It is a normal pattern. Sap sugar typically starts off fairly high, rises a little, then drops off as the season progresses. It can pop up a little after a good freeze. This is due to the physiology of the tree. https://mapleresearch.org/pub/mn2020sapsugar/Originally Posted by blissville maples
Most of the carbs in the wood rays are converted to and stored as starch, but as the spring gets close, a portion of that is converted to sucrose (more readily useable for tree metabolism) where it is important in providing some energy to fuel the growing tissues (roots, cambium, buds) and, to a slight degree, important in the sap flow process (although wood temperature is by far the larger driving factor in sap flow). Only a small portion of the starch gets converted, there is a lot left after the season is ended. Temperatures falling below freezing tend to push the enzymatic conversion of starch to sugar.
This isn't an abnormal thing in biology. If excess energy is made (or consumed), then a portion of the excess can get stored for later use. In people that's as fat -- in plants it is a starch. It may be converted later and get used, or may just hang around for a long time. The difference is that in trees the starches tend to be laid down within the rays of annual rings, but more can be added and some can be remobilized down the road as needed. Rays are the tissues that 1. are living and 2. cut across annual rings, so the mobility of the sugar is higher than many other compounds in the wood.
The sugar we collect by tapping comes from many annual rings of rays in the wood. In fact, the average age of the sugar molecules in maple syrup tends to be around 3 yrs, but ranges from less than 1 to 20-25 yrs. The hydraulic conductivity (the ability/rate/amount of sap to move through the stem) and carbohydrate levels tend to be highest in the outermost rings, then drop off as we get deeper. That's why drilling a taphole beyond a certain depth doesn't get us a lot more syrup yield. These reasons (higher sugar in the newest rings and higher hydraulic conductivity in newer rings) are why we've designed the barb spout https://mapleresearch.org/pub/innova...ystems-spouts/ currently being tested in several places in the U.S. and Canada. The new spout design allows better collection from shallower parts of the taphole and allows better collection during the frequent short thaw periods (when only the outermost portion of the wood thaws out) in which normal spouts do not run or run much.
At a certain point (decades), some amount of carbs are lost due to continued tree growth when the sapwood deep in the tree eventually converts to heartwood and is thus lost to the tree (and to sugarmakers).
Before the season starts, from the fall through the winter, the sap in xylem tends to be quite low in sugar (it's all being converted to starch and stored for future use). That's why when people try fall and early-winter tapping the sap tends to have very low sugar so syrup yields tend to be lower.
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