Most of the time we tend to think about forests as sources of important ecosystem service such as timber, firewood, water regulation, carbon stock, biodiversity. But there is a very important (and very sweet!) service that is delivered mostly by one tree species: maple syrup. Since the early 19thcentury – and even before by Native Americans – the precious sap of Sugar maple growing only in Eastern North American forests has been tapped for producing granular sweetener and syrup. However, since the tapping season is very much depended on weather conditions, scientists have started wondering whether current production could be maintained if temperature and drought events will keep increasing in the future.
Since I have recently moved to the country that produces 70–75% of the world’s maple syrup (Canada), I thought to give the readers of Forest Monitor a little insight about this sweet substance that comes out of a tree trunk! Maple syrup is a sweet, thick, viscous liquid that is made from the xylem sap of maple trees. The main species used for syrup production is sugar maple (Acer saccharum) but other maples such as red maple (Acer rubrum) or black maple (Acer nigrum) are suitable as well. For readers from Eastern North America this is probably nothing new. However, for the rest-of-the-world readers it might be interesting to know something more about this special production.
How to make maple syrup
Basically, during the cold winter months maple trees store starch in their stem and roots. When towards the end of the winter daily temperature start to rise, this starch is converted to sugar that start rising in the sap. This sap can be collected in buckets – 1 to a maximum of 4 depending on the diameter of the tree – by drilling holes into the tree trunks. The sap with an initial sugar concentration of about 2% is then processed by heating, typically in big wooden stoves inside shelters called “sugar shack”. When most of the water is evaporated (about 40 liters of sap for 1 liter of syrup) and when we reach a sugar content of about 60%, our syrup is ready for the pancakes! This is just a brief summary; if you are interested in the details you can read more about the processing of maple syrup in Wikipedia or in other articles.
As I mentioned before, maple syrup can be produced only in certain regions of north-eastern Canada and United States. The reasons for this are (1) the presence of these maple species; (2) climate. The flow of the precious sap is indeed climate-dependent. First of all, low winter temperatures are essential for triggering the formation of starch within the tree stem. Next, a combination of cold nights (at least below 0°C) and warm days (circa 3-7, or better 10°C) is needed to create a pattern of positive and negative pressure: positive pressure allows the sap to flow down from the branches and to be collected during the day while negative pressure refills the tree of sap from roots during the night. As you can imagine, this happens only for few weeks (or even days) during the year and since these particular climatic conditions occur only at northern latitude, the production of syrup is possible only in certain regions. Maples grow well also at southern latitudes but there winters are too warm to allow sap production.
What happen if climate warms up?
It can be a problem. Since the window of climate conditions for allowing sap flow is quite short, too warm temperature in late winter can cause sudden bud breaks, which means that the sap season is over before it begins. New climatic conditions would perhaps move sap production from southern latitude towards northern regions (read below). However, there is also the questions whether maple species would be able to migrate north as fast as the climate is changing.
For answering to some of these concerns, several studies have focused on the possible effects that climate change might cause to maple syrup production. For example, researchers from the University of Vermont have recently published a paper on the complex relationship between climate and sugar maple health in Vermont. This little state in the USA has a strong tradition for maple syrup production. Unfortunately, the region may be affected negatively by climate change, as more prone to drought and higher temperatures that at northern latitudes. Based on field data, researchers have calculated a summary index (called Forest Stress Index FSI) that consider all the possible stress (climatic, disturbances) that this species might be subjected. Then, they modelled it spatially across Vermont under future climate change scenarios. In a nutshell, their results showed that – although the impact of climate on sugar maple health vary geographically – the proportion of sugar maple across Vermont can be expected to experience moderate to severe climate-driven stress. Under the most severe climate change scenarios, about 84% of the tree population might experience severe stress (i.e., reduced crown conditions that could eventually cause tree mortality). These declines seem to be driven by increasing April and October minimum temperatures. This emphasise the vulnerability of this species to climate during the shoulder seasons. Knowing these possible scenarios, forest managers could take steps to protect and conserve sugar maple stands in vulnerable areas.
