Warm winters challenge the cultivation of temperate species in South America—a spatial analysis of chill accumulation

Winter chill accumulation plays a crucial role in determining the moment of bud burst in temperate fruit and nut trees, and insufficient chill can greatly limit yield potentials. To assess future cultivation options for such species in South America, we estimated winter chill through a spatial analysis. We used historical data (1980–2017) from 158 weather stations to calibrate a weather generator and produce temperature data for 10 historical and 60 future scenarios. We computed Safe Winter Chill (SWC, corresponding to the 10th quantile of a chill distribution) for the period 1980–2017 and for all historical and future weather scenarios and developed a framework to interpolate SWC for the continent using the Kriging method. To improve the interpolation, we applied a 3D correction model based on two co-variables (means of daily temperature extremes in July). Our results suggest important chill declines in southern Brazil and central Chile. By 2050 under the global warming scenario RCP4.5 (Representative Concentration Pathway), absolute SWC in these regions may reach a median of 18.7 and 39.6 Chill Portions (CP), respectively. Projections are most alarming for a strong global warming scenario (RCP8.5). In southern South America, adequate SWC levels of about 60 CP may be expected even under the RCP8.5 scenario. Our results highlight the need for climate change adaptation measures to secure temperate fruit production in important growing regions of South America. The procedure we developed may help farmers and practitioners across South America estimate future SWC to adapt their orchards to future challenges.

Chemical Budbreak Induction Methods to Increase Blackberry Yields under Inadequate Chilling Conditions

Blackberry production has recently expanded to the Southeast. In Florida, inadequate winter chill limits blackberry production by causing poor and erratic budbreak. To compensate, we developed chemical budbreak induction methods based on the field trial data collected over 3 growing seasons. This new 4-page article provides guidelines for the budbreak induction methods, with the aim of supporting the development of subtropical blackberry production. Written by Shinsuke Agehara and Syuan-You Lin.https://edis.ifas.ufl.edu/hs1419

Temperate Fruit Trees under Climate Change: Challenges for Dormancy and Chilling Requirements in Warm Winter Regions

Adequate chill is of great importance for successful production of deciduous fruit trees. However, temperate fruit trees grown under tropical and subtropical regions may face insufficient winter chill, which has a crucial role in dormancy and productivity. The objective of this review is to discuss the challenges for dormancy and chilling requirements of temperate fruit trees, especially in warm winter regions, under climate change conditions. After defining climate change and dormancy, the effects of climate change on various parameters of temperate fruit trees are described. Then, dormancy breaking chemicals and organic compounds, as well as some aspects of the mechanism of dormancy breaking, are demonstrated. After this, the relationships between dormancy and chilling requirements are delineated and challenging aspects of chilling requirements in climate change conditions and in warm winter environments are demonstrated. Experts have sought to develop models for estimating chilling requirements and dormancy breaking in order to improve the adaption of temperate fruit trees under tropical and subtropical environments. Some of these models and their uses are described in the final section of this review. In conclusion, global warming has led to chill deficit during winter, which may become a limiting factor in the near future for the growth of temperate fruit trees in the tropics and subtropics. With the increasing rate of climate change, improvements in some managing tools (e.g., discovering new, more effective dormancy breaking organic compounds; breeding new, climate-smart cultivars in order to solve problems associated with dormancy and chilling requirements; and improving dormancy and chilling forecasting models) have the potential to solve the challenges of dormancy and chilling requirements for temperate fruit tree production in warm winter fruit tree growing regions.

Increased autumn and winter precipitation during the Last Glacial Maximum in the European Alps

The culmination of the glaciers in the European Alps during the Last Glacial Maximum (LGM) is one of the most intensively studied paleoglaciological events, but its trigger and forcing remain incompletely understood. Here, we provide evidence that the timing of this glacier maximum coincided within age uncertainties with a 3100 yr-long interval of subsurface warming (26.6 to 23.5 ka BP) as recorded by an archive preserved in caves, cryogenic carbonates. This interval of sustained permafrost degradation during one of the coldest intervals of the last glacial period calls for a fundamental change in the dry Arctic-style precipitation regime. Instead, heavy snowfall during autumn and early winter led to the accumulation of a seasonal snowpack insulating the ground from the winter chill. Combined with thermal modelling, the data provide compelling evidence that the LGM glacier advance in the Alps was fueled by intensive snowfall late in the year, likely sourced from the Mediterranean Sea.

Foliar application of defoliants after winter chill accumulation changes phytohormone dynamics and improves budbreak in blackberry under subtropical climatic conditions

Inadequate winter chill causes poor budbreak in blackberry (Rubus L. subgenus Rubus Watson), limiting the commercial production in the subtropics. In ‘Natchez’ blackberry, our previous study found that, under inadequate chilling conditions, urea and lime sulfur (LS) applied as defoliants before chill accumulation advanced budbreak but did not improve final budbreak. In this study, we applied the two defoliants at the end of chill accumulation and examined their effectiveness in breaking dormancy, with a hypothesis that it can be enhanced with increased exposure to chilling. Field experiments were conducted over two growing seasons under subtropical climatic conditions. ‘Natchez’ blackberry was sprayed with urea or LS at 10%. Both defoliants were effective in both advancing and maximizing budbreak. Final budbreak reached 42.6% to 76.8% in the defoliant treatments, compared with 27.1% to 31.6% in the control. Advanced budbreak by defoliants increased early season yield by 35% to 88%. Although not statistically significant, defoliants increased total-season yield by 19% to 56%, compared with the control. Phytohormone profiling revealed no changes in abscisic acid and gibberellic acid 4, but increasing trends in jasmonic acid and indole-3-acetic acid (IAA) during budbreak. The LS treatment increased IAA accumulation by up to 377% compared with the control. These results suggest that urea and LS are highly effective dormancy-breaking agents for blackberry when applied after winter chill accumulation. These defoliants could be an important adaptation tool for subtropical blackberry production. Furthermore, increased IAA accumulation appears to be, at least in part, the mode of action for LS-induced budbreak.

