Agroclimatic requirements and phenological responses to climate change of local apple cultivars in northwestern Spain
Introduction
Apple (Malus domestica Borkh.) trees, like most woody perennial species that evolved in temperate or cold climates, spend the winter months in a dormant state that allows them to survive unfavorable conditions and avoid cold damage (Faust et al., 1997; Saure, 1985; Campoy et al., 2012). To break dormancy, trees undergo two distinct phases that ultimately lead to flowering: endodormancy, during which trees must fulfill cultivar-specific chilling requirements and ecodormancy, when heat requirements need to be satisfied (Lang et al., 1987; Egea et al., 2003). Chilling temperatures are important in fruit production, since they are needed for dormancy release, optimal flowering and satisfactory fruit set (Sunley et al., 2006; Campoy et al., 2011). In apple, the dormancy cycle is only regulated by temperature (Heide and Prestrud, 2005) and a sufficient amount of chill and heat is positively correlated with fruit weight, size and firmness (El Yaacoubi et al., 2020). The amount of chill that is required is cultivar-specific, and large variability has been reported among over 8000 apple cultivars and land races across the world. Nevertheless, most commercial cultivars have high to medium chilling requirements (El Yaacoubi et al., 2016; Parkes et al., 2020).
The apple industry plays a relevant economic and social role in Asturias in northwestern Spain, which contributes about 80 per cent of the total cider production in the country. The bulk of the orchards are composed of several local cultivars, which tend to be well adapted to the agro-climatic conditions of the region. Currently, apple production in Asturias relies on cultivars with medium to high chilling requirements (Dapena, 1996; Dapena and Fernández-Ceballos, 2007). So far, consequences of insufficient winter chill accumulation, such as delayed and irregular budburst (Erez, 2000), have rarely been observed in commercial orchards, except for a few occasions in years with particularly mild winters and/or cold early spring.
Global warming may compromise the fulfillment of trees’ agroclimatic needs during dormancy (Luedeling and Brown, 2011; Fernandez et al., 2020a). Mean global air temperature increased by 0.74 °C between 1906 and 2005 (IPCC, 2007) and numerous future climate scenarios project major changes in air temperature over the course of the 21 st century (IPCC, 2014). Since plant phenology is strongly influenced by air temperature, long-term phenological observations at specific sites that are combined with meteorological data can provide useful information on plant responses to climate change. In recent decades, advances in spring phenological events have been observed for many tree fruit species in many places (Guédon and Legave, 2008; Legave et al., 2008; Luedeling and Brown, 2011; Darbyshire et al., 2013; El Yaacoubi et al., 2014; Guo et al., 2015; Legave et al., 2015; Yong et al., 2016). These bloom advances are a result of a rise in air temperatures in spring, which has accelerated the fulfillment of heat requirements. However, in some regions, temperature increases in winter appear to have delayed the fulfillment of chilling requirements, sometimes to an extent that could not be compensated by phenology-advancing effects of warming in spring. In such extreme situations, warming during dormancy has been reported to result in delayed bloom dates (Harrington et al., 2010; Campoy et al., 2011; Luedeling et al., 2013a; Legave et al., 2015; Martínez-Lüscher et al., 2017; Bartolini et al., 2019).
Several models have been proposed for quantifying chill and heat accumulation. The most common concept of the dormancy season stipulates that chilling and heat requirements are fulfilled sequentially (Guédon and Legave, 2008; Luedeling et al., 2009; Darbyshire et al., 2013), but some recent studies have proposed more complex concepts that include an overlapping phase of both agroclimatic stimuli (Pope et al., 2014), or the possibility that budbreak can be triggered by various combinations of chill and heat accumulation (Harrington et al., 2010). To measure the accumulation of winter chill in deciduous trees, various models have been developed: the Chilling Hours Model (Hutchins 1932, as cited by Weinberger, 1950), the Utah Model (Richardson et al., 1974) and the Dynamic Model (Fishman et al., 1987a, b). For quantifying heat accumulation, the Growing Degree Hours Model (Anderson et al., 1986) is the most widely used model.
Since buds do not exhibit easily observable changes during dormancy, delineation of the chill and heat accumulation has long remained elusive, especially where no controlled experiments could be undertaken. In recent years, Partial Least Squares (PLS) regression has been used to overcome this limitation (Luedeling and Gassner, 2012). This statistical approach requires long-term temperature and phenology records. For each calendar day of the dormancy season, PLS regression can identify whether high temperatures tend to delay or advance bloom dates. In many climatic settings, this information can then be used to delineate the endormancy phase, when high temperatures should delay budbreak, and the ecodormancy phase, when high temperatures should result in advanced phenology.
