Rise and Shine: How Do Northwest Trees Know When Winter Is Over?

United States Department of Agriculture D E E P R A U R T LT MENT OF AGRICU Forest Service PNW Pacific Northwest Research Station INSIDE Tracking the Passage of Winter......................... 2 Budburst Now and in 2080................................. 4 FINDINGS issue one hundred eighty three / march 2016 “Science affects the way we think together.” Lewis Thomas Rise and Shine: How Do Northwest Trees Know When Winter Is Over? IN SUMMARY Trees bursting forth with new leaves signal the arrival of spring. Bud- burst for most temperate tree species occurs after a tree has been exposed to a sufficient number of chilling and forcing hours over the winter. Wait- ing until these chilling and forcing hours have accumulated is a survival mechanism. If a tree bursts bud prematurely, delicate tissue may be damaged by a late frost. Conversely, if a tree bursts bud too late in the spring, it will be unable to achieve substantial height growth before summer drought sets in. Although most Northwest tree species require a combination of chilling and forcing hours to promote budburst, the number of hours needed differs by species. To identify the chilling and forcing requirements of 11 common Pacific Northwest tree species, scientists with the U.S. Forest Service, Pacific Northwest Research Station exposed seedlings to various combinations of chilling and forcing temperatures. They tracked the timing of budburst and created possibility lines that describe the combination of chilling Climate change will lead to changes in the timing of budburst for many tree species. These maps show how and forcing hours required by each many days earlier budburst is projected to occur by 2080 within the native ranges of (A) western hemlock, species. As the climate changes, the (B) western redcedar, and (C) Pacific madrone. timing of budburst is also expected to change, so the scientists developed “Spring is sooner recognized by rees help mark the passage of the landscape models to predict when a plants than by men.” seasons: their new flush of leaves in species’ budburst would likely occur —Chinese proverb T spring; full leafy canopies in summer, in 2080. cones and seeds that litter the ground in fall, and bare branches and steadfast evergreen boughs signal winter. And even as the leaves begin to cover sidewalks, next year’s small KEY FINDINGS buds await the arrival of spring. • Trees have mechanisms that sense the passage of winter by tracking the accumulation In temperate climates, it is often assumed of chilling (cool temperatures) and forcing (warm temperatures). Species have evolved that winter dormancy is triggered by the cold. to require different combinations of chilling and forcing hours to promote budburst. However, “The initiation of dormancy in the fall is not temperature driven; it’s primarily triggered by shortening day lengths,” explains • Most tree species in the Pacific Northwest require a minimum amount of chilling to Connie Harrington, a research forester with successfully burst bud. Well-chilled plants require fewer hours of warm temperatures the U.S. Forest Service, Pacific Northwest for budburst than plants that have received less chilling. (PNW) Research Station. “Trees will go dor- mant even in warmer winters. Dormancy is • The number of chilling and forcing hours required by a tree species to promote bud- also triggered faster when the trees are under burst can be modeled as its possibility line. Comparing the possibility lines of different stress, such as when they experience extended species predicts the relative timing of budburst by species under a range of conditions. periods of drought.” • The timing of budburst will likely change in the future as the climate warms, but dif- Budburst marks the end of dormancy, when ferences in timing will vary by species and location of a population within its range. the growing leaves literally burst through their Although budburst is generally predicted to occur earlier in 2080 than today, budburst protective bud scales. Cultures around the in some tree species in low-elevation coastal locations in the southern portions of their world have tracked the budburst of a variety ranges may occur later due to a lack of chilling. As a result, these trees may be unable of species from one year to the next, and these to complete their height growth before droughty conditions occur. observations are examples of one of the oldest types of environmental sciences, phenology. This branch of science is derived from and out of dormancy,” Harrington explains. days. For many tree species, they “need to the Greek words phaino (to show or appear) “But what we still really don’t understand is experience a certain amount of chilling before and logos (to study). One famous phenology how plants keep track of the winter tempera- they are able to come out of dormancy,” says record is the flowering of the cherry trees in tures and know when to break dormancy.” Peter Gould, formerly a research