Climate Change Turning up the Heat on Pistachio Varieties
February 20, 2021 The pistachio industry should expect more low-chill winters in the future, according to Katherine Jarvis-Shean, a University of California Orchard Systems Advisor in Sacramento, Solano, and Yolo Counties.
In turn, “America’s pistachio,” the ‘Kerman’, is in jeopardy, she said, and in 30 to 40 years will not be appropriate for many parts of an ever-warming San Joaquin Valley.
Until then, dormancy-breaking chemicals may help, she told attendees of the Advances in Pistachio online short course, but over the long haul, new low-chill varieties will be necessary.
WHAT’S A CHILL HOUR?
Chill models have evolved over the years. The original version simply recognized any hour in which the temperature ranged between 32°F and 45°F.
Today the “best kid on the block,” Jarvis-Shean said, is the Chill Portions model. Also known as the Dynamic Model, it assigns different chill values for different temperatures and expands the range of temperatures considered effective for chill accumulation. For instance, maximum chill hours occur when the temperature is between 43°F and 47°F. Less chill value is recognized for temperatures until they reach either 32°F or 54°F, when chill value is no longer allotted.
“I think the most important thing that the Dynamic Model accounts for is how warm temperatures can subtract from chill accumulation,” Jarvis-Shean said.
WINTER CHILL IMPORTANCE
Budbreak in the spring is preceded by a big upswing in starch in the tree. If conditions are warm in the winter, a tree adjusts its starch-making systems to keep starch low and sugars stable in mid-winter. But then it takes more heat than normal years to achieve high starch pre-bloom.
Of significance, warm temperatures in winter (>65°F) have more influence on yield than cold temperatures (<45°F), Jarvis-Shean said, citing the work of University of California Farm Advisor Craig Kallsen.
“We talk about winter chill, but it’s actually heat during winter that is as important as cold during winter. We want a lot of cold, but we also want not a lot of heat,” Jarvis-Shean said.
Chill portions can easily be overshadowed by chill hours. The winter of 2013-14 was deemed “average” by a chill hours model, but in a chill portions model, it checked in as “unusually warm.”
“If we asked the trees that following spring what they thought of that winter, this is what they would tell us,” Jarvis-Shean said, pointing to a photo of the “classic symptoms” of low chill — blank wood, buds that never opened, and variation in size of leaves (“big up here, small down there”).
Why is it important to distinguish between chill hours and chill portions?
“This model comparison might seem like a real wonky academic concern … but if we want to start thinking about how we adapt to warmer winters and how we can do some intervention around those warmer winters, we need to be counting closer to how the trees count,” Jarvis-Shean said.
UC Davis provides growers with various chill calculators and do-it-yourself spreadsheets on the Fruit and Nut Research and Information (FNRIC) page of its website.
CENTRAL VALLEY FUTURE
California winters have been getting warmer the last 40 years, Jarvis-Shean said, citing data that indicates a 1.6°F to 3°F increase in maximum winter temperatures between 1970 and 2014 and a 1.6°F to 3.2°F increase in minimum temperatures during the same period. Regionally, the same statistics held true for the state’s Central Valley.
Meanwhile, winters have become less foggy in the Central Valley. Research performed at UC Berkeley shows the average number of foggy days have fallen from an average of three out of 10 days in the early 1980s to an average of two in the last 15 years.
Various UC climate models project that California winters will continue to warm heading into the decade of 2060-69 — up 3.1°F in the Sacramento Valley and 3.2°F in the San Joaquin Valley, Jarvis-Shean said.
Models also predict winter-to-winter variability. “So, we’ll still experience some cold winters and winters that we now consider average,” Jarvis-Shean said. “But we’ll also experience more low-chill winters and lower chill winters
than before.”
In terms of chill projections based on 90% of the years this century, chill portion days in the Sacramento Valley are expected to fall from 70 in 2000 to 59 at mid-21st century to 49 at the end of the century. In the North and South San Joaquin Valleys, respectively, the same figures are projected to be 71-61-51 and 64-51-42.
CULTIVAR REQUIREMENTS
Based on the chill and harvest of 2014, ‘Kerman’ requires 54 to 58 chill portion days. ‘Peters’ needs 60 to 65. Minus exact figures, Jarvis-Shean estimated that ‘Lost Hills’ and ‘Golden Hills’ require 10% fewer days than ‘Kerman’, ‘Gumdrop’ 15% less, and ‘Randy’ is “in the same ballpark” as ‘Kerman’.
By as soon as mid-century that does not bode well for ‘Kerman’ in the South San Joaquin Valley. By the turn of the century, ‘Kerman’ as well as ‘Lost Hills’ and ‘Golden Hills’ may have no presence in the entire Central Valley.
“I remember when people started talking about climate change and the end of the (20th) century, it seemed so far out and so abstract,” Jarvis-Shean said. “But we’re at 2020 now, and that orchard that you’re planting right now — that lasts 30 years — will take you well into mid-century.”
HELP ON THE WAY
Jarvis-Shean highlighted several studies of dormancy-breaking products that could help in result of a 20% chill decrease:
• Kaolin clay and calcium carbonate in the winter decreases bud and shoot heat
• Dormant/horticultural oil can increase and accelerate budbreak
• Early data shows ethylene and GA3 (gibberellic acid) may have similar results to oil
• Hydrogen cyanamide can increase and accelerate budbreak (although it is not yet registered)
On the international market there are several varieties that necessitate fewer chill hours. Females include ‘Mateur’ (206 chill hours, Tunisia), ‘Uzun’ (600, Turkey), ‘Halebi’ (650, Turkey), ‘Siirt’ (700, Turkey), and ‘Kale-Ghuchi’ (775, Iran). Males include Male-1 (500, Turkey) and Male-2 (750, Turkey). By comparison, California’s ‘Kerman’ (female) and ‘Peters’ (male) require 800 and 900 chill hours, respectively.
“There is some hope. The genetics are out there,” Jarvis-Shean said. “It just takes some time.”
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