In 1965, almost a century after it became a nation, Canada officially adopted its own flag with an 11-point red maple leaf in the center.1 The flag symbolizes the vast country’s unity and identity. But now, the emblematic sugar maple (Acer saccharum, Sapindaceae) trees and the syrup producers who depend on them face an uncertain future, largely because of climate change.2,3
Potential Effects of Climate Change
The sugaring season has always presented challenges, but now the season, which was not long to start with, is getting even shorter. It lasts just six to eight weeks or less, usually between February and April, depending on the latitude. Therefore, the loss of even one day can sometimes be significant. Harvesters also are experiencing a phenomenon called season creep, in which the season is starting and ending earlier than before.2
“In Vermont, maple trees are generally in good health,” said Mark Isselhardt, a maple specialist at the Proctor Maple Research Center (PMRC) at the University of Vermont (oral communication, June 24, 2019). “The most significant near-term threat is anomalous weather patterns. In 2012, we had multiple days above 70°F in March, which is very uncommon in Vermont. That wasn’t a risk to the trees’ health in the short term, but it significantly impacted syrup production…. You really need the critical temperature swing on either side of freezing.”
Until he retired in 2012, Barry Rock, PhD, professor emeritus in the Department of Natural Resources and the Environment at the University of New Hampshire, conducted research that monitored the timing of the color change of maple trees. Using new satellite data and satellite data dating back to the 1970s, he determined that the timing of the color change was standard until the mid- to late-1980s. Then, he said, the leaves of the maple trees started changing color later and later in the fall (oral communication, June 18, 2019).
“In New England, Columbus Day weekend, in early October, was considered the peak color display period, and people would come from all over to see the color change,” Rock said. “In recent years, on Columbus Day weekend, the trees are still green. They haven’t changed color at all. And now, the red color is not the dominant color. The trees are simply turning kind of brown and not having the really brilliant color displays.”
Whether the shift in color change timing is linked to climate change is unclear, Rock said, but it is possible, since it coincided with the start of some climate-related changes. It is also unclear how photosynthesis is involved, he said. As the days get shorter, an abscission layer (a waterproof barrier of thin-walled cells) forms at the bases of the petioles or leaf stalks, which stops water and nutrients from moving into the leaf. “Because of the removal of nutrients, the formation of the abscission layer should mark the beginning of color change, and we are not sure why it doesn’t now,” Rock said. “It’s a mystery!”
A 2019 study predicted climate change’s effects on two climate-sensitive aspects of maple syrup production: sap sugar content and sap flow. (Maple sap is boiled down to make syrup.) Over two to six years, the study authors observed sap at six sugar bushes (stands of maple trees) across sugar maple’s latitudinal range. Sap sugar content partly derives from carbohydrate stores that are influenced by the growing conditions of the previous year, including temperature and precipitation, which affect photosynthesis and respiration.4
Historically, on average, sap sugar content reportedly has been greater than two degrees Brix (°Bx, a measurement of the sugar content of a water-based solution) across most of sugar maple’s range. (One °Bx is one gram of sucrose in 100 grams of solution.) However, this study found that sap sugar content decreased by 0.1°Bx for every 1°C (1.8°F) increase in the previous year’s May-October mean temperature. So, under some climate scenarios, the authors projected that sap sugar content may decrease by 0.7°Bx by the year 2100.4
Reverse osmosis (which uses pressure to force water molecules through a partially permeable membrane) can help make up for lower sap sugar content, but it still means that more time and sap are required to make one gallon of syrup with the necessary 66-68°Bx.3 (This range is narrow because at less than 66°Bx the syrup is at increased risk for fermentation or mold growth, Isselhardt said. At more than 68°Bx, the syrup starts to crystallize.) Historically, the sap-to-syrup ratio has been about 40:1.4
