Effects of Meteorological Factors and Frost Injury on Flowering Stage of Apples and Pears Across Regions at Varying Altitudes

Abstract

Recent meteorological variability in winter and spring complicates predicting and managing frost damage in apples and pears. This study investigated the relationship between frost injury during the flowering stages of apples (‘Hongro’ and ‘Fuji’) and pears (‘Wonwhang’ and ‘Niitaka’) and weather conditions across regions at varying altitudes. Orchards were categorized into coastal, inland, mid-mountainous, and mountainous regions, and flowering stages and frost injury were analyzed. The flowering period of apples, from the onset of blooming to full bloom, averaged approximately 15 days, which was about 3 to 5 times longer than that of pears. Furthermore, the total flowering duration of apples was 1.5 to 2.0 times longer than that of pears. Additionally, flowering exhibited a tendency to be delayed at higher altitudes. However, orchards situated in mid-mountainous regions experienced earlier flowering compared to those in other regions. Among the two apple cultivars, the average frost damage was more severe in central flowers than in lateral flowers and was relatively higher in ‘Fuji’ than in ‘Hongro’. In pears, frost damage was most prevalent in the first to fourth flowers within the inflorescence, with ‘Wonhwang’ exhibiting relatively greater susceptibility than ‘Niitaka’. Across different cultivation regions, severe frost damage was observed in orchards located in mid-mountainous areas for both fruit species and cultivars. The severity of frost damage followed the order mid-mountainous, mountainous, plain, and coastal regions. Minimum temperatures were lowest in mid-mountainous and mountainous regions, while daily temperature differences were largest in mid-mountainous regions. Meteorological analysis (1981 to 2022) revealed consistent trends, with an increase in daily temperature range in recent years. These findings highlight the vulnerability of mid-mountainous orchards due to flowering stages overlapping with coastal and plain regions, exposing them to lower temperatures and larger temperature differences. Further studies on temperature variability are essential to mitigate frost damage risks.

1. Introduction

Cold resistance in fruit trees is influenced by factors including water content, stored nutrients such as carbohydrates [1], and meteorological conditions. Extensive research on minimizing freeze injury has been conducted, with strategies categorized into preventive and emergency measures [2]. For preventive measures, the selection of location and terrain before establishing an orchard is paramount. This includes considerations such as cultivating on slopes [3], installing windbreaks [4], deciding whether to use cover crops [5], and assessing soil characteristics [6] in relation to planting styles and soil management methods. Emergency measures include techniques such as burning [4], irrigation [5], air blowing [7,8] and trunk covering [9].

Frost injury during the flowering period is a type of damage that occurs at the lowest temperatures during the early growth stages of fruit trees, with the extent of injury varying annually. The response to methods applied to mitigate this injury has a significant impact on both the quality and yield of fruit. For pears, it is advantageous for the third to fifth flowers in the inflorescence sequence to set fruit, while for apples, fruit setting on the central flower results in a higher proportion of marketable produce [10]. However, the flowering period is short, and the number of blossoms that need to be prioritized for protection is predetermined, which limits the application of preventive or emergency measures [11]. Additionally, methods for predicting the flowering period are primarily based on the influence of spring temperatures, making it difficult to fully account for the effects of winter temperatures on the dormancy and germination variability of flower buds [12]. Therefore, as immediate countermeasures for frost injury during the flowering period are challenging, it is important to adopt a preventive approach through prior measures. Furthermore, establishing secondary emergency measures could serve as an effective complement to these preventive actions.

The apple cultivation area in Jeonbuk State in Republic of Korea spans approximately 2003 ha, with 74.6% (1495 ha) concentrated in mountainous regions (Jangsu and Muju) above 300 m in altitude. In contrast, pears are predominantly cultivated in coastal and plain regions such as Jeonju, Gimje, Wanju, and Gochang, accounting for about 65.8% (335 ha) [13]. However, frost injury during the flowering period has been reported as particularly severe in mid-mountainous regions, such as the Imsil and Namwon areas. Moreover, recent climate changes have raised concerns regarding low-temperature injury to fruit trees during various growth stages (dormancy, germination, and blooming). While research related to cold resistance, such as studies on winter minimum extreme temperatures and temperature variations [14], has been conducted, studies on frost injury during the flowering period in relation to specific cultivation regions remain limited.

