Sap Yield and Sugar Content of Boxelder and Norway Maple Trees in Northern Utah

Sap Yield and Sugar Content of Boxelder and Norway Maple Trees in Northern Utah Jesse Mathews1, Youping Sun1, Paul Harris1, Xin Dai2, Steven Price3, Darren McAvoy4, Michael Farrell1, and Kelly Kopp1 KEYWORDS. Acer negundo, Acer platanoides, maple sap, maple syrup, air temperature ABSTRACT. Maple syrup is a well-known natural sweetener made from the sap harvested from maple trees (Acer sp.). The North American scientific literature regarding maple syrup has predominantly originated in the Northeastern United States and Canada. However, the range of this Holarctic genus extends across the continent and all species produce sap with the potential for syrup production.

This study focuses on two maple species commonly found in Northern Utah, namely the native boxelder (Acer negundo) and the introduced Norway maple (Acer platanoides). Thirty trees of each species were tapped in Cache Valley, UT, USA, on 19 Feb 2022, and measured for daily sap yield and sugar content until the season ended 37 days later on 27 Mar 2022. The same trees were re-tapped on 1 Mar 2023 and taps were removed 41 days later on 10 Apr 2023. Average 2022 sap yields were 22.1 L for boxelder and 7.5 L for Norway maple per tree.

In 2023, average sap yields were 26.4 L for boxelder and 9.3 L for Norway maple per tree. Boxelder trees produced an average sap yield more than double that of Norway maple in both years. Sugar content was similar for both species ranging from 2.2% to 2.8%. Air temperatures were analyzed using data from Utah AgWeather System weather stations nearest to the trees, and air temperature had a significant impact on sap yield. It was found that an average daily air temperature of 0.5 C and a daily air temperature difference of ~10 C with a minimum air temperature close to 25 C and a maximum air temperature of ~6 C was the optimal condition for production. An analysis of the mineral nutrient concentrations in the sap and soil showed no correlation. These findings indicate that there is potential for using Utah’s maple species for syrup production.

M

aple syrup is a popular agricultural commodity produced mainly in Quebec, Canada, and the northeastern United States.

Quebec itself accounts for more than 90% of maple syrup production in Canada and 75% of production worldwide, with most of the remaining 25% being produced in the United States [Crops and Horticulture and Division Agriculture and Agri-Food Canada (CHDAAFC) 2021]. Within the United States, Vermont produced 40% of the country’s maple syrup in 2021, and other top-producing states that year include Maine (14%), New York (17%), Pennsylvania (5%), and Wisconsin (10%) [US Department of Agriculture (USDA), National Agricultural Statistics Service (NASS) 2022]. Sugar maple (Acer saccharum) is the most popular species to tap in these areas because of its higher sugar content (Perkins et al. 2022). Sugar maple does not survive well in Utah, in part because of to higher soil pH compared with its 52

native range (Boettinger 2009). Sugar maples also struggle in excessively dry soils, which occur frequently in Utah (Boettinger 2009; Perkins et al. 2022).

Many maples (Acer sp.) have problems with iron chlorosis in soils with a pH over 7.0, which makes it hard to uptake iron (Kuhns and Koenig 2002). This might be one contributing factor to the lack of historical maple syrup production and cultural familiarity with production in Utah. About 211,714 boxelder (Acer negundo) trees exist in the state that could be tapped (USDA, Forest Service 2022). Similar inventory estimates are not available for Norway maple (Acer platanoides), as they grow primarily in urban settings, but they are widespread and common in both parks and domestic landscapes in Utah. Little information has been published about the sap yield and quality of these maple species in the state.

Although tree characteristics can influence annual sap yield, climate conditions, mostly revolving around air temperature during the sap flow season,

