Objective 1. Identify the timing and environmental drivers of dormant-season sap flow in novel trees and the physiological mechanisms behind them. Knowledge of the timing of, and the environmental conditions that cause, dormant-season sap flow in novel species will help producers determine when and how long taps should be set out. A better understanding of the physiological mechanisms that drive dormant-season sap flow in novel species will help fill in scientific knowledge gaps and may further our understanding of dormant-season sap flow in maples, birches, and walnuts, the mechanisms for which are somewhat, but not completely, understood.
Objective 2. Determine optimal tapping depths of novel species. Though 4-cm commercially-available taps work well with maples, birches, and walnuts, preliminary work suggests that dormant-season sap flow depths vary among species. Shagbark hickory dormant-season sap flows at depths between 1 and 2 cm, with barely any flow deeper than 2.5 cm (Moore, unpublished data). Four-cm maple taps are not ideal for harvesting hickory sap because they are seated tightly in shallower sapwood depths and can block sap flow.
Objective 3. Determine the amount of nonconductive wood formed due to trees’ wounding responses to tapping, and whether mean annual diameter growth compensates for the loss of hydraulic conductivity. It is known that tapping sugar maples and paper birches causes the formation of a significant amount of nonconductive wood above and below tap holes, but that healthy sugar maples put on more than enough new girth growth each year to compensate for the loss of conductive wood if tapping guidelines are followed (van den Berg et al., 2012; van den Berg et al., 2017). This project would be the first to collect data towards developing such guidelines for other species. Species can differ widely in how they isolate wounds (Biggs, 1985; Dujesiefken et al., 2005). It is unknown how much nonconductive wood is formed in novel species because of tapping, and whether their annual stem growths compensate for this loss in hydraulic conductivity.
Objective 4. Determine the palatability of novel syrups. The marketability of novel syrup is directly related to how good it tastes.
The purpose of this project is to increase the resiliency and sustainability of the syrup industry by determining if sap from trees other than maples (Acer spp.), birches (Betula spp.), and walnuts (Juglans spp.) can be harvested and processed into syrup. The goal of this project is to develop a production manual that discusses all aspects of novel (meaning other than maple, birch, and walnut) syrup production from tree species that demonstrate an aptitude for syrup production. This production manual will cover all aspects of sap harvesting and syrup production, including the timing of and environmental conditions that drive dormant-season novel sap flows, optimal tapping depths, and all other relevant harvesting, storage, and processing techniques.
This project will directly benefit syrup producers in two ways. First, tapping different species offers season-extending opportunities. Preliminary work has shown that the timing of sycamore (Platanus spp.) and hickory (Carya spp.) sap flows differ from that of maple – both occur earlier in the year (Moore, unpublished data), and both are purported to yield edible sap that can be processed into syrup (Peterson, 1977). Indeed, birch (Betula spp.) sap runs later than maple sap, and one producer in . Extending the sugaring season will enable producers to benefit from their expertise and expensive infrastructure for more than just the 6- to 8-week maple sugaring season. If two species’ sugaring seasons do not overlap, the same equipment can be used for both species. Second, producing syrup from different species diversifies a producer’s portfolio and reduces exposure to poor years caused by bad weather or pest outbreaks affecting a single species. Sugar maple (Acer saccharum) is very sensitive to drought, pests, and pathogens, which have contributed to sugar maple decline (Bishop et al. 2015, Kosiba et al. 2017, Payette et al. 1996), so diversifying syrup production would be in a producer’s best interest.
This project also has implications beyond people who currently produce syrup. First, farmers are often busy with seasonal farm duties like preparing for crop and livestock production in Feb., Mar., and Apr., when maple, birch, and walnut sap flows, and thus they are often not able to produce these syrups. If novel species have sap flows that occur earlier in the dormant season (such as sycamores and hickories), they would present farmers with the possibility for syrup production that does not overlap with their late winter and early spring tasks. Second, landowners who are interested in syrup production but whose woodlots do not contain significant numbers of sugar maples, paper birches (Betula papyrifera), or walnuts may have the option to produce syrup if it is found that novel species present in their woodlots are viable for syrup production.
