By Connor Yearsley
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
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Maple, the Breakfast Staple
Ten maple species are native to Canada,
but 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.4,5 In 2017, Canada produced about 71% of the world’s
maple syrup: 12.5 million gallons valued at $494 million.6 The
main syrup-producing provinces are Quebec, Ontario, New Brunswick, and Nova
Scotia.5 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.6
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 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.4,5
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.5 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.7,8 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.9
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.”3,7,8 Today, maple
syrup is enjoyed in almost 60 countries around the world. Maple products also
include maple taffy, sugar, butter, and even liquors.5
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Changes in Production
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
this is linked to climate change is unclear, Rock said, but it is possible,
since the shift in color change timing 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. 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.10
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 2100.10
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 40:1.10
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.10
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.
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.11,12
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 expand the number of taps or
products during the next five years and only 10% considering retiring, selling,
or closing their business.11,12
Snowpack as a Blanket
Another
climate-related factor that may impact maple trees and sap 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
Under Pressure
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 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.
Conclusion
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.”
References
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.
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.
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: vtdigger.org/2012/10/03/maple-2/. Accessed March 18, 2020.