Fruit trees bloom earlier and earlier
Global warming is significantly affecting the flowering time of many plant species [1]. This especially concerns plants flowering in spring, as they are observed to flower earlier and earlier each decade [2]. This phenomenon has been very well documented in Japan where the dates of cherry blossoms have been recorded for centuries (Figure 1) [3, 4]. However, changes in flowering patterns are observed all over the world [2].
Figure 1: Dates of full bloom of cherry trees and March temperatures in Kyoto, Japan. A: Full flowering dates observed since the year 812 AD. B: Mean temperatures recorded in March in the period of 1881-2021 (blue; y-axis reversed) and full flowering dates observed in the same period. Figure adopted from Nikolaos Christidiset al. (2022) [3].
Why is this a problem?
Climate change manifests itself not only in rising temperatures but also in increasingly strong fluctuations in weather patterns [5]. For plants, one of the most dangerous phenomena are frosts occurring right after a period of warm temperatures which promote flower development. When forming very tight structures, buds can survive temperatures as low as -30°C. However, tolerance to frost drops quickly upon the buds’ transformation to flowers, which are highly susceptible even to mild frost [6, 7]. This problem affects many agriculturally important plant species. For instance, sweet cherry (Prunus avium L.) is one of the earliest flowering fruit trees, making it highly vulnerable to frost damage [7, 8]. In the case of stone fruits, such as apricots, plums, peaches and cherries, a single frost event can reduce the yield by 90% [9].
Economic and societal impact
Frost damage to flowers is the main climate-related cause of economic losses in global fruit production [10]. A week of spring frosts that hit the United States in 2007 resulted in losses estimated at over 2 billion US dollars [11]. A decade later, a wave of late-spring frosts that swept through Europe caused losses to the fruit sector of 3.3 billion euros [12]. This accounts for nearly 15% of the total fruit production in the EU that year [13]. This event affected nearly the entire continent, as the strongest effects were felt by growers from Italy, France, Germany, Poland, Spain, and Switzerland. Another series of overnight frosts that occurred in the spring of 2021 also had catastrophic consequences for European fruit growers. The most tragic damage was reported in France where, due to an extremely poor grape harvest, wine production was reduced by a third [14]. Farmers in the Netherlands are also well aware of this problem. For Jochem van Dijke, one of the cherry growers with whom we met, 2015 was especially difficult, as he lost 50% of his harvest. However, as he says, sometimes losses can be as high as 80% of the yield.
And the examples do not end here:
Figure 2: World map with examples of frost damage.
The effects of frost damage are felt not only by farmers but also by consumers, as low yields cause fruit prices to skyrocket (Figure 2).
Figure 3: Prices of cherries in Poland between 1995 and 2019 in US dollars per ton. Red points represent years in which frost events were recorded [15-19].
What are the protective measures used?
When spring frosts are predicted, fruit farmers use various methods to protect their orchards from getting damaged. One of the most applied solutions is sprinkling the trees with water which turns into a protective ice layer upon freezing. Alternatively, farmers can artificially raise the temperature around the trees by lighting big paraffin candles or using heat cannons. A similar, albeit much smaller, effect can also be achieved by covering the entire orchard with foil. However, none of these solutions are ideal - they are costly, provide only short-term benefits, and the effectiveness of most of them is highly dependent on weather conditions, such as wind [20]. Moreover, most of them have a negative environmental impact, as sprinkling requires large amounts of water, heat cannons are highly energy-intensive, and candles contribute to air pollution [20].
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