For us in the Northeast U.S., deciduous trees are impressionable indicators of seasonal change. Their growth cycle mirrors our own productivity patterns, as an active summer winds down into a quiet autumn that prepares us for a restful winter, until the warmth of spring re-energizes us for another summer to come. Like us, trees in temperate and boreal regions are sensitive to cold temperatures. Unlike migratory birds or retirees who simply take off and head south for the winter, these sessile trees have had to adapt to cold, dry, light-poor conditions. In late summer or early autumn, they stop growing and sheath their green shoots in hardened buds to protect precious apical meristems. The sight of foliage in warming hues as microscopic chlorophyll factories shut down reminds us, too, that a season of rest is approaching.

Like our own sleep, dormancy is essential to a tree’s health, development and wellbeing. Just as we enter various REM cycles each night, there are multiple stages of tree dormancy, which experts have termed paradormancy, endodormancy and ecodormancy. When a tree senses the shorter days of late summer through light-receiving photoreceptor proteins, it produces abscisic acid (ABA), a hormone that works to slow down metabolic activity and induce leaf drop. As a result, the tree slowly enters a deep sleep—endodormancy—when all growth is inhibited internally by the plant. Just as many of us tend to go “offline” during the holiday season, the tree becomes unreceptive to the outside world during endodormancy.

The quiet, complex metabolic processes that continue inside the tree during this stage are still not well understood by science. This may seem surprising, but even many of the molecular underpinnings of sleep regulation in human health and disease continue to elude us; we know that sleep disruption is associated with consequences for human health, but we do not fully understand which genes control the relationship between sleep disorders and disease. A central component of tree dormancy that greatly influences its health and productivity come springtime is the “chilling requirement.” A tree fulfills its chilling requirement once it has been exposed to sufficient above-freezing temperatures, at which point it is released from endodormancy. Each tree species has a specific chilling requirement (with some intraspecific variation, which emerges through cultivation or changes in response to latitude or climate). This determines when it enters the next stage of dormancy—ecodormancy—which is a lighter sleep that is poised to lift once environmental conditions in spring are favorable for bud break.

When a tree enters the ecodormancy stage, it is once again receptive to environmental signals to bloom. As spring approaches, it waits for critical cues—long days or warmer temperatures—to know when it is safe to open its buds. Bud break timing is everything: too early, and tender shoots are exposed to spring frosts; too late, and there is little time for flowers, fruit or new growth. The chilling requirement protects the tree from opening its buds prematurely during warm spells in the middle of winter by keeping it unreceptive to cues until the worst of the cold weather has passed. Meeting the chilling requirement is key for high blooming rates and uniform flowering in the spring—otherwise, bud break may be irregular and heterogenous.

Today, that chilling requirement is getting harder to fulfill. We are currently in the “Anthropocene,” a proposed geological epoch used to describe the period of time humans have had significant impact on Earth’s climate and ecosystems. The complex consequences of our changing climate play out differently across taxa, cultivar, and landscape. (See Arnoldia 76:2, “Dormant Vines, Future Wines,” to learn about how climate change is affecting viticulture). Investigating the molecular basis of dormancy—and how growth and sugar profiles change during the winter—is crucial in understanding how climate change will affect how trees wake and function in the spring. While energy metabolism slows to a minimum during dormancy, a complex array of signaling pathways among genes actively regulates the transition between its stages. In the past decade, a slew of studies have found that changes in primary and secondary metabolites in dormant buds are associated with dormancy progression. Many of these studies aim to characterize these pathways with the hopes of finding genetic modifications that can artificially regulate dormancy in buds to ensure that trees get the rest they need under warming conditions.

A team of researchers at the University of Georgia are working to understand which genes are “awake” during dormancy and what their functions may be. In 2022, they were awarded $2.7 million by the U.S. Department of Energy to investigate how wood-forming xylem cells function during colder months. The researchers have been characterizing a gene they believe is responsible for sucrose distribution during dormancy, and a small field trial has already revealed that the absence of this gene can cause trees to drop their leaves early and flush their leaves late. This preliminary result suggests that without the gene, carbohydrate reserves are negatively impacted. With this information, the scientists can build a foundational understanding of how sugar profiles are regulated at the molecular level throughout dormancy. With further research, they hope that genes or pathways like this one can be altered to protect buds under inconsistent weather patterns. For example, delaying bud break could prevent trees from waking prematurely during a warm snap in early spring.

The complex consequences of our changing climate play out differently across taxa, cultivar, and landscape.

As for human sleep, research on the consequences of insufficient rest has revealed that it increases risk of cancer, depression, and heart problems, and, according to a 2020 article in the Harvard Business Review, it decreases productivity in the workplace. In parallel, dendrologists have been studying the effects of disrupted dormancy in trees, often measuring productivity via flowering rate and fruit yield. Temperate fruit trees are popular study models for this work because the impact of warming temperatures on chilling requirements can be easily observed in irregular flowering patterns and lower, poorer-quality fruit yields. Threatened fruit production also has serious implications for economies, cultures, and food systems worldwide as temperatures continue to rise. While warmer weather can bring temperatures above the chilling range in some regions, slowing the chilling process, it can also raise ordinarily below-freezing temperatures into the chilling range, advancing the chilling process. Both scenarios affect the delicate timing of bud break and render the tree vulnerable.

