This has been a wimpy winter with less than half of the average number of below zero nights and less than half the average snowfall. We had a series of above-freezing days and freezing nights last week. It was sunny and 60 degees last Saturday. Sap started to flow in the sugar maple trees in February this year. Blame it on the El Nino phenomenon, but the sugar maple trees (Acer saccarum) respond to freezing nights and thawing days.
During warm periods when the temperature rises above freezing, pressure develops in the sugar maple trees, causing sap to flow out of a wound or a tap hole. When the temperature falls below freezing, negative pressure develops, drawing water into the tree through the roots.
The sap flows through the light-colored sapwood in the outer part of the tree trunk. Sapwood cells convey water and nutrients from the roots to the branches of the tree. Respiratory activity of sapwood cells produces carbon dioxide during the day. Carbon dioxide in the sap is also released as gas into the spaces between the cells. Osmotic pressure caused by the presence of sugars and salts in the sap adds to the overall pressure of sap flow in the tree.
At night and when the temperature goes below freezing, the carbon dioxide gas between the sapwood cells cools and contracts, some dissolves back into the sap and some of the sap freezes. This results in negative pressure, causing water in the soil to be drawn up into the roots and travel upwards into the tree. When the temperature rises above freezing the sap flow begins again.
A period of freezing nights and thawing days makes for a good maple sugaring season. Temperatures too warm or too cool during the normal six-week sugaring season reduce sap flow and the amount of maple syrup that can be produced.
Sugar maple sap has a high concentration of sugar compared to other tree species, although red maple, silver maple and box elder can also be tapped to make syrup. A single tap can produce about 10 gallons of sap each year and yield about a quart of finished syrup. The sugar in maple sap is the product of photosynthesis that occurred the previous year. Amino acids in the sap impart the delicious maple flavor that differs from pure cane sugar.
Trees growing with good moisture, plenty of light and nutrients produce higher yields of sap than trees growing in infertile, shaded or dry conditions. Tapping a healthy tree removes less than 10 percent of the tree’s sugar, not enough to adversely affect the tree. Family farms in Wisconsin and across the Great Lakes states and eastern Canada produce maple syrup for extra income or family use. Many have been producing syrup for generations. The Ojibwe of Wisconsin have harvested maple sap to make sugar for centuries.
University of Wisconsin climate scientists anticipate that future climate change will affect maple syrup production. The data indicates that we can expect a 4.5 to 6 degree increase in average spring temperatures in the coming decades. They project that the last spring freeze will come 12 to 15 days earlier, with a slightly wetter fall, a wetter spring and a reduced probability of snow in March. Based on this and other data we can expect the maple syrup season to begin and end earlier in the year and the number of cold days could reduce the season length.
The maple trees in our woods have produced lots of sap for syrup over the years. I hope that they remain healthy and growing. We really like the gnarly old trees for their shade, their brilliant colors in the fall and maple syrup on our pancakes.
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–Dan Wilcox, outdoor columnist