2014 Volume 83 Issue 2 Pages 117-121
Researchers have predicted that most of the current satsuma mandarin-producing regions will become unfavorable for growing this citrus crop by the 2060s owing to global warming. To offer satsuma mandarin growers information for making replanting decisions, we estimated future changes in locations suitable for the cultivation of tankan (Citrus tankan Hayata), the leading subtropical citrus in Japan. The results of fruit-freezing experiments suggested that the threshold air temperature for the cold tolerance of tankan fruit is about −2°C. The locations suitable for tankan were simulated on the basis of future annual mean air temperatures evaluated by the MIROC3.2-HiRes climate model under the SRES-A1B GHG emission scenario and future annual minimum air temperatures calculated from future daily minimum temperatures evaluated by this climate model and the current air temperature variability. The results of the simulation showed that most of the Pacific coastal area from the Kanto Plain and to the west will become suitable for tankan cultivation during 2031–2050, and that coastal areas of the current satsuma mandarin-producing regions in Japan should be suitable for tankan production by 2050. The inland area of southern Kyushu Island, despite its proximity to current tankan-producing regions, is predicted to experience temperatures causing a high frequency of cold injury to tankan fruits even during 2051–2070. Therefore, if satsuma mandarin production becomes difficult on account of increased global warming, tankan could be produced as a substitute in coastal areas of the current satsuma mandarin-producing regions. However, we predict that it will be difficult to produce tankan in inland areas of the current satsuma mandarin-producing regions.
Previous studies on the impacts of global warming on fruit production have predicted changes in yield (Santos et al., 2011; Stockle et al., 2010), fruit quality (Sugiura et al., 2013), and the particular regions suitable for fruit production (Kenny et al., 2000; Sugiura and Yokozawa, 2004; White et al., 2006). To make effective plans for adapting to climate change, farmers need to be confident that the projected changes will significantly affect their production, and there need to be technical options available to respond to the changes (IPCC, 2007). Compared to most crops, tree crops will require earlier action by farmers because such crops cannot be replanted easily and require several years to grow from nursery stock to the fruit-bearing stage.
For cereal crops, altering the planting time is a simple adaptation strategy to avoid heat stress (Seino, 1995). However, it is difficult to adjust the cropping season of perennial crops such as fruit trees. When production levels of existing tree species decline on account of climate change, converting to species that can adapt to higher temperature is an important management strategy.
Satsuma mandarin (Citrus unshiu Marcow.), a temperate citrus species (Kobayashi, 1978), is the leading fruit tree species in Japan (MAFF, 2013). Although it is one of the most cold-tolerant of the commercial citrus species grown in the world (Lindsey, 2008), high-temperature damage, such as sunscald, delays in peel coloring, and peel puffing, often occurs in warm years (Sugiura et al., 2007, 2012). According to our previous study of the effects of global warming on fruit production estimated on the basis of changes in annual mean air temperature, most of the current satsuma mandarin-producing regions in Japan will become unfavorable for growing this crop by the 2060s (Sugiura and Yokozawa, 2004).
If cultivation of satsuma mandarin becomes difficult in the future, converting to the cultivation of citrus species with high-temperature tolerance, such as subtropical citrus, would be an effective adaptation strategy owing to the relatively narrow technical gap between these two crops.
Tankan (Citrus tankan Hayata), a subtropical citrus species (Harada, 1963), requires a warmer climate than satsuma mandarin (Iwamasa, 1976). It is the leading sub-tropical citrus consumed fresh in Japan (MAFF, 2012). The suitable range of annual mean air temperature for the cultivation of satsuma mandarin is 15–18°C, whereas the suitable annual mean air temperature for tankan cultivation is more than 17.5°C (MAFF, 2010). Therefore, as judged only by annual mean temperature, tankan could be grown in regions where satsuma mandarin cultivation becomes difficult owing to warming. In fact, farmers are substituting tankan for satsuma mandarin in regions where its fruit quality has declined owing to recent warming (Yang et al., 2009).
