A friend of mine wants to know whether anyone is doing anything about sugar maples and climate change.
The issue, succinctly, is that sugar maples are a cold-climate species that only grow well in the cooler parts of North America. As climate change shrinks those areas, the range of the maples will shrink, too. That’s a problem most obviously because this is the tree that gives us maple syrup, but it also provides much of the fall color for which New England is famous–and leaf-peeping is a major driver of the tourist economy of the region. And beyond economics, sugar maples (like the also-climate-stressed paper birch) are part of the regional identity of New England (and doubtless of other regions I’m less personally familiar with).
Maple sugar sap production is already starting to drop in some areas, though the economic burden is being countered to some extent by improvements in collection methods. We’re not talking about some vague warning about the future; this is happening now.
So, my friend wants to know, are there any efforts underway to breed climate change-resistant sugar maples, just as there are efforts to breed disease-resistant American chestnut trees?
A good question.
Introducing the Sugar Maple
Let’s start by taking a look at the star of the show, the sugar maple, Acer sacharum, from whose unusually sweet sap maple syrup and maple sugar candies are made. The sap comes from the tree as sweetish water and must be concentrated by boiling (or freezing) in order to produce syrup. Further concentration allows crystallization into candy. The tree itself is large and long-lived, with furrowed bark and distinctive leaves with U-shaped sinuses (if a maple leaf is a hand, the sinuses are the parts between the fingers).
Those who aren’t plant geeks might be surprised to learn that not all maples are sugar maples, and that while some of the others also provide sweet sap, others don’t. The maples are a large group of trees native to North America, Asia, and Europe. In New England there are six native species (sugar, red, silver, mountain, striped, and ash-leafed), and at least two exotic species are also common (Norway and Japanese). The American maples all produce clear, sweet sap, though none as sweet as the sugar maple, but the Norway maple produces white, inedible sap.
The sweet sap runs in the early spring, when the plant mobilizes stores sugars in order to power the growth of leaves. It would be interesting to compare how quickly different species leaf out; is the sugar maple faster (less time fro bud-break to full leaf-out) than other trees because its sap delivers more fuel for growth?). It’s important to recognize that the sugar is food the plant stored for its own use; tapping trees stresses them, and there is a limit to how much sap a tree of a given size can spare. The sap flow itself is triggered and maintained by changes in temperature. If the winter is too warm, or if the spring does not include an extended period when nights are below freezing and days are above, the tree won’t produce. What effect reduced sap flow has on the tree’s own physiology, I haven’t learned–I don’t imagine it’s healthy.
Sugar maples are not only valuable to humans. Across much of Eastern North America, they are one of the two dominant species in older forests (the other being American beech). The trees collect calcium and concentrate it in their leaves, enriching the soil as the leaves fall and decompose, thus feeding other plants. Many animals depend on sugar maples; some are insects that feed on no other species (these insects go on to feed birds), while others are mammals and birds that use maples at certain times of year. Most charmingly, squirrels tap the trees for sugar by biting through the bark. The sap leaks out and evaporates, leaving behind sugar which the squirrels lick. Porcupines eat the leaves and buds in the early spring, when little else is available to them. Sugar maple seeds feed songbirds, squirrels, chipmunks, and mice, while the bark provides emergency food for deer, rabbits, and mice in winter. The list could go on.
Much of the above information comes from the wonderful The Book of Forest and Thicket, by John Eastman. Curiously, the author also states that sugar maple is in a decline across the southern part of its range, in part due to climate change–a striking assertion, given that the book was published in 1992.
Sugar Maples and Climate Change
A few years ago, I wrote an article imagining the climate through the lifetime of my nephew into his old age. Towards the end of the piece, I imagined him taking a vacation in Vermont with his wife and talking to a waitress who complains about the economy because the sugar maples are dying. While researching for the piece I did not find any actual discussion of sugar maples, but I did notice that the range of the sugar maple is essentially bounded by certain USDA zones, and that zones that do not currently support wild sugar maples are predicted to move north into New England by century’s end. I drew the logical conclusion.
