Maple syrup production involves the extraction of sugar-enriched, xylem sap from dormant sugar maple (Acer saccharum. Marsh.) stems. The sugar represents a portion of the trees stored nonstructural carbohydrates (NSC). Modern extraction methods utilize a vacuum pump to increase sap flow. Recent advancements have increased the yields of sap (and therefore the amount of NSC extracted) by 2-3 times. Maple sugaring is considered a sustainable process; resulting in a small volume of nonconductive xylem along with the removal of NSC. This pilot study investigated the impacts of vacuum and gravity sap extraction on sugar maple radial stem growth, twig growth and residual NSC concentrations in stem and twig xylem tissue. A group of 30 (previously untapped) sugar maple trees were placed into one of three treatments: gravity sap extraction (GSE-historical method), vacuum sap extraction (VSE-modern method) and no-extraction (control). Tapped trees had sap volumes and sap sugar content measured while trees in the control treatment remained untouched. Xylem tissue sample from the stem and twigs of all trees was collected at the end of the sap collection season. Samples were analyzed and residual NSC was calculated. The stem and twig growth was measured for all trees following one growing season.
Vacuum sap extraction (VSE) resulted in significantly greater mean sugar removal, 1.19 Kg (SE ±0.46) as compared with gravity sap extraction (GSE) method 0.48 Kg (±0.14). Residual stem NSC displayed a pattern of increased concentration with increased extraction. Twig residual NSC concentrations were highly variable, perhaps due to the highly dynamic late spring period, and no clear patterns were observed. Mean radial stem growth in the year following sap extraction was greater in untapped trees 2.93mm (± 0.58) than in VSE 1.99 mm (±0.44) or GSE 1.67mm (±0.12). Results raise the possibility that sap removal shifts sugar maple NSC source-sink relationships toward storage at the expense of growth.