Another interesting study was led by researchers of University of Michigan. They analysed the combined effects of anthropogenic nitrogen deposition and climatic conditions on the radial growth of sugar maple across the upper Great Lakes Region of the USA. Taking advantage of a long-term experiment in which nitrogen has been increased annually across a latitudinal gradient, they could demonstrate that temperature and consequent reduction of available soil water would be responsible of declining growth of sugar maple. In addition, maple growth was predicted to decline strongly under the most severe climate change scenarios. They concluded that, even if increased nitrogen might have positive effects on tree growth, nitrogen deposition would not be able to counteract the negative effects that increasing temperature and water limitation would have on maple growth and health. Thus, sugar maple growth is expected to decline across the studied region, with possible effects on maple syrup production.
Another interesting research has been recently published and it has a more ecosystem service perspective. Scientists from Ohio State University and USDA Forest Services explained very well in this paper how a changing climate can translate into changes in maple syrup production. Then, they used data from the extensive USDA Forest Inventory and Analysis (FIA) program to estimate the number of taps potentially placed into sugar maple trees across the Eastern United States and therefore to evaluate ecosystem production service and potential changes due to changing climate in the future. For this, they used species distribution models to assess the habitat suitability of the species to future environmental conditions.
They found that, according to FIA estimates, there are about 9 billion sugar maples growing in Eastern US but only about 4% of them (400 millions ca.) are large enough in diameter to be suitable for tapping. You can see from the figure below that maple syrup can effectively be produced only in a few states towards the north of the USA. Interesting to see that the small state of Vermont is the one with the highest number of taps and percent of tapped trees! With regard to future projections, the researchers showed that the number of taps need to be increased for maintaining current productions by the end of the century. This is also seen in the projected change in growing degree days to reach 75 (GDD75), used here as a direct measure for indicating sap production and optimal sap flow season.
These results show clearly an increasing in the length of maple syrup season, which is moving south to north. The reduction in season length are projected to be greatest along the southern edge of current syrup production and even if the loss of days seems to be less, the ratio of change is indeed greater than other northern states. But there are strong uncertainties and differences depending on the climate scenarios. In case of the milder (or middle) scenarios, habitat and conditions for sugar maple might even improve in northern areas, which may bring increased opportunities for syrup production when trees that are now too young will be ready for tapping.
So, is maple syrup doomed?
I do not think this will happen very soon, but we must stay on alert. Technology is advancing fast and it is likely that in the next decades increased level of tapping can still be sustained over the next few decades. But there are many other factors that can influence the occurrence of sugar maple (and therefore syrup production) on the landscape, such as soil components, pest outbreaks, natural disturbances, that’s why local- and large-scale studies on this regard would be certainly welcome.
However, rapid changes require rapid interventions. It is very likely that commercial syrup production will soon diminish at southern latitude and will probably move northwards. Management actions such as assisted expansion of the distribution of the species must then be carefully planned a long-term perspective if we want to maintain the provision of this delicious but very delicate ecosystem service.
Stephen N. Matthews & Louis R. Iverson (2017) Managing for delicious ecosystem service under climate change: can United States sugar maple (Acer saccharum) syrup production be maintained in a warming climate? International Journal of Biodiversity Science, Ecosystem Services & Management, 13:2, 40-52, Open Access.
Ibáñez, I. , Zak, D. R., Burton, A. J. and Pregitzer, K. S. (2018), Anthropogenic nitrogen deposition ameliorates the decline in tree growth caused by a drier climate. Ecology, 99: 411-420. doi:10.1002/ecy.2095
Oswald, E. M., Pontius, J., Rayback, S. A., Schaberg, P. G., Wilmot, S. H., & Dupigny-giroux, L. (2018). The complex relationship between climate and sugar maple health: Climate change implications in Vermont for a key northern hardwood species. Forest Ecology and Management, 422, 303–312.
Main photo: Maple syrup tapping in Quebec (Canada). Author of the photo: Marco Mina.