I should have known better.
Trees, in general, have narrower germination niches than growth niches, meaning that mature trees can survive circumstances that would kill seedlings–I learned that in grad school. In practice the narrower germination niche means that species can often be grown as specimen trees in lawns and parks in places where they can’t grow in the wild. Wild trees must start from seed, whereas those growing in lawns were usually sprouted in controlled environments in nurseries, then grown for years before being balled and burlapped for sale. By the time the tree is installed in a lawn, it’s already strong enough to withstand the local conditions.
Sugar maple will grow as a specimen tree well to the south of its current range (there is one in my mother’s back yard), so climate change alone is not likely to kill New England trees–but it could prevent a new generation of maples from establishing. Neither I nor my nephew are likely to see a Vermont without its classic autumnal red and orange, but he could well live to see the last generation of wild Vermont sugar maples sprout. Three hundred years later, those few lonely trees will die of old age without heirs, if things continue as they are now.
Research supports that second, corrected vision as closer to the truth.
In Michigan, where climate change is making the growing season drier, sugar maples are growing much more slowly than they used to. The lead researcher of that study was quoted by NPR as saying that under some climate change scenarios, that region would lose its sugar maples entirely; the new climate won’t kill mature trees, but no new young trees will survive to replace them. In the meantime, trees that don’t grow well don’t produce much sap, either.
A warm spring can cut the season short, and did so in 2012, when much of New England produced dramatically less maple syrup than in previous years because of heat waves in March. Climate change promises more heat waves, and just as trees that don’t grow well don’t produce a lot of sap, it’s hard to imagine that non-productive maple trees are growing much.
A study in Vermont found that climate-related stress has, over the last several decades, been equal in severity to pests and other more well-known problems; the authors note that 50 years from now, half the state’s sugar maples could be experiencing “moderate to severe climate stress.” I’m not sure what that means in practical terms, but some dieback combined with widespread declines in productivity and a general failure to breed (except in isolated refugia with colder climates) would seem to fit the bill.
In general, climate change could lead to widespread habitat loss for sugar maples, especially under the more dire warming scenarios.
Perhaps counter-intuitively, the loss of American sugar maples won’t necessarily translate into a lot of dry pancakes, despite many headlines to the contrary. For one thing, even in New England, a lot of pancakes are already being eaten with artificially-flavored “pancake syrup.” For another, according the the paper linked in the previous paragraph, most sugar maple trees that are large enough to be tapped currently are not, and as long as we avoid the most extreme warming, it should be possible to keep production up–while the number of sugar maples will decline, producers can tap a greater proportion of those that are left.
Of course, neither pancake syrup nor expanded tapping operations will help songbirds, squirrels, or porcupines very much.
My friend’s question is actually two or three questions, and they don’t necessarily have the same answers.
Is anyone developing sugar maples that can survive climate change and save New England’s forests? The most obvious way to answer that question is to look at whether anyone is developing cultivars or hybrids that can tolerate the new climate now moving towards them. But if the goal is indeed to save the maple forests–and that does appear to be my friend’s interest–we must also ask whether any of these cultivars could be released into the wild and whether they would indeed do all of the things we rely on sugar maples for. The possible third question hiding behind the other two is whether trying to propagate such a cultivar on a broad scale is even a good idea.
Breeding Sugar Maples
Is anyone developing a climate-proof sugar maple? The short answer is yes.
These are difficult questions to research because of the way search engines work–the information you get is not necessarily what you want but rather what the search engine’s algorithm thinks you want, and in this case breathless articles about dry pancakes tend to shoulder conservation genetics out of the way. I am therefore unable to say definitively whether there is a program that has as its stated goal rescuing the sugar maple from climate change.