In general, temperature decreases with increasing altitude, implying a greater probability of flowering-stage frost damage in apples and pears at higher elevations. However, severe frost damage has been consistently reported over several years in specific regions of Jeonbuk State, Republic of Korea. To elucidate the underlying causes, we classified cultivation altitudes into four categories for two apple and two pear cultivars. Additionally, we sought to determine the meteorological factors influencing the regional variations in frost damage severity. Therefore, this study was conducted to investigate the timing and severity of frost injury during the flowering periods of apples and pears in the cultivation regions of Jeonbuk State in Republic of Korea. This study aims to analyze the correlation between the meteorological characteristics of each cultivation region and to utilize the data as foundational material for identifying areas at risk of frost injury during the flowering period and for establishing countermeasures.

2. Materials and Methods

2.1. Experimental Design

Recurrent low-temperature damage during the flowering period of apple and pear orchards has been documented in specific regions of Jeonbuk State, Republic of Korea. This study focuses on research sites situated within major apple and pear production zones, as depicted in Figure 1, to identify the underlying causes of low-temperature damage by analyzing its relationship with meteorological factors. Geographically, Jeonbuk State is bordered by the sea to the west and mountain ranges to the east. The areas most susceptible to low-temperature damage are primarily located in the central regions between these two geographic features, at elevations ranging from 100 to 300 m above sea level.

2.2. Division of Cultivation Regions and Selection of Surveyed Orchards

In 2013, the cultivation regions in Jeollabuk-do Province in Republic of Korea were classified by altitude into coastal (altitude below 100 m), inland (below 100 m altitude and away from the coastline), mid-mountainous (100–300 m altitude), and mountainous regions (above 300 m altitude) [15]. The location of the surveyed orchards for each altitude-based cultivation region is shown in

Table 1. The locations of the apple and pear orchards surveyed for flowering stages and meteorological factors.

Fruit Species	Region Classification by Altitude	Location (Sampling Points)	Altitude (m)	Distance from Coast (km)
Apple	Coastal	Seongdeok-myeon, Gimje-si (1)	12	12
	Inland	Geumma-myeon, Iksan-si (1)	17	30
	Mid-mountainous	Deokgwa-myeon, Namwon-si (4)	121, 138, 141, 148	75
		Beonam-myeon, Jangsu-gun (1)	275	90
	Mountainous	Doosan-ri, Jangsu-gun (1)	520	92
Pear	Coastal	Seongsan-myeon, Gunsan-si (1)	10	9
	Inland	Won-dong, Jeonju-si (1)	25	34
		Iseo-myeon, Wanju-gun (1)	38	35
	Mid-mountainous	Deokgwa-myeon, Namwon-si (2)	141, 170	75
		Gwanchon-myeon, Imsil-gun (1)	252	63
	Mountainous	Sanseo-myeon, Jangsu-gun (1)	358	82

, with eight orchards selected for apples and seven orchards for pears. The surveyed apple orchards were located at altitudes of 12 m (35°49′54″ N, 126°49′43″ E), 42 m (36°00′33″ N, 127°02′42″ E), 138 m (35°29′51″ N, 127°21′47″ E), 140 m (35°31′25″ N, 127°21′00″ E), 141 m (35°30′48″ N, 127°21′39″ E), 174 m (35°30′43″ N, 127°22′14″ E), 277 m (35°33′32″ N, 127°25′20″ E), and 520 m (35°37′12″ N, 127°30′44″ E). The surveyed pear orchards were located at altitudes of 14 m (35°35′34″ N, 126°41′39″ E), 25 m (35°51′37″ N, 127°01′56″ E), 38 m (35°50′13″ N, 127°01′52″ E), 141 m (35°30′48″ N, 127°21′39″ E), 170 m (35°30′43″ N, 127°22′14″ E), 290 m (35°40′44″ N, 127°17′09″ E), and 358 m (35°36′21″ N, 127°25′57″ E). The test orchards were selected based on their similar cultivation styles and distinct blooming characteristics relative to their cultivation regions.