have long been identified as the main factor affecting annual changes in maple sap yields (Duchesne and Houle 2014; Duchesne et al. 2009; Kim and Leech 1985). Sap exudation occurs when winter temperatures fall below freezing at night and rise above freezing during the day, which causes freeze-thaw cycles, creates positive pressure inside the tree, and forces sap out through tap holes because of the lower atmospheric pressure outside the tree (Perkins et al. 2022; Stockie et al. 2022; Tyree 1984). Late winter weather conditions are monitored for daytime air temperatures above freezing that initiate the freezethaw cycle. In addition, observations are often made on a few trees tapped early to monitor sap flow initiation indicating when the remaining trees should be tapped, thereby, beginning the tapping season. As the season progresses toward spring, sap flows either become minimal or the tree begins to break dormancy leading to “buddy” sap with poor quality because of its off-tasting flavor (Camara et al. 2019). Because of the annual variability of seasonal air temperatures, the production season length, number of days of sap flow and collection, and timing can be inconsistent across years, often lasting only 4 to 6 weeks (Giesting 2020). Optimal sap production areas are found at latitudes 43 N, with latitudes within a few degrees above and below having comparable sap yields (Rapp et al. 2019). Northern Utah, generally considered all areas of the state from the Salt Lake area and north, is at a latitude close to 41 degrees north, which falls within the high sap production range. Utah receives less than 33% of the annual precipitation of the high syrup production states mentioned previously [US Department of Commerce, National Oceanic and Atmospheric Administration (NOAA) 2023], and soil moisture has been shown to affect sap yield (Perkins et al.

2022). It also has large areas of either thinly diffused forests or no forest at all, contrasting with densely diffused forests in the northeastern United States (USDA, Forest Service 2023). As mentioned previously, sugar maples do not grow well in Utah, in part because of the differences in soil in the state as compared with soil in the northeastern United States. This is significant because it is known that soil composition does influence sap composition

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(Watterston et al. 1963; Wild and Yanai 2015). Currently, it is unknown how Utah’s climate, season length, and effect of latitude influence sap production within the state. It is also unknown if the soil in Utah will affect the composition of sap produced in the state.

Maple syrup could provide an alternative agricultural product during the off-season for landowners. As of 2017, Utah had 32,495 agricultural producers in the state (USDA, NASS 2017). About 31% of the state’s agricultural sales, totaling $1.84 billion, are from crops, with the remaining 69% from animals (USDA, NASS 2017).

Agricultural producers’ work is highly seasonal, with seasons of intense work and income followed by periods of the opposite. Maple syrup production also follows a seasonal pattern, coinciding with the off-season for many cropReceived for publication 21 Aug 2023. Accepted for publication 13 Oct 2023.

Published online 5 Jan 2024.

1

Department of Plants, Soils & Climate, College of Agriculture and Applied Sciences, Utah State University, 4820 Old Main Hill, Logan, UT 84322, USA

2

Utah Agricultural Experiment Station, Utah State University, 4810 Old Main Hill, Logan, UT 84341, USA

3

Carbon County Extension, Utah State University, 751 East 100 North, Ste 1700, Price, UT 84501, USA

4

Department of Wildland Resources, S.J. & Jessie E. Quinney College of Natural Resources, Utah State University, 5230 Old Main Hill, Logan, UT 84322, USA

This research was supported in part by the US Department of Agriculture (USDA) National Institute of Food and Agriculture (NIFA) Hatch project UTA01381 and UTA01666, USDA Agricultural Marketing Service Acer Access and Development Program (21ACERUT1001-00, 22ACERUT101900), 2021 Utah State University (USU) Extension Grants Program (EXT00099), USU’s Center for Water-Efficient Landscaping, and the Utah Agricultural Experiment Station and approved as journal paper number 9724. We are also grateful for technical assistance from Alyssa Fenstermaker, Abby Nakai, and Macie Sanders, and the Open Access Funding Initiative at USU Libraries for the publication charge. We sincerely thank the Utah Farm Bureau Federation (Sandy, UT), Stokes Nature Center (Logan, UT), Providence City and Smithfield City, UT, and USU’s Landscape Operations and Maintenance. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agencies. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the USDA or the American Society for Horticultural Science and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

Y.S. is the corresponding author. E-mail: youping. sun@usu.edu.

This is an open access article distributed under the CC BY-NC-ND license (https://creativecommons. org/licenses/by-nc-nd/4.0/). https://doi.org/10.21273/HORTTECH05304-23

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producing operations in Utah. This does not consider landowners who are not agricultural producers with access to maple trees either on their own land or on public land, which would greatly increase the number of people who might be interested in producing maple syrup. With existing maple trees ready to be tapped, syrup production could commence immediately. Producing maple syrup during the off-season could serve as a potential source of revenue for these producers and landowners. By exploring the tapping potential of Utah’s maples, it is possible to initiate a maple syrup industry in the state.