Sap flow is influenced by freeze/thaw cycles in early spring. The 2019 study found that mean tapping season temperature was a good indicator of the timing and number of freeze/thaw cycles as well as total sap volume. Sap volume peaked at a January-May mean temperature of 1°C (33.8°F). The authors projected that, by 2100, the region of maximum sap flow may shift northward by 400 kilometers (249 miles). “Total syrup production is projected to decline over most of sugar maple’s range by the end of the century, except for the far northern range in Ontario and Quebec which project to have moderate to large increases in average syrup produced per tap,” the authors wrote.4
A 2014 survey assessed how maple producers in northern Vermont and the Adirondacks of New York perceive climate change and plan to adapt. Of the total estimated 1,200 commercial maple producers in this region, 264 completed the survey (178 in Vermont and 86 in New York). The respondents made an average of 1,337 gallons of syrup in 2014. The maximum made by a single producer was 32,500 gallons.5,6
Forty-two percent of respondents had no concerns related to climate change, while the main climate-related concerns of the other 58% were weather damage to the sugar bush, change in season timing, tree health, shorter season, and reduced sap flow. Producers with more than 3,000 taps expressed more concern about climate change than did smaller-scale producers.5,6
More than 70% of respondents said they had already modified their businesses. For example, almost a quarter said they were tapping earlier. Despite climate change, respondents were generally optimistic about the future of their businesses, with more than half planning to increase the number of taps or products during the next five years and only 10% considering retiring, selling, or closing their business.5,6
Snowpack as a Blanket
Another climate-related factor that may impact maple trees and syrup production is snowpack reduction. Snowpack acts as an insulating blanket and helps prevent soil and tree roots from freezing. A 2018 study found that almost 65% of the sugar maple basal area in the northeastern United States occurs in places with regular snowpack. But, under some climate change scenarios, the area of regular snowpack in these forests could shrink by 49-95% by 2099. To approximate diminished snowpack, researchers removed snow for the first four to five weeks of winter in an experimental forest in New Hampshire. They repeated this for five consecutive winters, stopped for one year to see if the trees recovered, and then collected core samples of sugar maple trees to analyze their growth rings.13,14
The growth rates of the trees decreased by about 40% (averaged across the six years of the study), and the trees did not recover one year after the end. It is unclear if this damage is permanent. Reduced growth rates suggest that, without insulating snowpack, maple forests may be less productive for sugar makers and will not sequester, or remove, as much atmospheric carbon dioxide, if the trees even survive. This may have other implications because maple forests enable improved water quality and flood mitigation, among other benefits. It is unclear if anything can be done to mimic the benefits of snowpack.13,14
Optimal maple sap extraction depends on freezing nighttime temperatures and mild daytime temperatures, which cause pressure changes. On warm spring days, sap can flow down from the tree’s branches and even laterally within the wood. It flows to the point of lowest pressure from all directions. Excessively high or low temperatures disrupt the pressure swings necessary for sap flow.2,15
For now, unpredictable weather largely is counteracted by widespread use of vacuum tubing systems that suck sap out of the tap. Instead of waiting for the sap to drip into galvanized metal buckets hanging from the taps, as in the past, the plastic tubing zigzags through the sugar bush, connecting each tree and running into the sugarhouse, where the sap is collected for reverse osmosis and then boiling. The food-grade tubing is cleaner than older methods, so producers can tap earlier with less chance that the tapholes will close before the sap flows.2,3
The vacuum system increases sap yield per tree, apparently without harming the tree or changing the taste of the syrup.3 Each inch of mercury (inHg) decrease in pressure corresponds to a sap yield increase of about 5-7%. So, generally, maple producers try to reduce pressure in the tubing as much as possible and quickly detect and fix leaks.16
According to Isselhardt, partly because of the shortened season, producers already would have seen decreased yields per tree if they were still collecting with buckets, especially if they were not tapping earlier since the sugaring season is starting earlier now. “If you have adopted modern tubing and vacuum and practices like how to manage the vacuum and how to keep the tubing clean, you are actually seeing more yield than you ever have,” he said.
