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

United States Department of Agriculture

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 ConnieHarrington 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 ConnieHarrington

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 ConnieHarrington 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. “Many people have this idea that and she had extra seedlings to contribute,” well known. But past trials often only studied the tree species groupings we see today have Harrington says. This study also examined the the effects of chilling in the optimum range; always been together. Part of climate change timing of budburst at the population level for they hadn’t really looked closely at what the education is understanding that these group- some species to determine whether differences shape of the relationship between chilling and ings will not shift together as a result of cli- existed among populations. forcing would look like across a wider range mate change. Each tree species will react in its of temperatures,” Harrington explains. The experiment spanned 3 years and was own way to a warmer climate, as it is geneti- conducted in the greenhouse and outside beds cally programed to react.” To see if the timing of budburst in other at the PNW Forestry Sciences Laboratory in Northwest tree species would be affected To learn how different species will react to Olympia. Because of the logistics involved by an increase or decrease in chilling and a changing climate, Harrington and Gould with monitoring a sample size of 800 seed- forcing temperatures, Harrington and Gould expanded upon one of their earlier studies, lings or more and moving 1- to 3-feet-tall exposed 11 common tree species to several which focused primarily on Douglas-fir and seedlings back and forth between chilling and combinations of chilling and forcing tem- the effects of chilling and forcing (warm- forcing temperatures, Harrington and Gould

3 studied a few species each year. The following tracked the number of hours the seedlings Exposing seedlings solely to greenhouse year, they replicated the treatments on another experienced in each temperature. The scien- temperatures, which prevented them from batch of seedlings to confirm the previous tists also monitored the seedlings throughout experiencing chilling temperatures, revealed year’s results. the summer, noting how the combination of that chilling affected the normal development chilling and forcing temperatures affected of the seedlings in the spring—as well as the For each species, a select number of seedlings their subsequent growth and appearance. timing of budburst. Most of the pine and true were assigned to one of five treatments. These fir seedlings that received very little chilling treatments ranged from a warm treatment in Although most conifer species have discern- did not grow a full complement of needles, which seedlings spent the fall and winter in a able buds at the tips of their branches, which and often the side buds burst but not the ter- heated greenhouse to a cold treatment where made assessing the timing of budburst easy, minal bud, which is responsible for height seedlings were outside all winter. Intermediate western redcedar was a bit more challenging. growth. As a result, they became more bush- treatments had the seedlings moved back and “Western redcedar doesn’t have a bud, and like because growth would shift from what forth between the warm and cold environ- so for the cedars, we would actually make a had been the main stem to one or more side ments to experience a range of chilling and mark on the stem using nail polish and mea- branches. The scientists also noticed that if the forcing temperatures. The trial started in sure from that mark,” Harrington says. “Once terminal bud and many of the lateral buds did October and each week on “moving days,” the length of the shoot was about a half inch not burst bud quickly or normally, the seed- seedlings in pots flagged with a certain color greater than it was when we started measuring ling developed fewer new needles. were moved from the greenhouse to the out- them in the fall, we concluded that the seed- side or vice versa. Temperature data loggers ling had initiated height growth.”

BUDBURST NOW AND IN 2080 he general public notices when Harrington says. “Our work is taking those pen from year to year and at different loca- the trees burst bud in the spring; observations and developing a mathematical tions on the landscape and, when combined “T people say things like ‘springs relationship—called a possibility line—that with climate projections developed by clima- a springing, plants are bursting bud,’” allows us to predict when budburst will hap- tologists, when it will happen in the future.” A possibility line is the combination of the chilling and forcing temperatures that are required for budburst. The area above the line represents the combination of temperatures where budburst is possible; the area below the line indicates where budburst is not possible. Each species has its own possibility line. For example, when western larch is exposed to many hours of chilling, it does not need as many hours of forcing temperatures for budburst to occur compared to other Pacific Northwest conifers. In contrast, when western hemlock experiences high levels of chilling, it requires more hours of forcing temperatures before budburst occurs. Madrone and redcedar were different than the other species studied because they did not require chilling for growth to begin and thus, they were capable of bursting bud under a wide range of forcing and chilling temperatures. To predict when budburst will occur under different climates, Gould and Harrington created models that projected budburst for the 11 tree species in 2080 based on their possibility lines. Sixty-five years isn’t that far off, given the lifespan of a tree, and the fact that some of the studied species are grown on harvest rotations of 40 to 80 years. What further complicates forest management is that the climate is not expected to change uniformly. Localized conditions may lead to greater or lesser change in certain areas. Species that can shift the timing of their budburst to stay in the sweet spot between frost and drought will have an advantage over species that can- For Douglas-fir, trees in some areas are projected to burst bud more than 60 days earlier by not shift their timing. Warmer winters may 2080 (left panel) while trees in locations with warmer winter temperatures are projected to cause a species’ range to constrict or expand burst bud later than they do currently (red areas in right panel).