An olive branch for the climate: Olea europea in fruit.

The Mediterranean Basin has been the subject of several important studies in the past few years, as one of the largest production zones of temperate fruits in the Northern Hemisphere and a climate change “hotspot,” according to the Intergovernmental Panel on Climate Change. Drought and rising temperatures in recent decades have affected bud initiation and differentiation, dormancy, flowering, fruit set, and development. A 2023 study in Tyre, Lebanon, found that the optimal fruiting of olive trees (Olea europaea) scales closely with temperature. The researchers used paleoand present-day data to define the optimal annual average temperature for olive flowering (16.9 ± 0.3 °C), which has remained consistent for at least the past 6,000 years. Their projections predict that temperature increases in Lebanon in the second half of the 21st century will threaten olive-growing areas and drive down productivity, as well as fruit and oil quality.

The olive tree is of great cultural and economic importance in the Mediterranean, which is why agronomists are focusing on the dormancy period that prepares the tree for a productive spring. While olive trees are not deciduous (i.e. they do not lose their leaves), they do require a dormant chilling period (ideally between 5–7 °C) to flower and bear fruit. During this time, the axillary buds that produce flowers and fruit in the spring are formed, and certain diseases and pest populations are brought under control. The 2023 study found that the olive tree is very sensitive to variations in temperature and precipitation during winter, which ultimately determine when and how it emerges from ecodormancy. A spring that is too wet or dry can also disrupt the optimal fruiting cycle and negatively affect olive production.

In Japan, a 2022 study in Frontiers in Plant Science used the Japanese pear (Pyrus pyrifolia) as a case study to similarly examine how warming temperatures are negatively affecting cold acclimation, dormancy progression, and floral bud maturation in temperate fruit trees. A physiological condition known as flowering disorder (or dormancy disorder) recently has plagued the pear trees, causing erratic flowering and bud break disorder. These changes were brought on by a marked increase in mean annual temperature in Japan since the 1990s. After several years of surveying trees in greenhouses and fields, the scientists compiled and analyzed their observations of symptoms, which included delayed blooming, flower bud abortion, fewer florets, smaller flowers, injured or dead flower buds, uneven bud break, and lower bud break rate. Similar phenomena have been observed in New Zealand, Israel, Brazil, and South Africa nearly 30 years ago during springs following mild winters.

The Japanese researchers suggest that changes in levels of dormancy-regulating genes render the pear trees more susceptible to flowering disorder. These widely-studied genes, called Dormancy-Associated MADS-Box (DAM), encode transcription factors that many scientists believe control dormancy progression in fruit trees, including apple, pear, peach, Japanese apricot, and kiwifruit. Their study shows that the trees are not chilled enough during dormancy to support the proper function of genes like DAM that regulate flower development. They also propose that the process of chilling exposure, as opposed to the total amount of chilling, may influence the metabolic processes. Further research into the metabolic regulation of endodormancy release and floral bud maturation would be instrumental in clarifying the physiological basis of this flowering disorder. These scientists hope that characterizing these regulatory pathways can help develop strategies to mitigate or overcome challenges with bud break during the changing climate of the Anthropocene.

Interestingly, the Japanese researchers also found that warm conditions lead to a decrease in carbohydrate accumulation in shoots, which reduces tree vigor. It is possible that the lack of chilling is affecting the function of a gene responsible for sucrose distribution during dormancy, similar to the one that the University of Georgia researchers genetically removed during their field trial that resulted in reduced carbohydrate reserves. While chilling requirements vary across species, there are several key genes, DAM included, that exist in one form or another across many species; these are known as orthologs—genes that evolved from a common ancestral gene and retain the same function in different species. This provides hope that certain developments can be extended beyond the single study species as they arise.

There is an urgent need for models and biomarkers that can help farmers precisely predict when a tree will reach its chilling requirement.

While scientists continue to characterize the molecular mechanisms of bud break and search for genetic modifications to confer resilience, agronomists and farmers will also need to artificially select species with chilling requirements that can thrive in warming conditions. To do this, there is an urgent need for models and biomarkers that can help farmers precisely predict when a tree will reach its chilling requirement. A study from the Humboldt University of Berlin in 2022 presented the first step in identifying the metabolites related to the timing and length of dormancy phases for ‘Summit’, a cultivar of sweet cherry (Prunus avium). The team hopes that these metabolites can be used as biomarkers to indicate the transition between endo- and ecodormancy, which would greatly aid in adaptive species selection.

As trees begin to wake this spring, let the blooming flowers remind you of the months of rest that made them possible. Proper sleep on the scale of days for us can have an impact on the scale of months or years; and, as Katherine May’s 2020 book Wintering recommends, we should also take time during our “winters”—be they seasonal or psychological—to retreat and take pause. The unseen molecular pathways that regulate tree dormancy, and those that regulate our own sleep, have been artfully engineered over millions of years to ensure a rejuvenating rest. If we can better understand these processes in trees, perhaps we can try to help them get the rest they need to wake well in the Anthropocene.

Christina Janulis is a budding science communicator interested in landscape history and ocean science who fell in love with the Arnold Arboretum during her time at Harvard.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

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