Climate regulation of fruit-set, orchard synchrony, and future production in apple agroecosystems across the Republic of Korea

A better understanding factors regulating agricultural production is a research priority, given the pace of environmental change and global human population growth. Fruit orchards may be particularly vulnerable to climate change owing to the strong temperature-dependency of reproduction in temperate trees. In this study, we explored climate influences on the reproductive dynamics of apple (Malus domestica), one of the most widely-grown, economically-important fruit crops worldwide. Observations of individual-tree fruit-set, a key indicator of final yield, were acquired for three apple cultivars in an eight-year census of ~44,900 trees in >5,500 orchards spanning wide climate gradients across the Republic of Korea. With maximum-likelihood models, we quantified temporal and climate-driven patterns in fruit-set, investigated evidence for spatially-synchronized production, and conducted simulations of orchard vulnerability to climate change. We found annual fruit-set oscillated around modal levels and was synchronized between orchards within 25 km. Higher temperatures had contrasting influences, reducing fruit-set during the cold-season (consistent with climate-induced phenological shifts), while increasing fruit-set during the season of bud initiation (prior spring). Simulations predict that higher future temperatures during bud initiation increase average fruit-set, despite the constraints of cold-season warming, but that such enhancements level-off by late-century and are accompanied by heightened volatility. Decelerating fruit-set and greater instability in production have implications for future societal needs, as robust supply systems depend as much on consistent production as on high average output. Our analyses also imply that future climate patterns promoting synchrony in fruit-set among orchards may accentuate the negative consequences of fluctuating production.