In a wonderful book I’ve written about before by Nalini M. Nadkarni called “Between Earth and Sky-Our Intimate Connection to Trees” she writes of how botanists and tree physiologists have been looking at how sap is produced within maple in the last couple of decades. Like many things, the wild world of maple syrup seems like a freak chance, a perfect random combination of physiology.
A tree’s goal, aside from reproduction, is to feed itself-it’s tough being an autotroph, and a whole lot of work. Photosynthesis takes place all summer long, making sugars for respiration, growth, reproduction, and a little extra. This extra, in a sugar maple, gets stored as starches within the sapwood of the tree. The sapwood, as the name sounds, is the area of the trunk and branches where water and sugars move around, located within the first couple rings of the wood.
As my winter term class hopefully remembers, Sugar maple is one of our “live slow, die old” species of trees. These trees are more shade tolerant, in life for the long haul, and have the foresight to save extra sugar for lean times, such as the introduction of shade or competition. Other tree species, such as Poplar, live fast and die young, and burn through all their sugar like a hyper 3 old, just as prone to growth spurts as an Aspen in the spring.
Early spring brings sun, a little higher in the sky, and better able to warm. Cats in our house know this, moving away from the woodstove and into little patches of sunlight on soft surfaces. Trees know this too, as the sun warms the bark and the wood. The air may be below freezing, but tree surfaces and interiors could be well above freezing. Once the wood gets to be above 40 degrees, enzymes turn these stored starches into sugars, mostly sucrose, and the sugar is now within the sap. This explains the magical sugaring temperature of 40, any warmer and this process stops.
The other freak chance miracle of maple is getting the sap flowing out of the tree. Not all trees can do this. Water is moved throughout the xylem of the tree by capillary action and transpiration, meaning the leaves need to be on the tree for water to move very effectively. That would ordinarily make for tough sugaring in March and April, except in Maple.
In maple trees the space around the wood fibers is filled with gas, not water like most plants. When the temperature drops, this gas contracts, making space for the sap laden with sugar in between the cells. So water can move upwards from the roots by capillary action without the benefit of transpiration from leaves. This water freezes at night between the cells.
The day brings warm temperatures, melting this ice and expanding the gas, forcing water down the branches into the stems and trunks of the tree. The taps put into tree trunks to collect sap pierce the xylem all this sap is moving through, and water flows down the tap into the bucket or plastic line.
Interested biology students should read another blog, The Botanist in the Kitchen, http://botanistinthekitchen.wordpress.com/2013/03/18/maple-syrup-mechanics/ , and a cool roadtrip would be the Proctor Maple Research Facility of my old school UVM.