2.3. Characteristics of Apple and Pear Cultivars and Their Cultivation Management

The apple cultivar ‘Hongro’ was developed in Republic of Korea through the hybridization of ‘Tsugaru’ and ‘Jonathan’. It is an early-maturing variety with a harvest period extending from late August to mid-September. In contrast, ‘Fuji’ was developed in Japan through the hybridization of ‘Red Delicious’ and ‘Ralls Janet’ and is a late-maturing cultivar, typically harvested from late October to early November. The pear cultivar ‘Wonhwang’ was developed through the hybridization of ‘Hwangkeumbae’ and ‘Nijisseiki’ and is classified as an early-maturing variety, typically harvested in August. In contrast, ‘Niitaka’ was developed through the hybridization of ‘Kikusui’ and ‘Nijisseiki’ and is a late-maturing variety, generally harvested from late September to early October.

The apple cultivar ‘Fuji’ and the pear cultivar ‘Niitaka’ are among the predominant commercially cultivated varieties in Republic of Korea. Additionally, in Republic of Korea, ‘Hongro’ and ‘Wonhwang’ have a relatively short distribution period, typically being consumed within 2 to 3 months after harvest. In contrast, ‘Fuji’ and ‘Niitaka’ are stored under controlled conditions and consumed until June or July of the following year.

In all surveyed orchards, apple trees were grafted onto M9 rootstock, whereas pear trees were grafted onto Pyrus pyrifolia rootstock. The test trees were 10 to 15 years old for apples and 15 to 25 years old for pears, and all orchards were equipped with irrigation systems. Additionally, the apple trees were all pruned in a spindled shape, and except for the mountainous areas (Y-shaped pruning), the pear trees were managed in a palmette shape. Notably, orchards in mid-mountainous areas, where frost injury during the flowering period is chronically severe, were intensively studied.

2.4. Determination of Flowering Period and Frost Injury Assessment

For this study, specific varieties of apples and pears were selected: ‘Hongro’ and ‘Fuji’ for apples; ‘Wonhwang’ and ‘Niitaka’ for pears. According to the Agricultural Science and Technology Research Survey Analysis Standard [16], the stages of flowering were defined as follows: first flowering occurs when about 10% of flowers are open, full bloom is when approximately 70–80% of flowers are open, and flower dropping is when 70–80% of flowers have fallen.

To evaluate the extent of frost damage in apples, each surveyed orchard was divided into five sections. From each section, 15 bearing branches were collected from three trees that were centrally located within the canopy and exhibited similar vigor. Frost damage was assessed in inflorescences that bloomed from terminal buds, with 15 inflorescences examined per replication across five replications. Additionally, frost damage was analyzed separately for central and lateral flowers within the inflorescence. For pears, the same methodology was applied, with 10 lateral branches collected per orchard. From each lateral branch, 3 inflorescences positioned in the middle were selected, and 30 inflorescences were examined per replication across five replications. Since pear inflorescences typically bloom sequentially from the first to the eighth flower, frost damage rates were calculated separately for flowers 1–4 and flowers 5–7.

To determine the presence of frost injury during the flowering period, a cutter blade was used to horizontally slice through the peduncles of inflorescences in sequence. This allowed for the visual inspection of internal browning in both the pistils and the peduncles. Specifically, the pistil condition was assessed by bisecting it, and flowers were classified as frost-damaged if complete browning was observed.

2.5. Meteorological Data Collection and Analysis

The regional meteorological data were collected from the Korea Meteorological Administration’s (KMA) Automatic Weather System (AWS) at the following locations: Jeonju (station number 146, altitude 60 m), Gunsan (station number 140, altitude 23 m), Buan (station number 243, altitude 12 m), Imsil (station number 244, altitude 247 m), Namwon (station number 247, altitude 133 m), and Jangsu (station number 248, altitude 407 m). These data were used for the analysis. An analysis of average climatic conditions was conducted using long-term average (1981 to 2010), along with minimum temperature and diurnal temperature range data over the past 12 years (2011 to 2022).