Sanitation is important. Put simply, a maple tree is like a dense collection of tiny straws (xylem vessels), Isselhardt said. When the taphole is drilled, many of those “straws,” which had been sterile, are severed and become inoculated with bacteria, yeast, mold, and more. The buckets or tubing are also not sterile, even when new. Early in the season, low temperatures reduce organism growth, but organisms grow more as temperatures rise. Because sap is mostly water with a little sugar, it is suitable for organism growth. “Whenever sap leaves the taphole, it comes into contact with things that are not sterile,” Isselhardt said. “During freezing events, that sap may be sucked back into the taphole.”
Eventually, as temperatures rise, the tissue is walled off, both by the tree’s responses and because the organisms form a gummy mass that plugs up the vessels. “If you can keep the tapholes clean, it limits that growth and keeps sap flowing,” he said. “Really, it just delays the inevitable, because eventually sap flow will stop. But sanitation can have significant benefits for sap production.” For example, using new taps and tubing each year, instead of reusing old taps and tubing (even if sterilized), may enable up to 100% more sap flow during the latter part of the season.17
Isselhardt noted that more sugar makers are tapping red maples (A. rubrum) than before. Red maples can compete on a wider range of sites than sugar maples, which are sometimes referred to as Goldilocks trees, and the reasons red maples were not tapped more in the past are not as relevant anymore, he said. For example, the sap of red maple trees typically contains less sugar than the sap of sugar maples, but now reverse osmosis concentrates the sap and helps make up for that difference.
“I think you are going to see more red maples in the mix, especially since producers who are certified organic need to keep a certain percentage of non-sugar maple trees in their woods, and red maples can count toward that diversity,” Isselhardt said.
Besides maple trees, some other North American trees also produce sap that can be reduced to edible syrup, though tapping them may sometimes be impractical. Examples include sycamore (Platanus occidentalis, Platanaceae) and multiple species of birch (Betula spp., Betulaceae) and walnut (Juglans spp., Juglandaceae).18 For additional income, maple producers and others could tap these species, if they are present and viable.
Harvesting Sap without a Tap?
To maintain yields in the face of challenges, maple producers may eventually consider using a harvesting method devised by the Proctor Maple Research Center (PMRC). Wanting to learn more about sap flow, PMRC researchers cut the crown off a maple tree and discovered a new way to extract sap. By attaching a sealed plastic bag over the cut stem, they realized that sap can be sucked directly from the top. This plantation method could enable maple production in farm fields and involves cultivated saplings instead of wild, mature trees.19 “It is very different, visually,” Isselhardt said. “The propagation is very different. It is provocative, I admit. People are not sure what to make of it.”
Because saplings can be planted close together, this method could enable greater yields per acre, even though yields per tree would be far less than with large, mature, natural trees. With the usual tapping method, traditional sugar bushes typically produce about 10-50 gallons of syrup per acre per season, with maybe 80 mature trees per acre. But PMRC researchers estimate that this new method could produce 400 gallons of syrup per acre per season, assuming the saplings are planted at a high density (about 6,000 saplings per acre). Also, typically, maple trees require about 40 years to reach tappable size, while saplings could be ready in seven years under the tap-less plantation method. Another potential benefit is that Asian long-horned beetles, which infest mature maple trees, apparently avoid maple saplings.19
The saplings regrow their cut crowns from previously dormant buds, at which point the stem could be cut again. However, this can be done a limited number of times before not enough stem remains to be cut. In that case, replanting and regrowth would be needed before cutting could resume again. Or, it is possible that with a multi-stem growth, one stem could be cut at a time. This method would shorten the lifespan of maple saplings like Christmas tree plantations shorten the lifespan of Christmas trees (from multiple genera in the Pinaceae family), Isselhardt said. Ultimately, the trees would be replaced.
