A global analysis of the comparability of winter chill models for fruit and nut trees

Many fruit and nut trees must fulfill a chilling requirement to break their winter dormancy and resume normal growth in spring. Several models exist for quantifying winter chill, and growers and researchers often tacitly assume that the choice of model is not important and estimates of species chilling requirements are valid across growing regions. To test this assumption, Safe Winter Chill (the amount of winter chill that is exceeded in 90% of years) was calculated for 5,078 weather stations around the world, using the Dynamic Model [in Chill Portions (CP)], the Chilling Hours (CH) Model and the Utah Model [Utah Chill Units (UCU)]. Distributions of the ratios between different winter chill metrics were mapped on a global scale. These ratios should be constant if the models were strictly proportional. Ratios between winter chill metrics varied substantially, with the CH/CP ratio ranging between 0 and 34, the UCU/CP ratio between −155 and +20 and the UCU/CH ratio between −10 and +5. The models are thus not proportional, and chilling requirements determined in a given location may not be valid elsewhere. The Utah Model produced negative winter chill totals in many Subtropical regions, where it does not seem to be useful. Mean annual temperature and daily temperature range influenced all winter chill ratios, but explained only between 12 and 27% of the variation. Data on chilling requirements should always be amended with information on the location and experimental conditions of the study in which they were determined, ideally including site-specific conversion factors between winter chill models. This would greatly facilitate the transfer of such information across growing regions, and help prepare growers for the impact of climate change

Climate change effects on winter chill for tree crops with chilling requirements on the Arabian Peninsula

Fruit production systems that rely on winter chill for breaking of dormancy might be vulnerable to climatic change. We investigated decreases in the number of winter chilling hours (0–7.2°C) in four mountain oases of Oman, a marginal area for the production of fruit trees with chilling requirements. Winter chill was calculated from long-term hourly temperature records. These were generated based on the correlation of hourly temperature measurements in the oases with daylength and daily minimum and maximum temperatures recorded at a nearby weather station. Winter chill was estimated for historic temperature records between 1983 and 2008, as well as for three sets of synthetic 100-year weather records, generated to represent historic conditions, and climatic changes likely to occur within the next 30 years (temperatures elevated by 1°C and 2°C). Our analysis detected a decrease in the numbers of chilling hours in high-elevation oases by an average of 1.2–9.5 h/year between 1983 and 2008, a period during which, according to the scenario analysis, winter chill was sufficient for most important species in most years in the highest oasis. In the two climate change scenarios, pomegranates, the most important tree crop, received insufficient chilling in 13% and 75% of years, respectively. While production of most traditional fruit trees is marginal today, with trees barely fulfilling their chilling requirements, such production might become impossible in the near future. Similar developments are likely to affect other fruit production regions around the world

Climate change effects on winter chill for fruit crops in Germany

To quantify the effects of climate change on fruit production in Germany, this study aimed at determining long-term trends in winter chill, as calculated with the Chilling Hours and Dynamic Models (Chill Portions). An idealized daily temperature curve was used to convert daily temperature records from 43 weather stations, taken throughout the twentieth and late nineteenth centuries, into an hourly dataset, which was then converted to units of winter chill. Besides exposing temporal trends in winter chill, the data could be spatially interpolated, yielding contiguous maps of typical winter chill in Germany around 2010, as well as chilling losses since 1950. Throughout Germany, winter chill varied between 1700 and 3000 Chilling Hours or 125 and 150 Chill Portions. The areas of highest winter chill were located in the northern parts of the country. For the whole of Germany, there were no significant temporal trends. The extent of interregional variation in winter chill depended on the chilling model used. While the Chilling Hours Model showed strong declines in winter chill for the areas around Dresden and Leipzig, as well as for the Lake Constance region, the Dynamic Model did not detect such dramatic changes. More than a decline in winter chill, increased heat during the winter months might become a challenge to German fruit growers. As already experienced during the extraordinarily warm winter of 2006/07, warm temperatures during the winter can cause fruit trees that fulfill their chilling requirements relatively early to bloom prematurely. This can then lead to elevated risk of frost damage and hamper the homogeneity of flowering