Imitation and genuine maple syrups with generic labels are commonly carried by grocery stores throughout the United States; however, there is an increasing demand for locally sourced niche products. The growing demand for local products can be quantified by using the increasing number of farmers markets as a proxy measurement. Farmers markets serve as platforms where local producers gather to sell the products they make or grow, facilitating direct transactions between producers to consumers. According to the USDA Economic Research Service (USDA, ERS 2022), the number of farmers markets has steadily increased by 7% every year since 1994. This upward trend in farmers markets indicates a strong demand for local products including niche maple syrup products. Moreover, local products often command higher prices.

In Northern Utah, 70% of products at farmers markets had higher prices compared with products sold in stores (Curtis et al. 2019). One maple syrup producer in Utah, who worked with the authors in a separate study, sold their bigtooth maple syrup for $2.50 per ounce and completely sold out in an hour (Witt G, personal communication). Although this price is higher than the $0.41 per ounce in local stores (Sam’s Club 2023), there is potentially a demand for local syrup in Utah.

The objectives of this research were to evaluate 1) the sap yield and sugar content of two maple species during the 2022 and 2023 tapping seasons, and 2) the effect of soil mineral nutrients on sap nutrients. We hypothesized that maple species would exhibit different sap yields and sugar content and that mineral content would vary based on soil nutrients and location.

Methods Two years of data were collected using the traditional “bucket and spile” collection method (Mathews et al.

2023). Supplies included 5/16-inch plastic bucket spouts and 5-gallon plastic sap buckets with covers (CDL USA Maple Sugaring Equipment, St Albans, VT, USA). Sixty trees were tapped, with 30 of them being boxelder and 30 being Norway maple. Each tree had a single tap 1.5 inches deep and was drilled on the south side of the tree most convenient for collection.

Three locations were selected, with 10 trees of each species in each location. The locations (Logan, Providence, and Smithfield, UT, USA) were all within Cache Valley, UT, USA, and are spread over an area roughly 20 miles long (Fig. 1).

Year 1 data were collected starting on 19 Feb 2022, the date at which trees were tapped. The season ended on 27 Mar 2022 when the taps were removed because of increasing air temperatures.

The production season’s length for year 1 was 37 d, and sap flowed from the taps an average of 19 d. Year 2 early data were collected starting on 4 Jan 2023 with four trees being tapped (two trees of each species) to monitor for flow initiation. The remaining trees were tapped on 1 Mar 2023 and harvest continued until 10 Apr. The production season for year 2 was 41 d, with sap flowing an average of 24 d. Except for two boxelder trees and two Norway maples, the same 56 trees were used each year. Two boxelder trees and two Norway maple trees from year 1 were inaccessible during year 2, which was an abnormally high snowpack year [USDA, National Resources Conservation Service, National Water and Climate Center (NWCC) 2023] so replacement trees were used during year 2.

After taps were installed, sap yield was measured in milliliters (mL) every day using a graduated cylinder (Globe Scientific, Mahwah, NJ, USA). Any ice in the bucket was added to the cylinder during measurement. A refractometer (HI 96801, Hanna Instruments, Smithfield, RI, USA) was used to obtain a Brix reading, which is a measure of the percentage of sucrose in the sap, on four occasions in year 1. In year 2, sugar content was measured daily along with the sap yield measurements. For example, a reading of 2.3 Brix means that there is 2.3% sucrose in the sap. Brix is a good 53

Fig. 1. Maple trees in Logan, Providence, and Smithfield, UT, tapped in 2022 and 2023. Two boxelder trees and two Norway maple trees were used in 2023 when an alternative was required. These sites are located over a length of 20 miles.

Permission was obtained from local cities where needed to tap trees in their parks and along their streets.

indicator of total sugar content because sucrose constitutes most of the sugar in maple sap (Ball 2007). The Revised Jones Rule of 86 (Perkins and Isselhardt 2013) was used to estimate syrup yield based on the Brix and yield data. The equation used is as follows: S 5 87.1/X 0.32, where “S” is the number of gallons of sap to produce 1 gallon of syrup and “X” is the Brix value. Brix measurements will be represented as a percentage, which indicates the soluble solids concentration.