2.6. Data Analysis

The arithmetic means of the meteorological data and the degree of injury were calculated using Microsoft Excel 2019 (Microsoft Corp., Redmond, WA, USA), and data visualization was performed using SigmaPlot 14.0 (Systat Software Inc., Chicago, IL, USA). A multiple mean comparison test was conducted using SAS 9.4 (SAS Institute Inc., Cary, NC, USA) to analyze differences in frost damage across cultivation regions.

3. Results and Discussion

3.1. Flowering Characteristics by Cultivation Region

The study categorized the orchards by altitude and investigated the stages of first flowering, full bloom, and flower dropping [17], as outlined in

Table 2. The differences in the flowering periods of ‘Hongro’ and ‘Fuji’ apples and ‘Wonwhang’ and ‘Niitaka’ pears at various regions in 2013. Black circles represent first flowering, full bloom, and flower dropping, respectively. The white circle indicates the progression status of two distinct flowering stages. Coastal: altitude of less than 100 m ASL; inland: altitude of less than 100 m; mid-mountainous: altitude of 100–300 m; mountainous: altitude above 300 m.

Various Altitudes	Recorded Flowering Stages
	April (Day)															May (Day)
	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	1	2	3	4	5	6	7	8	9	10	11	12
‘Hongro’ Apple
Coastal	●	○	○	○	○	○	○	○	○	○	○	○	○	○	●	○	○	○	○	●
Inland		●	○	○	○	○	○	○	○	○	○	○	○	○	○	●	○	○	○	●
Mid-mountainous			●	○	○	○	○	○	○	○	○	○	○	○	○	○	○	●	○	○	○	●
Mountainous				●	○	○	○	○	○	○	○	○	○	○	○	○	○	●	○	○	○	●
‘Fuji’ Apple
Coastal		●	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	●	○	○	●
Inland		●	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	●	○	○	○	●
Mid-mountainous			●	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	●	○	○	●
Mountainous				●	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	●	○	○	●
‘Wonwhang’ Pear
Coastal				●	○	○	●	○	○	○	●
Inland				●	○	○	○	○	●	○	○	●
Mid-mountainous		●	○	○	○	○	●	○	○	○	●
Mountainous					●	○	○	○	○	○	●	○	○	○	●
‘Niitaka’ Pear
Coastal			●	○	○	○	○	●	○	○	○	●
Inland				●	○	○	○	○	●	○	○	○	○	●
Mid-mountainous			●	○	○	○	○	●	○	○	○	○	●
Mountainous						●	○	○	○	○	●	○	○	○	●

. ‘Hongro’ showed a first flowering period from April 16 to 19, with a variation of 1–3 days across different cultivation regions, while ‘Fuji’ bloomed from April 17 to 19, varying by 1–2 days. The duration from first flowering to full bloom was 13 days for ‘Hongro’ in coastal and plain areas, and 14 days in mid-mountainous and mountainous areas; for ‘Fuji’, it took 16, 17, 17, and 19 days, respectively. Additionally, differences in the progression of flowering stages were observed on the same day (April 20) across various cultivation regions (‘Hongro‘, Figure 2(A0–A3); ‘Fuji’, Figure 2(B0–B3)).