Each year, I receive e-mails from syrup producers wanting to make, and from people wishing to purchase, sycamore syrup. I ran a successful birch and sycamore syrup operation (The Crooked Chimney, www.crookedchimneysyrup.com) in Lee, NH, for several years before graduate school, and it is evident that there exists both a strong interest from syrup producers in producing syrup from different species and a strong consumer demand for these syrups. I began producing birch syrup commercially in 2010 as the first commercial birch syrup producer in the continental United States, and at that time, there was no demand for birch syrup because nobody in the region was familiar with birch syrup. After a couple of years of offering free samples at farmers markets, meeting with chefs, and networking with agricultural industry personnel, the media ran stories about birch syrup and our operation, and a remarkable demand for birch syrup developed which still exists today.
Birch syrup commands a much higher price per volume than maple syrup does (Farrell, 2013; Farrell, 2015) because birch sap is much more dilute than maple sap and consequently it requires more processing. It is likely that novel syrups would command a higher price too – preliminary work has shown that sycamore sap is about as dilute as birch sap (Moore, unpublished data). To our knowledge, there are no syrup producers making pure syrup from the sap of trees other than maples, birches, and walnuts. One producer (Paul Hovan, Tunkhannock, PA) taps sycamore trees and produces a blended syrup made from both sycamore and maple sap. One producer (Hickoryworks, Trafalgar, IN) makes syrup from shagbark hickories (Carya ovata) and tulip poplars (Liriodendron tulipifera), but they do not do so by harvesting and processing sap; rather, they harvest the inner bark of these trees, make an extract from it, and sweeten it with cane sugar.
To our knowledge, there is currently no literature available on harvesting sap and producing syrup from novel trees. A handful of accounts exist that discuss the possibility of producing syrup from different species (Farrell, 2013) and that mention that people have consumed sap from different species in the past (Łuczaj, 2008; Łuczaj, 2011; Łuczaj et al., 2014), but these accounts do not include methods for harvesting or processing sap. One paper discusses dormant-season sap flow in novel species (Essiamah, 1980), but this paper makes no reference to sap edibility or to syrup production.
In 2015, 3,434,000 gallons (valued at $125,890,000) of maple syrup were produced in the United States (United States Department of Agriculture, 2016), and birch syrup is an important commercial industry in Alaska and Canada (Cameron, 2001; Maher, 2006). If production methods for novel syrups were developed, it is likely that several sugarmakers would begin to produce novel syrups and that markets would quickly develop for these syrups.
Bishop, D., C.M. Beier, N. Pederson, G.B. Lawrence, J.C. Stella, and T.J. Sullivan. 2015. Regional growth decline of sugar maple (Acer saccharum) and its potential causes. Ecosphere. 6:1-14.
Cameron, M. 2001. Establishing an Alaskan birch syrup industry: Birch syrup – It’s the un-maple!TM. In: Davidson-Hunt, I.; Duchesne, L.C., and Zasada, J.C., editors, Forest communities in the third millennium: Linking research, business, and policy toward a sustainable non-timber forest product sector. Proceedings of the meeting, Kenora, Ontario, Canada, 1-4 Oct. 1999. General Technical Report NC-217, United States Department of Agriculture, Forest Service, North Central Research Station, St. Paul, MN. p. 135-139.
Essiamah, S.K. 1980. Spring sap of trees. Ber. Deutsch. Bot. Ges. Bd. 93:257-267.
Farrell, M. 2013. The Sugarmaker’s Companion: An Integrated Approach to Producing Syrup From Maple, Birch, and Walnut Trees. Chelsea Green Publishing. White River Junction, VT.
Farrell, M. 2015. Weighing the pros and cons of producing birch syrup. Cornell Small Farms Program. Cornell University. Viewed online 17 Apr 2018. <https://smallfarms.cornell.edu/2015/04/06/weighing-the-pros/>.
Kosiba, A.M., P.G. Schaberg, S.A. Rayback, and G.J. Hawley. 2017. Comparative growth trends of five northern hardwood and montane tree species reveal divergent trajectories and response to climate. Can. J. For. Res. 47:743-754.
Łuczaj, Ł. 2008. Archival data on wild food plants used in Poland in 1948. J. Ethnobiol. Ethnomed. 4:4.
Łuczaj, Ł. 2011. Edible wild plants used in Poland from the mid-nineteenth century to nowadays. Ethnobiologia Polska. 1:57:125.
Łuczaj, Ł, M. Bilek, and K. Stawarczyk. 2014. Sugar content in the sap of birches, hornbeams and maples in southeastern Poland. Cent. Eur. J. Biol. 9:410-416.
