except for Xinjiang and the Loess Plateau, the percentages of irrigation in the apple-producing regions were low. Thus, the apple shape index, hardness, soluble sugar, and peel anthocyanin relied strongly on precipitation [28
]. Of the five apple-producing regions, annual precipitation was the most abundant in the Southwest Highlands, followed by the Old Course of the Yellow River, whereas precipitation in Bohai Bay was most suitable. Moreover, precipitation in the Loess Plateau was slightly insufficient, and the natural precipitation in Xinjiang did not satisfy the normal growth of apples. The variation in precipitation does not clearly show regional differentiation characteristics. Precipitations in the Southwest highlands and Bohai Bay were slightly decreased but showed significant increases in other locations (except for Xinjiang); however, the precipitation variations in the other regions were not significant. Overall, the precipitation in all of the apple-producing regions tended to shift toward the optimal range (Figure 3
) for growing high-quality apples.
A sufficient number of sunshine hours is important to assure high-quality apples with regard to the apple shape index, peel anthocyanin, and sugar-acid ratio [10
]. Insufficient sunshine hours may affect fruit coloring and sugar accumulation; however, extensive sunshine hours can decrease the fruit shape index [22
]. Of the five fruit-producing regions, the sunshine in Xinjiang was the highest, whereas that in the Loess Plateau and Bohai Bay did not assure good apple quality. In contrast, the sunshine conditions in the Old Course of the Yellow River and Southwest highlands were slightly worse, especially in the case of the latter, which had fewer than 1,800 sunshine hours. The sunshine hours in all of the apple-producing regions were consistently decreased but with significant variations. In particular, the decrease in sunshine hours was most significant for the Old Course of the Yellow River, which already had insufficient sunshine hours. The decreasing rate reached –86 h/10a−1
. As Figure 3
shows, sunshine hours have gradually decreasing since the 1990s to a level below the lower limit of the optimal range, and the decreasing rate of the sunshine hours in the Southwest highlands (which had the fewest sunshine hours) also reached –26 h/10a−1
, which inhibited good apple quality. Although the sunshine hours in Bohai Bay were significantly decreased, the effect on its apple quality was not large. Overall, the decreasing sunshine trends in the Old Course of the Yellow River and Southwest Highlands might negatively affected apple quality.
Summer is a key stage for fruit to grow and convert sugars. Therefore, the average summer temperature is significantly correlated with the titratable acid content, fruit hardness, peel anthocyanin, and sugar-acid ratio [30
]. Temperatures exceeding the optimal range can result in a quick decrease in apple hardness and inhibit fruit coloring [25
]. The average summer temperatures in both Bohai Bay and the Old Course of the Yellow River were higher than those in the other apple-producing regions, which were also higher than the optimal temperature, especially in the case of the Old Course of the Yellow River with temperature of 26 °C. The temperatures in the other apple-producing regions were all within the optimal range. All of the average summer temperatures in the five apple-producing regions were increased, and the increases in the Loess Plateau and Xinjiang were most significant. Figure 3
shows that the average summer temperature in the Southwest Highlands gradually increased to a level above the upper limit of the optimal index since the 2000s. In sum, the increased summer temperatures might have negligibly affected the Loess Plateau but might have threatened other producing regions.
The optimal summer diurnal temperature range plays an important role in determining apple hardness, soluble sugar level, and the sugar-acid ratio [22
]. Within the optimal range, a large diurnal temperature range can significantly increase peel anthocyanin; if the temperature is too high, however, then it can hamper sugar accumulation and affect the conversion of peel anthocyanin, leading to decreased quality [22
]. The diurnal temperature ranges in the Loess Plateau and Bohai Bay were generally close to the optimal range, whereas those in the Old Course of the Yellow River and Southwest Highlands were below the lower limit of the optimal range. In addition, the diurnal temperature range in Xinjiang was above the optimal range. The summer diurnal temperature range in all of the regions tended to decrease, and these decreases might positively have affected the apple quality of the Loess Plateau and Xinjiang producing regions. Figure 3
shows that the average diurnal temperature range has gradually decreased since the 2000s to the optimal range in the Loess Plateau, whereas the diurnal temperature range in Bohai Bay, the old course of the Yellow River, and the Southwestern Highlands significantly decreased, which might have negatively affected apple quality.