Foliar Application of Defoliants before Winter Chill Accumulation Advances Budbreak and Improves Fruit Earliness of Blackberry under Subtropical Climatic Conditions

In subtropical blackberry (Rubus L. subgenus Rubus Watson) production, inadequate winter chill causes poor and erratic budbreak, whereas high temperatures and heavy rainfall deteriorate late-season fruit quality. We examined the effects of four defoliants [zinc sulfate (ZS), potassium thiosulfate (KTS), urea, and lime sulfur (LS)] on defoliation, budbreak, yield, and fruit quality of ‘Natchez’ blackberry grown under inadequate chilling conditions in two consecutive growing seasons. Plants were treated with defoliants at 187 kg·ha−1 via spray application (1870 L·ha−1) at the beginning of chill accumulation (late December). A nonionic surfactant (Agri-Dex) was added at 0.5% (v/v) to all treatments including the water control. Cumulative chilling hours (<7.2 °C) at the experiment site were 209 and 134 in the first and second growing seasons, respectively. Defoliation was only 40.2% to 55.5% in the control, but it was induced moderately by LS (69.7% to 84.7%) and severely by the other defoliants (81.7% to 94.7%). Budbreak was induced most rapidly by urea application, followed by LS, KTS, and ZS, advancing by 17 to 66 days compared with the control. Consequently, urea, KTS, LS, and ZS increased early season yield by 2.79, 2.55, 0.87, and 0.31 t·ha−1, respectively, compared with the control (0.12 t·ha−1). By contrast, the final percentage of budbreak and total-season yield did not show significant treatment effects. KTS caused cane dieback and increased bud mortality, resulting in the lowest total-season yield among the treatments. Importantly, defoliants had no negative impact on berry size and soluble solids concentration. These results suggest that urea, LS, and ZS are effective bud dormancy-breaking agents for blackberry and that they could be an important adaptation tool for subtropical blackberry production. Among the three defoliants, urea appears to be the ideal chemical option because of its consistent efficacy, favorable safety profile, and low application cost.

Revisiting climate change effects on winter chill in mountain oases of northern Oman

For centuries, traditional high-altitude oases in Oman have depended on the cultivation of deciduous fruit trees. This study explores the effects of climate change on winter chill (estimated as Chilling Hours—CH and Chill Portions—CP), a prerequisite to overcoming dormancy and initiating flowering, in three Omani oases. The results are compared with findings from an earlier study which reported a decrease in the numbers of CH in high-elevation oases by an average of 1.2–9.5 CH year−1 between 1983 and 2008. Location-specific weather data were obtained by merging 15 years of in situ recordings with 28 years of observations from an official weather station near the top of the investigated watershed. Between 1991 and 2018, scenarios of the past few decades show chill reductions by 75, 35 and 18% when estimated in CP at the oases of Masayrat ar Ruwajah (1030 m a.s.l.), Qasha’ (1640 m a.s.l.), and Al ‘Ayn (1900 m a.s.l.), respectively. Over the course of the twenty-first century, the lowest-elevation oasis at Masayrat ar Ruwajah is projected to lose virtually all winter chill, whereas, despite significant chill losses, conditions are expected to remain viable for some of the currently grown species in the higher-elevation oases. These projected changes will compromise the cultivation of temperate fruit trees in the near future, affecting the sustainability of Omani oases. Our methods support results from earlier work performed at these locations and provide an updated procedure for assessing climate change effects on temperature-dependent systems.

Exogenous Gibberellic Acid Advances Reproductive Phenology and Increases Early-Season Yield in Subtropical Blackberry Production

Inadequate winter chill causes poor and erratic budbreak in blackberry (Rubus L. subgenus Rubus Watson), limiting the commercial production in subtropical climates. We examined the effects of exogenous gibberellic acid (GA3) on the reproductive phenology, fruit number, yield, and fruit quality of three blackberry cultivars (‘Natchez’, ‘Navaho’, and ‘Ouachita’) grown under subtropical climatic conditions in two consecutive growing seasons. A single spray application of GA3 at 0 or 49 g·ha−1 was performed when plants were dormant in late December to late January. Exogenous GA3 advanced the onset of budbreak by 12 to 82 days, flowering by four to 20 days, and fruit ripening by 0 to 15 days. When pooling across the cultivars, it also increased early-season yield by 83% to 276% in two consecutive growing seasons and total-season yield by 60% in the second growing season. Among the cultivars, the yield responses to GA3 were most consistent in ‘Ouachita’, with early-season yield increasing by up to 499%. The average berry weight and soluble solids concentration were slightly reduced by GA3, but these reductions were not consistent in the two growing seasons and the impact on overall fruit marketability was small. These results suggest that exogenous GA3 is an effective bud dormancy breaking compound for blackberry, and it could be an important adaptation tool for subtropical blackberry production.