For both pear varieties, the mid-mountainous areas experienced the earliest flowering, while the mountainous areas had the latest. Specifically for ‘Wonhwang’, blooming commenced earliest in the mid-mountainous areas, and the period from first flowering to full bloom tended to extend by 1–2 days as the altitude of the cultivation region increased. For ‘Niitaka’, the total flowering period lasted 10–11 days, with no significant differences across regions. Similarly to ‘Wonhwang’, the first flowering in mountainous areas started 1–3 days later. The duration from the start of first flowering to full bloom for both varieties ranged from 2 to 5 days, and the time for the full flower to flower dropping was 8–11 days. For ‘Wonhwang’ (Figure 2(C0–C3)) and ‘Niitaka’ (Figure 2(D0–D3)), the first flowering stages on 30 April matched the survey data, similar to the apples. Notably, the average temperatures from 1 to 15 April (Figure 5A), which is before the stage of first flowering, showed a trend of decreasing with increasing elevation similar to apples [18], though there were some differences for pears. These differences may be attributed to factors such as the nutritional status of the pear trees and slope cultivation affecting the orchards [19,20].

With increasing altitude in the cultivation regions, the onset of first flowering was delayed by approximately 1–3 days more noticeably in apples than in pears [21]. For both fruit types and all four varieties studied, the period from full bloom to flower dropping lasted 4–6 days. Apples took on average about 15 days from the start of first blooming to full bloom, which is 3–5 times longer than pears, and the total first flowering period was also about 1.5–2.0 times longer [17]. While there may be variations in the first flowering period depending on the region and variety, ‘Wonhwang’ and ‘Nittaka’ exhibit shorter flowering periods than ‘Hongro’ and ‘Fuji’. This condition suggests a lower risk of frost injury during the blooming of the pears, yet there is a potential increase in the risk of cold exposure during the fruit-setting phase.

3.2. Analysis of Injury Severity During the Flowering Period of Apples by Cultivation Region

Weather analysis from five areas identified significant temperature fluctuations and lower minimum temperatures during the pre-blooming phase, specifically on April 12 (−3.8 to 0 °C), 13 (−4.1 to 1.9 °C), and 15 (−3.5 to 0.4 °C), as well as during the flowering period on April 19 (−1.0 to 3.1 °C) and 22 (−2.0 to 3.1 °C). These conditions, especially noted in the mid-mountainous (Imsil and Namwon) and mountainous (Jangsu) areas, are estimated to have consistently caused frost injury during these periods. Frost injury during the flowering period, as shown in Figure 3, appeared as a browning of the pistils and peduncle tissues compared to the normal inflorescences. The injury patterns of apples and pears exhibited similarity. An investigation into the extent of injury across the cultivation regions revealed that for ‘Hongro’ (Figure 4A), the injury to the central flowers blooming first was significantly higher at 18.1%, compared to 3.0% for lateral flowers. A similar pattern was observed in ‘Fuji’, with central flowers showing 23.6% injury versus 8.6% for lateral flowers. The injury rates for the central flowers of ‘Hongro’ varied by cultivation region, with 0.0% in coastal areas, 16.9% in plains, 24.9%, 23.1%, 13.8%, and 40.0% in mid-mountainous orchards labeled A–E, and 6.7% in mountainous areas. ‘Fuji’ (Figure 4B) showed similar patterns with injury rates of 11.1%, 20.2%, 16.9%, 36.4%, 48.4%, 32.0%, 20.0%, and 4.0%, respectively, indicating the highest injury rates in mid-mountainous orchards. Similar tendencies were observed in the injury rates of lateral flowers and the average injury within the inflorescences. Specifically, ‘Hongro’ had an average inflorescence injury rate of 6.0%, which was relatively lower than ‘Fuji’ at 11.6%. This result aligns with the findings of previous studies [20].

3.3. Analysis of Injury Severity During the Flowering Period of Pears by Cultivation Region

For ‘Wonhwang’ (Figure 4C) and ‘Niitaka’ (Figure 4D), injury trends and extents were similar across regions. Notably, in mid-mountainous orchards A and B, injury to the first to fourth flowers in ‘Wonhwang’ reached 70.0% and 72.0%, respectively, significantly higher than in other regions, with a similar pattern in average injury per inflorescence. The severity of frost injury in different cultivation regions followed the order coastal < plain < mountainous < mid-mountainous areas. For ‘Niitaka’, the injury was similar to ‘Wonhwang’, with the highest rate in mid-mountainous orchards A and B, at 59.8% and 64.8%, respectively, for the first four flowers. In particular, the rate of frost injury decreased progressively from the first to the seventh flower within inflorescences in mid-mountainous orchards, suggesting that significant frost injury occurred during an advanced stage of flowering around April 19 to 22. Additionally, damaged flowers of apples and pears either fail to fertilize or, even if fertilization occurs, result in a high incidence of malformed fruits, leading to a significant decline in quality [22,23]. Despite similar flowering periods among varieties within the same species, the variance in injury rates likely stemmed more from differences in cold resistance between varieties than from climate factors.