Sap samples of 50 mL were taken from five trees of each species at each of the three locations, resulting in a total of 30 trees being sampled during year 1. These samples were analyzed for mineral nutrient contents at the USU Analytical Laboratories (USUAL, Logan, UT, USA). In brief, aluminum (Al), arsenic (As), barium (Ba), boron (B), calcium (Ca), chromium (Cr), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), phosphorus 54

(P), potassium (K), silicon (Si), sodium (Na), strontium (Sr), sulfur (S), and zinc (Zn) concentration in the maple sap were determined using nitric/hydrogen peroxide following the protocol described in Gavlak et al. (2005). One milliliter of maple sap was mixed with 6 mL of nitric acid (HNO3) in a digestion tube, which was then subjected to a digestion block for 10 min at 80 C followed by cooling for 20 mins. A total of 2 mL of 30% hydrogen peroxide (H2O2) was added into the digestion tube that was placed again in the digestion block at 130 C for 1 h. The digestion tubes were mixed using a vortex stirrer and then cooled at room temperature. All digestion tubes were brought to a volume of 25 mL. The digest was analyzed using an Inductively Coupled PlasmaOptical Emission Spectrometry (iCAP 6300 ICP-AES; Thermo Scientific, Waltham, MA, USA) and reported as mg·kg 1. Soil samples were collected in the fall of 2022 using a soil probe

(Compact One-Piece Soil Probe; Oakfield Apparatus, Oakfield, WI). These samples were collected from the top 18 inches of soil at the base of the same 30 trees from which the sap samples had been taken. Soil samples were combined into six samples, each of the three locations having one sample for each of the two species. These samples were also sent to the USUAL for mineral nutrient analysis. Copper, Fe, Mn, P, K, S, and Zn are available elements in the soil test reports. At the end of the sap collection season in year 1, circumference at breast height was measured for each tree to determine the diameter at breast height (DBH). Elevation was also recorded for each tree tapped. These measurements were also taken in year 2 for the four new trees mentioned previously. The DBH was 42.6 ± 13.3 cm (mean ± SD) for boxelder and 42.0 ± 8.9 cm for Norway maple, and elevation above sea level was 1428 ± 14.0 m for boxelder and 1409 ± 7.9 m for Norway maple. No sap or soil samples were taken in year 2 for nutrient and mineral analysis. Daily average, maximum, and minimum air temperature data were downloaded after the season concluded from the nearest weather stations located at the Utah Agricultural Experiment Station’s Greenville Research Farm (North Logan, UT, USA) and Evans Research Farm (Providence, UT, USA) and managed by the Utah AgWeather System (Utah Climate Center, Logan, UT, USA). Air temperature differences were calculated using the minimum daily air temperature subtracted from the maximum daily air temperature and represents the fluctuation of air temperatures within a day.

The trees were grouped by location, with 10 trees of each species located at each site in a randomized complete block design, with each location serving as a block. For the statistical analyses, PROC GLIMMIX was used in SAS OnDemand for Academics: Studio (SAS Institute, Cary, NC, USA), with a predetermined significance level of 0.05. A linear mixed model was used to assess the impact of species, year, and location on maple sap yield, soluble solids concentration, and mineral nutrients.

Species, year, their interaction, and location were fixed factors in the model, and tree was the random factor. Year was treated as a repeated factor with compound symmetry covariance structure. Differences between the two species

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Table 1. Estimated daily sap yield, total seasonal sap yield, average % soluble solids, and syrup yield over two tapping seasons.

Season 1 started on 19 Feb 2022 and lasted 37 d until 27 Mar 2022. Season 2 started on 1 Mar 2023 and lasted 41 d until 10 Apr 2023. Season 2 had colder air temperatures, which caused ice to form in the buckets and influenced the yield and sugar content measurements.