Summer relative humidity is of great importance to the VC content, hardness, soluble sugar, and peel anthocyanin; furthermore, it is closely related to the occurrence index of orchard pest hazards [21
]. The relative humidity in the Loess Plateau and Bohai Bay were within the optimal range, whereas excessive precipitation resulted in relative humidity that exceeded the upper limit of the optimal range in the Old Course of the Yellow River and Southwest Highlands. The relative humidity in Xinjiang was clearly the lowest. Figure 3
shows that the relative humidity had slightly increased in Xinjiang since the 2000s, whereas the relative humidity tended to decrease in the other producing regions; these levels were suitable to grow high-quality apples in the Old Course of the Yellow River and Southwest Highlands.
3.3. Spatial Distribution Characteristics of the Major Climate Factors Affecting Apple Quality
Based on variations of the average climate factor values in the producing sub-regions, we compared the regional differences of the major climate factors affecting apple quality. Thus, 10 km × 10 km grids of five out of six climate factors showing abrupt changes affecting apple quality were calculated before and after the corresponding abrupt changes (Figure 4
, Figure 5
, Figure 6
, Figure 7
and Figure 8
). Significant spatial variations occurred before and after the abrupt changes for the climate factors that affect apple quality.
The average annual temperatures of all of the producing regions were significantly increased (Figure 4
), especially in the Northwest Xinjiang, Northwest Loess Plateau, and Southern Jiangsu regions with the increasing magnitudes exceeding 1 °C after the abrupt change. Overall, the optimal temperature distribution region for good apple quality was shifted toward the north. This result is because the variation in the average annual temperature may positively affect the Northern Loess Plateau producing region; in contrast, it may negatively affect Southwestern Bohai Bay and the Old Course of the Yellow River because very high temperatures lower fruit hardness, reduce acid content, and affect fruit coloring.
shows that after the abrupt climate change, the annual sunshine duration in the major producing regions was greatly reduced, with decreases in Bohai Bay and the Old Course of the Yellow River being the greatest, >150 h. Decreases in Gansu within the Loess Plateau and Southwest Highlands were relatively lower but approximately 50 h. The decreased sunshine hours resulted in less than 2000 sunshine hours in the Old Course of the Yellow River and Shandong within the Bohai producing region and less than 2200 h in Southern Hebei. The decreased sunshine hours in the above regions might have resulted in lower fruit sugar-acid ratios and more fruit coloring problems.
Comparisons of the variations in the spatial distribution of the average summer temperature during abrupt climate changes show that the average summer temperature was also significantly increased (Figure 5
). The spatial distribution characteristics of summer temperature before and after the abrupt change are similar to those of average annual temperature. The spatial distribution of the optimal regions with increased summer temperature show that the effect on the Bohai producing region was the most significant, resulting in an increase in summer temperatures above 25 °C in Liaoning, which is within the Bohai and Old Course of the Yellow River producing regions. This increased temperature greatly and negatively affected apple quality.
Although the average summer diurnal temperature range for all of the fruit-producing regions gradually decreased on average, regional differences existed (Figure 6
). The distribution of the average summer temperature after the abrupt climate change show that the values in Bohai Bay and the Old Course of the Yellow River were significantly decreased compared with those before the abrupt change, reaching above 0.5 °C; the values in Shaanxi, Ningxia, and Shanxi, which are within the Loess Plateau and Yun-Gui highland producing regions, were slightly increased. Based on the distribution of the optimal regions after the abrupt change, we conclude that the variations in the diurnal temperature range negatively affected Shandong, Southern Hebei, and Northern Henan but positively affected the major production regions along the boundaries of the Yun-Gui-Chuan provinces.
The spatial variations in summer air relative humidity are complicated. However, the overall variation magnitude was not large. The values in the Southwest Highlands, Old Course of the Yellow River, and Ningxia, which is within the Loess Plateau region, were decreased, whereas those in Liaoning, which is within the Bohai Bay producing region, and Southern Loess Plateau were slightly increased. A large increased magnitude in Southern Xinjiang of >10% was observed. The variation of air relative humidity in the summer may positively have affected the apple quality in the Southwest Highlands and the Old Course of the Yellow River.