However, there are cultivars that have different habitat requirements than wild sugar maple does: Crescendo, and Green Mountain are each both heat and drought tolerant; Bonfire is heat-tolerant; Legacy is resistant to drought; and Majesty is resistant to frost cracking–since climate change is causing increased frost cracking in paper birch (there are more freeze-thaw cycles in the spring, now), I’m guessing it could be a problem for maples, too. There are probably other cultivars with useful properties offered from other sites. Whether any of these were developed with climate change in mind I’m not sure, but they certainly fit the bill.
And there are plant breeders working on sugar maple with climate change in mind. The US Forest Service and various state agencies have been working on “improving” the sugar maple–meaning, developing domestic cultivars with commercially desirable traits–since the 1960’s at least. Their primary goal has been to increase sugar content in the sap (they have succeeded), but state agencies in New York are also continuing the research with the aim of storing sugar maple germplasm in case of catastrophic loss due to invasive insects or disease–possibilities they acknowledge are made more pressing by climate change since the trees are stressed and less resilient than they once were. Some of the same researchers are also looking at how climate change is impacting the tapping of sugar maples.
So the research is being done, more or less.
Saving Maple Forests
Curiously, I have not been able to find anyone discussing using sugar maple cultivars on a large scale to save either the American syrup industry or American sugar maple forests–of course, for me not to find something is no proof it doesn’t exist, but the lack of results is interesting. What I’m finding instead are discussions of using improved technology and improved forest management techniques to preserve the industry, possibly coupled with the use of red maple sap as an alternative source of maple sugar. Red maples are much more heat tolerant and is being used for syrup already. The syrup is said to taste a little different, but it’s quite good and suitably maply.
If indeed the climate-change-tolerant cultivars are not part of an organized push to save the forests, why not? I’m just speculating here, but it’s possible the issue is economic; changing management practices and using some red maple syrup are adequate stopgaps for now, and have the advantage of using trees that already exist. People do plant maples, including high-sugar cultivars and hybrids, with an eye towards sugaring, but a young maple won’t be ready to tap for 20 years or more. A major investment in a new cultivar might not be very attractive when there are options with a more immediate return.
Saving Maple Forests?
OK, you mind if I speculate a bit?
I can think of two reasons why mass propagation of the cultivars might not be a good idea. First, unlike the American chestnut, which has bee taken out by disease everywhere, wild-type sugar maple could hang on in parts of its range. That means that if cultivars are naturalized, they could end up competing with the wild type. What if the cultivars win?
A domestic cultivar could have all the marketable advantages of the original, but it’s not the same tree. And depending on what the differences are, the switch, if it happens, could matter a lot, especially given that sugar maple is a dominant species in many areas. Changing it, even slightly, could have vast consequences. For example, according to some remarkable research by Dr. Doug Tallamy, many herbivorous insects are highly specialized–even plants closely related to their host species are inedible to them, such that replacement of natives with exotics causes insect populations to crash–followed by songbirds, who need insects to feed their chicks. Last time we spoke, Dr. Tallamy didn’t yet know whether cultivars are typically also inedible to specialized insect herbivores–but since some cultivars are marketed for superior pest resistance, it is clear palatability for insects suffers sometimes. And wild-type sugar maples, remember, support huge numbers of insects.
Second, if wild-type sugar maple loses its habitat to climate change, so will a lot of the other species that inhabit the same forests. Perhaps a sugar maple cultivar could save some of them, but not all. Many aspects of how the forest functions will change. Arguably, the sugar maple-American beech forests we know will be gone from certain areas, replaced by something else. If the “something else” includes a sugar maple cultivar, I don’t see how that would be a problem (the danger is in the cultivar spreading to places where the wild type persists)–but I don’t see how that’s really a victory, either. At best, it would be a lessened scope of loss.
Realistically, our best hope lies in saving as much of the wild-type sugar maple forest as we can, and that means stopping climate change now.