Seasonal sap yield (L/tap)i Yr

Species

2022

Boxelder Norway maple P value Boxelder Norway maple P value

2023

Daily sap yield (mL/tap)i

% soluble solids

Estimated syrup yield (gal/tap)i,ii

2.2 ± 0.1 a 2.4 ± 0.1 a 0.14 2.8 ± 0.1 a 2.5 ± 0.1 b 0.03

0.16 ± 0.03 a 0.06 ± 0.01 b 0.0001 0.24 ± 0.04 a 0.07 ± 0.01 b <0.0001

Mean ± SE 462.4 ± 55.0 aiii 220.6 ± 26.7 b <0.0001 598.1 ± 69.8 a 292.6 ± 33.4 b <0.0001

22.1 ± 3.9 a 7.5 ± 1.3 b <0.0001 26.4 ± 4.7 a 9.3 ± 1.6 b <0.0001

1 mL 5 0.0338 fl oz; 1 gal 5 3.78541 L. Syrup yield was estimated following the Revised Jones Rule of 86 (Perkins and Isselhardt 2013) based on Brix and yield data using the equation as follows: S 5 87.1/X where “S” is the number of gallons of sap to produce 1 gallon of syrup and “X” is the Brix value of the sap being used. iii Least squares mean ± standard errors, same letters within year and column denote no significance between two maple species at a 5 0.05. i

ii

analyses. For both mixed model and regression analyses, log transformation was applied to the yearly and daily sap yields to stabilize group variability. Values of zero, meaning no sap flow, were removed before this transformation.

and between the two years were tested and claimed significant if P < 0.05. The effect of air temperature on sap yield was evaluated using regression analyses. Daily average, maximum, minimum air temperatures, and air temperature differences (maximum–minimum) were used, and all showed quadratic functions for the impact on sap yield. Optimal air temperature condition for sap production was calculated from the regression

Results and discussion SAP YIELD. Boxelder consistently had a higher seasonal sap yield than

2022 Temperatures Temperature (oC)

30.0

Start

End

20.0 10.0 0.0 -10.0

Average

Maximum

27-Mar

23-Mar

25-Mar

21-Mar

19-Mar

15-Mar

17-Mar

13-Mar

11-Mar

7-Mar

9-Mar

5-Mar

1-Mar

3-Mar

27-Feb

25-Feb

21-Feb

23-Feb

19-Feb

-20.0

Minimum

2023 Temperatures Temperature (oC)

30.0

End

Start

20.0 10.0 0.0 -10.0

Average

Maximum

10-Apr

8-Apr

6-Apr

4-Apr

2-Apr

31-Mar

29-Mar

27-Mar

23-Mar

25-Mar

21-Mar

19-Mar

17-Mar

15-Mar

13-Mar

11-Mar

9-Mar

7-Mar

5-Mar

3-Mar

1-Mar

-20.0

Minimum

Fig. 2. Daily air temperatures recorded in 2022 and 2023 from the weather stations located at the Utah Agricultural Experiment Station’s Greenville Research Farm (North Logan, UT, USA) and Evans Research Farm (Providence, UT, USA) and managed by the Utah AgWeather System (Utah Climate Center, Logan, UT, USA). [(1.8 × C) + 32 5 F].

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0.32,

Norway maple over the 2 years (Table 1). Across the 2 years, the seasonal sap yield was 24.2 ± 0.5 L and 8.4 ± 0.3 L per tap for boxelder and Norway maple, respectively. The average boxelder sap yield in this study is higher than the average sap yields in two separate studies, which were reported at 13.7 and 9.8 L per tap (Kort and Michiels 1997; Peters et al. 2020). No previously published data exist on sap yield for Norway maples to use as a comparison; however, sap yields per tap of other maple species were found with 5.7 L for red maple (Acer rubrum), 9.4 L for silver maple (Acer saccharinum), and 10.1 L for sugar maple, which are similar to sap yields observed in this study (Peters et al. 2020). It should be noted that in previous research, trees above 30 cm DBH were tapped, and season length was not reported, so a comparison between these studies can be problematic as these can both affect sap yield.

Neither year (P 5 0.11) nor location (P 5 0.22) influenced the total seasonal sap yield. The diameter at breast height and elevation were also found insignificant to affect the seasonal sap yield. It is a well-accepted industry standard that the minimum diameter for tapping trees is 10 inches (Blumenstock and Hopkins 2020). However, in some parts of Canada where boxelder trees are tapped, the minimum tapping diameter for that species is 15 cm (Blouin 1992). In this study, most of the tapped trees exceeded the 10-inches threshold, and all exceeded the 15 cm threshold, with the smallest tree being 18 cm and the largest 67 cm.