Sugar makers have not adopted this concept, largely because the fittings, or flexible caps, to cover the tops of the saplings are not commercially available yet. According to Isselhardt, people may think of saplings as perfect cylinders, but, in reality, they have irregularities that make it difficult to create a vacuum-tight seal that can fit over the cut stems. If and when the fittings become available, some people are interested in exploring the potential of this system, he said.
Despite problems, maple sap yield per tree is actually higher now than ever, because of improved technology and harvesting methods, but, according to some studies, syrup yields are expected to decrease across much of sugar maple’s range by the end of the century. It is unclear if better and more expensive technology and methods may be needed in the future, and, if so, how long those improvements could counteract climate change’s effects.
Based on climate model projections, Rock expects that in 100 years sugar maples will still exist in Canada, where their range may extend further north than present, but they will likely be gone from New England. Isselhardt, however, is more optimistic. “The concern is that sugar maples are going to be at a competitive disadvantage in the future,” he said. “But a lot of projections do not [account for] humans in the sugar bushes doing anything. Sugar makers favor certain species in their woods at the expense of others. So, it may take longer for maples to be choked out. To preserve maples, it may require the people who manage those woods to exclude other trees, like oak [Quercus spp., Fagaceae] and hickory [Carya spp., Juglandaceae], that want to come in.”
- The Red Maple Leaf: How Canada’s Flag Came to Be. Toronto Public Library website. February 13, 2015. Available at: http://torontopubliclibrary.typepad.com/trl/2015/02/a-red-maple-leaf-against-a-blue-sky-a-quintessential-canadian-symbol-and-one-recognized-throughout-the-world-even-the-leas.html. Accessed March 17, 2020.
- Pierre-Louis K. Syrup Is as Canadian as a Maple Leaf. That Could Change with the Climate. The New York Times website. May 3, 2019. Available at: www.nytimes.com/2019/05/03/climate/climate-change-maple-syrup.html. Accessed March 17, 2020.
- Brown S. Global Warming Pushes Maple Trees, Syrup to the Brink. National Geographic website. December 2, 2015. Available at: www.nationalgeographic.com/culture/food/the-plate/2015/12/02/global-warming-pushes-maple-trees-syrup-to-the-brink/. Accessed March 18, 2020.
- Rapp JM, Lutz DA, Huish RD, et al. Finding the sweet spot: Shifting optimal climate for maple syrup production in North America. Forest Ecology and Management. 2019;448:187-197.
- Survey of Maple Producers About Climate Change. University of Vermont website. Available at: www.uvm.edu/sites/default/files/Rubenstein-School-of-Environment-and-Natural-Resources/Maple_Survey_fact_sheet_5-20-16.pdf. Accessed May 27, 2020.
- Study Looks at Adaptability of Maple Producers to Climate Change. University of Vermont website. June 6, 2016. Available at: www.uvm.edu/newsstories/news/study_looks_adaptability_maple_producers_climate_change. Accessed May 27, 2020.
- Statistical Overview of the Canadian Maple Industry 2013. Government of Canada website. Available at: http://publications.gc.ca/collections/collection_2016/aac-aafc/A71-40-2013-eng.pdf. Accessed March 17, 2020.
- Canadian Maple Syrup. Government of Canada website. Available at: www.agr.gc.ca/eng/food-products/explore-canadian-food-products/canadian-maple-syrup/?id=1425998175145. Accessed March 17, 2020.
- Statistical Overview of the Canadian Maple Industry 2017. Government of Canada website. Available at: www.agr.gc.ca/resources/prod/doc/pdf/st_ovrv_maple_2017-eng.pdf. Accessed March 17, 2020.
- History of Maple Syrup. Harvest Connection website. Available at: www.harvestconnection-ny.com/img/resources/History_of_Maple_Syrup-001602.pdf. Accessed March 17, 2020.
- Colonialism, Maple Syrup, and Ways of Knowing. Active History website. July 9, 2018. Available at: http://activehistory.ca/2018/07/colonialism-maple-syrup-and-ways-of-knowing/. Accessed March 17, 2020.