Figure 4. Chilling injury at various regions of ‘Hongro’ (A) and ‘Fuji’ (B) apples and ‘Wonwhang’ (C) and ‘Niitaka’ (D) pears in 2013. Coastal: altitude of less than 100 m ASL; inland: altitude of less than 100 m; mid-mountainous: altitude of 100–300 m; mountainous: altitude above 300 m. Lowercase letters indicate the separation of means among various regions according to Duncan’s multiple range test at p < 0.05 (n = 5).

Figure 4. Chilling injury at various regions of ‘Hongro’ (A) and ‘Fuji’ (B) apples and ‘Wonwhang’ (C) and ‘Niitaka’ (D) pears in 2013. Coastal: altitude of less than 100 m ASL; inland: altitude of less than 100 m; mid-mountainous: altitude of 100–300 m; mountainous: altitude above 300 m. Lowercase letters indicate the separation of means among various regions according to Duncan’s multiple range test at p < 0.05 (n = 5).

3.4. Analysis of Climatic Characteristics by Cultivation Region

Based on meteorological data from April to May, the research detailed in Figure 5 investigated temperatures including average, maximum, and minimum, diurnal range, daily sunlight hours, and average wind speed during the flowering period. For the average temperature (Figure 5A), Jeonju (plain area) exhibited the highest values, while Jangsu (mountainous area) recorded the lowest; however, the regional patterns were similar. In contrast, the highest daily temperatures (Figure 5C) showed that Imsil and Namwon (mid-mountainous areas), which generally have cooler average temperatures, experienced maximum temperatures that were 2–7 °C higher than Jeonju, as well as the coastal and other mountainous areas. Gunsan and Buan (coastal area), however, recorded the lowest maximum temperatures. The minimum daily temperatures (Figure 5E) in Jeonju, Buan, and Gunsan were approximately 2–4 °C higher than those in Imsil, Namwon, and Jangsu. During the flowering period from 16 April to 12 May, the mid-mountainous areas experienced 2–3 days with temperatures below 0 °C, while the coastal and plain areas had none. Additionally, during April and May, the mid-mountainous areas recorded 11–13 days below 0 °C, in contrast to only 1–2 days in the coastal and plain areas, with the majority occurring in April. Diurnal range variability also followed this pattern, with the largest fluctuations observed in Namwon, Imsil, and Jangsu. Despite lower minimum temperatures in mountainous orchards, apple and pear trees suffered less frost injury compared to mid-mountainous orchards, where earlier flowering periods are accelerated by relatively higher average and maximum temperatures, as indicated in Figure 5A,B [21]. Conversely, daily sunlight hours (Figure 5D) and average wind speed (Figure 5F) showed similar patterns across areas and were thought not to significantly influence injury levels.

Figure 5. Characteristics of meteorological factors at various regions based on AWS (Gunsan: an altitude of 23 m; Buan: an altitude of 12 m; Jeonju: an altitude of 60 m; Imsil: an altitude of 247 m; Namwon: an altitude of 133 m; Jangsu: an altitude of 407 m). Red and blue boxes represent the blooming period (first flowering to flower dropping) of apple and pear trees, respectively. Mean temperature (A); daily range (B); Maximum temperature (C); duration of sunshine (D); minimum temperature (E); mean wind velocity (F).