The average diameter [42.6 ± 13.3 cm and 42.0 ± 8.9 cm (mean ± SD) for boxelder and Norway maple, respectively] 55

Fig. 3. Comparative regression analysis of daily sap yield and different daily air temperature measurements obtained from the Utah Agricultural Experiment Station’s Greenville Research Farm (North Logan, UT, USA) and Evans Research Farm (Providence, UT, USA) and managed by the Utah AgWeather System (Utah Climate Center, Logan, UT, USA). The air temperature difference was calculated using the minimum daily air temperature subtracted from the maximum daily air temperature and represents the fluctuation of air temperatures within a day. [(1.8 × C) + 32 5 F].

and elevation (1428 ± 14.0 m and 1409 ± 7.9 m for boxelder and Norway maple, respectively) measurements were similar between species, ensuring a reasonable comparison for each species.

Year showed a significant impact on daily sap yield. Specifically, lower yields were observed in 2022 compared with 2023 (P < 0.001). This result is attributed to the variations observed in the daily average, maximum, and

Table 2. Estimated optimal daily air temperatures (average, maximum, minimum, and difference) for boxelder and Norway maple sap production. Air temperatures were obtained from weather stations nearest to the tapping locations.

These weather stations are located at the Utah Agricultural Experiment Station’s Greenville and Evans Research Farms. A comparative regression analysis of the air temperature measurements and sap yield was done to find optimal air temperature conditions for sap flow.

Species

Avg daily air temp ( C)i

Maximum daily air temp ( C)

Minimum daily air temp ( C)

Difference daily air temp ( C)

1.2 0.0

6.2 6.6

6.2 5.1

10.6 8.4

Boxelder Norway maple i

(1.8 × C) 1 32 5 F.

56

minimum air temperatures that were measured throughout the study period (Fig. 2). In addition, the difference in daily air temperatures (maximum – minimum) also impacted daily sap yield. Air temperatures were an average of 2 C lower in 2023 (Fig. 2), and as a result, there was ice in the collection buckets. This could affect yield as water expands by 9% in volume when frozen (Arthur et al. 2023). This was determined to be insignificant as there was most often only a thin layer on the top.

As shown in Fig. 3, optimal air temperature conditions existed to maximize daily sap yield for both boxelder and Norway maple. The estimated optimal conditions for each species are listed in Table 2. In addition, the trends of daily sap yield in response to air temperature measurements for the two species are similar but not identical. In general, 0.5 C is the optimal average air temperature for daily sap yield indicated by the peak of the curves. The best daily sap yield occurs when nighttime air temperatures are around 5 C and daytime air temperatures are 6 C, having an air temperature difference of 10 C. This is consistent with freezing air temperatures at night and above freezing air temperatures during the day driving sap flow (Perkins et al. 2022; Stockie et al. 2022; Tyree 1984).

SUGAR CONTENT. Sugar content was similar between species, with boxelder having an average of 2.46 ± 0.08% and Norway maple having 2.45 ± 0.09%. This observed boxelder sugar content falls within previously reported ranges of 1.3% to 3.5% (Blouin 1992; Conger 2007; Ellman and Ellman 2018; Peters et al. 2020). Norway maple sugar content was previously reported as 3.1%, which is higher than what was observed in this study (Blouin 1992). Other previous research reported sugar contents of 1.4% for red maple, 1.6% for silver maple, and 1.5% to 2.5% for sugar maple (Blouin 1992; Peters et al.

2020). The sugar content values found in this study are within the upper range of sugar content for maple species reported elsewhere. Soil nutrient contents are known to affect sap sugar content and could influence the sugar content in Utah as well (Watterston et al. 1963; Wild and Yanai 2015).

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Table 3. Least squares means of mineral nutrients and pH in the sap collected from boxelder and Norway maple trees. Sap samples were taken from thirty trees tapped, fifteen trees of each species, analyzed by the Utah State University Analytical Laboratories (Logan, UT, USA).