Satsuma mandarin has one of the earliest ripening periods among the world’s commercial citrus crops, with a harvest time from September to December in Japan. Other citrus species are harvested after January, and tankan ripens in February (Higa and Yonemori, 1977; Yang et al., 2009). Granulation, a type of cold injury characterized by dry and bitter flesh tissues, often occurs when fruits are exposed to severe cold during January and February, which are the coldest months of the year in Japan (Konakahara, 1984; Takebayashi et al., 1993). Therefore, when predicting future suitable locations for tankan production, we need to consider not only annual mean temperature, but also the temperatures that induce cold injury in this species.
Nakagawa et al. (1976) showed that cold injury of citrus fruit, such as granulation, resulted from freezing of the fruit, and the freezing temperatures varied by species. However, the freezing temperature of tankan fruit has not been measured experimentally.
In this study, we confirmed the threshold temperature for cold tolerance of tankan fruit by conducting freezing experiments. We also used a dataset of projected climate change to simulate future changes in the locations suitable for tankan production in Japan on the basis of both annual mean air temperature and extreme low air temperature causing cold injury. Our findings will provide satsuma mandarin growers better information for making replanting decisions.
To measure the threshold temperature for cold tolerance, we tested the freezing temperature of ripened tankan fruits in three experiments. These fruits were produced in Okinawa Prefecture and obtained through a commercial supplier, and each weighed approximately 120 g. Wet absorbent cotton was attached to the top and bottom portions of each fruit to avoid supercooling. A temperature sensor of 2-mm diameter (RT-31S; Espec Mic, Aichi, Japan) was mounted on the equatorial portion of each fruit to record changes in the surface temperature. Fruits were cooled to approximately 2°C over 30 min in a precooling step, and then sealed in a polystyrene box (4 L) for slow cooling. The box was put in a freezer set at −5°C (Exp. 1) or −8°C (Exp. 2 and 3) for 14 h. Following Nakagawa et al. (1976), the threshold temperature for cold tolerance of the fruit was defined as the initial temperature of fruit freezing, as judged by a rise in the surface temperature.
2. Simulation 1) Climate dataFor the current air temperatures, we used values of the daily mean and minimum air temperatures from 1981 to 2000 in the AMeDAS mesh data (Seino, 1993), a mesh climate dataset (each grid cell measures 45″ longitude × 30″ latitude, i.e. 1 km square) estimated using statistics at meteorological observation stations across Japan from 1978 onward. Projected air temperatures of each grid cell during 2011–2030, 2031–2050, and 2051– 2070 were calculated from Mesh Climate Change Data of Japan v. 2 (Okada et al., 2009), which is a mesh climate dataset (45″ × 30″) covering 1900–2100 estimated from climate evaluated by atmosphere–ocean coupled general circulation models. In this study, we used the projection calculated by the MIROC3.2-HiRes model (K-1 Model Developers, 2004) under the SRES-A1B GHG emission scenario in the dataset.
Monthly mean and minimum air temperatures were defined as the average of the daily mean and minimum air temperatures in each month. Monthly increases in air temperatures for the period 2011–2030 relative to 1981–2000 were calculated as the difference between the 20-year average of monthly mean and minimum air temperatures in 2011–2030 and those in 1981–2000 obtained from Mesh Climate Change Data of Japan v. 2. Daily mean and minimum air temperatures of each grid cell during 2011–2030 were calculated as the sum of current daily mean and minimum air temperatures (i.e., 1981–2000 in AMeDAS mesh data) and each monthly increase in mean and minimum air temperatures during 2011–2030. Daily mean and minimum air temperatures during 2031–2050 and 2051–2070 were determined similarly. The rate of increase in the average annual mean air temperature of all grid cells from 1981–2100 to 2051– 2070 was 0.47°C per decade.
2) Judging suitable locations for tankan cultivationThe suitable annual mean air temperature for tankan cultivation is at least 17.5°C (MAFF, 2010). Unsuitable locations for tankan cultivation were assumed to be those areas where the average annual mean air temperature for 20 years was less than 17.5°C, and suitable locations to be those areas where the average was at least 17.5°C, excluding regions that experience air temperatures that will often cause cold injury (granulation) in tankan fruit: namely, those where the average was at least 17.5°C, but in more than 4 of every 20 years the annual minimum air temperature was less than the threshold temperature for the cold tolerance of fruit.
