Accumulated winter chill is decreasing in the fruit growing regions of California

Abstract

We examined trends in accumulated winter chill across the fruit growing region of central California and its internal coastal valleys. We tested the hypothesis that global warming is in motion in California and is causing accumulated winter chill to decrease across the fruit and nut growing regions of California. The detection of potential trends in accumulated winter chill (between 0 and 7.2°C) was determined using two complementary climate datasets. The California Irrigation Management Information System (CIMIS) contains hourly climate data and is suitable for computing accumulated chill hours and chill degree-hours. But, its longest data records extend back only to the 1980s. The National Weather Service Coop climate record is longer, extending beyond the 1950s at many sites. But its datasets only contain information on daily maximum and minimum temperatures. To assess long term trends in winter chill accumulation, we developed an algorithm that converted information from daily maximum and minimum temperature into accumulated hours of winter chill and summations of chill-degree hours. These inferred calculations of chill hour accumulation were tested with and validated by direct measurements from hourly-based data from the CIMIS network. With the combined climate datasets, we found that the annual accumulation of winter chill hours and chill degree hours is diminishing across the fruit and nut growing regions of California. Observed trends in winter chill range between -50 and -260 chill hours per decade. We also applied our analytical algorithm to project changes in winter chill using regional climate projections of temperature for three regions in the Central Valley. Predicted rates of reduced winter chill, for the period between 1950 and 2100, are on the order of -40 h per decade. By the end of the 21st century, orchards in California are expected to experience less than 500 chill hours per winter. This chronic and steady reduction in winter chill is expected to have deleterious economic and culinary impact on fruit and nut production in California by the end of the 21st Century.

Continental‐scale phenology: warming and chilling

“…Numerous phenological studies have identified important direct effects of warming, such as earlier onset of spring plant growth and delayed onset of autumn senescence across temperate mid-latitude climates (Parmesan and Yohe, 2003;Root et al, 2003;Menzel et al, 2006;Schwartz et al, 2006). Yet, many woody plant species growing in these regions also paradoxically require a certain amount of exposure to cold temperatures (termed the fall/winter chilling requirement), in order to properly break their dormancy and be ready to respond to springtime warming (Sarvas, 1974;Cannell and Smith, 1983;Chuine and Cour, 1999;Baldocchi and Wong, 2008;Luedeling et al, 2009 Thus, with recent and projected future warming, a point will eventually be reached (likely different for every species), where plants in temperate climates will no longer be able to continue linearly expanding both ends of their growing season. It is highly likely that temperate plant species in many regions, especially those on the warmer (southern) extremes of their range, may already be inadequately chilled and no longer responding in the same way to additional spring warming (Zhang et al, 2007).…”

“…It is highly likely that temperate plant species in many regions, especially those on the warmer (southern) extremes of their range, may already be inadequately chilled and no longer responding in the same way to additional spring warming (Zhang et al, 2007). Therefore, comprehensive assessment of likely plant species responses (growing season change) to continued warming in temperate mid-latitude climates must include the potential impacts of insufficient chilling (Baldocchi and Wong, 2008;Luedeling et al, 2009).…”

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

“…Perennial fruit production systems, which are long-term operations that depend on seasonally low temperatures, may thus be among the agricultural or horticultural systems that are most vulnerable to climate change (Blanke 2008;Cannell et al 1989;Seguin 2003). In a recent study, Baldocchi and Wong (2008) have investigated trends in chilling conditions in California, finding a pronounced decrease in the number of chilling hours at most sites. While they expect these changes to have deleterious consequences by the end of the twenty-first century, other regions might feel detrimental effects of reduced winter chill at a much earlier date.…”

“…In many parts of the world, fruit growers use winter chill models to identify appropriate tree cultivars for their production sites. This practice is common in most subtropical growing regions, such as Israel (Erez et al 1979), South Africa (Linsley-Noakes and Allan 1994), Spain (Ruiz et al 2007) and California (Baldocchi and Wong 2008). Growers in these regions generally know very well how much …”

“…Numbers in parentheses indicate the minimum chilling requirement for the species according to Noel (2007) and Baldocchi and Wong (2008) winter chill they can expect in the majority of years and select their crops accordingly. Information about historic levels of winter chill is derived from experience, reflecting conditions of the recent past, whereas the potentially productive years of the cultivars that are selected based on this information in most cases extend decades into the future.…”