Boxelder

Mean ± SE (mg·kg21)iii

Itemi

iv

Ca Fe K Mg Mn Na P S Zn As Al B Ba Cr Cu Si Sr pH

DRIii

Norway maple

110.99 ± 16.15 0.93 ± 0.11 191.90 ± 30.66 15.25 ± 1.13 0.25 ± 0.05 0.35 ± 0.35 20.55 ± 3.93 2.21 ± 0.38 0.01 ± 0.01 0.02 ± 0.02 0.67 ± 0.09 0.08 ± 0.03 0.11 ± 0.03 0.01 ± 0.00 0.02 ± 0.01 6.27 ± 0.85 0.20 ± 0.04 6.65 ± 0.01

183.17 ± 17.85 0.82 ± 0.27 170.85 ± 11.63 23.72 ± 4.76 0.97 ± 0.15 4.37 ± 2.73 6.29 ± 1.36 1.22 ± 0.46 0.27 ± 0.01 0.01 ± 0.00 0.58 ± 0.17 0.11 ± 0.01 0.19 ± 0.05 0.01 ± 0.01 0.02 ± 0.01 36.08 ± 2.89 0.32 ± 0.08 6.53 ± 0.00

P value

mg/d

0.10 0.57 0.39 0.18 0.04 0.28 0.03 0.08 <0.01 0.47 0.49 0.33 0.08 0.46 0.80 0.01 0.16 <0.0001

1,000 8 3,400 400 2.3 1,500 700 N/Av 11 N/A N/A N/A N/A 0.035 0.9 N/A N/A N/A

i

Calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), phosphorus (P), sulfur (S), zinc (Zn), arsenic (As), aluminum (Al), boron (B), barium (Ba), chromium (Cr), copper (Cu), silicon (Si), and strontium (Sr). ii DRI 5 daily recommended intakes (National Institutes of Health, National Library of Medicine 2023). iii 1 mg·kg 1 5 1 ppm, 1 mg 5 3.5274 × 10 5 oz. iv Standard errors represent 15 measurements (three locations × five samples per species at each location). v Not available.

Location was found to impact the sugar content of sap, with Logan having significantly lower sugar content at 2.17± 0.08% than Providence and Smithfield did at 2.57 ± 0.10% and 2.65 ± 0.10%, respectively. There also were differences in sugar content between the years, with 2023 having a higher sugar content (P < 0.001).

It is worth noting that the average air temperature in 2023 was 2 C lower compared with 2022 and was more frequently below the freezing point (Fig. 2). As a result, ice sometimes formed in the buckets. When water freezes, solutes such as sugars freeze last due to a phenomenon known as freezing point depression (HyperPhysics 2023; Purdue Chem 2023). This could have led to the concentration of sap and potentially higher sugar content readings in 2023. It is challenging to control or quantify the extent to which this affected the sugar content reading. Sugar content could have also been affected if sap evaporated and concentrated. Small changes in sugar content can have a substantial influence on the amount of syrup produced. For example, a 2.0% sugar content requires 43 gallons of sap to

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produce 1 gallon of syrup, whereas 2.4% only requires 35 gallons (Perkins and Isselhardt 2013).

Although sap yield measurements were taken in milliliters, syrup production is reported in gallons of syrup produced per tree (every tree had only one tap), as the USDA reports maple syrup production in gallons of syrup produced per tap (USDA, NASS 2022). The impact of species is consistent regardless of year (Table 1). Boxelder had consistently higher syrup production at 0.16 and 0.24 gallons per tap in 2022 and 2023, respectively, almost triple the syrup production of Norway maple. Not surprisingly, year affected the syrup production.

The year 2023 resulted in significantly higher syrup production than 2022 (P 5 0.005) due to the higher sugar content in sap obtained and the longer tapping season in 2023. Comparing these results with the 2022 and 2023 average syrup production in the United States of 0.353 and 0.311 gallons per tap (gal/tap), respectively (USDA, NASS 2022), boxelder trees had lower production with an average of 0.19 ± 0.03 gal/tap. As mentioned earlier, the colder conditions in 2023 resulted in more ice formation in the buckets, which did

affect sugar content readings. Consequently, this affected the amount of syrup produced per tree as well. For future studies, it could be beneficial to separate the ice and sap in a bucket and compare the sugar content of each component individually, and when they are combined. This would help quantify the impact of ice in the sap on sugar content readings.

NUTRIENTS. Except for Mn (P 5 0.04), P (P 5 0.03), Zn (P < 0.01), Si (P 5 0.01), and the pH of the sap (P < 0.001), there were no significant differences in the nutrient composition between the two species (Table 3). Boxelder contained lower concentrations of Mn (0.25 mg·kg 1), Zn (0.01 mg·kg 1), and Si (6.27 mg·kg 1) as compared with Norway maple (0.97 Mn mg·kg 1, 0.27 Zn mg·kg 1, and 36.08 Si mg·kg 1, respectively). However, Boxelder had a higher P concentration (20.55 mg·kg 1) than Norway maple (6.29 mg·kg 1).

