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Temperate deciduous trees can only be productive where winters are cold enough to meet their chilling needs. In the Mediterranean region, chill has traditionally been sufficient for many species, but this may change as temperatures increase. We explored the region's present and future suitability for temperate trees by quantifying chill for the Sfax region in central Tunisia, one of the warmest regions where temperate nuts are commercially grown. We assessed climatic risk by calculating historic chill (since 1973) and using a weather generator calibrated with local weather data (1973-2015) to produce 101 years of chill estimates (computed with the Dynamic Model) and 3 past and 72 future scenarios (for 2041-2070 and 2071-2100, using two representative concentration pathways: RCP4.5 and RCP8.5). For almonds and pistachios, we compared available chill during the chilling period with the species' estimated chilling requirements, and we computed the date by which sufficient chill was expected to have accumulated. Our findings indicated severe chill losses for all future scenarios. For all species, the current chill period is no longer expected to be sufficient for meeting chilling requirements in the future. Chill needs may still be fulfilled later in the year, especially for low-chill almonds, but this would result in delayed phenology, with possible adverse effects on productivity. Temperate nut production is thus unlikely to remain viable at this site, highlighting an urgent need to identify locally appropriate adaptation options. This challenge is likely shared by other warm production regions of temperate fruits and nuts around the world.

Calculating many agroclimatic metrics, e.g., chill or heat accumulation in orchards, requires continuous records of hourly temperature. Such records are often unavailable, with farm managers and researchers relying on daily data or hourly records with gaps. While procedures for generating idealized temperature curves exist, interpolating hourly records has long been a challenge. The SolveHours procedure combines measured hourly temperatures, idealized daily temperature curves and proxy data to fill gaps in such records. It first determines daily temperature extremes by solving systems of linear equations that express the typical relationships between hourly temperatures and daily temperature extremes for every hour. After filling gaps in this record with bias-corrected data from proxy stations or by linear interpolation, SolveHours uses these data to generate an idealized temperature curve. Deviations of recorded hourly temperatures from this curve are calculated, linearly interpolated, and added to the idealized curve to obtain a gapless record. The procedure was compared to alternative gap-filling algorithms using an 8-month dataset from an orchard near Winters, CA, in which half the records were replaced by 500 gaps of random length. The SolveHours procedure achieved ratio of performance to interquartile distance (RPIQ) values of 6.7 (when using temperature extremes from a proxy station) and 8.2 (with temperature extremes measured on site), with root mean square errors of 1.6 and 1.3 °C, respectively. It outperformed all other algorithms in reproducing recorded accumulation of Chill Portions and Growing Degree Hours. The SolveHours procedure is implemented in the chillR package for the R programming environment ( https://cran.r-project.org/web/packages/chillR/vignettes/hourly_temperatures.html ).

Most trees from temperate climates require the accumulation of winter chill and subsequent heat during their dormant phase to resume growth and initiate flowering in the following spring. Global warming could reduce chill and hence hamper the cultivation of high-chill species such as cherries. Yet determining chilling and heat requirements requires large-scale controlled-forcing experiments, and estimates are thus often unavailable. Where long-term phenology datasets exist, partial least squares (PLS) regression can be used as an alternative, to determine climatic requirements statistically. Bloom dates of cherry cv. ‘Schneiders späte Knorpelkirsche’ trees in Klein-Altendorf, Germany, from 24 growing seasons were correlated with 11-day running means of daily mean temperature. Based on the output of the PLS regression, five candidate chilling periods ranging in length from 17 to 102 days, and one forcing phase of 66 days were delineated. Among three common chill models used to quantify chill, the Dynamic Model showed the lowest variation in chill, indicating that it may be more accurate than the Utah and Chilling Hours Models. Based on the longest candidate chilling phase with the earliest starting date, cv. ‘Schneiders späte Knorpelkirsche’ cherries at Bonn exhibited a chilling requirement of 68.6 ± 5.7 chill portions (or 1,375 ± 178 chilling hours or 1,410 ± 238 Utah chill units) and a heat requirement of 3,473 ± 1,236 growing degree hours. Closer investigation of the distinct chilling phases detected by PLS regression could contribute to our understanding of dormancy processes and thus help fruit and nut growers identify suitable tree cultivars for a future in which static climatic conditions can no longer be assumed. All procedures used in this study were bundled in an R package (‘chillR’) and are provided as Supplementary materials. The procedure was also applied to leaf emergence dates of walnut (cv. ‘Payne’) at Davis, California. Electronic supplementary material The online version of this article (doi:10.1007/s00484-012-0594-y) contains supplementary material, which is available to authorized users.

Flowering phenology of tree rhododendron (Rhododendron arboreum Sm.) was monitored in situ along elevation gradients in two distinct ecological settings. Observations were carried out in Gaoligong Nature Reserve (GNR) in China and in the Kanchenjunga Conservation Area (KCA) in Nepal. Using the crown density method, flowering events of the selected species were recorded. Flowering duration and synchrony were determined within each site and along the elevation gradient in each study area. Our observations showed high synchrony throughout the elevation gradient, especially for peak flowering. Mean 15-day soil temperature, soil parameters (soil moisture, nitrogen, organic matter and pH), age of the observed trees, and site characteristics (litter cover, canopy cover, inclination) were related to mean initial and peak flowering dates using partial least squares regression (PLS). Results differed between the two sites, but winter temperature was the most important variable affecting the regression model for both initial flowering and peak flowering at both sites. After temperature, soil moisture was the most important variable for explaining initial flowering dates. The distribution of tree rhododendron indicates that it is able to grow in a wide range of habitats with different environmental conditions. The recent trend of rising winter-spring temperature and the detected bloom-advancing effect of high temperatures during this period suggest that tree rhododendron might expand its distributional range in response to global warming.

Climate change has affected the rates of chilling and heat accumulation, which are vital for flowering and production, in temperate fruit trees, but few studies have been conducted in the cold-winter climates of East Asia. To evaluate tree responses to variation in chill and heat accumulation rates, partial least squares regression was used to correlate first flowering dates of chestnut (Castanea mollissima Blume) and jujube (Zizyphus jujube Mill.) in Beijing, China, with daily chill and heat accumulation between 1963 and 2008. The Dynamic Model and the Growing Degree Hour Model were used to convert daily records of minimum and maximum temperature into horticulturally meaningful metrics. Regression analyses identified the chilling and forcing periods for chestnut and jujube. The forcing periods started when half the chilling requirements were fulfilled. Over the past 50 years, heat accumulation during tree dormancy increased significantly, while chill accumulation remained relatively stable for both species. Heat accumulation was the main driver of bloom timing, with effects of variation in chill accumulation negligible in Beijing’s cold-winter climate. It does not seem likely that reductions in chill will have a major effect on the studied species in Beijing in the near future. Such problems are much more likely for trees grown in locations that are substantially warmer than their native habitats, such as temperate species in the subtropics and tropics.