The analysis of historic chill accumulation trends is a decisive step towards a better understanding of the impacts of climate change in a particular region. In a context of global warming, chill trend estimations can be very sensitive to the choice of chill model (Luedeling et al., 2009; Fernandez et al., 2020b). In the particular case of Asturias, the potential impacts of climate change on locally available winter chill may include changes in the timing of phenological events, which may have implications for agricultural management. Significant warming during the chilling phase could reduce the number of suitable cultivars for cider production in the region. Another important factor to consider is the possibility of increased frequency of adverse weather events such as late spring frosts, which can be associated with shifts in budbreak dates. While late damaging frosts have traditionally been rare in the study region, advances in spring phenology may lead to earlier appearance of advanced flowering stages, which are more frost-sensitive than fully dormant buds (Westwood, 1999). Anticipating future production risks related to the dormancy season would be facilitated by accurate knowledge of the flowering times of each local cultivar. Reliable characterization of chilling and heat requirements is also important for adapting agricultural practices to possible new constraints, as well as for the design of new orchards and as guidance for future breeding strategies.
To provide information for risk assessment and strategic decisions on the composition of future orchards, we pursued two objectives. First, we analyzed temperature trends in Asturias over the past 41 years to examine how changes in chill and heat accumulation have affected the phenology of apple cultivars in northwestern Spain. Second, we determined the start and end dates of the effective chill and heat accumulation periods to quantify chill and heat requirements of local apple cultivars using Partial Least Squares (PLS) analysis.
Section snippets
Study area
The study was carried out at Servicio Regional de Investigación y Desarrollo Agroalimentario (SERIDA) in Villaviciosa, Asturias, northwestern Spain (43.46 °N, 5.43 °W, 10 m above sea level) (Fig. 1). Villaviciosa is located in an area known as “Comarca de la Sidra”, the most important cider apple growing region in Spain. The climate in this region can be defined as temperate and humid oceanic climate. Temperatures are mild in winter, summers are not dry or very hot and the annual rainfall is
Temperature trend and variability
Annual and seasonal temperature trends were analyzed using meteorological data collected in Villaviciosa during the period 1978–2019. This location has a mean daily temperature of 13.35 °C, with mean daily minimum and maximum temperatures of 8.62 °C and 18.34 °C, respectively (Table A1 in the supplementary materials). Over the past 41 years, the average daily temperature increased significantly (τ = 0.48, p < 0.001) by 1.21 °C, at a rate of 0.30 °C per decade. The warmest year was 2014 (14.3
Temperature response of bloom dates and chill and heat accumulation
The observed temperature changes have important consequences for apple cultivation, because temperature is the primary driver of phenological development (Walther et al., 2002; Chmielewski et al., 2004). Temperatures at SERIDA (Villaviciosa) have been increasing at a faster rate (+0.30 °C) than the mean global land surface temperature, which has only risen by approximately 0.18 °C per decade since 1981 (NOAA, 2019, Global Climate Summary). Our analysis also revealed that the pace of temperature
Conclusions
Our results represent an advance in assessing the possible influence of climate change on apple phenology in mild humid climates such as that of northwestern Spain. Under local climate conditions, winter chill accumulation did not show a significant decrease despite temperature increases by 0.30 °C per decade since 1978. Our results indicate that local apple cultivars have shown a high degree of phenotypic plasticity to respond to gradual changes in the environmental conditions. However, their
Author’s contributions
AD and ED designed the study. ED collected phenology and meteorological data. AD performed the analysis with input from EL, JAE and ED. AD wrote the manuscript and all authors contributed to interpretation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank María Dolores Blázquez, Mercedes Fernandez, Paulino Dapía and María José Antón, for their contribution in collecting phenological data over the last three decades. We also thank Eduardo Fernández, Rodrigo Martínez and Alejandro Nuñez for advising with the analysis and figures. Funding was provided by an FPI-INIA fellowship to AD (CPD-2016-0190), AEI-MNECO through project RTA2017-00102-C03-01 and RFP2015-00022. Financial support has been provided by PRIMA, a program supported under H2020
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