Boxelder sap had a pH of 6.65, and Norway maple sap had a pH of 6.53. Several beneficial nutrients (Harvard Health Publishing 2021) were found in the sap, including Ca, Fe, K, Mg, Na, P, S, and Zn. Although these are beneficial to human health, one cup of sap would contain less than 1% of the daily recommended intake for these nutrients [National Institutes of Health (NIH), National Center for Biotechnology Information 2019].

The heavy metal As was contained in the sap at a 0.01 to 0.02 mg·kg 1 concentration, which is well below the unsafe threshold of 3 mg·kg 1 (US Food and Drug Administration 2023).

At this concentration, a person would have to consume 150 L of sap to reach unsafe levels. Other potentially toxic metals found in the sap were Cr, Cu, and Mn. These metals are not toxic or a problem in the sap as Cr is only toxic in a form that is a byproduct of industrial manufacturing [NIH, Office of Dietary Supplements (ODS) 2021a], Cu is only toxic at levels above 10 mg (NIH ODS 2022), and Mn is only harmful in amounts over 11 mg (NIH ODS 2021b). A person would have to consume 500 L of boxelder or Norway maple sap to reach the toxicity level for Cu, and 44 and 11 L of sap to reach the toxicity level for Mn for boxelder and Norway maple, respectively.

No previous studies have reported the nutrient compositions of boxelder and Norway maple sap, but nutrient compositions of sugar maple sap have 57

Table 4. Correlation coefficients (r) and corresponding P values for copper, iron, manganese, phosphorus, potassium, sulfur, and zinc concentrations in both sap and soil samples. Thirty sap samples were taken from two maple species, 15 trees of each (five trees at each location × three locations), and the mean values of five sap samples each location were used for correlation. Soil samples were taken from those same 30 trees and combined into six samples where each of the three locations had one sample for each of the two species. r P values

Copper

Iron

Manganese

Phosphorous

Potassium

Sulfur

Zinc

0.26 0.63

0.48 0.34

0.27 0.61

0.87 0.03

0.59 0.22

0.10 0.86

0.11 0.84

been published. Nutrients in the sap of sugar maples, listed from highest to lowest concentration, are K, Ca, Mg, and Mn (Lagac e et al. 2015; Perkins and van den Berg 2009). The sap analyzed in this study generally followed this trend, although there were notable differences in nutrient compositions.

For example, Norway maple exhibited a higher average concentration of Ca than that of K, and both K and Ca concentrations in the sap are higher than those reported in sugar maples (Lagac e et al.

2015; Perkins and van den Berg 2009).

The other nutrients are similar in amount, although neither study reported finding any concentration of As in the sap.

The samples revealed that both sap and soil shared the following nutrients: Cu, Fe, Mn, P, K, S, and Zn. Correlation analyses indicate that there is no correlation between sap and soil nutrient levels for all available elements, except for P (correlation coefficient 5 0.87, P 5 0.03, Table 4). It is worth noting that correlation coefficients are 0.59 for K and 0.48 for Fe, but none is detected to be statistically significant.

This is likely due to the large variability usually observed in soil measurements.

Conclusions From the 2-year study, it can be concluded that boxelder is a more productive species compared with Norway maple, with more than double the sap yield. The sugar content, measured in Brix, was similar for both species, ranging between 2.2% and 2.8%, but these readings were affected by ice in the buckets during the second year of the study. Boxelder trees therefore have significantly more potential for syrup production compared with Norway maple. Anecdotally, the syrup produced from each species has a slightly different taste, and producing syrup from each species could be beneficial for that reason. Air temperature had 58

a significant impact on sap yield. Average daily air temperature of 0.5 C and a daily air temperature difference 10 C with maximum air temperature of 6 C and minimum air temperature close to 5 C were correlated with higher daily sap yields. New producers wishing to sell maple syrup as a new income source can focus on tapping boxelder trees as they will produce more syrup for the same amount of effort. These findings show that the native maple species boxelder growing throughout Utah could provide a feasible new source of maple syrup.

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