Maher, K.A.C. 2006. Factors influencing birch sap production in Alaskan birch (Betula neoalaskana Sarg.). University of Alaska-Fairbanks School of Natural Resources and Agricultural Sciences. Viewed online 2 May 2018. <https://www.uaf.edu/files/ces/aknfc/resources/workshops/06FactorInfluenceBirchSap.pdf>.
Payette, S., M.-J. Fortin, and C. Morneau. 1996. The recent sugar maple decline in southern Quebec: Probable causes deduced from tree rings. Can. J. For. Res. 26:1069-1078.
Peterson, L.A. 1977. Edible Wild Plants: Eastern/Central North America. Peterson Field Guides®. Houghton Mifflin Company. New York, New York.
United States Department of Agriculture. 2016. Northeast Maple Syrup Production. United States Department of Agriculture. <https://www.nass.usda.gov/Statistics_by_State/New_England_includes/Publications/Current_News_Release/2016/Maple.pdf>.
During the fall of 2018, sensors for measuring sap flow were built in preparation for the first season of data collection (early Jan. of 2019 through early Apr. of 2019). These sensors were constructed following the protocol of Burgess et al. (2001). In the original proposal for this project, it was stated that sap flow would be monitored at 0.5-cm intervals throughout the entire sapwood in each tree. Since this would require many more sensors than were budgeted for – for example, a basswood with a sapwood depth of 8 cm would require at least 5 sensors (3 depths are measured per sensor) – we realized that this original plan was too ambitious due to time and money constraints. Thus, sap flow will instead be monitored at 6 different depths (0.5, 1.0, 1.5, 2.5, 4.0, and 6.0 cm), which is still much more intensive that most sap flow studies. The sapwood depths at which sap flow will be measured were carefully chosen to provide high resolution at shallower depths while still being able to generate accurate sap flow radial profiles to a depth notably greater than the depth of taps used in the syruping industry (tapholes are generally made to depths of between 3.8 and 5.1 cm). A total of 60 sap flow sensors were built and will be installed in early Jan. of , and sap flow will be monitored continuously until leaf-out occurs in the spring.
As alluded to above, there will be two sensors in each tree. A shorter sensor will measure sap flow at the shallower depths (0.5, 1.0, and 1.5 cm), and a longer sensor will monitor sap flow at deeper depths (2.5, 4.0, and 6.0 cm). These short sensors are widely used in our lab for sap flow data, and thus their design is well-developed. These longer sensors are not. An important component of sap flow sensors is the heater, which sends out a heat pulse every 15 min (the movement of this heat pulse throughout the tree is measured by temperature sensors and used to estimate sap flow). Heaters consist of a resistive nichrome wire to which voltage is applied. For longer sensors, which require longer heaters, it was important to ensure that the amount of heat generated per unit length of sensor was consistent with that of the shorter sensors to ensure that data from short and long sensors can be compared impartially. To accomplish this, a slightly lower voltage (8.3 V instead of 12.0 V) should be applied to short heaters, as long heaters are nearly 1.5 times longer than short heaters. An additional resistor of 23.1 Ω was installed in series with each short heater such that the nichrome portion of the heater (which provides the heat pulse) will receive 8.3 V instead of 12.0 V. Additionally, last year, we verified that sap flux density data from sap flow sensors were closely correlated with sap yield from standard maple tapping practices (r > 0.80; Moore, unpubl. data) and that the sensors performed well in the cold, demonstrating that sap flow sensors can be used during the dormant season to accurately predict sap yield.
During the fall of 2018, 30 study trees (5 trees of each species) were identified in Lee, NH. The 6 species included in the study are American sycamore (Platanus occidentalis), shagbark hickory (Carya ovata), American basswood (Tilia americana), quaking aspen (Populus tremuloides), American beech (Fagus grandifolia), and American hophornbeam (Ostrya virginiana). Healthy trees with full live crowns and whose diameters at breast height are at least 20 cm (for hophornbeams, which do not grow very large, the minimum diameter is 12 cm) were selected. Sap flow sensors will be installed in these selected sample trees in Jan. 2019.
Burgess, S.S.O., M.A. Adams, N.C. Turner, C.R. Beverly, C.K. Ong, A.A.H. Khan, and T.M. Bleby. 2001. An improved heat pulse method to measure low and reverse rates of sap flow in woody plants. Tree Physiol. 21:589-598.