4 depending upon its chilling requirements for dormancy. LAND MANAGEMENT IMPLICATIONS That’s one reason why these modeling projections are valuable for land managers and • Many tree species may continue to experience sufficient chilling in their current rang- sivilculturists, like Robyn Darbyshire: “As es so range contractions are unlikely to occur by 2080. However, individual trees that we learn more about how the physiology experience fewer chilling hours may have reduced growth and poor form. of these species differ in respect to climate change, this might affect how we might tend • Tree growth may be unchanged or enhanced in areas where the timing of budburst those stands after they are established, such as is predicted to occur early enough in spring for trees to complete their height growth doing more management to maintain one spe- prior to summer drought. Mid- to high-elevation areas will experience more chilling cies on the landscape versus another.” hours in the future, which may result in improved growth. Yet modeling a species’ possibility line does not fully reveal when budburst will occur, • Some species may exhibit poor growth and poor form where predicted climate change because each population within a species is expected to result in winter and fall temperatures too warm to provide effective has its own timing. This timing is a result of chilling. populations, through natural selection, being adapted to their local environmental condi- • Although each species has a general possibility line, populations within species may tion. Harrington and Gould studied many also have distinct possibility lines. populations of Douglas-fir, but only two to three populations of the other species. For “With information about future climates and better adapted to future climate in terms of silviculturists, having possibility lines for an understanding of the population’s possibil- bursting bud at the right time.” populations, will help them decide which seed ity line, you may choose a seed source that is source to use for reforestation. “There are differences amongst populations,” Gould says. “We must consider the distinctive characters and the general nature of plants from the point of view of their morphology, their behavior under external conditions, their mode of generation, and the whole course of their life.” —Theophrastus, Greek scientist considered the father of botany

FOR FURTHER READING Gould, P.J.; Harrington, C.A.; St. Clair, J.B. 2011. Incorporating genetic variation By combining into a model of budburst phenology of information on the coast Douglas-fir (Pseudotsuga menzie- chilling and forcing (warming) tem- sii var. menziesii). Canadian Journal of peratures associ- Forest Research. 41: 139–150. http://www. ated with budburst, treesearch.fs.fed.us/pubs/39618. scientists developed possibility lines for Harrington, C.A.; Gould, P.J. 2015. Tradeoffs each tree species. between chilling and forcing in satisfy- Points on or above ing dormancy requirements for Pacific the line for a species indicate combina- Northwest tree species. Frontiers in Plant tions of chilling Science. 6: 120. http://www.treesearch. and forcing where fs.fed.us/pubs/48756. budburst is possible. Points that fall Harrington, C.A.; Gould, P.J.; St. Clair, J.B. below the species’ 2010. Modeling the effects of winter envi- possibility line indi- ronment on dormancy release of Douglas- cate combinations fir. Forest Ecology and Management. 259: of chilling and forcing where budburst 798–808. is not possible. Parks, N. 2010. The future of spring budburst: looking at the possibilities, based on science by Connie Harrington, Brad St. Clair, and Peter Gould. Science Findings 128. WRITER’S PROFILE Portland, OR: USDA Forest Service, Pacific Andrea Watts is a Seattle-based science writer. She can be reached at [email protected] and Northwest Research Station. 6 p. http:// a portfolio of her published work is available at http://wattswritings.wordpress.com. www.fs.fed.us/pnw/sciencef/scifi128.pdf.

5 PRSRT STD US POSTAGE PAID PORTLAND OR FINDINGS PERMIT N0 G-40 U.S. Department of Agriculture Pacific Northwest Research Station 1220 SW Third Avenue P.O. Box 3890 Portland, OR 97208-3890 Official Business Penalty for Private Use, $300

SCIENTIST PROFILES CONNIE HARRINGTON is PETER GOULD is a research COLLABORATORS a research forester with the analyst with the Washington Pacific Northwest Research Department of Natural Resources. George McFadden, Bureau of Land Station. Her research focuses While working at the Pacific Management on studying plant responses Northwest Research Station, he Washington Department of Natural Resources to environmental conditions. was a research forester. She also studies silviculture Marianne Elliott, Washington State University Gould can be reached at techniques, such as variable density thinning. Washington Department of Natural Resources Harrington can be reached at 1111 Washington St. SE USDA Forest Service Olympia, WA 98504 Pacific Northwest Research Station Phone: (360) 902-1369 Forestry Sciences Laboratory E-mail: [email protected] 3625 93rd Ave SW Olympia, WA 98512 Phone: (360) 753-7670 E-mail: [email protected]

“USDA is an equal opportunity provider, employer, and lender.”