Figure 5. Characteristics of meteorological factors at various regions based on AWS (Gunsan: an altitude of 23 m; Buan: an altitude of 12 m; Jeonju: an altitude of 60 m; Imsil: an altitude of 247 m; Namwon: an altitude of 133 m; Jangsu: an altitude of 407 m). Red and blue boxes represent the blooming period (first flowering to flower dropping) of apple and pear trees, respectively. Mean temperature (A); daily range (B); Maximum temperature (C); duration of sunshine (D); minimum temperature (E); mean wind velocity (F).

3.5. Analysis of Climatic Characteristics by Cultivation Region for the Past 12 Years and the Long-Term Average

An analysis of the average minimum temperature range for April and May from 1981 to 2010 (Figure 6A,B) showed that the plain area (Jeonju) had the highest minimum temperatures, approximately 2 to 5 °C higher than other areas, followed by the mid-mountainous areas (Namwon and Imsil) and the mountainous area (Jangsu), indicating a trend of decreasing temperatures with increasing altitude. Meanwhile, the diurnal range showed the lowest fluctuations in the plain area (Jeonju), with increasing range observed in the mountainous (Jangsu) and mid-mountainous (Namwon and Imsil) areas. During the flowering period in mid-to-late April 2013, for ‘Wonhwang’ and ‘Niitaka’, there was a notable difference of over 4 °C between Namwon and Jeonju. This pattern persisted through April and May, leading to varied frost injury across the cultivation regions. Similarly, the data for the past 12 years (2011 to 2022) on minimum temperatures and diurnal ranges across the cultivation regions (Figure 6C,D), compared to the long-term average (1981 to 2010), showed that minimum temperatures were highest in the plain area, followed by the mid-mountainous and mountainous areas. Conversely, diurnal ranges were greatest in the mid-mountainous areas, followed by the mountainous area and then the plain area.

During the flowering period (April and May) for apples and pears, the weather data of the past 12 years showed an increasing trend in average minimum temperatures compared to the long-term average, with the mid-mountainous areas experiencing a more significant rise than other cultivation regions. Conversely, while the average diurnal range has shown a decreasing trend over the past 12 years, the range itself expanded by 1.3 to 2.7 times. In May, the average diurnal range varied across different cultivation regions, as detailed in

Table 3. Comparison of minimum temperature and daily range at various regions based on AWS (Jeonju: an altitude of 60 m; Imsil: an altitude of 247 m; Namwon: an altitude of 133 m; Jangsu: an altitude of 407 m) of the past 30 years (1981–2010) and the past 12 years (2011–2022).

Month	Period	Classification	Minimum Temperature (°C)					Daily Range (°C)
			Inland (Jeonju)	Mid-Mountainous		Mountainous (Jangsu)		Inland (Jeonju)	Mid-Mountainous		Mountainous (Jangsu)
				(Namwon)	(Imsil)				(Namwon)	(Imsil)
April	1981–2010	Mean	6.7	4.0	3.0	3.1		12.8	15.9	15.6	14.8
		Range	6.6	6.9	6.8	7.1		1.4	2.8	2.3	2.3
	2011–2022	Mean	7.6	5.4	4.1	3.7		12.2	14.2	14.8	14.4
		Range	7.8	8.2	8.0	8.2		3.8	6.6	5.6	6.1
May	1981–2010	Mean	12.4	10.3	9.1	9.1		12.0	14.2	14.1	13.1
		Range	3.8	4.1	3.6	3.3		1.3	2.1	1.7	2.0
	2011–2022	Mean	13.1	11.3	9.7	9.4		12.2	13.8	14.6	14.0
		Range	6.1	7.2	7.4	6.1		4.6	5.9	5.7	6.0

.

