Maple (Acer spp.), birch (Betula spp.), and walnut (Juglans spp.) syrup production relies on sap flow and sap pressurization that occur during winter dormancy. Since, during winter dormancy, these three species of hardwoods are leafless, evapotranspiration is not the principal driver of sap flow and sap pressurization in them. Other mechanisms, which may be physical or biological and are based on xylem anatomy or physiology, cause sap flow and sap pressurization in these three genera. Preliminary data show that other temperate, deciduous, woody angiosperms, such as sycamores (Platanus spp.), beeches (Fagus spp.), basswoods (Tilia spp.), ashes (Fraxinus spp.), kiwiberries (Actinidia spp.), hickories (Carya spp.), hophornbeams (Ostrya spp.), hornbeams (Carpinus spp.), alders (Alnus spp.), poplars (Populus spp.), sassafrases (Sassafras spp.), willows (Salix spp.), and tulip poplars (Liriodendron spp.), exhibit sap flow patterns during the winter dormant season, although the sap pressures developed in the sapwood of trees in these genera are nowhere near as large as those in maples, birches, and walnuts.

Figure 1. A sassafras (Sassafras albidium) outfitted with heat pulse sap flow sensors during January of 2020. One of these sensors monitored sap flow at sapwood depths of 0.5, 1.0, and 1.5 cm beneath the cambium, and the other monitored sap flow at sapwood depths of 2.5, 4.0, and 6.0 cm beneath the cambium. An insulative cover, which is folded back in this photo, ensures that sensor measurements are not affected by exposure to direct sunlight. This tree was located in New London, CT.

Heat-pulse sap flow sensors are used to monitor sap flow in these deciduous hardwoods (Fig. 1). Heat-pulse sap flow sensors are more appropriate for monitoring sap flow during the winter, when temperatures often drop below 0 ° C, than heat-dissipation sap flow sensors. Heat-dissipation sensors apply continuous heat to the sapwood and may prevent freezing events from obstructing sap flow. Furthermore, heat-pulse sap flow sensors are designed to monitor low levels of sap flow and the downward movement of sap, which winter-dormant-season sap flow in many deciduous trees is partially characterized by. Other environmental variables, such as air temperature, soil temperature, precipitation, and solar radiation, are monitored continuously along with sap flow. These environmental variables are examined alongside sap flow data to determine the environmental conditions that drive winter-dormant-season sap flow in these deciduous hardwoods (Fig. 2). Ultimately, once the environmental drivers of winter-dormant-season sap flow are understood for these species, it is our hope to discover the physical or biological mechanisms, and the anatomical and physiological characteristics of the sapwood, that bring about these sap flow patterns. Future work will incorporate sap pressure sensors to allow for a more holistic view of sap movement patterns in the sapwood.

Figure 2. American beech (Fagus grandifolia) and London planetree (Platanus x acerifolia) sap flux densities. Sap flux densities were measured at a sapwood depth of 1.0 cm beneath the cambium and at breast height (1.4 m above the ground). Five trees of each species were monitored. Air temperature measurements are also included on a secondary vertical axis, with a dashed red line representing the freezing point of water. It is evident from this figure that, for both American beech and London planetree, freeze-thaw cycles drive winter-dormant-season sap flows. These data are from February and March of 2019, and all the trees were located in Lee, NH.

In addition to addressing these basic science knowledge gaps, it is also important to consider the applications of this research. Syrup production is possible from trees other than maples, birches, and walnuts, but production methods are not widely available for these species. Hopefully, the results of this research will help to increase the number of genera tapped for syrup production. Tapping novel species for syrup production may give syrup producers an opportunity to extend their sugaring seasons and get more use out of their expensive syrup-producing infrastructure. Maple and birch syruping seasons are known not to overlap very much (if at all) – birch sap flows after maple sugaring season has ended – and other species may provide other similar opportunities for season extension. Furthermore, tapping novel species for syrup production may also provide consumers with new, interesting flavors, and producers will undoubtably find it easy to market new types of syrup.

This work and related projects have been funded NH Agricultural Experiment Station, USDA- SARE, USDA-AFRI, USDA-ACER, and the North American Maple Syrup Council.