Modelling spatio-temporal changes of Plant Hardiness Zones in peninsular Spain and the Balearics (1952-2022)

The temperature increase associated with climate change has affected distribution ranges for many crops. In countries with strong agricultural sectors such as Spain, the change in crop range may be estimated with indices such as Plant Hardiness Zones (PHZs), which classify territories according to their minimum absolute temperature (Tmin) as a proxy for frost damage. The classification is based on a number (class) and a letter (subclass), with higher numbers indicating higher Tmin values, and letters (a and b) establishing subclasses within the number, with a used for colder conditions within the number class threshold. By using a non-linear regression model (Gaussian SVM) to predict Tmin based on physical parameters of 3,794 weather stations, a daily model covering the period 1952-2022 was generated at a 1Km resolution for peninsular Spain and the Balearics. For each year and pixel, the absolute minimum was chosen, and its value assigned to the corresponding PHZ class, resulting in a series of 71 PHZ maps. The PHZs present in the study zone range from 5b (-26.1 to -23.3ºC) to 11b (7.2 to 10ºC), although not all of them are present all years. The most common PHZs were 8b, 9a and 9b, with around 100,000km2 each and corresponding to a minimum temperature range of -9.4 to -1.1ºC. A major change in area of the PHZs was detected using a Mann–Kendall Test, highlighting a significant increase (99% confidence) in the area occupied by the 10a and 10b zones (-1.1 to 4.4ºC). The extent of zones corresponding to temperatures lower than 8b (up to -6.7ºC) showed a decreasing trend, contrasting with those above. The shift towards warmer PHZs was more evident in mountain environments, expanding the potential range of subtropical crops. These changes highlight a change in climatically suitable land for crops in the study area and an increasing adequacy for less frost-tolerant crops over time.