forester with Japan, and the Chinese are credited with hav- the PNW Research Station and now a research ing the oldest phenology records, which date During dormancy in the Pacific Northwest, analyst with the Washington Department of back to 974 B.C. trees experience a wide range of temperatures, Natural Resources. from below-freezing to mild 55 °F degree However, observing when a tree bursts bud does not reveal how the tree knows when winter is over. “There have been thousands of studies on dormancy on many plants, includ- TRACKING THE PASSAGE OF WINTER ing fruit trees and agricultural plants, and ust as dormancy is not triggered by daily temperatures are not necessarily a lot which genes are activated as part of going into cold temperatures, its end is not trig- warmer than they might have been during a J gered by the first warm days in spring. few days in January. Yet there is a mechanism Purpose of PNW Science Findings Budburst occurs when conditions are favor- that keeps the tree from bursting bud too soon. able for growth and there is little risk of the Trees have evolved this mechanism to keep To provide scientific information to people who delicate new growth being damaged by frost. them from growing at the wrong time of year,” make and influence decisions about managing “When trees are bursting bud in April, the Harrington says. land. PNW Science Findings is published monthly by: Pacific Northwest Research Station USDA Forest Service P.O. Box 3890 Portland, Oregon 97208 Send new subscriptions and change of address Connie Harrington information to: [email protected] Rhonda Mazza, editor; [email protected] Cheryl Jennings, layout; [email protected] Science Findings is online at: http://www. fs.fed.us/pnw/publications/scifi.shtml To receive this publication electronically, change your delivery preference here: http://www.fs.fed.us/pnw/publications/subscription. shmtl United States Forest Seedlings are moved from a heated greenhouse on the left to lath house with open sides on the right to Department Service expose them to different combinations of chilling and forcing temperatures. of Agriculture 2 Connie Harrington Connie Harrington True firs are quite sensitive to lack of chilling. Trees exposed to less chill- Western pines are also quite sensitive to a lack of chilling. Note the reduced ing (yellow and red flagged pots) had less new growth for the year (bright needle area on the new growth on the seedlings on the left in the yellow and green needles), whereas trees exposed to more chilling (blue and white red flagged pots. The plants with low chilling also are more likely to have flagged pots) had more new growth. multiple tops. Climatologists predict the Pacific Northwest ing temperatures) will experience warmer summer and winter on the timing of its temperatures in the future, which means the budburst. Although timing of budburst will be even more critical. other researchers had Harrington explains, “As the climate becomes observed that the warmer, you want trees to burst bud a little timing of budburst Connie Harrington earlier than they do now so that they can still is affected by the take advantage of the good growing condi- length of time a plant tions before the summer dry period. But you experiences chilling don’t want them bursting bud so early that or forcing tempera- they could be damaged by frost.” tures, Harrington and Gould’s study was Most Pacific Northwest tree species require the first to model the chilling temperatures to promote normal bud- timing of budburst burst. If trees have not spent enough hours with temperature data during the fall and winter under chilling in a way that allowed temperatures, the timing of spring budburst them to predict the could be delayed. This could reduce growth timing of budburst because the trees may not have enough time to over a large landscape. Madrone has a low chilling requirement. It burst bud normally and grew complete their normal period of height growth well in all treatments. Their models included before the summer drought sets in. Not only developing what they could this potentially affect forest health, but termed “possibility lines,” which graphically peratures. “In this study, we wanted to include also the diversity of tree species found across depict the tradeoffs between chilling and as many major species as we could, such as the landscape. forcing temperatures on budburst. western hemlock, western redcedar, western “Not all tree species are going to react the larch, and several species of pine and true firs. “People who studied dormancy in the past same way to a changing climate,” says We also had the good fortune of including were often trying to determine the optimum Robyn Darbyshire, regional silviculturist for Pacific madrone because there was a research- temperatures for chilling, so the temperature the Pacific Northwest Region of the Forest er in Puyallup who was working on madrone range which was most effective was pretty Service.

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