In Exp. 1 the surface temperature of fruit dropped to about −3.5°C, but the fruit did not freeze (Fig. 1). However, in Exp. 2 and 3 the fruit did freeze, and the surface temperature began to rise due to the heat of solidification when it had reached −4.5°C in Exp. 2 and −4.7°C in Exp. 3. These results indicate that tankan fruit began to freeze when the surface temperature dropped to approximately −4.5°C. Nakagawa et al. (1976) showed that cold injury of citrus fruit resulted from extracellular freezing of the fruit, and the fruit surface temperature of any citrus variety was 2.5°C lower than the air temperature owing to radiational cooling under weather conditions that might cause cold injury. Therefore, the threshold air temperature for the cold tolerance of tankan fruit is about −2°C.
Surface temperature of tankan fruits during cooling treatment. The freezer used in the treatments was set at −5°C (Exp. 1) or −8°C (Exp. 2 and 3), and the surface temperature was recorded every 30 min (Exp. 1) or 1 min (Exp. 2 and 3).
According to standard air temperatures for fruit tree cultivation reported by MAFF (2010), which were summarized from observations in orchards across Japan, granulation of tankan fruit occurs at temperatures lower than −2°C. This temperature is only slightly higher than the value of −3°C reported for iyo (Citrus iyo hort. ex Tanaka), hassaku (Citrus hassaku hort. ex Tanaka), and navel orange (Citrus sinensis Osbeck var. brasiliensis Tanaka). Therefore, the threshold air temperature for cold tolerance of tankan fruit was assumed to be −2°C in the following simulation.
2. SimulationUnder the current climate regime, locations judged as suitable for tankan cultivation were distributed across all of Okinawa Prefecture, most of the remote islands belonging to Kagoshima Prefecture, and parts of the coastal area of southern Kyushu Island (Figs. 2A and 3A). At present, 54% of tankan in Japan are produced on the remote islands of Kagoshima Prefecture, 31% in Okinawa Prefecture, and 15% in southern Kyushu Island (Kagoshima Prefecture, 2009; MAFF, 2012). Most of the estimated suitable regions covered the current main tankan-producing regions; however, small parts of the current regions in southern Kyushu Island were judged as areas where cold injury is likely to occur.
Predicted change in the spatial distribution of regions suitable for tankan production under (A) current climate (1981–2000) and (B–D) predicted climate during (B) 2011–2030, (C) 2031–2050, and (D) 2051–2070.
Predicted change in the spatial distribution of regions suitable for tankan production. An enlarged view of southern Kyushu Island under (A) current climate (1981–2000) and (B) predicted climate during 2051–2070.
The predicted changes of suitable locations for tankan cultivation indicate that the suitable area will gradually expand northward (Fig. 2B–D). Parts of coastal areas in southern Shikoku Island and Kii Peninsula are predicted to become suitable for tankan cultivation during 2011– 2030. Most of the Pacific coastal area from the southern Kanto Plain and to the west was classified as suitable during 2031–2050, which means that coastal areas of the current satsuma mandarin-producing regions in Japan may be suitable for tankan production by 2050. Suitable regions were also predicted to exist on the Sea of Japan side of western Japan during the period 2051–2070.
Under current air temperatures, the regions prone to cold injury were distributed only in small parts of southern Kyushu Island (Figs. 2A and 3A), but not the remote islands. Our predictions of future conditions indicate that suitable regions will increase in coastal areas over time, with the regions prone to cold injury expanding in inland areas. For instance, in inland areas of southern Kyushu Island, in spite of being close to current tankan-producing regions, there would be a high frequency of cold injury even during 2051–2070 (Fig. 3B).
The development of fruit from flowering to ripeness requires an accumulated temperature that is characteristic for each variety and species (Kumashiro and Suzuki, 1994; Takaya, 1965). The fruit developmental period of tankan, which flowers between late March and early May (MAFF, 1991) and is harvested in February, is about 10 months. Therefore, the accumulated temperature required to ripen tankan can be approximated by annual mean air temperature, and suitable locations for tankan cultivation can be judged from the annual mean air temperature with high accuracy. However, our findings show that judging suitable locations from low temperature is also important, because tankan fruit is vulnerable to cold injury during midwinter.
We conclude that if increased global warming causes declines in satsuma mandarin production, then sub-tropical citrus such as tankan could be produced as a substitute in the coastal areas of current satsuma mandarin-producing regions. However, it may be difficult to produce tankan in the inland areas of these regions.