- Acer saccharum. Native American Ethnobotany Database website. Available at: http://naeb.brit.org/uses/search/?string=acer+saccharum. Accessed March 27, 2020.
- Reinmann AB, Susser JR, Demaria EMC, Templer PH. Declines in northern forest tree growth following snowpack decline and soil freezing. Global Change Biology. 2018;25(2):420-430.
- Moran B. Not So Sweet: Climate Change Means Slow-growing Sugar Maples, Study Finds. NPR website. December 7, 2018. www.npr.org/sections/thesalt/2018/12/07/673713824/not-so-sweet-climate-change-means-slow-growing-sugar-maples-study-finds. Accessed March 18, 2020.
- How Sugar Maple Trees Work. Massachusetts Maple Producers Association website. Available at: www.massmaple.org/about-maple-syrup/how-sugar-maple-trees-work/. Accessed March 18, 2020.
- Perkins TD, Isselhardt ML, Wilmot TR, Stowe B. A Summary of research to improve vacuum in maple tubing systems. Maple Digest. 2016;55(1):11-19.
- Idlebrook C. Maple Researchers Say Vermont Syrup Yields Can Be Maintained, Even as Season Shrinks. VTDigger website. October 3, 2012. Available at: http://vtdigger.org/2012/10/03/maple-2/. Accessed March 18, 2020.
- 22 Trees that can be Tapped for Sap and Syrup. Wild Foodism website. Available at: http://wildfoodism.com/2014/02/04/22-trees-that-can-be-tapped-for-sap-and-syrup/. Accessed March 18, 2020.
- Remaking Maple. University of Vermont website. November 6, 2013. Available at: www.uvm.edu/uvmnews/news/remaking-maple. Accessed March 18, 2020.
Maple, the Breakfast Staple
Ten maple species are native to Canada, but, of those, syrup producers historically have favored sugar maple and black maple (Acer nigrum) due to their sap’s higher sugar contents. Canadians take pride in maple production, which spans Canada’s entire history, from before European contact and settlement to the present.7,8 In 2017, Canada produced about 71% of the world’s maple syrup: 12.5 million gallons valued at $494 million USD.9 The main syrup-producing provinces are Quebec, Ontario, New Brunswick, and Nova Scotia.8 Despite a large production increase over the last decade, Canada’s world share has decreased due to greater competition from the United States, which previously led the world in maple syrup production.9
Maple trees accumulate starch in the fall that converts to sugar in the spring and mixes with meltwater that is absorbed by the trees’ roots to make sap, which is distributed through the wood to feed growing leaf buds. Maple sap is boiled down to make syrup and contains about 97% water plus minerals, organic acids, and taste precursors. The syrup derived from the sap rarely spoils when made at the correct density. It is a good source of riboflavin, manganese, and phenolic-derived antioxidants, giving it a nutritional edge over some other sweeteners. To ensure the longevity of their trees, Canada’s syrup producers take a limited amount of sap from each tree each season and leave the rest for the tree’s own internal nourishment.7,8
Maple sugar reportedly was the first type of sugar produced in North America, where it remained the main sweetener until the price of cane sugar (Saccharum spp., Poaceae) declined in the late 1800s.8 Indigenous people taught early French settlers how to obtain maple sap and reduce it to syrup. Haudenosaunee (Iroquois) traditions describe piercing maple trees for “sweet water” to cook venison.10,11 They also have used the sap to make a drink for home consumption and longhouse ceremonies. The Meskwaki have used maple sugar instead of salt for cooking.12
The earliest known settler records of maple sugaring were by André Thevet in 1557 and Marc Lescarbot in 1606. Lescarbot noted that indigenous people “get juice from the trees and distill it down into a very sweet and agreeable liquid.”3,10,11 Today, maple syrup, which often is used as a breakfast topping, is enjoyed in almost 60 countries around the world. Maple products also include taffy, sugar, butter, and even liquors.8