Bigleaf Maple Sap Flow in Western Washington

Abstract

Bigleaf maple (Acer macrophyllum) is common on well drained soils in mixed conifer forests and along river terraces in Western Washington. Protected riparian buffers often include bigleaf maple stands, and producing syrup from the tree’s sap can provide revenue and incentivize maintaining forest cover on land that cannot be harvested for timber. Washington’s small forest landowners can face economic difficulties managing their land, and managing forests near streams is often complicated and avoided due to riparian buffer zones and other regulations. Development of forest land accounted for 400,000 acres of deforestation in Washington between 2007 and 2019, much of which resulted from economic pressure on forest landowners. Bigleaf maple sugaring could encourage mixed-species forest ownership and yield an ecological low-impact income from land that might otherwise be a net drain on cash for maintenance and property taxes. Bigleaf maple sap flow is not well documented, and there are key differences between its sap flow and that of eastern North America’s sugar maple (Acer saccharum) that must be understood if bigleaf maple syrup production is to become commercial in scale.In this study, tree-level bigleaf maple sap flow data from the University of Washington’s Pack Forest near Eatonville, WA was collected for three tapping seasons under high vacuum pressure. A positive correlation between tree diameter at breast height in centimeters and seasonal sap volume in liters was found (sap (l) = 0.22 * DBH (cm) + 1.04). Multi-stem clumps over 30 cm in diameter produced two to three times more sap per tap than individual trees of the same diameter, though taps on clumped stems were more variable. The seasonal sugar content of tapped stems ranged from 0.7% to 1.9%, averaging 1.3%. Sugar content varied with tree diameter but not canopy health, canopy position, or growth form. Sugar content was more similar within stems of the same clump than between clumps or between seasons. Freeze-thaw events were associated with high sap volume, though sap still flowed in low volumes on warmer days. A linear model relating daily weather from PRISM data at a 4km2 resolution to daily sap volume per tap was used to estimate statewide average sap flow potential per tap for each January 1 to March 1 tapping season from 2010 to 2020, and the ten season-long estimates were averaged. The model generally over-predicted sap volumes collected at eight sap collection sites in Western Washington by a factor of two, though harvested sap volume was within 14% of predicted for the most experienced and the only commercial syrup production site. The model therefore probably represents an upper limit on sap harvests achievable with tapping experience and high-power vacuum pumps. If only small, private forest land parcels with electricity are tapped, between 11,693 and 350,785 liters of bigleaf maple syrup could be produced annually in Western Washington depending on the percentage of feasible tapping sites that are utilized. These estimates range between $926,000 and $27.8 million based on 2022 bigleaf maple syrup retail prices. If bigleaf maple sugaring continues to develop, the industry could approach that of some of the Northeastern states’ maple harvest in terms of value.