While reports on frost injury to fruits due to early leafing and blooming prompted by mild winter temperatures are prevalent [24,25], studies on the risk of spring frosts are scarce [26]. Crop frost hardiness decreases progressively after dormancy [27,28], varying with the growth stage of the flower buds and weakening notably just before blooming [29,30]. Meteorologically, plants are more vulnerable in fluctuating temperatures than in steadily declining cold, with temperature descent speed playing a role [14]. Not just minimum temperatures, but average temperatures and diurnal ranges are also critical factors [31]. Despite a rise in average temperature, a variability under 5% could increase frost injury risk by offsetting up to 5.5 °C rises [32]. Therefore, as seen in Figure 4 and Figure 5, frost injury in mid-mountainous apple and pear orchards was higher than in mountainous areas due to significant impacts from extreme cold and large temperature ranges during advanced flowering stages. Weather analysis over the past 12 years compared to long-term average norms indicates increased weather variability [33], including unusual weather events and region-based differences [21]. This variability, differing from typical patterns, is expected to heighten concerns about potential frost injury due to unpredictable meteorological phenomena in the future [26].

To avoid frost injury during the flowering period, it is recommended to analyze terrain and microclimate to identify high-risk areas, choose cold-resistant varieties [34], and increase resistance through robust plant health [35]. However, these methods are challenging to implement swiftly in urgent situations [11]. Global warming is expected to increase the adaptability of major apple-producing areas to higher temperatures, potentially leading to greater frost injury when it occurs [36], despite anticipated lower risks of frost exposure. This could adversely affect annual growth cycles [37]. Additionally, weather factors such as average temperature and diurnal range can show significant variability within small orchards, with location-specific differences reaching up to 6 °C [38]. The temperature differences observed between the sun-exposed and shaded areas of the apple canopy are consistent with findings in a similar context [39]. This highlights the need for further research into how microclimate, topographical characteristics [40], and the physiological state of the plants relate to frost injury [41].

4. Conclusions

This study analyzed the relationship between frost injury during the flowering period and meteorological factors for apples (‘Hongro’ and ‘Fuji’) and pears (‘Wonhwang’ and ‘Niitaka’) cultivated at various altitudes in Jeonbuk State in Republic of Korea. The theoretical foundation of our study is based on reports of recurrent low-temperature injuries during the flowering period in regions where such damage occurs persistently, despite the low cultivation altitudes. It was revealed that the timing of flowering stages across various cultivation regions was delayed by approximately 1–3 days at higher altitudes, with this effect being more pronounced in apples than in pears. All four varieties of both fruit types experienced a 4–6 days period from full bloom to flower dropping. Specifically, apples required an average of about 15 days from the first flowering to full bloom, which is 3–5 times longer than pears, and the overall flowering period was also about 1.5–2.0 times longer. Frost injury was most severe in the mid-mountainous areas for both apples and pears. Apples showed more significant injury in the central flowers, while pears exhibited higher injury rates in the earliest blooming flowers within the inflorescences. Weather analysis showed that during the flowering period in mid-mountainous areas, the lowest temperatures and the largest diurnal ranges were recorded, both of which significantly contributed to the occurrence of frost injury. In mountainous areas, despite lower minimum temperatures than mid-mountainous areas, slower blooming progression and lower average temperatures resulted in relatively less frost injury. Our study findings indicate that mid-mountainous regions, where flowering stages are accompanied by complex conditions such as significant diurnal temperature variations and low temperatures, are more prone to chronic low-temperature damage compared to higher-altitude mountainous areas with consistently lower temperatures.

Furthermore, weather data comparison between the long-term average (1981 to 2010) and the past 12 years (2011 to 2022) indicated a decrease in April’s average diurnal range. However, the variability in the diurnal range expanded by 1.3 to 2.7 times, signaling increasing meteorological variability. This trend suggests that frost injury during flowering periods may become more frequent in the future. These results underscore the necessity of integrated strategies to prevent frost injury that consider cultivation regions, weather conditions, variety’s frost resistance, and blooming physiology. Particularly in mid-mountainous areas, selecting cold-sensitive varieties and implementing preventative and emergency measures to minimize frost injury are crucial. Additionally, further research should continue to explore the relationships between microclimate, topographical characteristics, and the physiological states of plants in relation to frost injury. Our study highlights that factors beyond low temperatures can influence injury during the flowering period, emphasizing the importance of diverse environmental conditions. These findings are expected to provide valuable insights for future research in this area.